Testing a Capacitor Microfarad Rating that has a Resistor

J

Janglers

New Member
Hi all, first I am not an electronics guys so sorry if I have any errors in what I am trying to explain. I have a single phase 220v air compressor that has 3 capacitors. Two are start and one is the run. I am using a clamp on meter that has a capacitor setting to check each one (klein cl-800). 2 out of the 3 tested within spec of the uf range on the label. The third however, has 2 resistors soldered onto the terminals. This one is the same as the other start capacitor, expect for the soldered resistors. When I check it with the probes, its out of spec. But I am thinking that the resistors are throwing those numbers off and wanted to sk if that was the case?

When testing them I took off all the quick connect wires, except for the one with the solder points. But they were disconnected from the other cap.





I tested the one with the resistors and solder like this. No connection to the other one.

The two start caps have the same label only small thing I did notice is that on top one has a 20 and the other a 25, not sure why that is.
 
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Is anything wrong with the compressor?, if not why are you testing them?.

Assuming the compressor is faulty, then you should unsolder the resistors (at one end) and then check the capacitor.
 
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Thanks for the reply Nigel Goodwin, yes I have been having issues with the thermal overload in the mag switch tripping intermittently. So the resistor is indeed throwing my reading off. Ok I wanted to confirm that. I was seeing some like 400+ reading on the uf. I think I also read that caps, when they fail, don't go up number wise.
 
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This is only about the capacitor with those 10K resistors soldered :

Disconnect the terminals as in picture 3.
Apply 12V from any battery or supply to its terminals and a voltmeter to the terminals, any polarity.
- If the voltage probed decreases slowly when you disconnect the 12V, capacitor is good.
- If voltage probed decreases fast the capacitor is bad-open circuit.
- If sparks when applying 12V, the capacitor is bad-shorted circuit.

Edited : the formula for the discharge time to 4.5V from initial 12 V = R C
In your case R = 5000, C is in Farads.
 
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OK so I have a 12volt car battery. So apply the positive to either terminal from the battery via a wire. Then remove it and then on multimeter see if the voltage drops slowly? No negative from the battery to the cap correct?

Since you mention 2 possible failure scenarios, let me ask you this, would a capacitor still function if one of the bad states existed? I ask because the compressor I am having a problem with works sometimes, and not others. I am seeing some arching at the magnetic switch and the heater packs.

Thanks for the test procedure.
 
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I know its late, but the resistors are only there to bleed down the capacitors. they have nothing to do with anything else. If they read wrong capacitance, they are bad. Are you reading the 2 start caps with wires disconnected? if not, you might get 400 mfd if they are wired in series
 
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I need advice on testing capacitors, not the little tiny ones on circuit boards, but can types and motor run/start type. Youtube gave me some info on how to test with basic multimeter, I have 2 identical caps and getting different readings, yet not sure which one is bad. I can get a better meter with capacitance testing, that will help. Going further down the rabbit hole of capacitor testing videos I learned that the multimeter cap testing is not 100% definitive as it does not measure cap leakage. Using a Heathkit IT-11 capacitor tester would let me detect leakage, but not sure what the max uf rating is on that machine. If a tester has a max rating of 60uf, is there a way to test a 600uf capacitor with it? Heathkit IT-11's run about $300ish on ebay, any suggestions on better units to watch for? I'm tired of guessing if caps are bad.
 
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The Run Cap ~ 235 uF nom. is about 20~25 ohms @ 220V 60~50 Hz. The bleeder-discharge resistors are irrelevant to the test and operation, as they are relatively high-R.

It fails your test, so replace it. Then move the resistors to the new one.

This is your Run Cap (with the bleeder R's.) The Start Cap is doubled up to boost the alternate phase current during the start surge.

AC motors have orthogonal poles with primary on one and Run cap phase shifted to simulate commutation on the 2nd poles so the pole coils are not alternating torque and fight each other when the cap is weak and thus can trip the breaker more easily.
 
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