[Amps] Question on Step Start Relays

[TexasRF@aol.com]

In a message dated 8/17/2005 9:37:49 P.M. Central Standard Time,  
dennisberry at worldnet.att.net writes:

There  has been a lot of recent discussion on Step Start circuits.  Would one 
of  the 'hockey puck' or solid state relays work OK in this application?   
They are usually available with 25 amp capability at a decent cost.  I'm  
curious if the solid state relays are susceptible to any RFI/EMI pick-up from  an 
amplifier?  Any experience with one of these devices?

Thanks  for any comments,

Dennis

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I have used solid state relays for several years in this application with  
excellent results. As usual though, there are some gotchas. 
 
Most of us use capacitor filter high voltage power supplies. These power  
supplies characteristically have peak current requirements five to ten times the  
average current draw. This is because the filter bank has to be recharged d
uring  the short period of time that the ac half cycle voltage is between the 
filter C  voltage and the peak of the ac cycle.
 
At full legal power limits, a solid state relay rated at 25A will be under  
stress due the high peak currents during key down periods. I have had 25A units 
 fail under these conditions. After switching over to 40A units I have seen 
no  more failures.
 
Also, there is a voltage drop across the solid state relay that generates  
some heat. The relay must be mounted in a manner that the heat is absorbed for  
long life; you can't hang one out in space like a mechanical relay.
 
I have had no rfi issues but my antennas are some distance from the power  
supply location and that probably helps.
 
73/Gerald/K5GW