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Copyright © 1994-2008
Reproduction of this document in whole or in part is permitted if both of the
following conditions are satisfied:
1.This notice is included in its entirety at the beginning.
This document describes techniques for the testing of capacitors using
a multimeter without a capacitance test mode. Information on safe
discharging of high value or high voltage capacitors and a discharge
circuit with visual indication of charge and polarity is also included.
There is also general information on capacitors, capacitance and ESR meters,
and other related topics.
Where equipment is AC line connected or uses high voltages, special
precautions are required both for personal safety and to prevent damage to
circuitry from careless actions. In addition to the specific safety issues
with respect to capacitors discussed below, read, understand, and follow the
recommendations provided in the document:
Capacitor Testing Precautions
WARNING: make sure the capacitor is discharged! This is both for your safety
and the continued health of your multimeter.
A pair of 1N400x diodes in parallel with opposite polarities may help protect
the circuitry of a DMM. Since a DMM doesn't supply more than 0.6 V generally
on ohms ranges, the diodes will not affect the readings but will conduct should
you accidentally put the meter across a charged cap or power supply output.
They won't do much with a charged 10 F capacitor or high current supply where
you forgot to pull the plug but may save your DMM's LSI chip with more modest
goof-ups.
This approach cannot be used with a typical analog VOM because they usually
supply too much voltage on the ohms ranges. However, my 20 year old analog
VOM has something like this across the meter movement itself which has saved
it more than once.
CAUTION: For this and any other testing of large capacitors and/or capacitors
in power supply, power amplifier, or similar circuits, make sure the capacitor
is fully discharged or else your multimeter may be damaged or destroyed!
However, a VOM or DMM without capacitance ranges can make certain types of
tests.
For small caps (like 0.01 uf or less), about all you can really test is for
shorts or leakage. (However, on an analog multimeter on the high ohms scale
you may see a momentary deflection when you touch the probes to the
capacitor or reverse them. A DMM may not provide any indication at all.)
Any capacitor that measures a few ohms or less is bad. Most should test
infinite even on the highest resistance range.
For electrolytics in the uF range or above, you should be able to see
the cap charge when you use a high ohms scale with the proper polarity - the
resistance will increase until it goes to (nearly) infinity. If the capacitor
is shorted, then it will never charge. If it is open, the resistance
will be infinite immediately and won't change. If the polarity of the
probes is reversed, it will not charge properly either - determine the
polarity of your meter and mark it - they are not all the same. Red
is usually **negative** with (analog) VOMs but **positive** with most
DMMs, for example. Confirm with a marked diode - a low reading across
a good diode (VOM on ohms or DMM on diode test) indicates that the positive
lead is on the anode (triangle) and negative lead is on the cathode (bar).
If the resistance never goes very high, the capacitor is leaky.
The best way to really test a capacitor is to substitute a known good one.
A VOM or DMM will not test the cap under normal operating conditions or at its
full rated voltage. However, it is a quick way of finding major faults.
A simple way of determining the capacitance fairly accurately is to build
an oscillator using a 555 timer. Substitute the cap in the circuit and then
calculate the C value from the frequency. With a few resistor values, this
will work over quite a wide range.
Alternatively, using a DC power supply and series resistor, capacitance
can be calculated by measuring the rise time to 63% of the power supply
voltage from T=RC or C=T/R.
The best technique depends on what the cap is used for. A lot of
electrolytics are said to be "leaky" when they are really partially
open and just not doing their job. Electrolytics that are actually
electrically leaky are not as common. You can take each capacitor out of
circuit and test it with a cap checker or even a VOM, but in-circuit
testing is faster. I don't like to grab for a soldering iron unless I'm
pretty sure the part is bad. Time is money.
I first do a visual inspection and see if any electrolytics are
bulging (they -are- leaky and usually get hot), or physically leaking
(corrosion around terminals). Bulging caps in a switching power supply
are a dead giveaway, but can point to leaky diodes as well. Next, if the
unit will power up, I look for signs of filter caps open... hum bars in
picture, hum in audio, flickering displays, low B+ but nothing gets hot,
etc. You can tell quite a lot by just being observent and a makling a few
simple checks. Try all controls and switches... you may get other clues.
What works and what doesn't?
If you have an obvious fault... like a reduced vertical scan on a TV
set or monitor for example, to find the cap that is starting to open up,
you can bridge each of them with another cap, one at a time and see if
it corrects the problem. (Experience has taught me that bad electrolytics
will not -usually- kill vertical sweep completely.) In a TV set that is
several years old or more, there could be more than one cap dried out
(open). Check them all.
"Popping" filters (as it used to be called) by bridging the original
with a like value is not good practice with solid state electronics. The
shock to a live circuit is likely to damage other components, or it could
shock the circuit into working again... for awhile. Then you get to sit
there like a fool and wait for it to act up again... minutes or weeks
later. For small electrolytics, I use a trick of bypassing each one with
a small 0.1 to 0.47uF capacitor while the set is running. If I see -any-
change in the performance, I KNOW the original is not doing its job
(greatly reduced in value or open). Of course if you hit the timing caps,
it will upset the vertical oscillator a bit... that's normal. For bigger
electrolytics like the one used to feed the yoke or power supply main
filters, the only effective way to check them is by substitution with the
same or larger capacitance. Turn the set off, connect the new cap into
the circuit and power it up again.
As I stated before, leaky caps are actually quite rare... but it
does happen. They usually upset a circuit a lot more than open ones.
Things tend to get hot quickly if the cap is a filter in a power supply.
Shorted tantalums and electrolytics in power supplies can literally
explode. Obviously, leaky caps must be removed from the circuit to
substitute them for test purposes.
Most of the other types of small capacitors: mylar, disc ceramic,
etc. are pretty rugged. It is rare indeed to find them bad. It happens
just often enough to keep a tech humble.
All an ohm-meter tells you is if the cap is shorted or not if it is an
electrolytic of fairly large value it can tell you if a cap is open. I am a
tech in a large industrial controls company in the factory service center. We
consider any electrolytic cap to be suspect if it's code date is over five
years old. We have a Fluke 97 and it is useless for in circuit tests. All a
meter like a Fluke 97 can tell you is if the Cap is on the way to being open
from electrolyte loss or if it is shorted. Actually not all you need to
know. Several other facts you need to know are what is the conductance
(internal leakage resistance), it sometimes varies with voltage. You also
need to know what a caps power factor is in some cases. That is its ability to
pass A.C. This is especially important in computer equipment that has to pass
harmonics and noise to ground. Switching power supplies like are found in
almost all PC's these days use high frequency voltage converters to regulate
voltage. The harmonics and noise produced by this rapid switching heats DC
filter caps and causes them to loose moisture from their imperfect seals. This
effect causes the capacitor to gradually open or drop in capacitive value.
If you are talking about other types of capacitors you can test their value
with a meter but I have seen caps that look good with a meter but break down
under voltage. Special cap meters exist that test all these parameters and let
you judge whether the cap is good or not but the best test short of that is
to replace the cap and see if it works or not. Feel free to ask if that isn't
what you wanted to know.
Actually sometimes the best test is to use a oscilloscope to look at what the
cap is doing in the circuit.
Many capacitance meters don't test anything else but are probably more
accurate than a cheap DMM for this purpose. A meter of this type will
not guarantee that your capacitor meets all specifications but if it tests
bad - very low - the capacitor is bad. This assumes that the test was made
with the capacitor removed (at least one lead from the circuit - otherwise
other components in parallel can affect the readings.
To more completely characterize a capacitor, you need to test capacitance,
leakage, ESR, and breakdown voltage. Other parameters like inductance aren't
likely to change on you.
