I'm testing a 50/12μF start/run capacitor out of an AC unit and got, what are to me, odd results.

I tested it with a VOM, qualitatively looking at the rate of change in voltage and resistance. The VOM works at 0.5V. The voltage had the expected slow drop for both capacitors. The resistance went to ∞ immediately.

I then redid the test with a 9V battery and got the same results, but in the 0.5-9V range.

Finally, I redid the test with just the VOM and observed a gradual increase in resistance.

I do not know what is going on. Could this be a temerature issue? I did the test in a garage at ~50°F. Did the 9V version of the test warm up the capacitor enough for it to function in the 0-0.5V range? I know this test is usually done on a hot summer day when the AC is broken.

 

I'm testing a 50/12μF start/run capacitor out of an AC unit and got, what are to me, odd results.

I tested it with a VOM, qualitatively looking at the rate of change in voltage and resistance. The VOM works at 0.5V. The voltage had the expected slow drop for both capacitors. The resistance went to ∞ immediately.

I then redid the test with a 9V battery and got the same results, but in the 0.5-9V range.

Finally, I redid the test with just the VOM and observed a gradual increase in resistance.

I do not know what is going on. Could this be a temerature issue? I did the test in a garage at ~50°F. Did the 9V version of the test warm up the capacitor enough for it to function in the 0-0.5V range? I know this test is usually done on a hot summer day when the AC is broken.

What do you mean the VOM works at 0.5V?

What do you mean a gradual increase in resistance?

 

This type of capacitor has a very strong change in capacitance dependent on the voltage applied. The best way to test them is to apply line voltage and measure the current they pass.

Xc = 1/ 2PiFC
I = VACrms/Xc

Always be aware that they could fail by exploding. Protect yourself!

 

Thanks, I'll test it in the unit. If I get no results it'll narrow down the problem to a circuit board I think has gone bad.

I'll also be sure to be a few feet away with my meter.

 

I do this for my day job. Ask a lot of questions if you need to.

You should be able to see the capacitor pass current during the start-up, with an amp-clamp meter, from several feet away. Diagnosis is usually not about how much. It's ususally about if it conducts at all. See a start surge and it's probably good.

 

Thanks #12. The problem is that it does not turn on, at all. The power cord is good, the fuse is good, the switch is good, but nothing happens. I am trying to test everything before buying a ~$40 part. I think its the PWB display assembly but I don't have an extra on hand to test it out.

 

Electronic controls on a window air conditioner?
No compressor, no fan?
You shouldn't even be checking capacitors.
What is it using for a thermostat?
Does it have a remote control?
assuming you are in the USA?

Talk to me. It will save you barking up the wrong tree.