[Amps] Solid State Relays Again

[Peter Voelpel]

I guess with transformers the relay should switch in on the peak voltage.
As current peaks 90 degree behind that should be the best moment with least
surge current.

73
Peter



-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of TexasRF at aol.com
Sent: Montag, 10. Februar 2014 18:50
To: donroden at hiwaay.net; amps at contesting.com
Subject: Re: [Amps] Solid State Relays Again

But, unlike a mechanical relay, the SSR typically fails with a short  
circuit. The power supply could be energized when expected to be off. There
is a  
bit of residual current leakage in the off state that can leave a few 
hundred  volts present on a hv power supply.
 
Of course the relay can fail with welded contacts and have the same  danger.
 
A capacitor input type filter will cause much higher current peaks (5X not  
unusual) than the average current. This makes over sizing the SSR current 
rating  very important. In addition, there are small losses that are subject

to peak  current squared heating. At 100A and only .01 ohms,  this amounts 
to 100  watt peak power loss and the SSR does get warm. This makes attention

to adequate  heat sinking important as well.
 
After loosing a couple of 70A rated SSR's in a 70cm kw amplifier I  went 
back to the original mechanical relays. 
 
Can someone please explain why turning on/off at a zero crossing is a  
problem? Seems that could still happen at times even with the random
on/off.
 
73,
Gerald K5GW
 
 
 
In a message dated 2/10/2014 8:58:26 A.M. Pacific Standard Time,  
donroden at hiwaay.net writes:

Quoting  KA4INM <ka4inm at gmail.com>:
> and an instant disconnect if the  protection circuit detects too much  
> screen current or a sharp  increase in (negative) power supply current
> (an arc) surely make them  well worth bothering with.
> -- 
>   Ron   KA4INM

If for no other reason.

Don  W4DNR


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