Okay, so today I wanted to mess around with MOSFETs, specifically how they behave in the “linear” region. I’ve played with them as switches before, but I wanted to see how they act like a variable resistor. It’s kinda neat, actually.
Getting Started
First things first, I grabbed an N-channel MOSFET from my parts bin – an IRFZ44N, ’cause that’s what I had lying around. I also needed a power supply, some resistors, a potentiometer (for easy adjusting), my multimeter, and of course, some jumper wires. My workbench is always a glorious mess, but hey, that’s part of the fun, right?
The Circuit Setup
The setup is pretty basic. I connected the MOSFET’s drain to the positive side of my power supply (set to a low voltage, around 5V, to start – safety first!). Then, I put a resistor (I used a 220-ohm one) between the source and ground. This resistor is important; it limits the current.
Now for the control part: I wired up a potentiometer as a voltage divider. One end to the power supply’s positive, the other end to ground, and the wiper (the middle pin) to the MOSFET’s gate. This lets me smoothly adjust the gate voltage.
- Power Supply (+) -> MOSFET Drain
- MOSFET Source -> 220 Ohm Resistor -> Ground
- Potentiometer (End 1) -> Power Supply (+)
- Potentiometer (Wiper) -> MOSFET Gate
- Potentiometer (End 2) -> Ground
Experimenting and Observations
I started with the potentiometer turned all the way down (minimum gate voltage). The MOSFET was “off,” and my multimeter showed almost no current flowing through the drain-source. The voltage across the drain and source (Vds) was close to my supply voltage.
Then, I slowly started turning the potentiometer, increasing the gate voltage. And here’s where the magic happens! I could see the current (measured with my multimeter in series with the resistor) gradually increasing. The Vds started dropping, which means that the resistance across the MOSFET(Rds) was decreasing.
I played around with the potentiometer, watching how the current and Vds changed. It really did behave like a variable resistor! It’s not perfectly linear across the entire range, of course, but within a certain gate voltage range, the relationship between gate voltage and drain current is reasonably linear.
Some Key Takeaways
It’s important to remember not to crank up the gate voltage too high, or you might exceed the MOSFET’s maximum ratings and poof goes your component. And use proper sized heat sink.
This little experiment really helped me visualize how a MOSFET works in the linear region. It’s one thing to read about it in a datasheet, but it’s another thing entirely to see it in action on your workbench. I’m definitely going to use this in some future projects where I need a digitally controlled variable resistor.