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Re: [Amps] open frame relay rating

To: Steve Flood <>, <>
Subject: Re: [Amps] open frame relay rating
From: Pat Barthelow <>
Date: Sat, 29 Mar 2008 10:06:09 -0700
List-post: <">>
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 :

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

> From:
> To:
> 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

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