EMRFD Message Archive 14469

Message Date From Subject
14469 2017-12-29 16:36:09 Ravi Miranda Sizing of WV of capacitors for an LPF

How does one size the Working Voltage of a capacitor for an LPF, is
this different for SSB/AM/FM?

Say, Vcc is 13.8V and the output is 35Watts (approx).

Any suggestions?

Have a great weekend.


I'm here to add more value to the world than I'm using up.
14470 2017-12-29 17:13:14 Tayloe, Dan (Noki... Re: Sizing of WV of capacitors for an LPF

P = Vpeak*Vpeak/(2*R)


For 35w and 50 ohms and re-arranging terms, Vpeak = sqrt(2*50*35) or 59.1v.  I would go with 100v rated parts.


  • Dan, N7VE


14471 2017-12-31 12:50:50 arfghans Re: Sizing of WV of capacitors for an LPF
There may be a bit more to consider, if you want to be thorough. The main thing is a mismatch condition, especially an open circuit, where the voltage will be higher. Also, some filter topologies with higher section Q will produce un-obvious increases in voltage (and current) in some filter elements. Try simulating your filter (and amplifier) in Spice. Apply various loads and see what voltages and currents arise in the filter elements. 

I went thru this process in the design of high-power bandpass filters, and it was a real eye-opener. With 1500W into mismatches (3:1 SWR), some capacitors saw 4 kV rms and circulating currents reached 27A... No wonder the high-power filters from 403A and VA3AM are huge, expensive, and get hot when run at full power.

Gary NA6O