[Amps] soft start circuits

[Ian White, G3SEK]

PAUL HEWITT wrote:

>I normally use these as the HV supply master mains switch in the 
>amplifier and have never had a dv/dt problem.

What makes me nervous about SSRs as safety-critical circuit breakers is 
that if ever they do fail, they will almost certainly remain switched on 
(forever). In safety engineering terms, they are not 'fail-safe'.

Mechanical relay contacts may also weld shut, of course, but that's 
nowhere near as certain to happen. More than a century of industry 
experience makes it much more likely that the spring will pull the 
contacts safely apart.

(Maybe I've spent too much time around safety engineers. They are a 
simple, primitive tribe, who like technology that's obvious and sure to 
work... or if it doesn't work, is guaranteed to fail safe.)


>If MOV's make you nervous, spark gaps are also suggested.

MOVs don't make me nervous at all - I use them, and love 'em. The key 
difference is that we don't use MOVs as circuit breakers!

[...]


> I think I've posted this before but here it is again, Solid-state 
>Relay Handbook with applications, by Anthony Bishop, (ex Royal Air 
>Force, HI), from Crydom Company.  This is an excellent book(224pp) and 
>available from Mouser for under $20.  It covers all types of switching 
>for various loads and control side interfacing, (and MOV's, HI). 
>Everything you wanted to know about SSR's but were afraid to ask!!

Thanks, Paul - I do need to find out more about SSRs, and will 
definitely get that book.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek