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Simple over voltage protection circuit, using mosfet.

Hello guys. I'd like to know if I can use a mosfet (as shown on sample schematic). The idea is to turn off my circuit (connected to V-OUT) if more than 15V is applied. Please let me know if this works. Thanks.
 

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Diver300

Well-Known Member
Most Helpful Member
That should work. You may want to add a capacitor on the output so that there is no significant voltage rise in the time it takes the MOSFET to turn off, if the input rises quickly.

Also, you could add a high value resistor between Vout and the base of the transistor. If the input voltage rises slowly, you don't want the MOSFET running in it's linear region as it will get hot. The suggested resistor will add hysteresis, which should stop that happening.

To test the circuit, supply it via a diode from its normal voltage. When you connect the higher voltage, do so after the diode. That way there will be no break in the supply when the higher voltage is applied.
 
Got it. That makes perfect sense, will add a large cap to the output. For the resistor across Vout and base of transistor, I'm not sure of the value, something like 100K will do?
 
In my case I prefer to shut down the circuit, it's more like a protection since Vin isn't supposed to go above 14V. The circuit protects it at 15V or more. I don't know how to calculate the value of the resistor across the base of transistor and Vout, if you can, please let me know. Thanks.
 

Diver300

Well-Known Member
Most Helpful Member
Got it. That makes perfect sense, will add a large cap to the output. For the resistor across Vout and base of transistor, I'm not sure of the value, something like 100K will do?
The hysteresis will be approximately the output voltage multiplied by the ratio of the base resistor (R29 on your diagram) and the new resistor.

If the output is 15 V, R29 is 1k and you add a 100k resistor, the hysteresis will be about 0.15 V.

If this is to be on a vehicle, you should set the voltage a bit higher than 15 V so that common surges don't cause the circuit to turn off. Maybe go for 16 V. A circuit like that is very useful for protection against load-dump that can go as high as 60 V.
 

crutschow

Well-Known Member
Most Helpful Member
That Zener has a tolerance on its breakdown voltage of 14.25V to 15.75V.
You add about 0.65V base-emitter voltage to that before Q5 turns on, thus the maximum turn-off point for that circuit could be as high as 16.4V.

Below is the LTspice simulation of the circuit using a low-cost LM431 adjustable reference in place of the Zener, so you can set the voltage to the desired value with pot U2.
The output (yellow trace) is cutoff when the voltage reaches 15V and turns back on at about 14.4V (for about a 0.6V hysteresis caused by R4).

1666452104989.png
 

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