# Open a circuit when voltage exceeds a certain value?

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!

• You don't specify the power or voltage of the alternator you're using. Where I come from (high power land) we have overvoltage / overfrequency sensing relays hooked into the shunt trip of a circuit breaker. May 27, 2014 at 15:20

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.

• Thanks! It's not exactly a binary system but I don't see why it'd be a problem. So basically, M1 is held in conduction by the pulldown R1 and rapidly shutdown by M2 forcing it to high when the voltage from the voltage divider crosses the saturation gate voltage? May 26, 2014 at 19:37
• @MisterMystère precisely and if you make a comment tomorrow I'll be at work and able to fill you in on the MOSFETs. May 26, 2014 at 19:48
• Great, thanks. I don't have to wait until then to accept the answer. Quick and clean. May 26, 2014 at 19:50
• Last time I had to do something like this, I put some hysteresis into it, but I'm a worrywart about things. May 26, 2014 at 19:53
• @MisterMystère I've changed the values in the circuit dude. May 28, 2014 at 12:58

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.

• Thanks, that's a good idea. However I had forgotten to mention the fact that such system should be self-disengaging. I updated my post with a third requirement. May 26, 2014 at 18:23
• Er, the "circuit to be protected" shouldn't be connected directly to the power source -- it needs to be switched through the relay contacts! May 26, 2014 at 18:26
• @Cornelius Triacs basically behave the same as SCRs when you put DC through them. They latch on, provided the holding current is exceeded (in either direction, in the case of a triac). May 26, 2014 at 19:22