ESR testers, which are for good for quick troubleshooting, are designed to just
measure the Equivalent Series Resistance since this is an excellent indicator
of the health of an electrolytic capacitor. Some provide only a go/no go
indication which other actually display a reading (usually between 0.01 and
100 ohms so they can also be used as low-ohms meters for resistors in
non-inductive circuits). See the section: What is ESR and
How Can It be Tested?.
Note: always place the test probes on the capacitor terminals themselves if
possible. Any wiring between your meter and the capacitor may affect the
readings. Although your user manual may state that you can test capacitors
in-circuit, other components in parallel with the capacitor can screw up
the readings - usually resulting in an indication of a shorted capacitor or
excessively large uF value. Removal is best. Unsoldering only one of the
pins is adequate if you can isolate it from the circuit.
Substitution is really the best approach for repair unless you have a very
sophisticated capacitance meterd.
The March 1998 issue of Popular Electronics has plans for a digital
capacitance tester with a range from 1 pF to 99 uF.
The May 1999 issue of Popular Electronics has plans for an "Electrolytic
Meter" which will accurately measure the capacitance and allow the
determination of some of the other characteristic of large value capacitors -
up to several hundred thousand uF. This is basically a time constant based
tester using a constant current source.
First, you need an AC ripple current source. Then, you tune to the frequency
of interest (120 Hz for rectifier power supply filter capacitors is usual) and
apply both the AC current and a DC voltage bias. Measure the phase shift
between the current and the voltage (for a perfect capacitor, this is 90
degrees) and measure the induced voltage (for a perfect capacitor, this
is I*2*pi*f*C).
Take the tangent of the difference of the phase shift and 90 degrees. (This
is 'tan(delta)' and appears on the spec sheet for the capacitor...)
Then remove the AC, and crank the DC bias up to the voltage surge rating;
measure leakage current. Ramp the DC bias down to the working voltage rating;
measure leakage current.
Raise temperature and repeat the capacitance, phase shift, and working-voltage
measurements at the max temperature the capacitor is rated for.
Yes, it DOES sound rather elaborate, but that's the test that the
manufacturers use.
This doesn't mean that every one of the 250 capacitors in your TV needs to be
discharged every time you power off and want to make a measurement. However,
the large main filter capacitors and other capacitors in the power supplies
should be checked and discharged if any significant voltage is found before
touching anything - some capacitors (like the high voltage of the CRT in a
TV or video monitor) will retain a dangerous or at least painful charge for
days or longer!
A working TV or monitor may discharge its caps fairly completely when it
is shut off as there is a significant load on both the low and high voltage
power supplies. However, a TV or monitor that appears dead may hold a charge
on both the LV and HV supplies for quite a while - hours in the case of the
LV, days or more in the case of the HV as there may be no load on these
supplies.
The main filter capacitors in the low voltage power supply should have
bleeder resistors to drain their charge relatively quickly - but resistors
can fail. Don't depend on them. There is no discharge path for the
high voltage stored on the capacitance of the CRT other than the CRT beam
current and reverse leakage through the high voltage rectifiers - which
is quite small. In the case of old TV sets using vacuum tube HV rectifiers,
the leakage was essentially zero. They would hold their charge almost
indefinitely.
(From: Edwin Winet (ewinet@softwareresearch.org).)
Some of us work in areas where capacitors are huge, unusual or sometimes both.
Many people believe that only "big" capacitors can kill you, knock you
across the room, blow a hole in you, or get your attention. Here are a
couple of comments:
When a capacitor is safely discharged, do not stop there. Some capacitors,
due to their ability to leak---are "dead" after being safely discharged with
a "bleeder resistor" of the right value for the job. Using a resistor that
is under-rated - wattage-wise - can result in the bleeder going open circuit
DURING a discharge sequence LEAVING some energy! High voltage capacitors, or
worse yet, high energy-high voltage capacitors require correct wattage AND
correct resistance to be bled safely. Also, high microfarad low voltage
capacitors can vaporize a screwdriver and spray metal in your eyes. (Adequate
voltage margin is also essential for resistors used in high voltage circuits.
--- Sam.)
Certain types of capacitors are made of VERY good materials, which can hold a
charge for YEARS! Putting away charged capacitors of this type is an
invitation to disaster!
Low inductance capacitors that are used in energy pulse circuitry, many times
are of the oil-filled high energy/high voltage type. This type can give a
MOST un-pleasant surprise AFTER it has been completely drained by a safe
bleeding technique. After the capacitor has been bled, IMMEDIATELY short
it, from terminal to terminal AND to the external metal can (if applicable)!!!
These capacitors RE-charge from their internal fluid and can STILL deliver a
lethal, as they "recover" a certain amount of energy! this type of
capacitor, or any capacitor of any high (enough) energy value MUST be LEFT
shorted.
Be particularly leery of any capacitor with a broken off lead that is sitting
in a drawer! Sometimes, these units break off during testing and don't get
thrown out - but remain charged - to kill or shock years later.
Lastly, the word "electrocution" is used in many high voltage device writings.
That's bad, because it was only intended for the "electric chair", short
for electro + execution.
Then check with a voltmeter to be double sure. Better yet, monitor
while discharging (monitoring is not needed for the CRT - discharge is
nearly instantaneous even with multi-M ohm resistor).
Obviously, make sure that you are well insulated!
Clip the ground wire to an unpainted spot on the chassis. Use the discharge
probe on each side of the capacitor in turn for a second or two. Since the
time constant RC is about 0.1 second, this should drain the charge quickly
and safely.
Then, confirm with a WELL INSULATED screwdriver across the capacitor
terminals. If there is a big spark, you will know that somehow, your
original attempt was less than entirely successful. At least there will
be no danger.
DO NOT use a DMM for this unless you have a proper high voltage probe.
If your discharging did not work, you may blow everything - including
yourself.
Reasons to use a resistor and not a screwdriver to discharge capacitors:
This discharge tool will keep you safely clear of the danger area.
A visual indication of charge and polarity is provided from maximum input
down to a few volts.
The total discharge time is approximately:
WARNING: Always confirm discharge with a voltmeter before touching any high
voltage capacitors!
For the specific case of the main filter caps of switchmode power supplies,
TVs, and monitors, the following is quick and effective.
(From: Paul Grohe (grohe@galaxy.nsc.com).)
I've found that a 4 watt 'night light' bulb is better than a simple resistor
as it gives an immediate visual indication of remaining charge - well down to
below 10 V.
Once it stops glowing, the voltage is down to non-deadly levels. Then leave
it connected for a little while longer, and finish it off with the `ole
screwdriver.
They're cheap and readily available. You can make dozen 'test-lamps' out of an
old 'C7' string of Christmas lights (`tis the season!).
Editor's note: where a voltage doubler (or 220 VAC input) is involved, use two
such bulbs in series.
(From: Dave Talcott (75711.1537@compuserve.com).)
I built the capacitor discharge tool. I had all the parts to hand except
for the series resistor, for which I used a 2 watt axial unit, since the power
dissipation is not critical. I decided to package it in probe form for
convenience. Except for the series resistor, which lives in a counterbore,
everything is surface mounted and communicates through a LOT of cross-drilled
holes. A piece of heat-shrink tubing holds everything in place. The only
tricky part was making two small recesses to locate the LEDs. The probe tip is
a short piece of solid copper wire salvaged from some Romex house wiring and
ground to a point.
(From: Ian Field (ionfieldmonitors@ic24.net).)
The version of the checker that I have, also contains a miniature 12 V
battery for continuity checking - any resistance less than about 22K will
produce some glow. It's handy for quick checks of semiconductor junctions -
in general if it produces a slight glow it's leaky, but transistor B/E
junctions have an inherent zener voltage, so there is usually some glow.
