[sdiy] ASM Capacitors/Bench Power Supplies

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: Re: [sdiy] ASM Capacitors/Bench Power Supplies
Date: Thu, 31 Jul 2003 10:49:56 -0600

> At 03:32 AM 7/31/2003, Neil Johnson wrote:
> >Steven,
> >
> >1. Polystyrene capacitors are needed for the oscillators because of their
> >excellent low leakage and stability.  As is the way, they are therefore
> >more expensive than polyester or ceramics, which are fine in other places
> >(I use polyester for audio path, and ceramic for decoupling).
> 
> Does anyone know of a reason not to use NP0 ceramics for the integrating 
> cap? I briefly tried both NP0 and polystyrene in my last VCO and didn't see 
> any difference in waveshape or tracking.  The NP0 has a smaller 
> tempco.  Also it seems to me that the high-grade mica units, although 
> expensive, should work better than polystyrene.

In general, ceramics is bad due to leakage, memory effects, tempco etc.
If this is critical for NP0 in oscillators I can't say, but memory effects you
want to avoid and leakeage is certainly a low-freq problem. I think tempco
might be the thing where NP0 can be better than some plastics.

> Also, PEN (polyethylene naphthalate) and PPS (polyphenylene sulfide) look 
> interesting.  Mouser has some CD PPSs in chips, the only ones I have seen 
> readily available.

Look at leakage and memory effects. I would assume that memory effects is
much lower for any plastic than for ceramics or electrolytics due to the lower
relative capacitance and hence less strain in material.

Cheers,
Magnus