cyamamot@girtab.usc.edu (Cliff Yamamoto) (12/22/89)
Observation : I've read and seen circuits where a large value electrolytic & a small value mylar capacitor are wired in parallel for filtering. My understanding is that the small value mylar is for high frequency filtering and the large electrolytic for low frequency filtering. Problem : I want to filter the AUDIBLE whining noise from the alternator in my car (Yes I know about other methods, but I want to try stopping the problem at the source). Question : How are these capacitor values calculated? I'm asking because I was going to pluck down $30 for a 61,000 uFD 50 Vdc electrolytic capacitor to use as a filter across the alternator output. But then I remembered that a large valued capacitor does not filter ripple for ALL frequencies (it would probably just filter lower audible frequencies, not the irritating higher pitched whining sound). Again, I'd like to ask those analog and power supply gurus out there how one selects the proper values capacitor(s) for filtering ripples that have frequencies in the audio as well as RF ranges. How are these parallel combinations of capacitors actually chosen? Is there any kind of formula to use? Are there any pros and cons? Thanks for any info! Cliff
jeffw@midas.WR.TEK.COM (Jeff Winslow) (01/04/90)
In article <7152@chaph.usc.edu> cyamamot@girtab.usc.edu (Cliff Yamamoto) writes: >Again, I'd like to ask those analog and power supply gurus out there how >one selects the proper values capacitor(s) for filtering ripples that have >frequencies in the audio as well as RF ranges. How are these parallel >combinations of capacitors actually chosen? Is there any kind of formula >to use? Are there any pros and cons? For every capacitor you can draw an impedance curve with respect to frequency. Real capacitors have series inductance (leads, and plates wound in a cylinder) and series resistance (leads, plates, some dielectric losses?), so, above a certain frequency they don't act like a capacitor. Electrolytic caps have high ESR's relative to capacitive reactance near the series resonant point, so they have broad minima in their impedance curves. Film and ceramic capacitors tend to have much smaller ESR's relative to capacitive reactance at the resonant point, and so can have quite a sharp minimum. The thing to do is probably to get the impedance-frequency curves of a whole lot of parts you think might be useful, and try to overlap selected ones to get an impedance that stays below what you need to filter out your ripple. (Of course, this is going to depend on the output impedance of your alternator and connection to it.) Watch out that you don't start getting parallel resonances when you do this - say, from the ESL of the electrolytic combined with the C of a film capacitor. You could play around with SPICE to see what your impedance curve might look like, but since no SPICE I know of models frequency-dependent resistance, the simulation will be pretty approximate. Jeff Winslow