poly/film caps are good for....

rwgast_logicdesign

so i have a massive amount of random metal film, poly sterene, and polyester film caps. i was wondering what these are suitable for besides audio.

im pretty sure the .1ufs arent good for decoupling as they dont filter hi frequency like ceramic right? right now im working on an hbridge circuit based on l293e i was thinking maybe adding some caps as power resivoirs before the ic would be a good idea. also have alot of small caps suitably sized for xtal feedback, but i always see ceramic also used there. od usually just use low esr electros and x7r type ceramic maybe a tant in most designs becuase its what i know.

basically i have tons of film and poly sterene and just looking for a usevothervthan audio which i dont do much of

Mike Green

Metal film / Polystyrene, and Polyester film capacitors are mostly for audio frequencies along with some low frequency RF use because they're relatively large physically and they're made of rolled up thin metal films with one or another plastic films as insulators. This gives them a fairly high inductance which makes them unsuitable for high frequency use. Ceramic capacitors also have thin metal films and insulating layers of ceramic, but these are small and have relatively little inductance, thus they're suitable for high frequency use including high speed digital bypass / power storage functions.

Tracy Allen

Film capacitors are good for timing circuits, also things like analog sampling, and integrators and differentiators. Say you need to generate a linear ramp of voltage, or you want to use the RCTIME type of command to measure an input voltage. Film capacitors do not change capacitance much when the voltage across them changes, and that is important when you need a linear ramp or measurement. They can be had in values with pretty good accuracy, say +/- 2%, so you don't have to trim your circuit so much. They generally are more stable against things like temperature changes. They do not store charge (soakage), so when your circuit samples a voltage, you can be sure it won't drift back toward its previous measurement. They have low leakage, so they don't behave like they have a resistor in parallel. They don't exhibit microphonics and sensitivity to vibration.

There is a lot of variation in types of film, too, but there are plenty of articles available to compare the advantages and shortcomings of different types. You are right, they are not a good choice for bypassing. But there you have it, you can add timing and signal processing to your list of potential uses. Oh, surface mount stacked film capacitors are readily available now that get around inductance problems that come from the leads.

rwgast_logicdesign

So how about crystal feedback? Mike says no to hi frequency but just what is hi frequency?