[Amps] open frame relay rating

[Pat Barthelow]

Hello Steve,

There have been extensive past discussions on high power relays, in RF Amp service, on this reflector.  I think consensus was that contact ratings of ordinary relays, were often for DC or near DC currents, and that considerable de-Rating needed to be done in High power RF service. I think you are correct in your calculations of required current, as long as you allow some extra headroom, over the calcualted current values.    So 5.5 amp RF current service may translate to a standard DC control relay contacts needed to be DC rated at 10 or more amps. There were also other important physical design parameters, capacitance, isolation, RF voltage exposure, etc.  

Also, receive modes causes problems because relays depend to some degree on a minimum current through their contacts to keep them clean.  Receive service obviously does not have that minimum current, so that is sometimes a problem.
I bought an open frame relay  from a surplus vendor that was claimed to be used in a commercial/military amp  (Collins) at a KW RF level.

Take a look at the RNF 100-SP at :
http://www.surplussales.com/Relays/rfopen.html

The silver plated contacts to me looked really tiny,  and the armature contact were distinctively hemispherical, and only contacted at a very tiny area, I guess to keep the PSI pressure up... I was surprised at it's ratings..

All the best,
Pat Barthelow
aa6eg at hotmail.com
http://www.jamesburgdish.org

> From: kk7uv at bresnan.net
> To: amps at contesting.com
> Date: Sat, 29 Mar 2008 09:54:22 -0600
> Subject: [Amps] open frame relay rating
>
> I have several open-frame relays. If I use one for the T/R relay, what should the contact rating be for a 1500w amp? Is it as simple as 1500w / 50 ohms = SQRT of 30A or 5.5 Amps?
>
> Tnx,
> Steve, KK7UV