Watcher:
Any recommended part values for the "crowbar" above?
I'm not sure the R or C are needed - just make sure the zener leakage at the
working voltage is well below the minimum trigger current of the SCR. Note that
SCR gate will be a volt or so above ground so the zener voltage will be a bit less
than you might think - check the datasheet for the SCR in question (it just has to
be big enough to handle the current, so most devices will be fine).
Actually I found the site that crowbar circuit came from and it admits the C prevents the crowbar
from firing with short pulses, which is simply wrong - the whole point of the circuit is to prevent
the output voltage exceeding the breakdown voltage of the load, short spikes are just as able
to destroy as long spikes.
Just the zener and SCR are needed I conclude. In fact I'd add a small resistor in series with the
SCR gate to limit the current through it, perhaps 100 ohms or so. However I'd also then check
the circuit functions as expected with a 'scope to back up my intuition in case I've missed something.
For really demanding applications I would suggest a high speed comparator, flip-flop and MOSFET would
be better to respond really fast (SCRs are not exactly snappy switches).
Remember that there is a cap across the supply line (C2), and maybe more downstream. They should kill spikes.
A small cap on the gate might be needed if the crowbar circuit is used in a switching supply (HF more hash).
I would replace the zener/gate resistor for a 100ohm trimpot (gate to wiper).
And pick the zener 1-1.5volt less than supply.
Then you can trim the crowbar firing voltage, and have some gate series resistance.
Leo..
Capacitor C is there to stop triggering when the supply powers up.
Going from 0 to Vreg has in some instances the same gate trigger effect as Vreg to Vunreg when failure occurs.
Thanks all for your responses. Good to know about crowbar circuit protection. Also there was a recommendation to use 7805 instead of LM317, is that means 7805 is short circuit protected? i.e., even if its failed, there are no chances of input = output?
would replace the zener/gate resistor for a 100ohm trimpot (gate to wiper).
In the context, that suggestion is an absolute scream!
@Paul,
Forgive my ignorance but I'm not seeing the humor in this. What is your point ?
Is it that less than 100 ohms might not be enough current limiting for the zener ?
Or is it that there is no need for adjustment in this application ?
raschemmel:
Forgive my ignorance but I'm not seeing the humor in this. What is your point?
Whilst regulators can fail of themselves, at least two people agreed with me that the trimpot is the weakest link in the use of a variable regulator, failure of contact of whose slider will necessarily cause over-voltage.
You then suggested a crowbar circuit using - a trimpot!
Two completely different things.
While adjusting the pot of the voltage regulator, contact bounce can/will spike up the voltage.
While adjusting the crowbar pot, nothing bad will happen.
Zeners come in selected voltages only 3.9volt, 4.7volt, 5.1volt, etc.
Some finetuning could be needed when 5.5volt is ok and 6volt is not.
As suggested, ditch the LM317 use the 7805.
No crowbar circuit needed.
Leo..