To sum it up, a FET is a transistor that acts as a resistor, with a built-in capacitor for controlling the FET’s resistance. You open the FET by charging and discharging its gate – at its simplest, you can imagine the gate as a capacitor. You can imagine a FET as a resistor you can control, and its resistance can go as low as a fraction of an ohm (open), or to an infinitely high resistance for the purpose of your design (closed). These are all under the transistor umbrella, but when people say “transistor”, they usually mean BJT, and when people say “FET”, they usually mean “MOSFET”. They’re all field-effect transistors, however, siblings to the other kind of transistor that’s abundant – BJTs (Bipolar Junction Transistor), popular enough that we typically just refer to them as NPN or PNP transistors. When talking about a FET, people usually mean a MOSFET, and that’s what this article is going to talk about, too – other types aren’t as popular for the usual hacker purposes, and I don’t know much about JFETs to begin with. There’s different kinds of FETs – MOSFETs, JFETs, and a few dozen less popular but still abundant ones. For this article, you can use this schematic as a reference of how FETs are typically used in a circuit. It’s the most popular usecase for a FET, and if you’re wondering more about high-side switches, I highly recommend this brilliant article by our own, where he shows you high-side switch basics in a simple and clear way. This circuit is called a high-side switch – it enables you to toggle power to a device at will through a FET. If you want to control it from a 3.3 V MCU that can’t handle the high-side voltage on its pins, you can add a NPN transistor section as shown – this inverts the logic, making it into a more intuitive “high=on, low=off”, and, you no longer risk a GPIO! This circuit uses a P-FET – to turn the power on, open the FET by bringing the GATE signal down to ground level, and to switch it off, close the FET by bringing the GATE back up, where the resistor holds it by default.
Here’s a simple FET circuit that lets you switch power to, say, a USB port, kind of like a valve that interrupts the current flow.
#Fet transistor for multiple leds how to#
You might not be familiar with how a FET works, how to use one and what are the caveats – let’s go through the basics. Whether you want to control a couple of powerful LEDs, switch a USB device on and off, or drive a motor, somewhere in the picture, there’s usually a FET doing the heavy lifting.
If you ever work with a circuit that controls a decent amount of current, you will often encounter a FET – a Field-Effect Transistor.
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