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question about capacitors

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Teletech

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Sometimes when troubleshooting circuits, I will check a cap using my meter and it appears that it is working (even out of circuit). Although when I find that all other components are good, I will replace that capacitor in question and WALA!! the device now works. What is up with that? I check them out of circuit with an ohm meter, watch the deflection, switch leads and watch the deflection that way. I even check them with a capacitance meter to see if they read the proper value and they will. I'm not saying this always happens, sometimes I find a shorted cap or one that is open. Is there a sure fire method of testing these rascals? Another question: To power l.e.d.'s with 120vac, you need a cap in the circuit. I am told that they have to be non polarized, but all the non polarized caps I ever see are only up to 50 volts. Where can I find a 150 volt one or is there another way?
 
You need a Capacitance meter that tests for ESR and Dielectric Absorbtion. Both these factors can Serverly affect the performance of a cap.

I use a Sencore LC102 Capacitor and Inductor Analyzer.
Not at all Cheap, But it works good.

Make Your Own non-polarized cap with two regular caps back to back. Two 100 uf caps makes a 50 uf cap. Total Voltage = Voltage of one of the caps (or the Lowest voltage cap if there not the same voltages.)
 
I do have plenty of the higher voltage polarized caps. So you are saying to connect them in series? If so, which way would you put the positives and negatives of the two caps to make it non polarized?
 
Hi Teletech,

If you need to connect LED via a capacitor to AC mains, the proper way to do it is to use a capacitor rated for class X operation, not just any non-polarised capacitor of rated voltage.

They are there for a reason.
 
Teletech said:
To power l.e.d.'s with 120vac, you need a cap in the circuit.
You also need a diode in the circuit.

Regarding using back-to-back polarised capacitors - you can either connect neg to neg or pos to pos, doesn't matter which way.

BUT I wouldn't advise doing it if you are messing around with the mains. The leakgae current of polarised capacitors is too high.
And, as a nother correspondent says, the proper way to do it etc etc.

However, in general terms, if you need a certain value of C with a voltage rating above those you have or can get, you can use two capacitors of twice the value, both of equal voltage, in series.
The resultant value of C is half that of their individual values but the voltage rating is now double.
 
Someone told me how to use my scope and a signal generator to check ESR and it works really well.
 
Teletech said:
Someone told me how to use my scope and a signal generator to check ESR and it works really well.

Yes, you could do it like that - you should use a frequency about 100KHz, that's the frequency ESR is usually quoted at.
 
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