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Old 22nd October 2005, 04:58 PM   #1
mrsmith is offline mrsmith  United Kingdom
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Default whats the best way of testing electrolytic capacitors using a multimeter?

I am wondering if there is a technique for testing circuit board mounted electrolytic capacitors in (or out of circuit if nessecary) using a multimeter?

I may have a few bad caps on my amp. somewhere on my board and wondering the best way t test useing a dmm

anyone please advize? best way to test them with a dmm?

Thanks
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Old 22nd October 2005, 05:19 PM   #2
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Not really.

What makes you think you have faulty caps? If I have a problem like this, I normally swap out all of the electrolytics in one go. After all, if one has reached the end of its life, then its likely the rest are nearly there as well.
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Old 22nd October 2005, 05:56 PM   #3
anatech is offline anatech  Canada
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Hi mrsmith,
Pinkmouse is right. A multimeter may tell you about a complete failure, open, short or very leaky. That's all. There are some ESR meters that can give you some of the story. I use a combination of HP 4263A ($$$$$), older cap checkers that apply high voltage and common sense.

Like pinkmouse said, "if in doubt, take them out"! Well, I said that, but it's cheaper to replace them when they are old rather than worry about it too much. That's where he was going.

Remember one thing, caps need to be pulled out of circuit to check them in any meaningful way, may as well stick a new one in.

-Chris
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Old 22nd October 2005, 07:33 PM   #4
mrsmith is offline mrsmith  United Kingdom
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oh ok thanks for the advice,

can i ask a quick question, i have some exect caps same uf and voltage,

the origanal caps in amp are blue bodied cans, but the spare ones i have are black bodied cans.

is there a diffrence or not?

thankyou
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Old 22nd October 2005, 08:30 PM   #5
anatech is offline anatech  Canada
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Hi mrsmith,
No difference, just a different make. Just as long as you haven't had them in storage since the Seventies!

Go ahead and use them. Mind the polarity.

-Chris
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Old 1st November 2005, 09:44 PM   #6
dimitri is offline dimitri  United States
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Quote:
if there is a technique for testing circuit board mounted electrolytic capacitors in (or out of circuit if nessecary)
yes, you can do it, but not with multimeter, you need so-called in-circuit ESR meter, which measure the series resistance (best models measure it at several frequencies). The usual elco failure is not capacitance loss, but substantial increase in ESR with time...
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Old 2nd November 2005, 02:10 AM   #7
anatech is offline anatech  Canada
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I'm sorry dimitri,
I have to disagree with you. In circuit testing will not show leakage when it occurs. Circuit effects may change other readings too.

The only way to test capacitors (or any other component, save a fuse maybe) is out of circuit. It's the only way you can be sure of your reading. What happens if there is another cap in parallel somewhere else?

-Chris
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Old 2nd November 2005, 02:36 AM   #8
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Before acquiring my cap/ind meter, I checked cap in the following way:

1. Voltage range

Put a suitable resistor in series with the cap and the power supply. Crank the voltage up slowly and observe the current into the cap. The current should come down to a very low level(leakage current) when the cap is fully charged up. If the current is still high for sometime, you got a leaky cap. Toss it.

2. Capacitance check - a very rough method.

I prefer to use analogy meter to do this. Switch the meter to ohm range. Which range to use is depending on the capacitance. Connect the +ve test probe to the +ve terminal of the cap, negative probe to the negative terminal. The initial reading should be zero as the cap is at short-circuit state. The meter needle will then move slowly towards the high resistance reading and eventually at infinite. You can use this technique to compare the cap you want to test with another known good one of known value. The needle movement speed difference will give you a good idea the capacitance difference.

Hope this helps.
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Old 2nd November 2005, 02:37 AM   #9
dimitri is offline dimitri  United States
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Chris, leakage is parallel resistance to cap, I'm speaking about the series one ...

http://www.fullnet.com/u/tomg/esrscope.htm
http://www.e-repair.co.uk/tipsgeneral101.htm
http://ludens.cl/Electron/esr/esr.html
http://www.qsl.net/iz7ath/web/02_bre..._esr/fig03.gif
http://personalpages.tds.net/~rcarlsen/ce/esrmeter.txt
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Old 2nd November 2005, 12:37 PM   #10
anatech is offline anatech  Canada
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Hi dimitri,
Yes, I know. ESR is but one failure mode. Mostly a low voltage thing. High voltage caps can to either or both, leakage is dangerous.

My point is simply that you can't accurately know a component is good or bad in circuit. I've been burned too many times by believing what others claim to be true. Over 30 yrs. experience has shown me that you can't take shortcuts.

Putting my $$ where my mouth is, I bought an HP 4263A - new, with three sets of leads. It's my second one (the first was an HP 4261A). This is after using the other so called capacitor testers. I didn't spend that money because I like to spend. It was needed for what I do.

The other testers (ESR, etc ..) found some bad caps. The HP finds more. What the HP doesn't catch, a meter with HV supply and high resistance (220K) finds. The only thing I check in circuit is leakage in tube circuits. Well, I lied, I look for ESR on supply caps with a 'scope while the unit is in operation too.

-Chris
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