# Mosfet reverse input protection vs. diode question

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#### Triode

##### Active Member
I'm curious if there is something here I'm missing. For my electric racer team, I designed a reverse polarity protection circuit almost exactly like this one on a 24V line. If the image doesn't show up, it's a P-channel MOSFET with a 10k resistor connecting the gate to ground, and a 10V Zener on the output to protect the diode from exceeding the Vgs of 20V. The Vth to switch is 4v.

Everyone who looked at it said this was "over-complicated" and to just use a diode to short reverse voltage. My argument would be that this shorts the power back to the battery. At that point, if the diode can take enough current to protect the circuit it is short circuiting the battery. I can put a resettable fuse on the line, but then how is it more simple than this p-channel MOSFET solution? I like that the fet doesn't just short out the voltage, it blocks it. But maybe I have the concept wrong.

#### crutschow

##### Well-Known Member
You circuit will work as shown to protect the load from reverse polarity.
It's advantage is that it has a much lower forward voltage than a series diode.

You can replace the zener with a 10kΩ resistor with a 10kΩ resistor from the gate to ground ( or a 100kΩ resistor with the 100kΩ shown) to reduce the maximum Vgs to 1/2 the supply voltage or 15V.
Resistors are cheaper and more reliable than a zener.

#### Triode

##### Active Member
Thanks for the confirmation. I was starting to think I was going crazy. I still wonder how none of them had seen that circuit before, though these are mostly academics so maybe it never came up.

#### crutschow

##### Well-Known Member
Here's a writeup I did on that circuit.

#### Triode

##### Active Member
That's great. It actually helped me clearly explain one of the arguments against it. "won't the Vgs be zero because the source is 0 and the gate is zero?" the body diode takes care of that for us, I think.

#### crutschow

##### Well-Known Member
That's great. It actually helped me clearly explain one of the arguments against it. "won't the Vgs be zero because the source is 0 and the gate is zero?" the body diode takes care of that for us, I think.
Yes, upon the application of a normal supply voltage, the body diode conducts until Vgs becomes sufficient to turn on the MOSFET.

#### Diver300

##### Well-Known Member
Reverse parallel diodes will do damage unless there is a current limiter. That damage might just be a blown fuse.

It is important to make sure that the fuse blows before the diode. It is also a very good idea to make sure that is the case at all supply voltages. In some cases, with a low battery and reverse parallel diode, the fault current is low and the fuse might take so long to blow that the diode has burned out.

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