[R-390] Paper Capacitor Replacement II

[Charles P. Steinmetz]

Barry wrote:

>With all this talk about capacitors, I thought I'd ask about HV 
>polypropylene caps.  What are some of the characteristics to watch 
>out for with this construction?
>
>The reason I ask is I have a General Radio C bridge that lost its 
>standard capacitor (0.5uF at 800V).  I have some polypropylenes from 
>WIMA that I'm considering using to replace it.  Particularly, I have 
>a 0.33, a 0.15, and 0.011 that I planned to put in parallel and then 
>finish with silver micas to get as close to 0.5uF as possible.

For a capacitance bridge, you would really like a polystyrene 
cap.  Polypropylene may be the next-best of the common dielectrics, 
but still not as good.  It would be unwise to add silver micas to 
make up the value, due to their inferior dissipation factor and 
dielectric absorption -- better to leave the cap below value and 
apply a correction factor to your measurements.  You will want to 
know the value of the capacitor you install to better than 1%, so you 
will need to measure it with a calibrated meter or bridge before you 
install it in any event.

Best regards,

Charles