Over-Voltage Protection – Crowbar Circuit for 35V OVP Parts


Please excuse my confusion, because I really don't understand what the "150mA max continuous cathode current" means on the datasheet for the LM431. I'm also not sure it'll work in the crowbar circuit I intend to build. (I suspect it will, seeing as it's rated for 36V).

I have the following circuit diagram, based on a Wikipedia description of a crowbar circuit:


simulate this circuit – Schematic created using CircuitLab

D3 leads to a(n) LM3481 used as a SEPIC (Pg 26) to output 7V for a load that draws between 500mA to 1A (1.25A to be on the safe side).

This leaves me with a number of questions, assuming I want the crowbar to reject (short out) input voltage higher than 35V:

  1. If the input voltage at D1 is higher than 35V (unlikely, but possibly as high as 40V), will the LM431 become damaged or will the fuse burn out first?
  2. Will a polyfuse/PTC work for F1? What part type/specifications should I look for?
  3. To what should I connect the wiper for R1, GND? Does R1 need to be a POT or can I use a fixed resistor?
  4. What are the values for R1, R2 and R3?
  5. DI1 is a TRIAC. What specifications should I look for?
  6. Does the 150mA mentioned in the data sheet have anything to do with the load after D3? What is it actually telling me?
  7. I know that I can use Vout ~= (1 + R1/R2)Vref to work out what R1 and R2 should be, but I don't know how I work out Vref. Is that Vin from D1 or from the LM431?

I have seen this as a proposed solution, but apparently the MOSFET doesn't short to ground (according to one of the comments).

Best Answer

Something like this, then?


simulate this circuit – 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.