Okay, so I wanted to mess around with MOSFETs, specifically the depletion mode ones. I’d played with enhancement mode MOSFETs before, but depletion mode was new territory. I grabbed a few different types from my parts bin – some N-channel and some P-channel, just to have some variety.
Getting Started
First things first, I needed a basic circuit. I sketched out a simple setup on a piece of paper. I decided to start with an N-channel depletion MOSFET. I connected the source pin to ground, and the drain pin to a resistor, and then that resistor to a positive voltage source (I used a 9V battery for this). The gate, I left unconnected for now.
- Power Source: 9V Battery
- Resistor: 1k ohm (just to limit the current, nothing fancy)
- MOSFET: N-channel depletion type
Initial Test
I applied Power. And? Current flowed! That’s the cool thing about depletion mode MOSFETs – they’re “normally on.” Without any voltage on the gate, they conduct. I used my multimeter to measure the voltage across the resistor, and sure enough, it showed a voltage drop, confirming current was flowing.
Playing with the Gate
Now for the fun part. I wanted to see how applying a voltage to the gate would affect the current flow. I used a potentiometer, also connect the positive to source. I connected the middle wiper pin of the potentiometer to the gate of the MOSFET.
By turning the potentiometer knob, I could adjust the voltage applied to the gate. As I started applying a more negative voltage to the gate, I watched the voltage across the resistor on my multimeter. it begin to decrease, and the light became dim!
I kept turning the knob, making the gate voltage more and more negative. Eventually, the voltage across the resistor dropped to almost zero, The light was off. I had successfully pinched off the channel and stopped the current flow!
Switching to P-channel
Next, I swapped out the N-channel MOSFET for a P-channel one. The basic circuit was the same, to be convenient, I decided to flip the battery, connect the source to the positive voltage, and ground is connected to resistor and the drain.
With the P-channel MOSFET, it’s the opposite: you need a positive gate voltage to turn it off. So, I adjusted my potentiometer accordingly. As I increased the gate voltage, the current flow decreased, eventually reaching zero when the gate voltage was sufficiently positive. I had done it!
It was such a simple experiment, but it really helped me understand how depletion mode MOSFETs work. It’s one thing to read about it, but actually building the circuit and seeing it in action is a whole different level of understanding.