Also schottky-barrier diodes give a reverse leakage glow - this does not
mean they're faulty, check the Vf with the diode-check on a DMM before
binning! Any zener diode above 10-11 V can be given a quick test for S/C,
lower Vz will produce some glow - again check Vf before binning.
These checkers are getting hard to obtain, most of the component stockists
here only carry vastly over complicated (and expensive) versions with
built-in measurement computer and LCD - these wouldn't last 5 min's around
flyback circuitry! Some Automotive accessory shops have a simpler version
with no battery - always check that it's stated to be capable of measuring
AC or DC at 4 to 380 V before parting with money! The internal circuit should
contain the LED's, a 15 ohm resistor to limit the maximum surge current when
the PTC is cold and the special PTC film-thermistor. The battery can be
added with a button from a VCR front panel - but don't blame me if you kill
yourself because you didn't insulate the added components properly! There is
a more complicated non-battery version with 2 LED's close to the front of
the handle to indicate polarity and a row of LED's along the length of the
handle to indicate the voltage-range. This version contains 2 special PTC's
and a discrete-transistor bargraph circuit - there might be room to add a
battery inside the case. As for the special PTC this is the only place I've
seen them - one possibility that might be worthy of looking into is the
Siemens PTC SMPSU startup thermistor for TDA4600 control chips, this usually
has a series resistor of at least 270 ohms and is more likely to turn-up in
European TV set's, but I have seen it in early Matsushita IBM displays and a
few others (possibly Tandon) the PTC thermistor is always blue and looks
like a very-miniature copy of the Philips white-plastic PTC degauss
thermistor.
When troubleshooting electronic equipment, electrolytic capacitors, in
particular, may degrade resulting in a significant and unacceptable increase
in ESR without a similar reduction in uF capacity when measured on a typical
DMM's capacitance scale or even a cheap LCR meter.
Here are some Web sites that discuss ESR testing in some detail and some
include complete information on building your own ESR meter:
There are commercial ESR meters and kits available ranging from $50 to $200
or more. Here are a couple of sites to check out:
There devices can generally be used to measure really low resistances of
non-inductive devices or circuits as well (they use AC so inductance would
result in inaccurate readings). Since their lowest range is at least 10
times better than a typical DMM (1 ohm full scale - 0.01 ohm resolution),
they can even be used to located shorted components on on printed circuit
boards.
Note: always place the test probes on the capacitor terminals themselves if
possible. Any wiring between your meter and the capacitor may affect the
readings. While usually not a problem, very low resistance components in
parallel with the capacitor may result in a false negative indication - a
capacitor that tests good when in fact its ESR is excessive.
(From: Larry Sabo (ac274@FreeNet.Carleton.CA).)
I find my ESR meter invaluable for finding high ESR caps, and have never
seen a shorted cap that hadn't exploded. It's such a pleasure to zip
through the caps in a power supply that's duff and find the ones that have
had it, all without touching the soldering iron.
There have been days I wish I had the LC102 for it's leakage measuring
capability, but in my limited experience the 10% figure seems high. The
LC102 commends itself for the inductance ringer, too, but you sure pay a
premium. I'll build Sam's gizmo first.
BTW, I built my ESR meter from a kit purchased from Dick Smith Electronics
in Australia, for $A 52.74 + $A 25.00 for delivery. It took about 8 hours
to assemble, but I'm a fuss-ass.
Before I bought my ESR meter I too wondered--what exactly did it measure?
Nevertheless, having heard so much about the meter, I went ahead and bought
one. It works, and that's the real bottom line.
A recent question about what exactly in being measured (DF or Q) piqued my
interest again. I think I have the answer -- 'think' being the operative
word. Here's my interpretation.
In summary, the ESR is indeed related to Dissipation Factor (DF), but it is
not the same. A DF measuring device might not as readily identify a bad
capacitor as does the ESR meter because the reading varies and is not direct,
as described below.
Capacitors may be thought of as having pure capacitance (C) and some pure
resistance (R), the two being in series. An ideal capacitor would have only
C, and no R. However, there are the leads and plates that have some
resistance and constitute real R. Any R in series with C will reduce the
capacitor's ability to pass current in response to a varying applied voltage,
as in filtering or DC isolation applications, and it will dissipate heat which
is wasteful and could lead to failure of the component. As with ESR, a lower
DF (or higher Q, it's inverse) may be equated with better performance, all
other things being equal.
Now I get a bit more mathematical, but only using basic electronic theory and
formulas so I hope most will be able to follow this.
DF is defined as Rc/Xc, the ratio of the R in the capacitor (Rc) to the
reactance of the capacitor (Xc). The higher the Rc, the higher the DF and the
"poorer" the capacitor. So far so good.
The reactance (Xc) is a function of frequency. Xc=1/(2*pi*f*C). So, as the
frequency goes up, Xc goes down. Now look back at the formula for DF. DF is
an inverse function of Xc. As Xc goes down, DF goes up, and vice-versa. So
DF varies proportionately with frequency.
Here's an example using the ubiquitous 22 uF, 16 V electrolytic that seems to
be at fault too often in many switched mode power supplies.
At 1000 Hz, this capacitor has an Xc of 7.2 ohms. If the series Rc is only
0.05 ohms (pretty good), then the DF is 0.0069.
At 50,000 Hz, this same capacitor would have an Xc of only 0.14 ohms. At this
frequency, the DF is 0.36, again good.
Now, change the Rc from 0.05 to 25 ohms. At 1000 Hz, DF = 3.4. At 50,000 Hz,
DF = 178.
So we see that DF is a function of the test frequency. The higher the
frequency, the higher the DF. DF is a measure of the capacitor "quality", but
the figure is valid only at the frequency of the test. (A good capacitor,
with an ideal Rc of zero, will have a DF of zero regardless of frequency.)
DF can indeed be used to identify a bad capacitor, but the user must interpret
the level of measured DF that would indicate a bad component. Any 'go/no go'
tables of DF values would be valid only at the specified frequency. As an
alternative, the user can calculate the Rc by first measuring both DF and C,
and then, knowing the test frequency, determine if the Rc is
excessive. (Rc=DP*Xc).
The ESR meter measurement system, however, does not appear to be a function of
Xc. It measures the voltage across the capacitor resulting from the
application of a very short pulse of current. This short pulse is not enough
to charge the capacitor so the voltage being measured across the capacitor's
leads is primarily a function of Rx, which is not frequency sensitive. And,
with the 'tables' of typical ESR (=Rc) that is provided with the ESR meters I
have seen, there is no need to do any further calculations.
The ESR meter is not going to be reliable with very small capacitors. In this
case, they will become more fully charged by the applied current at the time
the meter samples the voltage. Even if the Rc is an ideal zero ohms, the meter
will now read the voltage built up on the capacitor and interpret it as a very
high (possibly off-scale) ESR. Thus its advantage, and main purpose, is in
testing electrolytics which tend to be larger value capacitors.
(Note: The inability of the ESR meter to test low value capacitors is true
only if the meter does not distinguish between in-phase and quadrature
voltages, and it does not. If it did sense only the in-phase voltage that is
produced across Rx (i.e. in-phase with the applied current), then it would not
be sensitive at all to the delayed (minus 90 degrees) voltage built up on the
capacitor's plates.)
All testing I have done with small capacitors (less than 0.001 uF) seems to
suggest that the (Bob Parker) ESR meter is not phase discriminating and Bob
Parker has confirmed this. This is not a great disadvantage. The objective
of the ESR meter is to identify capacitors that have gone bad. This is more
the case with electrolytics where the dielectric compound tends to dry up.
Smaller capacitors usually are not electrolytic and therefore tend to be
relatively stable. Faults in the latter (e.g. ceramic, mica, polystyrene)
are more likely to be open, shorted, or leaky, all of which will be detectable
by capacitance or resistance measuring devices.)
(From: Roy McCammon (rbmccammon@ieee.org).)
