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Capacitor ESR Testing: do you? Should I? How to?...

Doug3004

Sep 5, 2014
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Lately I have come across a desire to be able to test a capacitor, and can't do it with the one meter I have.

The capacitor in question is a round-can 440v 40/5 µF castor-oil-filled capacitor from an outside condenser of a central-AC unit. The indoor AC unit blows air, but it isn't cold.... Casual observation showed that the condenser unit fan will not start on its own, but it spins freely and will run if I spin the fan with a stick--and then the AC produces cold air like normal. The outdoor condenser unit was replaced only ~4 years ago, so I am guessing the cap just failed.

After shutting off the power to the AC system, I removed the capacitor and will just go shopping for a new one tomorrow. I got curious tho and decided to try testing the capacitor, since I had one DMM that could do that (I thought).

The best meter I have (and the only meter I have that has a capacitance setting) is an Extech EX420. The Extech meter can't seem to detect any value at all with either side of the HVAC cap, it shows a change (or a difference using the range function) of .08 µF and .09 µF for the two values. Which looks like "a margin of error equal to nothing". Since the motor acts like the cap is bad, this would make sense on its own.

At this point I realized that I couldn't remember ever using the capacitance setting for anything, so I decided to test it against some new presumed-good capacitors. I grabbed three different electrolytics of 100, 47 and 4.7 µF and tried checking them. The EX420 would not register values at all when used the normal way. If I used the [range] button to zero-out the stray capacitance of the leads, it would sometimes hit almost right on the labeled values, but other times it just says "OL". It seems to take 3-4 seconds to show anything on the display, which is really rather slow compared to how fast it can show volts, amps or ohms. (-I leave it on the auto-ranging setting usually-)

So then I tried a couple different values of ceramic caps--and the same meter (without using the range setting) hit right on the labeled values, in maybe 1/4-second. It might waver 1 or 2 counts, but it quickly settles right on, or very close, to the stated values on the caps themselves. So it measures ceramic caps correctly, and just as fast as it can show volts, amps or ohms.

Investigating the matter further online let me to the subject of the problems of testing caps, and of ESR testing for electrolytic caps, that I'd never heard of before.

I have found the cheaper meters that do this, the "auto-detect" transistor testers based on an ATMega328. They usually only have a 14-pin ZIF socket and a [test] and [off] buttons. There is open-board ones for ~$10-$15 around, and a couple in a rather generic looking cases for ~$25 or so.

What I am wondering here is, is there any device better quality than that? There's a few "transistor testers" with only 3 leads that claim to do ESR testing, but they only have a digit readout and no buttons to select a mode, so I don't understand how that would work.... I've looked at a few lower-cost multimeter-style RCL meters, but none of them seem to have the ability to measure ESL. I'm thinking that there must be a better/higher-quality machine for doing this, but I can't seem to guess what it's called.

I have also seen a few pages where the opinion is that since you can't test a cap properly without removing it from a circuit anyway, that it's usually just easier and faster to replace it than it is to cut it out, test it and put it back in if it's good. The principle of the situation annoys me though--that you can have this part and you have NO way to really tell if it's working or not.

Do you have anything to test ESR with? Is it one of the above two (cheaper) ATMega328-type things, or is it something better?

....The funny part about this situation is that just TWO DAYS AGO, I decided to order a couple new DMMs, just because the one I tend to use the most out of habit is pretty old and rather inexpensive nowadays. And both of the new ones also have a capacitance function, tho neither measures ESL... but I'd at least have had another different meter to try testing with. :|
 

73's de Edd

Aug 21, 2015
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Sir Doug3004 . . . . . .

EXTREMELY likely that the 40 ufd start portion has opened up or is now 0.1 ufd., since it will run with a "push" and the 5 ufd is maintaining the running speed.

73's de Edd
 

Doug3004

Sep 5, 2014
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Sep 5, 2014
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Update:
1. I bought another motor cap and the AC works now.
2. The 40 µF side was for the compressor motor, and the 5 µF side was for the fan. Both sides measured totally dead, so I don't know how the compressor was still starting and running... ?
3. When I tested the new capacitor on the Extech EX420 I have, the meter updated about once per second. The first value was some tiny odd number but the 2nd value was correct. If I kept holding it on longer, the meter stayed on the correct value. So the meter does test C correctly.

The hardware store also tested the old capacitor with their own DMM; it looked to be a typical ~$30 generic China brand. It had a model number but no name brand visible at all.

My problems here--
1--I only had one meter to check capacitance with
2--I only had one cap to test, that I assumed at least one half of it was already failed (but not both halves...)
3--I didn't know what type of capacitor it was, so I didn't know if the meter was having issues measuring an electrolytic or not. ...I searched for a while and finally found mentioned on the wikipedia page for capacitors that motor run capacitors are usually paper or plastic film types. So a normal DMM *should* be able to test them with no problems--if they are still good, of course.

The cheapest (LCR) meter that I can find so far that includes an ESR function is the Mastech MS5308, that costs around $175 - $200. It probably works really well, but that's quite too expensive to pay for as minor as this issue is for me.

I also decided to order one of the Mega328 ESR meters. It's not an industrial-quality solution but it doesn't cost much and it tells more than the DMMs will.
 

Alec_t

Jul 7, 2015
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If you want an el cheapo home-brew ESR tester there are many schematics on the web. Accuracy may not be great, but for many tests a ball-park figure is good enough.
 

BobK

Jan 5, 2010
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You have not given any reason why you need to measure ESR.

There are plenty of cheap component testers, about $15 that will test transistors, diodes, resistors, capacitors and inductors. I have one and I am quite happy with it.

Bob
 

debe

Oct 15, 2011
278
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Oct 15, 2011
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278
Heres 3 different testers ive built over the years. Two of them are the cheep Ebay component testers which ive found very usefull. The other is just an ESR Capacitor tester which ive built about 20yrs ago & still use. The capacitor in the test is a motor run capacitor, & the value shown is what is on the capacitor. It is a good capacitor.TESTER.1.JPG TESTER.2.JPG TESTER.3.JPG
 
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Y2KEDDIE

Sep 23, 2012
259
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Sep 23, 2012
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259
Typically, the best way to test motor start and run capacitors is to replace them. The motors are usually integral with the compressor and need to be replaced as a single unit. The capacitor is the least expensive external part and substitution is the best test.

An VOM/ohmmeter test to see if the needle moves up scale and then comes back to rest works well. The value of capacitance, leakage, ESR, are of little value.

As the capacitor ages and looses value the motor struggles more to start. If the load is light you may not notice it struggle.
 
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