Voltage Rating for Unpolarised Capacitor Made From Two Polarised Capacitors

[The Electrician]

For those that think it is ok to make an AC capacitor without adding the diodes will have to convince me and themselves that the capacitor maker thinks it is ok to reverse bias the electrolytics repeatedly, cycle-by-cycle, allowing the capacitor to repeatedly break down.

Read page 13 of this CD Application guide...

On page 13 of your linked reference under the heading "Non-polar and motor start capacitors" the capacitor maker says "The two capacitors rectify the applied voltage and act as if they had been partially bypassed by diodes."

The capacitor maker does not say that actual diodes need to be used, but it certainly wouldn't hurt to do so.

 

[Grossel]

Mike - based on your second simulation, could you try to simulate what happens if the capacitors have different capacitance? And also another if there is a resistor (say 1k) in parallell to one of the caps.

I ask because I dont have the software to simulate at here, and I have an idea of using that particular setup to see if I can pick out faulty capacitors.

 

[MikeMl]

Grossel

Is this for testing electrolytics?
Will you always have two in-series? Are you comparing a known good one to a questionable one?
What excitation voltage? 10 to 50Vpp @ 50Hz?

A preliminary look suggests that a four-arm bridge, where two arms are resistors, and the other two are series-connected electrolytics (with protection diodes around them) shows some promise...

 

[Grossel]

Yes, Mike - that was a good idea using resistors/capacitors bridge.

If I start looking around have several small transformers (picked out from old radios etc.), so can say 14Vpp 50Hz, and put a 2 ohm serie resistor to voltage generator. Electrolyts with 220µF / 180µF capacitance - and the other test : electrolyts with 220µF / 220µF||100ohm.

for the second half of the bridge, 10k and 10k.

That I think will be a good test.