EMRFD Message Archive 5689
Message Date From Subject 5689 2011-01-06 16:48:38 g0kla High voltage capacitors While we are on the subject of amplifiers, I need to order some capacitors for the Low pass filters. A quick calculation gives voltages of up to 350v peak for a 2:1 SWR. So 500V or 1KV capacitors are needed.
At first I looked at Silver Mica capacitors, thinking they would be the ones needed to handle the voltage, but they are $2-$4 each at mouser. Is there any reason why I can not use 1KV ceramic disk capacitors for this? They seem to be pennies and available for the values that I need.
5692 2011-01-06 21:26:06 R Wall Re: High voltage capacitors Hi Chris,
I purchased my capacitors from:
Roderick Wall, vk3yc.
5693 2011-01-06 21:44:02 R Wall Re: High voltage capacitors Chris, I’ve also purchased Mica Capacitors from:
Roderick Wall, vk3yc.
5694 2011-01-07 08:42:34 Tim Re: High voltage capacitors I would be reluctant to use non-NP0/C0G ceramic disks in a tuned circuit (even a low pass).
Most cheap 1kV ceramic disks or MLCC's will by X5R, X7R, Y5P, Z5U. These can be good for bypass purposes but will not be so good in a tuned circuit. It's not just the tempco, those dielectrics will change properties depending on applied voltage too (nonlinear dielectric properties).
Note that in a low-pass the capacitors are set up as sort-of bypass capacitors and to be honest I have used X5R/X7R/Y5P/Z5U type ceramic disks in low pass filters without major incident - but I knew those low-pass sections were not particularly critical to begin with.
Today many of the NP0/C0G ceramic caps you find will be MLCC's. Good dielectric material (e.g. porcelain ceramic) MLCC's are capable of outperforming silver micas in every respect in demanding high current RF circuits. But most of the MLCC's you see listed in Mouser or Digikey will not be the the good dielectric stuff, and when you price out the good dielectric leaded MLCC's you will probably find them comparable to silver micas of the same spec, maybe even more expensive.
BTW, the really good high voltage good dielectric RF MLCC's these days are available primarily in surface mount, if for no other reas