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crowbar ovp

throbscottle

Well-Known Member
I decided to use a fuse + zener for over-voltage protection, in the case of a regulator failing s/c or other steady-state ov.

Is a zener the best choice for this? Or should I be looking at a TVS?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The standard crowbar circuit has always been a zener and a thyristor, just a zener on it's own would tend to fail S/C - and is a common occurrence where just zeners are used (and are intended to fail in that way).
 

JimB

Super Moderator
Most Helpful Member
Have a look at this:

PSU with Crowbar.png

A high current (20A 12v) PSU which if the regulator failed, could apply about 20v to an expensive HF radio.

The crowbar modification is highlighted in red.

JimB
 

throbscottle

Well-Known Member
That's a good idea, I might do that. Would mean I could use a polyfuse like I wanted to and not a wire one (not enough volts to trip the polyfuse, with zener option).
So, TVS not worth bothering with at all for this, then?
 

JimB

Super Moderator
Most Helpful Member
So, TVS not worth bothering with at all for this, then?
Probably not.

With the zener and thyristor, the trip point is well defined, when the thyristor is turned on it creates a short circuit and blows the fuse.
When the fault is clear and the fuse replaced (or cooled in the case of a polyfuse), the whole thing is ready to go again.

Also consider that TVS is a Transient Voltage Suppressor, something to remove spikes, not to deal with a hard high current source that has gone overvoltage.

JimB
 

throbscottle

Well-Known Member
To be honest I would never have considered a TVS for that exact reason, but that I found reference to them being used in this way when searching for info on the subject, so thought I'd ask.
I've got a small SCR somewhere anyway - only needs a couple of hundred mA so should be good.
Thanks for the help :)
 

crutschow

Well-Known Member
Most Helpful Member
Here's the LTspice simulation of a slightly more complicated SCR crowbar circuit that uses a programmable TL431 reference as a comparator, if you want a more stable, accurate, and adjustable trip point than a Zener gives.
You adjust the R2/R4 resistive divider to give a REF voltage of 2.5V at the desired trip point (adjusted for a 6V trip in the simulation).

1611066508780.png
 

gophert

Well-Known Member
Most Helpful Member
Motorola (ON Semiconductor) used to make a programmable crowbar tripping IC. Two resistors for the programming and an SCR for the short circuit tripper. If you need a few, let me know. I'm not sure they are still available.

DAE92EF4-6830-4FB7-B205-F8DD1B12E687.jpeg
 

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