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I'm designing a circuit which includes a fan being controlled and I've seen on a few websites that I should include a protection diode across the fan (as shown in the diagram below).

However, after searching and trying to think about it, I can't see what the purpose of the diode is? If it's in that direction it's not allowing any current to travel down that branch of the circuit anyway as it will always be reverse biased?

Does anybody know/can explain if this is necessary/correct to use a diode in this way for a fan/motor circuit?

schematic

simulate this circuit – Schematic created using CircuitLab

Froot
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  • Its explained [here](http://electronics.stackexchange.com/questions/80137/how-diodes-protect-h-bridge-dc-motor-driver?rq=1). It is there to protect the driver against inductive loads. – Wesley Lee Feb 02 '17 at 21:15
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    Can't have looked very hard then - it gets mentioned every time someone puts up a question involving switch any sort of inductive load. – JIm Dearden Feb 02 '17 at 21:18

3 Answers3

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The problem comes when you try to switch the load off.

You don't show a switch in your circuit, but if you had one say in the low side of the fan and you switched it off, without the diode there would be no path for the inductive current except for the parasitic capacitance at the switch node.

Motors are inductive loads and an inductor can't change its current instantaneously. V=L*di/dt so the V can spike extremely high if the current is forced to change quickly.

By putting the diode across the load you provide a path for the current to continue, with a small voltage across it to discharge the inductor slowly.

John D
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Does anybody know/can explain if this is necessary/correct to use a diode in this way for a fan/motor circuit?

you can think of a motor as an inductor. all inductors have one important attribute: current through an inductor cannot change suddenly, as just voltage across a capacitor cannot change suddenly.

so when your device switches off the motor / inductor, the current through it will want to continue to flow, create a huge voltage across your device, potentially killing it.

the diode provides a path for that current. thus the name "free wheeling" diode.

dannyf
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It's a flyback diode. When you disconnect the power from an inductive load, such as a motor, the load will induce a voltage to resist the change in current. If nothing in the circuit allows the back EMF current to flow, the inductor will induce a voltage so high that something breaks down and allows that current to flow.

The diode will allow the current to flow in the same direction and at the same strength as before the motor got disconnected and drain the inductor's magnetic field.

Oskar Skog
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