[AMPS] Capacitor voltage rating?

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Mon, 12 Oct 1998 11:28:36 +0100

Fraser Robertson wrote:
>I am building a HF LPF to handle 150W.  In a 50R system that equates to 87V
>rms, or 245Vpp.
True, but the filter sections are impedance-transforming L networks so
the impedances inside the filter can be several times higher than 50R. 

>I have some polystyrene capacitors rated at 630V dc.  Would these be ok?
>How do I de rate the dc voltage for RF?  Presumably it depends on the
>dielectric.  Would I be better off with silvered mica caps, readily
>available at 500V dc rating?
Yes, much better! Silvered micas also have the current-carrying
capability for the kinds of power levels you're using (but they die at a
kilowatt on 144).

>I appreciate that the voltage could go higher into a non 50R resistive load.
>They seem to get away with very small capacitors in modern black boxes.

"Get away with" is probably pretty accurate. However, those boxes also
tend to use VSWR protection.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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