[R-390] Another Capacitor Question

[Charles P. Steinmetz]

Barry wrote:

>In this application, the system can apply up to 600VDC across the 
>capacitors (several in parallel).  I happened on some 1000V 
>polystyrenes for the bulk of the 0.5uF and need to finish with some 
>smaller value polystyrenes.  I can get some 1000V capacitors for 
>this, but they're pricey ($10 each in lots of two).  I can readily 
>get 630V polystyrenes, though.
>
>Is this too close to the max rating?  I usually get capacitors rated 
>at well beyond the anticipated applied voltage (1.5x or better) and 
>would do so here if it weren't for the lack of reasonably-priced units.

Is the 630 V a DC rating?  630 V is most commonly an AC rating, which 
does not translate directly to any particular DC rating (AC rated 
caps are generally rated for DC voltages much lower than the AC 
rating).  In the case of polystyrene caps, an AC rating means that 
the intended use is in high-Q resonant circuits, which can generate 
hundreds or thousands of volts with a DC power supply voltage of 24 V 
or less.  Even if the 630 V is a DC rating, that is uncomfortably 
close IMO -- it would only take a line voltage excursion of +6 V to 
put you over the edge (and remember that today's typical line 
voltages are already 5-10 V higher than they were when your bridge 
was made).  The original cap was rated for 800 Vdc.

You are rebuilding a very nice piece of test equipment history -- get 
the 1000 V caps and stop trying to cheap out!   ;-)

Best regards,

Charles