Need help for FET crowbar circuit

[fjpompeo]

FET crowbar circuit

Hi all,

I need a crowbar to protect my HAM rig so was thinking to use the classic SCR+ZENER but looking ahead this circuit is very sensitive to RF then a prefer something more accurate like a voltage comparator (e.g. LM311 refered with TL431) driving a power MOSFET instead of a SCR. Is there any problem?

Thanks,

Fernando

 

[dick_freebird]

Comparator and MOSFET may be slow to react, and probably
will not "crowbar" the same way - SCR will pull in and hold
until current is removed. The comparator / FET scheme may
trigger, but then release when the FET has discharged
the load, only to see the load pop back up (motorboating
in and out of action). Seen this in designing ESD protection.
You might need a low voltage housekeeping supply so you
can have a comparator and a latch (plus you need the FET
gate voltage to be ~5V ("logic level") or ~10V (standard)
where the SCR only needs the trigger current and away
you go.

Maybe you just want to add a shunt capacitor to the trigger
resistor shunt, maybe choke the thing too, to get RF under
control. There ought to be some space between RF band, and
the tolerable actuation-time of the crowbar unless this is
really low frequency and really fragile components.

 

[fjpompeo]

Very thank you for the information. What about to build a MC3423 using MC393 and TL431 or even discrete comparator circuit? I don't know way but MC3423 is very rare here in Brazil so I'm looking for a replacement for this circuit.