Note that "equivalent series resistance" is not necessarily the same as "series
resistance".
"Series resistance" is just the resistance in series with the capacitance.
This is what most of the descriptions have delt with, and with high currents
and frequencies as you tend to see in a switchmode supply, the "true series
resistance" is just what you want to know.
"Equivalent series resistance" is the resistance that you would have to place
in series with a pure capacitance to produce the same loss. It may be
frequency dependent. A cap with a resister in parallel has an esr. At a
single frequency, you cannot tell the difference between a cap with a parallel
resister and a cap with a series resister. For example, at 100 Hz, a 1 uF and
10 ohm in series has a reactance of 10 + J1591, as does a 1 uf in parallel with
253K, hence both have an ESR of 10 ohms.
You need to know just what your meter is doing. Its best that the measurement
relate to your use.
(From: Pete Culf (pete@topden.com).)
Here are a couple of basic analog ESR meter schematics:
Mark Zenier (mzenier@eskimo.com) has an
ESR
Meter Schematic which is about as simple as it gets.
The ultimate in cheap if you have an oscilloscope would be:
99 Cent ESR Test
Adapter.
(From: Ron Black (ron.black@pstbbs.com).)
An inexpensive way (for the cost of a resistor) to measure the ESR of a
capacitor is to apply a squarewave signal through a resistor in series with
the capacitor under test. Monitor the waveform on the capacitor using an
oscilloscope. When using a sensible squarewave frequency (a few kHz - not
one where the inductance of the circuit becomes an issue) there will be a
triangle waveform with a step at the squarewave transition times. The
amplitude of the step will proportional to the ESR of the capacitor.
Calibrate things by adding a known small value ESR simulating resistor in
series with the capacitor. This doesn't have to cost anything if you have
a squarewave generator, or can build one cheaply.
(From: Gary C. Henrickson (gary@aloha.net).)
Motivated by the discussions on the virtues of ESR testing, I ordered a
genuine ESR meter. While waiting for it's arrival, a large pile of dogs were
accumulating in my shop.
To crank out these repairs quickly in the meantime, I constructed an 'ESR
meter' by cabling a (50 ohm) function generator output to the scope input and,
via a T-connector, on to a set of test leads.
With the test leads shorted, mere millivolts displayed on the scope. Across a
good capacitor, mere millivolts. Across a sick capacitor, mucho volts. The
defective caps stuck out like a sore thumb.
Wow, this is too easy. Instant in-circuit (power off) fool-proof testing of
electrolytics. I wish I had thought of this 50 years ago.
I used 100 kHz and 5 V p-p. With scope set at 0.2 V/div you can also check
diodes surrounded by low ohm transformer or inductor windings.
(Editor's note: to avoid the possibility of damage to semiconductors due to
excessive voltage, use a lower amplitude signal - say 0.5 V p-p - for
in-circuit testing. This will also prevent the most semiconductor junctions
from conducting and confusing your readings.
(From: Bert Christensen (70461.2507@compuserve.com).)
I have been reading the various messages about ESR checkers and while I don't
doubt their value in electronic servicing, I think that the use of these
devices adds an extra and IMHO unneeded step. My method of diagnosing possible
electrolytic fault is to use just a scope. Remembering that electrolytics pass
AC or signals through them, a scope should show *the same* waveshape on both
sides of the cap. If the cap is a bypass cap to ground, then the waveshape
should just be a flat line on both sides; if it is a coupling cap, the
waveshape should be the same on both sides.
There are some exceptions, one being a cap that is used for waveshaping in a
vertical circuit but such applications are few. Most electrolytics are either
coupling or bypass.
Using 'my' scope method has several advantages. The main one is that it tests
caps dynamically in the circuit they are used in and using the actual signals
applied to them in real life. The method is fast because you just have to go
from one to another (if you are using the scatter-gun approach) using just the
scope prod. But, best of all, it seamlessly integrates a total dynamic approach
to servicing using the set's own signals or lack thereof. If you are tracing a
video circuit, you can find an open cap, an open transistor, or a defective IC
using the same piece of equipment.
I have been running a service business for over 40 years. Most of my business
today is doing tough-dog service for other service companies.
But, I must admit that sometimes I fix sets just by changing the caps that are
swollen. ;-}
(From: Clifton T. Sharp Jr. (clifto@megsinet.net).)
I still do just enough work that I'll one day break down and buy an ESR meter
(I always give in and indulge myself with the toys of my "trade"). For now,
though, the quickie method I use is the oscilloscope. It goes something like
this:
Set on; scope positive lead. Significant difference? If not, note this
location and go to next cap.
(From: Tony Williams (tonyw@ledelec.demon.co.uk).)
It is always best when measuring a component parameter to lean the measurement
method towards some sort of emulation of the application to which the
parameter is important. This is particularly true of power components,
because the parameter-value may vary with operating conditions. It is
essential with magnetics, less so with electrolytics, but a good habit anyway.
Hold the cap charged up and find some way of applying repetitive square
*current* pulses to it, an Amp or more each time, depending on the expected
ESR.
If the cap has no ESR then a scope across its terminals will show that each
current pulse results in a nice smooth ramping triangle. If the cap has an ESR
then each triangle will be preceded by a small vertical step. If the current
is known, measurement of that step gives you the ESR-value. You can
cross-check the accuracy of the method by seeing the effect of increasing the
"ESR" as low-value R's are put in series with the cap, 0.01 to 0.1 ohms.
Be careful about the placement of the scope leads, you don't want to measure
the IR-drop in the wiring.
If the size of each step+triangle is small compared to the steady voltage on
the cap then the known "constant-I" discharge pulse can be approximated with
no more than a resistor and switching Fet.
(From: Oliver Betz (list_ob@gmx.de).)
If you want to the decoupling capability, you maybe want to know the ESR only
at the series resonant frequency. It's quite simple:
Use a sine generator, connect some coax cable to its output, at the end of the
cable put 47 ohms in series and connect the resistor to one end of the cap,
connect a kind of detector in the same way (47R - cable - detector) to the
same lead. Other end of the cap (and coax shields) to a small ground plane.
Detector can be voltmeter, scope or spectrum analyzer, depends on your
equipment and resonant frequency. Spectrum analyzer with tracking generator
eliminates need for separate generator, makes measurement easy and enables you
to measure even very small capacitor values.
Tune to minimum signal at the detector. With a scope you can check also phase
shift (thanks for this hint, Winfield!), cap should be only resistive (no
phase shift). ESR can be easily calculated now.
(From: George R. Gonzalez (grg@umn.edu).)
After seeing all the glowing recommendations for ESR meters on the
sci.electronics.repair newsgroup, I decided to look into this. Being a cheap
sort, I first tried setting up my own ESR meter using stuff lying around the
shop: Function generator set to 2 volts p-p, 100 kHz sine wave, hooked up to
a BNC tee, one side of the tee goes to some clip leads, other side to the
scope, set for 0.1 volts/cm, 10 us/cm sweep.
With the clip leads dangling free, the scope trace is almost invisible, as
it's zooming up and down 20 cm 10 times across the screen. With the clip
leads shorted, I get about 0.3 cm of a sine wave. With a 1 ohm resistor
across the clip leads, I get about 1 cm of sine wave.
I put a GOOD 2 uF capacitor across the clip leads, we see about 0.5 cm of sine
wave. Tests with various good electrolytics all give less than 1 cm of sine
wave.
Now we can just hop along a circuit board bridging the electrolytics as we
go along. A good electrolytic is going to show 1 cm or less of deflection.
Many old ones with 1970's date codes will show 2 or 3 cm. Probing around a
suspect old pc board revealed that 80% of the caps gave more than 2 cm
of deflection!
