Why won't my FET turn on?


#1

If you have just wired up your first simple FET circuit, you are most likely attempting to use an n-channel FET as a switch, and the odds are 50/50 whether you have wired your circuit with the FET as a high-side or low-side switch. If you wired it as a high-side switch, and are trying to turn it on using the same voltage you are switching with it (not uncommon), then you are most likely scratching your head wondering why your FET didn’t turn on.

As an example, consider the following simple circuit with a high-side n-channel FET attempting to pass power to a 5V DC motor when the button (S1) is pressed.

If wired as shown, upon pressing the button, at best you could expect a fraction of a second movement from your motor (most likely nothing)… but why?

This is a classic misunderstanding with first time FET users. It’s commonly said that, “the gate voltage controls the FET,” and this is true, but most forget that voltage is a differential measurement that has to be made relative to something, and in the case of FETs that something is the FET’s source. So what’s needed is to have sufficient voltage at the gate relative to the source.

What happens in the case above when the button is pressed that prevents this? Well, when the button is first pressed, the source of the FET is most likely sitting at GND potential, but an instant after the button is pressed and the gate rises to 5V (relative to GND), the source potential will begin to swing up toward 5V as well. As the source potential continues to rise, it will eventually hit a point where the potential difference between the gate (at 5V) and the source (approaching 5V) is no longer sufficient to keep the FET turned sufficiently ON, and the FET will act more like a resistor than a switch, thereby starving the motor from the current necessary to run properly.

So how do you fix the problem? In this case it’s easy, rewire the n-channel FET as a low-side switch instead.

When your FETs seem to be misbehaving, always be sure to check that you are providing the gate-to-source voltages you think you are, and always check their datasheets for the necessary threshold voltages to turn them ON/OFF without damaging/destroying them (also other things like drain-to-source current and voltage ratings).


Why won't my FET turn off?
#2

You probably also want to put a flyback diode in parallel with the motor to make sure the mosfet isn’t damaged when it shuts off.


#3

Correct, for a motor or large inductive load of any kind where a large current is attempted to be stopped abruptly that would be a good idea. In this case I was trying to avoid unnecessary complication and just stick to explaining the singular concept of the article topic without going off on any tangents. Making it a mystery load instead of a motor seemed too boring and non-visual of an example to me I suppose.


#4

Thanks for the graph.