Simply put, its impossible to flow loads of current through a microcontroller to control something such as a solenoid, large relay, contactor, or Motor. That’s why we have Metal Oxide Silicon Field Effect Transistors (MOSFETs). There are two types of MOSFETs, N-Channel and P-Channel, lets focus on the N-Channel MOSFET. As a hydraulic comparison, a MOSFET is much like a sink in a bathroom or a kitchen. The gate is like the valve that turns the water on. The source comes from the water pipe, and drains into the drain.
MOSFET Hydraulic Comparison
I made a simple circuit and also drew up a quick schematic in Eagle CAD to show how one is hooked up. S1 is like a momentary push button switch, but in the real world this would probably be replaced by a microcontroller, otherwise you are just connecting a switch to make a switch work, might as well just avoid the MOSFET and go with a switch. R2 is a current limiting resistor for the LED. If the positive supply voltage is +5V, then R2 would be 330Ohms, just use Ohm’s Law. Because a capacitance forms at the gate of the MOSFET you need a pulldown resistor from the gate to the ground. Without this resistor the gate floats on and always remains on even if the 5V threshold is no longer reached at the gate (even if S1 is off). If the gate threshold voltage is 5v, a 10k Ohm resistor works well. Also in the real world you probably wouldn’t be using a MOSFET to switch an LED on and off, MOSFETS are good for high power switching such as turning a motor on and off. Giving a pulse width modulation (PWM) signal to the gate, you can control the speed of a motor. If the Duty cycle is at 100%, motor will be at full speed, 50% duty cycle, half the speed. Some MOSFETs have a built in protection diode, some don’t. If your controlling a motor with the MOSFET, it would be nice to have this diode. Motors act as a generator sometimes and if the motor feeds voltage back into the MOSFET, it can damage it.
N-Channel Mosfet Sample Circuit
Sometimes when prototyping you might burn out a MOSFET. MOSFETs when burned will either be completely open or closed. Using an Ohm meter if it has infinite resistance across any of its pins or little resistance (practically conductive), its probably a bad MOSFET and should be replaced.
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