I'm trying to understand MOSFET's and am having a hard time understanding why we should put a diode between the hot and ground wires when using a motor. It is only when using a motor, right? I wouldn't have to do this if I'm using LEDs?

I believe it's because a motor can generate static which can make it's way back into the circuit?

So I've made a circuit which will control 4 propellers. P1, P2, P3, P4 are the + and - pins that connect to the propeller.



Between the + and - pins I've added a diode. Would anyone be able to let me know if it's in the correct place?

Also if it is in the correct place I don't understand how it does anything. Surely the electricity will just ignore it and take the route through the motor?

Thanks for anyone who takes the time to help me.

 

Correct place...wrong polarity.

The way you have them, they will simply bypass the motors, you want them in the other way round so they can suppress the back EMF generated by the motors.

And no...you would not need them for LEDs.

And no, the electricity will not ignore the diodes, they will create a direct short and destroy your MOSFETS.

 

Thanks @ElectricSpidey. Really appreciate your time.

I thought a diode should always point it's negative end towards ground?

But maybe this is different because we're protecting the circuit from the electricity going in the opposite direction. So if the motor generates EMF, it will get to the diode, but not get past it because of it's polarity?

 

The diode is to absorb the reverse voltage spike from an inductive load, such as a motor, when it is switched off.

Also you should have the fan connected between 5V and the MOSFET drain with the MOSFET source connected to ground.
You have it connected as a source-follower so the fan voltage will be equal to 5V minus the MOSFET gate-source voltage required to turn it on, which typically is a couple volts or more.

 

Thanks @crutschow. Sort of like this?



I think the names source, drain confuse me. Gate I seem fine with.

 

Nope. I think I made a mistake. That would short circuit I think. So basically the gate goes at the end of the circuit?



Which I think might work, according to circuitjs simulator :S



Maybe this is a better way to represent it?

 

That looks correct now.

The gate is where the signal is applied.

The source is connected to the supply rail.

The drain is connected to the load.