Steve, I would make that decision once you get to the root cause of the PTO instability. Between now and then if you want to try a Polystyrene cap to simply rule out the dog-bone cap, I think it's fine. Both Polystyrene and silver mica are both highly temperature stable.
Keep in mind that the self inductance of a Polystyrene cap is typically greater than that of a comparable silver mica type. Also, Polystyrene caps typically have a red tracer on one end. This end is connected to the outer foil. Why is this important? In applications where one end is attached to circuit ground or near +Vcc potential, the capacitor will be significantly more stable as the result of distributed capacity effects (e.g., a grounded coil cover). The tracer does not imply that it is a polarized cap -- it only distinguishes the end connected to the outer foil. Analog Devices has produced a nice summary of capacitor dielectric types: http://www.analog.com/library/analogDialogue/Anniversary/21.html Paul, W9AC ----- Original Message ----- From: Steve Wedge To: drakelist@zerobeat.net Sent: Sunday, July 31, 2011 2:49 PM Subject: [Drakelist] Capacitor type for PTO C146 ("The Dogbone") 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? Steve Wedge, W1ES/4 "I can't complain, but sometimes I still do." - Joe Walsh If the above message appears, it came from Steve's Son of Laptop! ------------------------------------------------------------------------------ _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
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