On Tue, 27 Jun 2000 07:38:01 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
>I've asked before on this topic without an answer - maybe lucky this
time.
>What's the failure mode for SSRs? Do they fail short circuit or open?
Hi Peter
They generally fail shorted... if they open, it is because they are
broken! :-)
I understand a stuck relay but, I dislike melted sand.
I've shied away from using them myself for the HV control...even
though I have a drawer full of big ones, yet, I seemingly have no
compuctions against using them for low power control: fans, heaters, grid
supplies. etc.
>Working on the basis that 'Murphy rules - OK?', I've always assumed that
the
>failure mode is short circuit, and then figured that mechanical relays
can at
>least be seen to be stuck closed (well, sometimes) and gone for those.
Plus a
>neon indicator across the bottom electrolytic of the HV stack, a circuit
breaker
>on the AC feed so the amp can be totally isolated, and final line of
defence, a
>shorting stick. Even so, I never feel happy about touching the HV lines
after
>all that....but if the SSR fails short, it could be inconvenient at
best.
I vaguely recall an interesting safety feature (not sure if it was from
a Ham publication or one of the electronic trade magazines) that used a
(sonic) beeper to warn if there was HV present in the abscence of the
acutating signal. This could be used with either an SSR or a
conventional relay. I'll have to dig through the files and see if I can
find it -- though, I think it used a FET driven from the control signal
(with a short RC delay to preclude nuisance beeping during a "normal"
shutdown) to short the beeper which was driven from the HV (low end of
the bleed?).
73,
Marv WC6W
*
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