Crowbar circuit design

[Sidewinder]

OK, HELP PLEASE!

I am so rusty on doing component level design it's unbelievable.

Done a few searches and come up with nothing concrete yet.

I need to quickly make a 1 off crowbar circuit to protect a nominal 12V supply, voltage maximum 14.8V, (Vcc) I need to blow the supply fuse if it over voltages, which is 7.5A (F1).

There are a few ways, would like to have it quite accurate if possible, so the TL431 circuit suits.

As per attached.

I'm going through trying some designs with RS Design Spark, & I have the TI datasheet for the device.

However, if anyone has any good suggestions for component values, or component codes, that would be great!

I can breadboard it & pot it once done as it only needs 2 connections supply Vcc & ground.

Thanks in advance.

 

[ProDave]

It's a long time since I have built one, but try something like this, don't over complicate it.

https://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit

 

[Sidewinder]

I looked at that, but, I need to trigger the over voltage quite precisely, I'm not sure if that circuit will give me what I need, I need to allow up to 14.8V, but trigger between 15 & 16V.

Plus I need to blow a 7.5A fuse.

I have found a more simplified circuit since with C1, R1, R2, R3, a Triac & the TL431. 

View attachment Crowbar1.bmp

Canoe, just been checking out the TL43xx datasheet and came up with that basic circuit, I think I've posted a bmp of basically the same one.

 

[Sidewinder]

I'd just plumped for a BT139 triac which is 20A, I wanted a bit of headroom on the triac, as I want the crowbar to survive the overvoltage, but the fuse to expire!

I'd failed to find a fully built crowbar ic so far

 

[Sidewinder]

That one seems to be EOL/Obsolete

 

[Sidewinder]

It looks like I've gone around in a circle from the TL431 to the MC3423 & back to the TL431!

 

[Marvo]

Did you make your crowbar? I'm a bit late, been away for a couple of weeks. If this is a standard 12-14V PSU application ie there's no RF component then the common or garden SCR crowbar with a tight tolerance zener trigger should be fine. They're a bit hit and miss in PC 5V rail applications but you should be fine with 12V. The advantage of an SCR is it usually fails in short circuit if it gets damaged. You'd just need a single clamping resistor and a shunt cap to damp spikes that could accidentally cause it to trigger.

 

[Sidewinder]

Bits are due today, have another issue with the job now.

Need to look at it again & may need to add some more bits as well as the crowbar.