Alright, so today I wanna share a bit about a hands-on session I had with an n-channel depletion MOSFET. It’s not the kind of MOSFET you might reach for every single day, like those common enhancement mode ones, but it definitely has its place, and I had a specific idea where I thought it would shine.
My Little Project Idea
I was tinkering around with a concept for a circuit, nothing too fancy. The main thing was, I needed a switch that would be “on” by default. You know, power flows through it normally, and then I could actively turn it “off” when needed. Most MOSFETs, the enhancement types, are the other way around – they’re off until you poke the gate with some voltage. Then I remembered these depletion mode guys. They’re supposed to conduct even with zero volts on the gate. Sounded perfect for what I wanted to achieve, making things a bit simpler, or so I thought.
I rummaged through my box of components, pretty sure I had a few of these n-channel depletion MOSFETs lying around from some old project or a sample pack. Found one, I think it was something like a J113 or a BF256, one of those classic JFET-like depletion MOSFETs. Grabbed it, a breadboard, and was ready to see it in action.
First Power-Up – Wait, What?
So, I got to wiring it up. My plan was simple: control a small LED. I connected the source to ground, and the drain to my LED (with a current-limiting resistor, of course) which then went to the positive supply. For an n-channel depletion MOSFET, it should be conducting with the gate at 0V, right? So, initially, I just connected the gate to the source, effectively setting Vgs to zero.
Turned on the power. And yup, the LED lit up. Nice, phase one complete! It was on, just as expected. Now, the interesting part: turning it off. For an n-channel depletion type, you need to apply a negative voltage to the gate, relative to the source, to pinch it off and stop the current.
This is where it got a little less plug-and-play than I sometimes wish things were. I hooked up a variable power supply to try and feed a negative voltage to the gate. I started dialing it down, more negative, more negative. The LED dimmed, sure, but getting it to turn off completely and cleanly felt a bit… mushy. It wasn’t the crisp on/off I was used to with the enhancement MOSFETs I typically use for switching.
Digging In a Bit
I sat there for a moment, thinking, “Hmm, this isn’t quite how I pictured it.” It’s always like that, isn’t it? You have a mental model, and then the real world throws a little curveball. I figured I was missing something subtle or my setup wasn’t ideal.
So, I did what I probably should have done from the get-go: pulled up the datasheet for the specific part I was using. Datasheets, man, they’re your best friend. It clearly showed the characteristics: conducts at Vgs = 0V, and you need to drive Vgs negative to a certain point (the Vgs(off) or pinch-off voltage) to fully turn it off.
- I realized my “mushy” feeling was probably because I wasn’t being precise enough with the negative voltage, or maybe my quick-and-dirty setup for the negative supply wasn’t stable enough.
- It also reinforced that these depletion mode devices really are ‘normally on’. You have to actively work to turn them off, which is the opposite of an enhancement mode N-channel where you actively work to turn it on with a positive Vgs.
Making It Work My Way
Armed with that refreshed understanding, I adjusted my approach. I made sure I had a solid negative voltage source for the gate. This time, I paid more attention to the actual voltage I was applying. As I carefully increased the negative voltage on the gate (making it more negative than the source), I reached a point where, boom, the LED went completely off. Cleanly. Aha! So it does work as advertised.
Bringing the gate voltage back towards zero (or just disconnecting my negative supply and tying gate to source again) made the LED spring back to life. It was a good reminder that while the concept is simple, the practical application sometimes needs a bit more care, especially when you’re dealing with negative voltages relative to another point in your circuit.
So, What’s the Deal?
In the end, I got my ‘normally on’ switch behaving exactly as I wanted. The n-channel depletion MOSFET did its job perfectly once I understood and correctly applied its control requirements. It wasn’t some super complex part of the project, but it was a great little practical exercise.
It just goes to show, even with components you kinda know, revisiting them hands-on can teach you or remind you of the nuances. For anyone looking to use these, especially the n-channel depletion types, just keep that negative Vgs for turn-off firmly in mind. It might seem obvious on paper, but when you’re at the bench, it’s easy to get into the ‘enhancement mode’ habit. They’re super useful for certain applications, like current limiters, or as active loads, or, like in my case, a simple switch that defaults to ‘on’. Definitely a good tool to have in the shed.