Crowbar Circuit for 35V OVP - What parts should I use?
Something like this, then?







simulate this circuit &ndash; Schematic created using CircuitLab

\$Q_2\$ is a big BJT easily capable of causing the fuse to burn out in the case of an over-voltage situation. (You could try something smaller, of course. But this BJT is way more than you need to make sure.)

Here, \$R_1\$ and \$R_2\$ are used to set the threshold voltage to trigger on. I've set the threshold just a little high, just by way of an example. But you can adjust these, as needed, to hit the point you want.

\$R_3\$ is designed to provide about \$7-8\:\text{mA}\$ for the base of \$Q_1\$. Assuming \$Q_1\$ is driven into saturation, this should still mean at least \$75\:\text{mA}\$ into the base of \$Q_2\$ (and probably a lot more than that.) I might consider adding a base resistor for \$Q_2\$, but since the point is to blow the fuse I'm not sure it's really needed. If you do add it, then something like this would be fine:







simulate this circuit

That should blow the fuse.

The main thing here is to adjust the values of \$R_1\$ and \$R_2\$ to get the trigger voltage you want. Or, use a \$10\:\text{k}\Omega\$ potentiometer in there, as appropriate, to allow an adjustable setpoint.

The voltage across the LM431 shouldn't exceed your trip-point setting, less a diode drop or so. You could add more circuitry to make absolutely certain to protect the LM431. But this arrangement may be okay for your needs.