[R-390] Paper Capacitor Replacement II

[Barry]

> 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

My first attempt at replacing this is an array of polystyerene caps; however, since they're only rated at 250V, I had to put them in series (actually 9 caps, 3 sets of three in parallel and then those three in series).  In this application, I can't very well use balancing resistors, so putting them in series this way was only a temporary device.

I'm going to put the three polypropylenes on a board and have a local metrology lab tell me what their total capacitance is to as close a tolerance as they can give.  From there, I plan to pad the board with caps needed to get to 0.5uF at (hopefully) better than 0.05% (0.01% if possible).  The reason I was going for SMs is I can get them in very tight tolerances.  Once the lab gave me a value, I could then get very, very close with the SMs without having to go back and bother them for more measurements.

I don't know what GR used in the original cap.  I emailed with the original designer (Henry Hall) and he seemed to recall it used a combination of cap types to keep the tempco as constant as possible.  I think it uses polystyrene as one of the types but not sure about that.

Thanks for the reply!

Barry - N4BUQ