Now this isn't always a bad thing. You have to use a little judgement. If
the electrolytic is in a high-impedance circuit, such as coupling two voltage
amp stages, a few ohms isn't going to hurt too much. But if it's a bypass
capacitor on a Vcc line, it could be significant. Just realize that a circuit
may appear to work just fine even with caps with many ohms of ESR.
I usually replace these caps anyway, as they're only going to go downhill
from here.
I can't tell you how much time this little set-up has already saved me!.
Before I had to unsolder one lead of the capacitor, hook it up to the cap
bridge, twiddle the dials until I got a semblance of balance, or if it was a
bad capacitor, I'd waste even more time trying to find the missing null.
Now I can just probe the caps in-circuit, and mark the bad ones with a big
red magic-marker for later replacement. It's quick and great for morale.
This method works well with caps in the range of 1 to 500uf, with medium or
high ESR. But it doesn't have enough oomph to drive BIG caps. For this
you'd need a generator with a lower output impedance.
Next experiment-- we'll hook up a tranny from an old dead SMPS to lower the
generator's output impedance so we can test those big PS capacitors. Stay
tuned....
BTW, this isn't meant to take away from built ESR meter sales! It may even
increase them as once you see how wonderful this technique is, you may want
to buy a dedicated ESR meter.
Electrolytic capacitors like to be kept cool! If there's anything that these
capacitors can't stand, it's heat. It causes them to dry out.
Electrolytic capacitors exist in (at least) two different temperature
ratings: 85 C and 105 C. The latter are obviously more temperature resistant.
Unfortunately they also tend to have a higher ESR than their 85 C counterparts.
So in an application where the heat is due to I^2 * ESR dissipation, the 105 C
type may actually be a *worse* choice! If the heat is due to a nearby hot
heatsink then 105 C is indeed a better choice.
From: Ralph W. M. (ralphwm@aol.com).)
While it seems true that 105 C electrolytics have about 50% greater ESR when
new, compared to similar 85 C electrolytics, IMO, that is not significant in
the circuit. If you would (could) perform a long term experiment and install a
85C and a 105 C in the same circuit, and measure the ESR after 1,000 hours, I
would expect to see the ESR of the 105 C part after aging/usage, now to be less
than the 85 C.
Electrolytics have a shelf life. Electrolytics can go bad (i.e., dry out) on
the shelf even though they were never used/turned on even once.
Technically, a "stale" electrolytic (more than one year after it was
manufactured) would have excessive DC leakage, and should be properly re-formed
before using it. In practice, I have never found this to be a problem 99% of
the time (only exception is critical timing/direct coupled circuits; very rare
these days). The worst I have even noticed, when installing a stale
electrolytic, was that the circuit was slightly unstable for 15 minutes, but
cleared up and was fine thereafter and NEVER "bounced". (all bets are off if
something so old it has "whiskers" is tried though).
How old is too old? I would offer that up to 5 years on the shelf, in
practice, should not be a problem. But 10 years stale MIGHT upset things a
bit.
Technically, if you read electrolytic specification sheets, you will find that
the best (i.e., lowest) DC leakage is not until it has been ACTUALLY used for
at least 10% of the total projected lifetime, (i.e., a 1,000 hour @105C
electrolytic would not achieve the lowest DC leakage until it was used for 100
hours @ 105C (or used for 600 hours @ 65C; but that conversion is another
story).
In practice, IMO, the vast amount of circuitry designs/type of circuits being
currently designed, have built into it enough tolerance for above average DC
leakage, that (these days), excessive/drifting DC leakage is rarely a problem.
As far as "exercising" seldom used equipment; couldn't hurt.
Problem: Electrolytic capacitors are leaking after a period of time causing
computer failure.
Questions:
I saw a very revealing graph in the Sprague catalog concerning
life tests at +130'C plotting ESR versus time. It turns out that for a 10 uF
cap, the ESR actually drops during the first 1,500 hours or so. The interesting
part is that from 1,500 hours out to 5,000 hours the value then about doubles.
On the other graph the results of a 47 uF cap shows no change in ESR
over the whole life time test. However, it's uF value drops by about 2.5%.
The 10 uF electro on the other hand shows little capacitance change (less
than 1%).
If we extrapolate these results we can see a general trend of the larger
value caps losing capacitance over time but their ESR remaining pretty
constant and the smaller caps keeping their value but their ESR increases
over time. Thus this sort of makes some sense to me as to why those little
1 uF caps are so notorious. Comments welcome...
(From: Ralph W. M. (ralphwm@aol.com).)
I know a lot of people try to improve reliability by increasing the volt of the
replacement electrolytic. And some companies like Sony issue modification
upgrades increasing volt rating. And yes, SOME, (but NOT all) electrolytic
manufacturers recommend increasing volt rating to improve reliability of
ORGANIC electrolytics. But in my opinion, I would not, and do not.
To improve reliability, I first upgrade the temp. Or I might chose to upgrade
to a Low ESR electrolytic. Sometimes, circumstances or logistics prevents the
proceeding, and I will increase the uF up to 200% of original IF it is
filtering or decoupling application.
Basically, any improvement in lifetime from increasing volt rating would simply
come from the larger case size allowing the electrolytic core temp to be
perhaps 5C cooler, i.e., the temp reduction comes from the larger case size
being a better "radiator". I estimate that increasing the volt rating of the
replacement part would not achieve greater than 50% lifetime improvement; BUT
at the expense of greater/worse DC leakage, (the greater DC leakage might not
be a problem).
On the other hand, I have read some component manufacturers who recommend
increasing the uF to improve reliability, and I estimate that 2X the original
uF will result in at least 200% improvement (maybe 400%), in component
operating lifetime.
And, to anticipate a possible question, i.e., "what if you tried to restore the
"1.5X" cap that was operated at a lower voltage to it's original volt rating by
trying to carefully, and slowly increasing the applied voltage in order to
restore the dielectric". Maybe, I don't know, never performed such an
experiment. At a minimum it would require a lot of labor on something that
cost relative pennies.
(From: Steve Bell (service@bell-electronics.freeserve.co.uk).)
From experience I see no problem fitting capacitors of slightly higher
voltage ratings. I keep a full range of high frequency low ESR 105 deg caps. I
find, as an example, when I replace a 47uf 35V cap, it is with a 47uf 50v
device. Due to improvements in capacitor manufacturing, the replacement
fitted is usually the same size, possibly smaller, and usually has a lower
ESR than the original did before failing..
Where problems might occur is if someone fits a much higher voltage cap in a
critical area, such as a monitor switchmode power supply or video circuit.
Higher voltage capacitors have higher ESR that may be unsuitable for the
circuitry.
(From: Robert Macy (macy@california.com).)
The higher voltage electrolytic has a higher esr value.
The ripple current will be the same for both caps and the higher esr means
more power dissipated into the cap, drying out the electrolyte and shortening
the capacitor's life a lot.
Without going into an extended discussion (see below):
(From: David Sherman (davids@virtual-cafe.com).)
I have been "into" electronics for at least 20 years and learned electronics
originally on old WWII military surplus gear which was cheap at the
time. Since then I have been a degreed EE and professional engineer and an
avid junk collector. To really old military gear designers often went to a lot
of expense to avoid electrolytics. They' use a big 2-section choke and a
couple 4uF oil-filled paper capacitors in a power supply rather than just one
big electrolytic because the electrolytics in those days tended to "dry out"
and fail with age.
In early consumer electronics I have often found bad electrolytics. The first
thing to do on that old stuff is look and see if anything has leaked out of
the capacitors. Next, power it up. It's not unusually at that point for
something that has been idle to blow a capacitor with a puff of steam! Then
you know which one's bad. Signal capacitors (coupling, emitter/cathode bypass,
etc) are not usually a problem because they don't have so much voltage on them
as power supply capacitors. After replacing any blown capacitors (and maybe
other that look just like it) fire the thing up again. If it doesn't work,
check DC voltages across all electrolytics. Even if you don't know what
they're for, they should all have DC of the proper polarity and usually within
a fair fraction of the working voltage printed on them. Also feel to see if
any is hot. I think you're getting the idea.
