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

I know what a mosfet is but as a newbie to 3D printing I see references to...

"Has a fuse and mosfet installed"

The Question: What is a a mosfet used for in 3d printing?

Thanks for any help.

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.

Thanks Martin_au. Perfect answer!

One "off topic" question if, as a newbie, I may ask...

Is this forum principally for commercial/industiral 3D Printing or "hobby" or some of both?

Thanks again for the help.

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Originally Posted by 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.

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IMO, generally hobby printing.

FETs also are used to control the NEMA stepper motors in these printers.

Before FETs were invented, we used BJTs, bijunction transistors, but those have a power penalty - You get a voltage drop of, say. 0.5V across the BJT. At 21A as for that heater, this would mean that almost 11W goes to heat the BJT, which is not helping you print!

FETs act lore like a controlled relay, where BJTs act more like a controlled resistor, FETs will have an on resistance in the milliohms range, so at that 21A heater current you'd heat the FET up with about 1/10th of a watt of heat, about 100 times better.

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Is this forum principally for commercial/industiral 3D Printing or "hobby" or some of both?

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We regularly cover most aspects up to light industrial.

On everything - martin only speaks for himself.

You tend to find that once you hit the area where machines start to cost over $100,000 people tend to start contacting manufacturers directly.

That said I've covered a decent number of inquiries about the high end rapid prototype machines over the years.
And a fair number of our members have experience with industrial machines.

I don't think we've ever not answered a question :-)

Are you perhaps running 24V through a 12V bed?

What resistance are you getting for the bed? What voltage is the printer using?

The electronic boards in most 3Dprinters 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.