TODay I Learned about MOSFETs

Because really I didn’t learn about MOSFETs today, more like a couple months ago and I’m steadily forgetting all I learned. So TODay really stands for The Other Day.

If you are like me and work in a company that dabbles in electrical engineering but have absolutely no background in all that stuff, then welcome to the club! This is true on-the-job-training. Two-ish months ago – I was working on lighting some stuff up without relying on the dismal current + voltage our selected sensor gave off. (I still need to brush up my knowledge of current and voltage too. What’s the difference between the two?!)

Thankfully, we’d had some experience with that sort of thing, lighting up hundreds of circuit stickers using just a micro:bit and a custom PCB. What we used was an n-channel MOSFET transistor. And despite all my reading up on them, I still had no idea what exactly I was using. All I knew was that these teeny things of which I could fit a dozen on my thumbnail essentially worked like relay switches.

What are MOSFETs?!

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. Phew what a mouthful. MOSFET is certainly easier to roll off the tongue. Transistors are basically electronic switches with three terminals. One to connect to current in, one for current out, and one for control.

Click here to find out the difference between transistors and relays.

Image result for mosfet
Don’t mix your terminals up or you’ll have to redo entire PCB designs like me.

The three terminals of the MOSFET are the Gate, Drain and Source. When the potential difference of the Gate-Source is 0, the Drain-Source resistance is super high, becoming practically an open switch. And conversely, when the potential difference of the Gate-source is high, the Drain-Source resistance lowers and acts like a closed switch.

An N-Channel MOSFET

So let’s say in the above n-channel MOSFET, if there is no current running through GATE and the voltage is currently 0, Gate and Source have the same voltage of 0, there’s no potential difference there. And thus Drain – Source stays ‘open’.

But if you increase the voltage of Gate to say 3V, then there’s a potential difference of 3V between the Gate and Source, and Drain – Source ‘closes’ and current can flow through the LED.

Yay! Understanding and knowledge! Wow, I wonder how genius the people who invented these things must be. Just figuring out how they work is enough for my puny brain. And that was just the N-channel MOSFETs. Now I have to figure out P-channel.

Lots of love to this website : where sudden clarity shone through the gloomy clouds in my head.


Also if anyone is wondering, but heck, even if you weren’t wondering! The n-channel mosfet is the tiniest 3 legged component up in the featured image. It can literally hide in the space under my nail (the white part of course, not the pink part, that’s gross to think about.)

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