Capacitance measurement with multimeter

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mike_relay

Well-known member
Joined
Jan 17, 2005
Messages
112
Location
Los Angeles, CA
I have an old organ from that I bought at the thrift store, and I have been trying to figure out why it is so noisy.
One thing that I have been doing is trying to check the capacitor values.
I am using a a Fluke 112 multimeter, and all of the capacitor values were pretty good, but the electrolytics I checked were reading some crazy values.
One was supposed to be 500yF and I was reading 1000yF, then 3000yF, and it kept on going up...Could ALL of the electrolytics be bad? All the other caps seemed to check out fine.
 
I have often had really old organs come through my shop where all the electro's were well up in value, often double. Just have to replace them, as they are dried out.
 
Are you checking these in the circuit?

Are you able to do that and get accurate measurements?

Joel
 
[quote author="Mbira"]Are you checking these in the circuit?

Are you able to do that and get accurate measurements?

Joel[/quote]

I am checking them in the circuit, so that is why I was a little unsure...
Is that a viable way to do it? Is there a better way?

I'll try to change one and see if I get the the correct result.

Thanks,
mike
 
You are probably measuring more than one cap at a time due to paralell caps .
If two 1000uf caps are in parellel, you wil get 200uf, etc. Also, the battery in the voltmeter is probably slowly reforming the capacitor's electrolyte. You can see the same effect by putting a lytic cap across your ohm meter, (use neg lead to neg terminal). The ohms will dive down and then slowly build up until it stabilizes.

Inductors can be checked in circuit, but caps should be removed to get an accurate reading.
 
Don't forget that the tolerance of electrolytic caps is appalling- and that is especially true with older units. Also, caps of value above 470uF are often used as PSU decoupling. The value doesn't matter so much, as long as the circuit "sees" some capacitance. The problem with older capacitors is that they tend to go short-circuit or open-circuit. I prefer open-circuit faults, but they always seem to be s/c :wink:

In some ways, an older analogue meter is more useful when testing caps. Set to x10 or x100 Ohms range, you see the needle dip to 0R and then rise to infinity-Ohms (hopefully!) This is the capacitor charging up and then blocking DC. The speed of the needle swing is proportional to the capacitance C and the internal resistance of the meter R. Whilst it isn't an easily measurable time constant, it's easy enough to connect a brand new 1000uF cap to your meter, check out the time it takes to charge, and then compare the suspect 1000uF cap. Check the voltage polarity of your Ohms range with another voltmeter- often the red lead becomes -Ve under Ohms testing.

If the needle fails to move, the cap is o/c. If the needle doesn't reach infinity-Ohms (where's Keef when you need him- he's got that infinity-symbol shortcut :? ) it's s/c or leaky.

Another way to test is use a 9V battery, 2k2 resistor and an LED. This basic continuity tester works in a similar way to the meter- good cap: LED lights and then slowly dims. Bad cap- LED is always on, bright or dimly depending on level of s/c, or never comes on at all. Again the RC constnat holds true, so you can compare speed of dimming.

Don't try this for caps below 100uF- it all happens quite quickly! The s/c tests still stand though. And don't forget that static meter testing out of circuit doesn't test fully enough, because in the actual circuit it's possible that the capacitor will be biased by a larger voltage than from your test meter- this may be affecting the performance of the cap. So it may test okay with a small AC signal when testing on a DMM, or a small DC voltage doing the needle-swing test, but once biased by a 300V HT, the dieletric may say goodbye!

Replace any suspect devices- they're cheap enough nowadays to prevent major headaches.

Mark
 
and another tip for everyone not having a capacitance meter but a Multimeter:

You can use the short circuit testing (don't know how you really call it in english) which produces a beeping sound, if the resistance is below a certain value.

If you connect the multimeter to the cap it should give a beep while it's charging up. Then, connect it the other way round (and vice versa if you want. It's not great for the cap to charge it the wrong way, but those low voltages shouldn't harm for a few times) - now the length of the beep is a measure for the capacitance.

So you can at least roughly verify the capacitance if you have some reference caps with confirmed capacitance at your hands. Thats how I repaired a few devices through finding some bad electrolytic caps already.

Cheers
Dominique
 
just scanned this quickly and failed to notice if anyone mentioned having the cap DIScharged when testing. i've seen meters do funky things when the caps are charged.. they *shouldn't* but they do.
 

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