Open a circuit when voltage exceeds a certain value?

Question

General question

I would like to protect a load against overvoltage by cutting one of its power lines when the voltage exceeds a set value.

All the examples I have seen online present a way to short circuit the supply but not open it. I can't short circuit the supply nor use a Zener for regulation as the current drawn would be too high (and would limit it under normal conditions).

1. How can I drive a series transistor to cut one of the lines when a control voltage (from a comparator) is driven high (or low)?
2. How can I make sure that this protection is unengaged (i.e. supply is on) when the protection unpowered?
3. How can I make sure it can automatically disengage itself when the voltage goes back within allowed range?

Application

In my case, an alternator generates a voltage which is rectified and fed in a big capacitor before supplying a voltage regulator. I am designing the circuit for a certain alternator rate, but I want to protect it in case it goes faster. 2) comes from the fact that the protection circuit will be powered by the regulator past it... Which means, if it's engaged when unpowered the entire system will never power on!

Answer 1

I've designed one recently. Here is my memory test: -

^{simulate this circuit – Schematic created using CircuitLab}

I can't remember what the MOSFETs actually were so sorry for this but it isn't rocket science to simulate this circuit in LTSpice (e.g.) and try a few different MOSFETs. Mine turned off M1 when the input voltage got beyond about 30V - it was protecting a 42V (AMR) switcher on the output side for excessive voltages coming in. M1 is normally on when there are a few volts applied to Vin and as Vin rises past 30V (ish) M2 starts to conduct and shuts off M1.

Both transistors were rated in excess of 100 volts because the generator could lose control easily and shove out well over 50V (this would fry my switcher).

Reading your question, mine as a pretty similar application to yours!

EDIT - I've changed the values in the circuit to reflect what I used in my prototype.

Answer 2

The "crowbar" circuits you've seen online use a fuse and when the voltage exceeds a limit, this fuse gets blown because of a triac or SCR that connects it directly across power supply.

Although they are much slower than semiconductors, relays can be used to disconnect the power source. Here is an example by Tony van Roon:

Of course a NC (normal closed) relay will be used and its switch will be connected in series with the power source before the circuit.