Now, about salvaging old capacitors. Ones made since, maybe, 1970 are FAR
better than ones from the '40s and '50s and are all worth saving unless they
have goo leaking from them or the rubber plug is bulging out (kind of like
evaluating an old can of beans!). I have never found one in post-1970 gear to
go bad from storage. If you want to be sure before installing it in a circuit,
simply apply the rated working voltage from a variable power supply (right
polarity, of course) and let it sit with it for a few minutes. If you can set
the current limit on the supply to a low value, it will prevent a potentially
gooey explosion. Applying the DC voltage is actually a good thing. It's called
"forming" the capacitor and it builds the insulating oxide film on the
aluminum foil.
(From: John Popelish (jpopelish@rica.net).)
There are at least two distinct wear-out mechanisms at work in electrolytics.
One is electrolyte loss by leakage from the container. This is made worse by
poor seals and heat, so varies a lot depending on the quality of the original
package and things like ambient temperature and internal heating by ripple
current. If they are stored in cool conditions, they can stay wet for a lot
longer than 10 years. The second is oxide deterioration, and this has a
thermal and a bias component. Heat speeds up the deterioration during
storage, and lack of bias voltage also speeds up the loss. I always plug very
old equipment into a variac the first time I bring it up, and apply no more
than about 70% line voltage for a while, and check for caps heating up. If
everything looks good, I will slowly raise the line to full voltage over about
an hour. This allows some oxide recovery to occur without catastrophic thermal
rise. I have not had to replace caps wholesale unless reliability was very
important (where a later malfunction would be a lot more costly than all the
capacitors).
Electrolytic caps have one metal plate and one liquid plate. The dielectric
between them is a very thin layer of oxide that is formed on the metal plate
after it has been etched to make its surface very spongy and porous. This
etching process multiplies the surface area of the metal by many times
(increasing the capacitance which is proportional to surface area), but means
that the oxide is formed over a very rough surface. So some of the oxide is
wrapped around very sharp edges and over points. This is a chemically less
stable situation compared to oxide formed over a smooth surface or inside a
hollow. Same for oxide formed over metal grain boundaries. Over time, some
of this oxide either breaks off, cracks, or reverts to metal and oxygen atoms,
resulting in thin spots in the insulating layer.
If the cap is stored with a DC bias, these thin spots draw current which
liberates atomic oxygen from the electrolytic which reoxidizes those weak
spots as they form. If it is stored without applied voltage, all these spots
need reforming at once when the cap is put in service. This makes them leak
excessive current, produce lots of gas, and give off heat. If the leakage is
bad enough, the cap may self destruct. If large and expensive caps,
especially high voltage types, are going to be put in service after extended
storage, they can be more gracefully reformed by applying a voltage in series
with a current limiting resistance. And they should be checked for acceptable
leakage current at rated voltage before being used. I think modern
electrolytic caps are expected to last about 10 years in cool storage. Higher
temperatures shorten their life.
If you were going to be reforming lots of similar caps, you could build an
adjustable DC supply that had both voltage regulation and a current limit that
could be set to values appropriate for various sized caps. For one or two, I
have used a Variac upstream of a simple unregulated supply. The point is to
allow some forming current to flow, but limit it to less than what would cause
a noticeable temperature rise in the cap. For a small tubular cap, this on
the order of a tenth of a watt. Divide that by the applied voltage, and you
have some idea of the current limit needed. For large (fist sized) caps, you
might allow the internal dissipation to approach a watt. These power levels
would not raise the cap's temperature so that you would notice it with your
fingers (though they might cause some quite measurable hot spots at small
areas within the cap).
(From: Dbowey)
My recollection of forming electrolytics is that a time-stepped voltage
was applied. The timer was me, and I increased the Variac output to a power
supply over the span of a day or two, starting with 10% of the rated voltage
and ending up with 100%.
(From: Jack Schidt (jack@wintel.net).)
This works well. NOS electrolytic caps should always have this done prior
to using. Often, for old gear, NOS or used caps must be used for economics
or availability.
Since I do a lot of tube equip repair, I built a small isolated tripler- to
easily deliver 450V for the supply electrolytics. I used all new mylar
caps.
I modified your procedure slightly by setting the tripler to the working
voltage of the cap unloaded, putting a 2 M or so resistor in series with the
cap, and connect it to the supply.
For really large (1000 uF+) caps, I use a few hundred K; you want the
applied current to be more than the average leakage current of a good cap.
Check the cap voltage periodically with a DVM or VTVM, disconnecting the
meter probes immediately after measurement. If you use a high voltage, low
leakage transistor as an emitter follower, you can leave the meter connected
at all times. I recommend this.
Often you see an older cap get to a particular voltage, then drop
dramatically, as its dielectric breaks down, then the process repeats.
These should be tossed, as the dielectric clearly has thin spots, and will
fail in service.
Some will charge up completely in a few hours [t=RC], some a few days, and
some never get off the ground. Toss those that do not charge.
However, exploding capacitors aren't all THAT common in properly designed
equipment.... (Well, except for that EPROM programmer that had a tantalum
electrolytic installed backwards at the factory. Six months later - K-Blam!)
(From: Gary Woods (gwoods@wrgb.com).)
If you look in a DigiKey catalog, they detail the 'Vent Test' in which an
electrolytic cap is overloaded in a specified way and the can fails expelling
the material *only* through that scored portion. Sounds like material for
another urban legend; like the supplier who carefully tested each incoming
fuse for blowing in a specified time at a specified overload. Of course, the
people trying to *use* those fuses didn't appreciate how nicely they passed
these tests!
You can do a vent test by hooking up an electrolytic to your 'suicide cord'
and plugging it into 110 VAC. Entertaining. (I did NOT recommend you do
this, and am NOT liable!)
For small capacitors - say, 1 uF or less - a non-electrolytic type will very
likely be satisfactory if its size - these are usually much larger - is not a
problem.
There are several approaches to using normal polarized electrolytic capacitors
to construct a non-polarized type.
None of these is really great and obtaining a proper replacement would
be best. In the discussion below, it is assumed that a 1000 uF, 25 V
non-polarized capacitor is needed.
Here are three simple approaches:
The increased leakage in the reverse direction will tend to charge up the
center node so that the caps will be biased with the proper polarity.
However, some reverse voltage will still be unavoidable at times. For
signal circuits, this is probably acceptable but use with caution in
power supply and high power applications.
However, there will be some non-linearity into the circuit under transient
conditions (and due to leakage which will tend to discharge the capacitors)
so use with care. The diodes must be capable of passing the peak current
without damage.
(From: Ralph W. M. (ralphwm@aol.com).)
First off, you need to identify/specifiy the particular Tantalum you're
speaking of. There are both SOLID, and ORGANIC tantalum's. If it is the
familar epoxy teardop case style, it is the solid variety; any other package
could be either solid or organic, (and they are NOT the same).
Yes, Solid tantalums can explode. But this is either rare in the case of
manufactured equipment in original condition, OR someone modified, the
circuit and selected/sized it improperly. Solid Tantalums are VERY intolerant
of spikes/surges; BUT organic electrolytics are tolerant of spikes/surges;
(BUT organic electrolytics are NOT a direct subsitute for solid tantalums!!!).
Solid tantalums are VERY stable in regards to:
Solid tantalums also have VERY low impedance, at low frequencies; (organic
tantalums do not).
