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Re: [Amps] thyristor controlled soft start?

Subject: Re: [Amps] thyristor controlled soft start?
From: Steve Thompson <>
Date: Sun, 23 Mar 2008 16:56:17 +0000
List-post: <">>
>> Out of curiosity, how does a zero crossing solid state relay work?  I had
>> thought it had a triac inside, but your comment about voltage drop makes
>> me question that assumption.
> Yes, normally they use either a TRIAC, or two antiparallel SCRs. And 
> they do have a significant voltage drop. These thyristors by itself will 
> always switch off on the zero crossing (of current). True zero crossing 
> solid state relays have some circuitry that produces a window for the 
> firing pulse that falls into a small time span just after zero crossing, 
> so that switch-on is on the voltage zero crossing and switch-off of 
> course remains on the current zero crossing.

A comment, in case it's not self evident - a zero crossing switch 
won't give you a soft start function. It turns on at (almost) zero 
voltas and stays on until the current falls to zero. To use a 
triac for soft start you need to increase the conduction angle 
over a number of cycles to gradually charge up the psu capacitors.

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