Capacitor tester

Geo-TH,In

Well-known Member
The question of how do I test a capacitor has been asked many times here and there are many good responses.

I don't recall anyone posting using an amprobe to test caps. So I calculated the capacitive reactance of a 10 mfd using 60 hz. Then divided 120vac by Xc. Came up with 0.452 amps. So for every 10 mfd a good cap will use 0.452

. Some may question the work use, so lets say, 0.452 amps will pass through the cap. I then connected a 23, 35 and 190-220 mfd cap to 120 vac.(the start cap has a range of 190 to 220 mfd stamped on it. My test shows it appears to be close to 200). Within the accurary of the caps and meter, I was surpised how close the currents were to the calculated values.

The 23 and 35 cap, run caps, also passed the spark test. The 190-220 is a start cap, electrolytic cap. If I recall, could be wrong and I sure someone will tell me I'm wrong either way, electrolytic caps are by designed are leaky and should only be used for a few seconds at a time during start. If anyone questions this, please leave one plugged in while filming it. Wait for it to blow.

As for me, I use amprobe while cap is on motor and look for current. If I have current, good chance cap is good. No current and voltage is applied to cap, good chance cap is bad. I will simply replace cap. Have buckets full us used ones.

Been playing with electrical devices for 45 years. Never saw a need to buy a cap tester. Substition works best for me. Amprobe also tells me if start cap is being removed after motor starts.

Please try using an amprobe method before you tell me it won't work.
a210283.jpg

a210284.jpg

a210285.jpg
 
Pretty slick Geo.! That is almost thinking too much for the average tree mechanic but to an electrician that makes plenty of common sense. The old wire jump or if you want to blow up a screw driver works too but you are checking the motor functions and the cap and start fields and switch all at once. Bravo. Love my fluke 179 meter and I have a for real Amprobe clamp on. Next time I mess with a motor at work I will need to give that a try. We just go out with a 10MFD and change it out. Works about 90% of the time. Next is the motor cause most times they have fryed it already! Who in their right mind thought it would be a good idea to run TWO hp. 480 3* pumps on commercial dish machines? When the ceramic seals go bad and start to leak on these buggers guess where the water goes??? Thanks
 
jeffcat,
If you want to charge the cap to 155v and get a big spark, you need to put a diode in one side of the wire. This will make a half wave power supply, 155v is the peak voltage for 120vac. Without using a diode, the spark test is determined by when you unplug the cap so the voltage could be anywhere from 0 to 155 volts.

I've always had a special interest in electronics ever since I was a kid. Took electronics in college. Even took classes to repair tube TV's back when they could be fixed.
George
 
>As for me, I use amprobe while cap is on motor and look for current. If I have current, good chance cap is good. No current and voltage is applied to cap, good chance cap is bad. I will simply replace cap. Have buckets full us used ones.

The reason that test works is that capacitors generally fail open in the real world. They actually first short out, then quickly open due to high current. It's more common to find shorted capacitors in electronic devices, where the current is limited by other devices and isn't high enough to blow out the shorted cap.
 
Interesting idea. I've often needed a tester for electrolytics in tube radios and amps. They're usually bad from age anyway but it would nice to be able to check them beyond shorting.
 
I wouldn't use this method on electrolytic used in radio. They are designed for DCV only. I used to have a tester for those. It would measure the cap value and tell if they were leaky. My test is for caps on motors.
 
Thanks. I agree that the method would not be quite suitable for my needs but I like the method you've figured out with a little brain work and commonly available tools.
 
Mark,
The reason for my post is to come up with a simple way for people to test capacitors. Do you have a better way to test caps and measure the capacitance using common tools most people have, like an amprobe? Each microfarad is = to 0.0452 amps. Yes, there meters to measure capacitance, but most people can't afford the good ones. HF EVOM will only measure caps up to 20 mfd. Can't see buying one with such a limited range. HF amprobe on the other hand is surprisingly accurate and cheap.

A 60 mfd run capacitor on my small 1.5 hp air compressor was neither open or shorted, it changed capacitance. The only one in 45 years that I've seen do that. It was also the only run capacitor I've see that big, 60 mfd. Most run caps on 3 hp central AC's are only 35. It was very easy to trouble shoot the air compressor by the substitution method. This cap passed an ohmmeter test. Geo
 
Centex Farmall.
I'll ponder a way to test DC electrolytic caps. If I recall tube TV's and radios came with a schematic that also had the required voltages at different points. The problematic electrolytic caps were in the power supply and not so easy to remove. So when the cap because leaky, the power supply voltage wasn't high enough. Most tubes used either 150vdc or 300vdc, so knowing what the PS voltage was very critical in trouble shooting bad caps.

Later manufacturers would leave schematics out and you then had to buy them. I think they were called sam's photo fact.

I have a working 1956 Rock-O-La juke box. The tube amplifier is still working.

When I got out of the business of repairing tube TV's, I had boxes of vacuum tubes I gave away to a person like you who was into tube radios. I also gave away my tube tester too. Can you imagine what those boxes would be worth?
Geo.
 
The tricky ones have two or more caps combined in one housing with different voltage and capacities. Occasionally a stronger system will have some rated as high as 450VDC. Fortunately you can still buy the combined caps. They're not giving them away but they're somewhat reasonable.

I've actually had decent luck finding schematics on the internet. I can't remember the name but they're basically the same idea as the sam's photo facts. You can find them by Mfg and chassis number. The little clock radios and such will have a tube diagram and that's about it. The bigger pieces, like you are saying, will frequently have a schematics stapled to the inside (if it hasn't fallen apart).

Since you used to repair the old tube stuff you are sitting on a wealth of knowledge. Tubes had a bit of a renaissance in the mid 90's driven by the audio crowd. Now you can get all kinds of parts and they're still making new tubes in Russia.
 
Interesting old school idea - making do with what you have. Too much work for me though. I don't think there's hardly any dmm's out there for $50 and up that won't test a capacitor. Probably just my warped way of thinking but I don't believe a person needs to be messing with tricity with a $5 meter. I put that in the same league as an old shop guy I knew who used his finger to test for voltage on a 575 three phase circuit. Sure you can do it but you're playing Russian roulette without a gun. JMO
 
No need to get defensive, George. As long as folks realize you can't apply 120 Vrms to a capacitor that isn't rated to handle it, your method is fine.
 
That was on "How do they do it" just last week. Look up how much they get for a "matched set" of those suckers!! Couple of car payments.
 
Repaired car radios and TVs part time in a little shop in Mt holly, NJ. Did it for almost ten years till I moved away from that area. Never assume a cap didn't have something in it and always short it out. Safe. The worst is when you play with solid state color TV drivers. They cook along at 22- 25,ooo volts. Marty the owner got nailed one time and you could see little wisps of smoke come out of his ears.
 
FYI,
The picture tube of old tv is also a capacitor. They too have 25kv+. It will reach out and grab you Need to discharge before getting close.
 
Side note - and you probably know this, but for those that don't - those old metal cased capacitors are very likely full of PCBs.

Nasty stuff. Be careful not to pop one.

If you're experimenting with a circuit, best to not even risk it and use modern equivalents instead. Just too easy to wire one incorrectly, blowing out a cloud of toxic vapor that'll stay with you for the rest of your life - they accumulate in the body.

There's a good reason they stopped using the stuff.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top