Today, I wanted to mess around with NPN MOSFETs, so I grabbed a few from my parts bin. I’ve used these things a bunch of times before, but it’s always good to refresh the basics, right?
Setting up the Test Circuit
First things first, I needed a simple circuit to test the MOSFET. I decided to go with a basic setup:
- Power Source: My trusty bench power supply, set to 5V.
- MOSFET: An IRF520, because that’s what I had on hand.
- Load: A simple LED with a 220-ohm current-limiting resistor. Nothing fancy, just to see if the MOSFET is switching.
- Gate Control: A 10k-ohm resistor to pull the gate down, and a simple jumper wire to connect the gate to the 5V supply when I wanted to turn it on.
So, I put all that together on a breadboard. The power supply positive went to the LED, the other side of the LED to the resistor, then that to the drain of the MOSFET. The source pin of the MOSFET went to the ground of the power supply. The 10k resistor connected the gate to ground, and I held the jumper wire in my hand, ready to tap it to the 5V rail.
Testing the MOSFET
With everything wired up, I flipped on the power supply. The LED was, of course, off. The MOSFET was not conducting because the gate was pulled low (to ground) by that 10k resistor. Good, that’s what I expected.
Next, I touched the jumper wire from the gate to the 5V rail. Boom! The LED lit up bright. This is the MOSFET doing its job. By applying a positive voltage to the gate (relative to the source), I created a channel between the drain and source, allowing current to flow through the LED.
I pulled the jumper wire away, and the LED went off. The 10k resistor quickly pulled the gate voltage back down to zero, turning off the MOSFET and stopping the current flow. I repeated this a few times – touching the jumper, LED on; removing it, LED off. Worked like a charm every time.
Observations and Key Takeaways
It’s a simple test, but it reinforces a few important things about NPN MOSFETs:
- Voltage Controlled: The gate voltage is what controls the current flow between the drain and source. It’s like a voltage-operated switch.
- Normally Off: With no voltage on the gate (or the gate connected to ground), the MOSFET is normally off. No current flows (except for a tiny leakage current, but we’ll ignore that for now).
- Easy to Drive: It doesn’t take much current to drive the gate. That’s why I could just use a jumper wire. This makes them great for controlling higher-power circuits with low-power signals (like from a microcontroller).
- Gate Resistor: That 10k resistor is there for a good reason. It makes sure the gate is pulled down to ground when I’m not actively driving it high. Without it, the gate voltage could float, and the MOSFET might turn on and off randomly.
This was a quick and dirty little experiment, but it was a good reminder of how these useful components work. Might use this setup later for a more complex project I have been planning, but this little win will do for now!