The statement that solid tantalums have lower DC leakages when compared to
organic electrolytics has become a misnomer, i.e., 20 years ago that would
basically be true, but not today. Currently the DC leakages of Solid
tantalum's is similar to the average organic electrolytic; there are some
organic electrolytics that have approx 50% LESS DC leakage, (after allowing
from 2 to 5 minute "warm up"), (BUT solid tantalums have both VERY stable DC
leakage, and NO "warm up").
Within the past 2 weeks or so (current date: 11-August-1997), probably
prompted by an article in EE Times, I set Excite to dig for 'supercapacitors'
and 'ultracapacitors'. I did find that when you use the 'More Like This
option' enough, it gives you the same hits.
Anyhow:
What I found was fascinating to an old-timer. Capacitor technology is
now at the point where it can do load-leveling to extend the life of
electric vehicle (EV) batteries. The high power needed for EV acceleration
can be provided by an ultracapacitor. The ultracap. can also absorb energy
for regenerative braking, to limit the otherwise very high charging
current for the battery.
Noted in passing was a Mazda experimental EV that uses ultracaps. this
way; it is called, believe it or not, the Bongo Friendee. No kidding.
(I have a collection of 7 or 8 other such names...)
Mentioned were capacitors of 1,800 farads at 2.3V. Yup, we're now in the
kilofarad era, folks! The capacitor bank comprised a total of 80, in
groups of two in parallel, 40 groups in series. Total voltage was 92.
Other specifications noted in passing:
Ultracaps. are now in the 0.1 to 8 kWh (kilowatt-hour) range.
Some are made of carbon aerogels (that must not be news...)
Maxwell has an 8-cell assembly rated at 24V, bipolar, 4.5 Wh/kg. The same
company also has a monopolar cell (monopolar?) rated at 2,300 F, 3V; 5
Wh/kg. This one can provide over 100 A !
Some ultracapacitors apparently (pretty sure) do not use electric double
layer technology. They use oodles of alternating layers of conductor and
dielectric, stacked 'to the thickness of a credit card'. Some keen mind(s)
have found out how to make a dielectric layer that is 'intrinsically free
of defects'. These caps, fairly sure, use metal conductors; they have
quite-low inductance.
Multilayer thin-film caps can be made up to 25 cm^2, to 1,200 V (!), and
store 10 joules / cm^2 with applied voltage just below breakdown.
Also noted, but considering the topic, maybe a repeat: Carbon aerogel
caps can go to 40 F /cm^3; work excellently as cold as -30 C, and can
manage power over 7kW/kg. Self-discharge is in weeks.
I found this info. utterly fascinating. When I get a decent job, I'm
getting myself a 100F Elna.
BTW, did you hear that a DMM uses a supercap. for power? I think the
figures are that a 3 minute charge will run it for 3 hours.
Class X caps are for across-the-line use. Class Y caps are for
line-to-safety ground. These caps are constructed to be "self-clearing".
That is, if the device develops a short, the energy dissipated in the
short will "blow" the short away. A typical line input filter will have
a single Class X cap from line to neutral or from line to line and a
Class Y cap from each line to ground or from line to ground and neutral
to ground. No regulatory agencies require their use. However, you may
find you need them to meet EMI/EMC regulations and to meet your own
EMI/EMC susceptibility requirements. UL, CSA, VDE, and other safety
agencies will require that you use proper components to meet safety
standards (which is always good practice) and to receive permission to
use their safety markings. As for the precise differences between the
types (X1, X2, Y1, Y2), I suggest you contact cap manufacturers such as
Vishay-Roederstein for their catalogs and applications books.
The common ones from photographic flash units are electrolytic capacitors but
those in more specialized applications may be other types which can have much
shorter pulse durations.
Note that photoflash capacitors may have mediocre temperature ratings like
55 °C instead of the 70 to 105 °C normally found in consumer
electronic equipment. Thus, they may not be appropriate for use as service
parts replacements for general electronics even though the uF and voltage
ratings match.
-- end V2.44 --
All Rights Reserved
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DISCLAIMER
We will not be responsible for damage to equipment, your ego, blown parts,
county wide power outages, spontaneously generated mini (or larger) black
holes, planetary disruptions, or personal injury that may result from the use
of this material.
Introduction
Scope of Thie Document
Capacitors may not be considered the superstars of electronic equipment
(except perhaps in devices like xenon flash units and pulsed lasers), but
more like the helpers and extras. However, they play a vital role in virtually
everything that uses electrons in some way. A defective 2 cent capacitor
in a TV or monitor can render it useless.
Safety Consideration
Basic Capacitor Safety
While accidental contact with capacitors on a 3.3 V logic board isn't going
to result in a shocking experience, this is not true of many common types of
equipment including TVs, computer and other monitors, microwave ovens; the
switchmode power supplies in some VCRs, laptop computers, camcorder battery
chargers; electronic flash and other xenon strobes; laser power supplies, and
many other consumer and industrial devices.
Basic Capacitor Testing
Testing Capacitors with a Multimeter
Some DMMs have modes for capacitor testing. These work fairly well to
determine approximate uF rating. However, for most applications, they do
not test at anywhere near the normal working voltage or test for leakage.
Normally, this type of testing requires disconnecting at least one lead
of the suspect capacitor from the circuit to get a reasonably accurate
reading - or any reading at all. However, newer models may also do a
decent job of testing capacitors in-circuit. Of course, all power must
be removed and the capacitors should be discharged. This will generally work
as long as the components attached to the capacitor are either semiconductors
(which won't conduct with the low test voltage) or passive components with
a high enough impedance to not load the tester too much. The reading may
not be as accurate in-circuit, but probably won't result in a false negative -
calling a capacitor good that is bad. But I don't know which models are
better in this regard.
Ray's Notes on Capacitor Testing
(This section from: Raymond Carlsen (rrcc@u.washington.edu)
Gary's Comments on Capacitor Testing
(From: Gary Collins (collgra@preferred.com).)
What About Capacitance Meters?
Simple capacitance scales on DMMs just measure the capacitance in uF and
do not test for leakage, ESR (Equivalent Series Resistance), or breakdown
voltage. If the measurement comes up within a reasonable percentage of the
marked value (some capacitors have tolerances that may be as much as
+100%/-20% or more), then in many cases, this is all you need to know.
However, leakage and ESR frequently change on electrolytics as they age and
dry out.
More About Capacitor Testing than You Probably Wanted to
Know
(From: John Whitmore (whit@hipress.phys.washington.edu).)
Safe Discharging of Capacitors in TVs, Video Monitors, and Microwave
Ovens
Why This Matters
It is essential - for your safety and to prevent damage to the device under
test as well as your test equipment - that large or high voltage capacitors
be fully discharged before measurements are made, soldering is attempted,
or the circuitry is touched in any way. Some of the large filter capacitors
commonly found in line operated equipment store a potentially lethal charge.
Capacitor Discharge Technique
The technique I recommend is to use a high wattage resistor of about
5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical -
a bit more or less will be fine but will affect the time it takes to fully
discharge the capacitor. The use of a current limiting resistor will
prevent the arc-welding associated with screwdriver discharge but will
have a short enough time constant so that the capacitor will drop to
a low voltage in at most a few seconds (dependent of course on the
RC time constant and its original voltage).
The discharge tool and circuit described in the next two sections can be
used to provide a visual indication of polarity and charge for TV, monitor,
SMPS, power supply filter capacitors and small electronic flash energy
storage capacitors, and microwave oven high voltage capacitors.
Capacitor Discharge Tool
A suitable discharge tool for each of these applications can be made as
quite easily. The capacitor discharge indicator circuit described below
can be built into this tool to provide a visual display of polarity and
charge (not really needed for CRTs as the discharge time constant is
virtually instantaneous even with a muli-M ohm resistor).
Again, always double check with a reliable voltmeter or by shorting with
an insulated screwdriver!
