Okay, so today I wanted to mess around with MOSFETs, see how they actually work in different modes. I’ve read the theory, but you know, it’s different when you get your hands dirty.
Setting Up the Experiment
First, I grabbed a basic N-channel MOSFET from my parts bin – an IRF540, I think. Nothing fancy. I also needed a few resistors, a breadboard, my trusty multimeter, and a power supply.
I hooked up the MOSFET on the breadboard. The source pin went straight to ground. For the drain, I used a 1k resistor connected to the positive rail of my power supply (I set it to 12V to start). The gate, well, that’s where the fun begins. I used a 10k resistor to connect the gate to a separate, variable voltage source – this let me play around with the gate voltage easily.
Cut-Off Mode – It’s Off, Duh!
I started with the gate voltage at 0V. I connected that variable supply’s negative output to the same ground as everything else. Makes sense, right? Everything needs a common reference.
I switched on the power and, unsurprisingly, nothing much happened. I checked the voltage across the drain and source (Vds) with my multimeter. It read pretty much the full 12V. The MOSFET was acting like an open switch, completely blocking the current flow. We expect that, the gate voltage is lower than the threshold of the MOSFET, thus the MOSFET is not “opened”.
Linear/Triode Mode – Acting Like a Resistor
Now for some action! I slowly started increasing the gate voltage (Vgs). I was watching the Vds on my multimeter. Once Vgs went past a certain point (around 2-3V for this MOSFET, I think – it’s called the threshold voltage, but who cares about names, right?), I saw Vds start to drop.
The more I increased Vgs, the lower Vds became. The MOSFET was now letting current through, but it was also acting like a variable resistor. The higher the gate voltage, the lower the resistance between drain and source. It’s like I was turning a knob to control the current flow.
I made sure to keep an eye on the power dissipation. In this mode, the MOSFET can get pretty hot because it’s dropping voltage and passing current. I didn’t want to burn anything (or myself!).
Saturation Mode – Fully On!
I kept cranking up Vgs. Eventually, I reached a point where increasing Vgs further didn’t really change Vds much. Vds had dropped to a very low value, maybe a few tenths of a volt. At this time the Vds is smaller than the (Vgs-Vth).
This is saturation mode. The MOSFET is fully “on,” acting like a closed switch. It’s conducting as much current as the circuit (and the power supply) will allow. The resistance between drain and source is at its minimum. This is the mode you’d use if you wanted the MOSFET to act like a simple on/off switch for, say, an LED or a motor.
Observations and Messing Around
- It’s all about Vgs: The gate-source voltage is the key. It controls how the MOSFET behaves.
- Threshold Voltage Matters: Nothing really happens until Vgs gets past that threshold.
- Linear Mode is Tricky: It’s cool to control current, but watch out for heat!
- Vds changing: When I change the Vgs the Vds is also changing, and the saturation happens when Vds is smaller than the (Vgs-Vth).
- Saturation is Simple: The MOSFET is basically just a wire at this point.
I played around a bit more, swapping out the drain resistor for an LED (with a suitable current-limiting resistor, of course!). I could control the LED’s brightness by adjusting Vgs, which was pretty neat.
This was just a simple experiment, but it really helped me understand how a MOSFET works in practice. It’s one thing to read about it, but actually seeing the voltages change and controlling the current flow makes it all click. Next time, I might try a P-channel MOSFET or maybe build a simple amplifier circuit. Who knows!