Depending upon the maker and the value of capacitance, a SM will exhibit about +30 to 100PPM. Polystyrene is a little more consistant at around+125PPM. Both cap types are popular when combined with iron powder cores of low mu because they buck the mild negative TC of the core. Polystyrenes are hard to find these days. They have a low melting point which is not so good for surface mount boards. There is also the mass to consider. A large sized cap will take longer to thermally stabilize. That may be good or bad depending upon the mass of the other components you wish to buck. Does anyone know what dielectric comprised the original dogbone? I would stick with that if possible. If the dogbone is ceramic, though, good luck determining it's actual TC as ceramic can have many tempcos.
Dennis AE6C On Sun, Jul 31, 2011 at 2:01 PM, Garey Barrell <k4...@mindspring.com> wrote: > Steve - > > Stick with the 'dog-bone'. Hard to beat a silver mica for temperature > stability. > > PLUS........ > > EVERY capacitor has 'some' temperature sensitivity, just a matter of how > much, and _which way_! > > I'm going from memory here, but as I recall a silver mica has a very slight > negative tempco, while polystyrene caps have a slightly positive tempco. > Obviously this has been taken into account and compensated for by 'the > ladies' at Drake! > > 73, Garey - K4OAH > Glen Allen, VA > > Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line > and TR-4/C Service Supplement CDs > <www.k4oah.com> > > > > Steve Wedge wrote: > >> I'm looking at options for replacing the "dogbone" 3000 pF SM cap in the >> PTO. I've long heard the polystyrene caps are stable and of high quality. >> What's your opinion of using a polystyrene cap in place of the SM unit at >> C146? >> > > ______________________________**_________________ > Drakelist mailing list > Drakelist@zerobeat.net > http://mailman.zerobeat.net/**mailman/listinfo/drakelist<http://mailman.zerobeat.net/mailman/listinfo/drakelist> >
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