What is a 3D Printer "mosfet" used for?

[Martin_au]

The electronic boards in most 3D printers use 5 volts. The heaters and beds generally use 12 or 24 volts. The mosfet is a switch (gate) that uses the 5 volt signal from the board to turn on and off the 12/24 volts that drives the heaters. Most mosfets can handle higher voltages for controlling the gate, so the use of the 12/24v from the board's heater terminals to control the mosfet is also common (if in doubt, check the datasheet).

Most boards have several mosfets already installed so they can drive the heaters directly from the board. However, the board mosfets usually aren't rated for very high currents. In some cases, the board mosfets are underpowered relative to the bed. These are situations where an external mosfet that can handle higher currents is required.

A typical mod is to attach an external mosfet, and control it via the old bed heater connections (with the mosfet using the 12/24v from the bed terminals on the board as the switch.

As an example, I've got a 500W bed on one of my machines, which draws 500W/24V = 21 amps. However, my board (SKR 1.4T) is only specced to provide 15 amps. I've installed one of these - https://www.digitmakers.ca/products/...ing-controller - which is good for 30A, next to the board, and am controlling it from the board's heatbed terminals.