Alright, let’s talk about figuring out the threshold voltage on a MOSFET. I had this N-channel MOSFET lying around, an IRF540N I think, and needed to know roughly when it starts to turn on for a little project I was tinkering with. Datasheets are fine, but sometimes you just gotta check it yourself, you know?
Getting Started
First thing, I gathered my gear. Nothing fancy.
- My bench power supply, the old reliable one.
- The MOSFET itself, obviously.
- A breadboard for quick connections.
- A couple of multimeters – one for voltage, one for current.
- Some jumper wires.
- A resistor, maybe 1k ohm or something, for the drain side, just to limit current and have something to measure across if needed.
Hooking Things Up
So, I popped the MOSFET into the breadboard. Remember the pins: Gate, Drain, Source. Easy to mix up if you’re not careful. I connected the Source pin straight to the ground of my power supply. Then, I connected the Drain pin through that 1k resistor to the positive output of the power supply. I set the supply voltage to something reasonable, like 5 volts. It doesn’t need to be high for this test.
Now, the important part: the Gate. This is what we control. I hooked up another variable voltage source (could be a separate supply, or just a potentiometer divider from the main supply, but I used a second small supply I had handy) to the Gate pin. The negative lead of this gate supply also went to the common ground.
I put one multimeter, set to measure DC voltage, across the Gate and Source pins. This measures Vgs. The other multimeter, set to measure milliamps (mA), I put in series with the Drain pin, right before the resistor. This measures the drain current, Id.
The Actual Test
Okay, setup done. I double-checked my connections because letting the magic smoke out is never fun. Both power supplies were on, but the gate voltage was initially set to zero volts.
As expected, with 0V on the gate (Vgs = 0V), the multimeter measuring drain current read basically zero. The MOSFET was off.
Then, the careful part began. I started slowly, really slowly, increasing the voltage on the gate using my second supply. I watched both multimeters like a hawk. The Vgs voltage reading went up… 0.5V, 1.0V, 1.5V… still nothing happening on the drain current meter. Zero, zero, zero.
I kept inching the gate voltage up. Around 2.5V, maybe a bit higher, I saw the drain current meter flicker. Just a tiny bit, like 0.1mA or something minuscule. That’s the sign! That’s the start of conduction.
I nudged the gate voltage just a tiny fraction more, and the current started to climb more noticeably. But I was interested in that very beginning point where it just started conducting.
So, I noted down the Vgs reading right when I first saw that stable, non-zero drain current. For this particular IRF540N I had, it seemed to be around 2.8 volts. I backed the voltage down and repeated it a couple of times just to make sure I wasn’t seeing things. Yep, consistently around 2.8V, maybe 2.9V, the current started to flow.
Wrapping Up
So, that’s it. The threshold voltage (Vt or Vth, whatever you call it) for the MOSFET I tested was roughly 2.8V. This is the gate-source voltage needed just to start turning the thing on. Of course, to get it fully on and conducting lots of current, you need to push Vgs much higher, maybe 5V or 10V depending on the device and the load. But knowing that turn-on point is super useful for designing circuits. It’s a pretty straightforward test, just need to be a bit patient with turning that knob!