Capacitor Discharge Indicator Circuit
Here is a suggested circuit which will discharge the high value main filter
capacitors in TVs, video monitors, switchmode power supplies, microwave
oven capacitors, and other similar devices quickly and safely. This circuit
can be built into the discharge tool described above (Note: different value
resistors are needed for LV, HV, and EHV applications.)
Adjust the component values for your particular application.
(Probe)
<-------+
In 1 |
/
R1 \ 2 K 25 W (LV) Unmarked diodes are 1N400X (where X is 1-7)
/ 100 K 25 W (HV) or other general purpose silicon rectifiers.
\ 1 M 10 W (EHV)
| Resistors must be rated for maximum expected
+---------+--------+ voltage. For non-continuous use, the wattage
__|__ __|__ | can be lower.
_\_/_ D1 _/_\_ D5 /
| | \ R2
__|__ __|__ / 100 ohms
_\_/_ D2 _/_\_ D6 |
| | +-----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ D3 _/_\_ D7 _\_/_ LED1 _/_\_ LED2 without an internal resistor.
| | | + | -
__|__ __|__ +-----------+ Use different colors to
_\_/_ D4 _/_\_ D8 | indicate polarity if desired.
In 2 | | |
>-------+---------+--------+
(GND Clip)
The two sets of 4 diodes (D1 to D8) will maintain a nearly constant voltage
drop of about 2.8-3 V across the LED+resistor as long as the input is greater
than around 20 V. Note: this means that the brightness of the LED is NOT an
indication of the value of the voltage on the capacitor until it drops
below about 20 volts. The brightness will then decrease until it cuts
off totally at around 3 volts.
Voltage Checkers
Whereas a multimeter is intended to measure voltages (and other things),
a checker is used mostly to just produce a quick indication of the presense
of voltage, its polarity, and other basic parameters. One use is a quick,
but reliable indication of the status of the charge on a BIG capacitor. An,
example of a simple version of such a device is the "capacitor discharge
indicator circuit" described above.
Equivalent Series Resistance (ESR) and Related Parameters
What is ESR and How Can It be Tested?
ESR (Equivalent Series Resistance) is an important parameter of any capacitor.
It represents the effective resistance resulting from the combination of
wiring, internal connections, plates, and electrolyte (in an electrolytic
capacitor). The ESR affects the performance of tuned circuits (high ESR
reduces the Q factor) and may result in totally incorrect or unstable
operation of devices like switchmode power supplies and deflection circuits
in TVs and monitors. As would be expected, electrolytic capacitors tend to
have a high ESR compared to other types - even when new. However, due to
the electrochemical nature of an electrolytic capacitor, the ESR may indeed
change - and not for the better - with time.
More on ESR, DF, and Q
(From: Michael Caplan (cy173@FreeNet.Carleton.CA).)
Simple ESR meter schematics and plans
Electronics magazines have published various ESR meter schematics over the
years. Here is one that is unique is being able to test caps on live
equipment though I'm not sure what great advantage this is:
"The January 2003 issue of Television magazine has an article about a
live - in circuit electrolytic ESR tester. The battery operated
project by Ian field is based on a TL431 high gain comparator with the
input isolated via an optocoupler. It is designed for live testing. I
haven't built the thing yet as it's my habit to wait awhile and read about
any problems the other guys find before I try it, but in subsequent issues
I havn't read of any problems."
ESR testing without an ESR meter
While, the techniques described below can in principle be applied to any
capacitor, they will be most useful for electrolytic types. Of course,
make sure to observe the polarity and voltage rating of the capacitor
during testing! In addition, take care with the maximum voltage applied
to other components if you attempt to test caps in-circuit. It should be
small enough that semiconductor junctions do not get forward biased (a few
tenths of a volt max) and the impedance should be such that low value
resistors don't smoke!
If that doesn't catch it, a quick review of the "noted locations" often does.
This fixes 98% of cap problems. Not exhaustive or perfect, nor is it intended
to be. Close cover before striking. Probably causes cancer in laboratory rats.
Your mileage may vary.
Electrolytic Capacitors and Special Types
Cool Electrolytics - Temperature Rating Versus ESR
(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)
Care, Feeding, and Storage of Electrolytic
Capacitors
"I seem to recollect reading (or is it an old wives' tale?) that electrolytics
last longer if you apply a voltage across them every so often. This to me
implies that seldom used devices should be turned on every now and again to
make them last longer, not left sitting on the shelf. True or false?"
(From: Ralph W. M. (ralphwm@aol.com).)
Some Qs & As about Capacitor Failure
Here is a three part question concerning electrolytic capacitors. This is an
automotive computer application.
(From: Asimov (mike.ross@juxta.mnet.pubnix.ten).)
Comments on ESR and uF Ratings
(From: Asimov (mike.ross@m-net.axess.com).)
Voltage Rating of Electrolyitc Capacitors Versus
Reliability
Some of the questions go like:
"I am wondering if there is any problem with replacing a cap of lower voltage
rating with one of higher rating. For example, would a 2.2uf 50v cap
generally work OK as a replacement for a 2.2uf 16v cap which is used as a
filter in a 6 volt or 12 volt circuit? I never used to think twice about
doing this, but have seen some discussion recently leading me to question
whether an electrolytic will function properly if it is only operated at a
small fraction of its rated voltage."
Comments on Old Electrolytics and Failure
Mechanisms
A question that often (well, at least sometimes) comes up is what to do with
respect to electrolytics capacitors in really old equipment. Replace all?
What are These Scored Lines on the Ends of Electrolytic
Capacitors?
They are there to channel the debris in a known direction should the capacitor
turn into a bomb. Really :-).
Making Non-Polarized Capacitors from Normal
Electrolytics
You may find non-polarized electrolytic capacitors in some equipment - usually
TVs or monitors though some turn up in VCRs and other devices as well. Large
ones may be found in motor starting applications as well. These usually do
need to be replaced with non-polarized capacitors. Since polarized types
are generally much cheaper, the manufacturer would have used them if it were
possible.
- + + -
o----------)|-----------|(-----------o
2,000 uF 2,000 uF
25 V 25 V
It doesn't matter which sign (+ or -) is together as long as they match.
+---|>|----+----|<|----+
| - + | + - |
o-----+----)|----+-----|(----+------o
2,000 uF 2,000 uF
25 V 25 V
Note that initially, the source will see a capacitance equal to the full
capacitance (not half). But very quickly, the two caps will charge to the
positive and negative peak values of the input across the combination via
the diodes. In the steady state, the diodes will not conduct at all and
therefore it will be as though they were not in the circuit.
+12 V
o
|
/
\ 1K
/
- + | + -
o----------)|-----+-----|(-----------o
2,000 uF 2,000 uF
35 V 35 V
The resistor value should be high compared to the impedance of the driving
circuit but low compared to the leakage of the capacitors. Of course, the
voltage ratings of the capacitors need to be greater than the bias plus the
peak value of the signal in the opposite direction.
About Tantalum Capacitors
Supercaps and Ultracaps
(From: Nicholas Bodley (nbodley@tiac.net).)
What are those X and Y Capacitors in the AC Line
Input?
"I have noticed recently that so-called "X" and "Y" capacitors are used at the
input power section of power supplies. When I have looked into this further,
I find that there are various grades of X and Y - X1, X2, Y1, Y2, etc.
Apparently this is code or regulatory agency related.
(From: Paul Kasley (kasley@fnal.gov).)
Photoflash Capacitors
These are found not only in electronic flash units and strobes, but pulsed
laser power supplies and other fast discharge applications. They are designed
for rapid discharge with minimum losses and without self destructing. Thus,
the ESR and inductance are very low and the internal structure is set up to
survive very high peak currents (hundreds or thousands of amps).