I normally use these as the HV supply master mains switch in the amplifier
and have never had a dv/dt problem. If MOV's make you nervous, spark gaps
are also suggested. According to Crydom their product's surge current
rating is 10 times the rated RMS value, nonrepetitive peak current for one
line cycle. They do take a licking and keep on ticking. I also use these
to control filament transformers, not necessarily for inrush but to shut
down the tube in the event of air flow failure. I trust my life to these
things everyday as I drive through traffic signals controlled by them, so
far so good:-) 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!!
Cheers, Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://wd7s.home.att.net
>
> >>one minor bug is that the lightly loaded solid
> >> state relay has once operated by itself, probably due to a mains
> >> transient, and the power supply has powered up by itself, hardly
> >> a major concern in a fully enclosed low voltage power supply but
> >> not something you'd want to trust entirely. This glitch has only
> >> occurred twice in maybe 10 years but still not acceptable for a
> >> high voltage supply.
> >>
> >>
> >The SSR manufacturers recommend MOV's at the line side terminal to avoid
> >DV/DT turn-on.
>
> Even better would be a mains filter, then the master mains switch, and
> then the MOVs on the SSR.
>
> The filter protects the whole amp, including the transformer and
> rectifiers, from incoming mains spikes. The spikes are attenuated and
> stretched enough that they may not affect the SSRs anyway, and the MOVs
> are a good backup. Since MOVs can be 'worn out' by having to handle too
> many transients, it's best to connect them downstream of the master
> mains switch, so they only see mains voltage when the amp is actually
> switched on.
>
> Although Paul wrote earlier that he'd never had an SSR "weld shut", I
> still feel uneasy about that, because semiconductor devices do tend to
> fail short rather than open. I haven't tried SSRs yet, but I just wonder
> about the minimum guaranteed current breaking capacity for any load, any
> phase angle?
>
> OK, failing short-circuit is not likely to happen in a soft-start
> circuit, and wouldn't be a big problem if it did... but the mains relay
> that feeds the HV transformer is another story. That relay *must* open
> reliably for safety reasons. The bigger the current surge, the more
> important it is that the relay can stop it.
>
> Maybe I'm being old-fashioned, but where safety shutdown is involved, I
> somehow feel more confident in a device whose contacts physically move
> apart.
>
> (BTW, my standard shutdown test is to drop a piece of copper pipe into
> the anode compartment so it crowbars the HV. The protection circuit has
> to shut down the HV quietly, with no big bangs and without even blowing
> the mains fuse... and then do it again and again, to cover all phase
> angles of the mains cycle.)
>
>
>
> --
> 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
> Editor, 'The VHF/UHF DX Book'
> http://www.ifwtech.co.uk/g3sek
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