[Boatanchors] GB> Using ac motor start capacitors in dc power supplies - wvdc question

[Marvin Match]

On 29 Sep 2009 at 22:31, Greg Mijal wrote:

> 
> Hi:
> I just found a box of motor starting capacitors in my garage. PPD 
> film capacitor 15 uf @ 370 vac 50 - 60 hz.
> My question is how do you calculate the maxium DC voltage capability 
> of an AC rated capacitor and are there any trade-offs or gotchas! 
> involved with DC service.
> Thanks!
> Greg
> WA7LYO
> Kinston NC

Hmmm... I've used them with success. This is my analysis. You can 
decide if I'm right or wrong:

On each half cycle, the AC capacitor will see a peak that is 1.414 
times the RMS voltage applied. So, in the first half-cycle the 370v 
cap will see about 520 volts peak.

BUT, on the next half cycle, the cap will see minus 520 volts in 
addition to the plus 520 volts from the previous half cycle, so the 
DC breakdown voltage has to be at least 1040 volts for a cap rated 
370 volts AC.

So, an AC cap should be safe to 2.8 times it's AC voltage rating when 
used in a DC supply.

Gotchas? The only one I know of is that some AC caps are not just 
caps, but have a series resistor integral to the unit. These will be 
labeled "protected" or something similar, and I'm not sure that a 
series resistor is the only way to "protect" a motor cap.

Does this help?

Marvin
KA7TPH