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Precision adjustable crow-bar

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Jester

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I'm in need of a precise crow-bar circuit with 3.5V trip

The device should be capable of sustained 3A current

In an ideal world: An OTS circuit on a board or an instrument that can have the threshold set precisely (say within 0.1V or so) in the range of 1.8 to 20V or so

Are there any off the shelf ready made devices available?

If not suggestions on a fairly precise adjustable circuit?

Thank you
 

Simple method to turn on a transistor when a zener diode conducts at 3.5V.

9232927800_1463594744.png


The above schematic illustrates the concept. However it causes gradual turn-on. Your aim probably is for the crowbar to turn on sharply. Therefore the solution is to have a high-gain device (such as darlington/ sziklai arrangement).

Or else an SCR could figure in somewhere. It turns on suddenly.
It also causes the crowbar to stay on once triggered. (In some cases that is desirable.)

In an ideal world: An OTS circuit on a board or an instrument that can have the threshold set precisely (say within 0.1V or so) in the range of 1.8 to 20V or so
Click to expand...

To have it adjustable, there is the TL431. It works like an adjustable zener diode. However it may not operate down to 1.8V.
 

Jester said:
I'm in need of a precise crow-bar circuit with 3.5V trip

Are there any off the shelf ready made devices available?
Click to expand...
Check out the Motorola MC3425 integrated circuit, it works down to 2.6 (nominal) volts.

The biggest difficulty with these types of crowbar circuits is false triggering from noise, and Motorola have addressed that problem.
 

Brad, that's a clamp, not a crowbar.

Maybe a clamp is ok or do we actually need a crowbar which creates a hard short as soon as the voltage exceeds 3.5V? Does the short need to latch until all current is gone or does it just need to clamp down excessive voltage?

Given the 0.1V precision I picture a low voltage TL431 variant triggering a PNP triggering an SCR (Littelfuse makes SCR's for this current range in SOT-223 or DPAK).

An SCR with a zener triggering its gate is a simple standard crowbar topology but if 0.1V is really the spec that won't do it.

EDIT: It's always good when other people have the same idea:
crowbar5.png


https://axotron.se/index_en.php?page=26
 
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Thanks for your ideas.

I made a quick and dirty one, 10 turn pot driving a SCR.

To test I stepped from 3.3V to 5.5V and it looks like it takes about 4us to activate.

The objective is to protect a bunch of 3.3V IC's that got smoked when the buck converter I'm working on inexplicably jumped from 3.3V to about 5V

I wonder if 4us is fast enough to protect 3.3V logic, memory etc?
 

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