Crowbar Circuit problem

[Warpspeed]

See how you go.

Another way to make the SCR even less dV/dT susceptible would be to get more serious with R4.

As it is now, R4 only shunts about 7mA which is not a lot.
If you make R4 only ten ohms it will then shunt about 70 mA.
You will then need to increase your 56mA to something like 50mA + 70mA.
Decreasing R3 to perhaps 91 ohms should get you there.

The required total 120 mA is still well within the crowbar chip capability.

 

[pscammp]

Hello all,
Alas the 100nf cap didn't work either, connected it on the gate side of Rg (220R) to Vee, also tried it between IC Pin 8 and Rg and Vee, still the same circuit breaker trip on 12v instantaneously. Still works fine if the crowbar is connected to the DC-DC output before the DC-DC is powered up.

warpspeed,

I'll look at that too thank you, lets just see if anyone has any other brain waves to add to it if you don't mind.

Just in case it makes a difference, the circuit breaker I am testing on is a 1A one of these:



Any other thought's ?

Many Regards
Paul

- - - Updated - - -

Sorry,
Missed the timeout for editing the post due to the Mrs........Here is the link to the CB in case it makes a difference:

https://www.rapidonline.com/techna-tr11c1-0-1a-miniature-panel-circuit-breaker-26-5200

Cheers
Paul

 

[Easy peasy]

you may have problem in that the ic is issuing a pulse at hard power up, so try a cap bigger than 100nF, AND put a snubber across the line say 1uF and 22 ohm

 

[Warpspeed]

Try disconnecting the 220 ohm gate drive resistor to the SCR, but leave R4 connected between gate and cathode.
See if that trips when power is applied.
If it does, it can only be by dV/dT triggering within the SCR.

If all is well, it must be as Easy is suggesting, your crowbar chip is firing the SCR at power up, and you need to look in that direction.