Pardon the incoming stupidity. I’m a physics student, so something as electronically involved as this is a little outside of my wheelhouse, at least in this stage of my education.
I recently got a MOSFET, and the intention was to use it as a “signal amplifier” of sorts. I have an Arduino to put out a PWM signal, and the hope is that I can use a MOSFET to behave as a switch that turns on and off with the PWM signal. The MOSFET switch would then be connected to a 120V power supply, which runs a motor (really high voltage rating for a motor, I know, but it’s correct). That way, I would basically have a 120V PWM signal instead of the 5V PWM signal, and I could change the duty cycle to control the motor’s speed. Easy peasy. A MOSFET is just a device that I found other people using in this way online. Hypothetically (and idealistically), all I need is for the MOSFET to behave like a switch.
The problem is that MOSFET’s clearly don’t work the way I thought they do, because my setup isn’t working like I expected it to. I have a schematic below:
This circuit is somehow wrong, and I don’t know why.
There is a diode there inside of the MOSFET I have according to the spec sheet. I’m not sure why it’s there, and I don’t think it affects anything. If I had to take a guess, it’s that building up charge while the MOSFET is off with the voltages reversed would somehow hurt the semiconductor inside, but that’s just a guess without any physical knowledge of its inner workings behind it. First question: why is the diode there?
I see everywhere I look that there’s supposed to be a resistor between source and gate. Second question: Why is that? My guess is that it’s a means of discharging the gate, but I don’t know if that’s right or if that’s a good thing for the MOSFET, anyway.
Third question: why doesn’t this setup work? When the Arduino is unattached, there’s no voltage across the gate, so the motor should not be powered. I measure 0V on my multimeter when I do that. So far so good.
The problem comes when I attach a steady 5V from the Arduino to the gate. This does NOT turn it on (again, measured with a multimeter and not the actual motor), even though I thought it should. Why doesn’t this setup turn on?
NOTE: Yes, I know I should have a flywheel diode in there somewhere. That’s another question itself; I don’t know where I should put it, yet. However, that’s why I’ve been using a multimeter this whole time instead of watching if the motor started spinning.