MOSFETs

Warning Safety Warning: If it will burn your skin, don't touch it. Simple.

Smoothie has up to 6 MOSFET controls (6 on 5X, 4 on 4X, and 2 on 3X).

The MOSFETs act as switches to ground: loads must be connected between the power source and the MOSFET switched terminal.

When the MOSFET is switched on, power will be applied to the load.

When the MOSFET is switched off, power will be removed, because one load terminal will be essentially disconnected and current cannot flow.

The exception being inductive load ‘flyback’ switching transients, discussed above.

Connect your PSU to the power input connector for those FETs (providing power to the load), and connect your power-consuming element (be it heating element, spindle, etc.) between the power output terminal and the MOSFET terminal.

Smoothie connects/disconnects the element’s ground as needed to maintain temperature or as requested by G-codes.

There are three main pairs of MOSFETs on the board:

MOSFET Overview

Smoothieboard v2 has a simplified and safer MOSFET architecture compared to v1:

Output Type Count Current Purpose
Low-Current FETs 4 ~5A each Hotend1, Hotend2, Fan1, Fan2
Bed FET 1 (2× parallel) ~10-12A Heated bed
SSR Outputs 2 Milliamps Solid-state relay control

All MOSFET outputs are powered from the VFET power input (2× XT30 connectors, 30A total).

Key Safety Feature: High-Side PFET Watchdog

Enhanced Safety
The 4 low-current FETs share a common +VFET rail controlled by a high-side P-channel MOSFET (PFET) watchdog. The firmware can instantly kill power to all 4 outputs if a thermal runaway or fault is detected.

How it works:

  • All 4 low-current FETs (hotends, fans) connect to a shared +VFET power rail
  • This rail is controlled by a high-side PFET watchdog
  • If any low-side FET fails shorted (stuck ON), the high-side PFET can still disconnect power
  • Provides an additional layer of protection against thermal runaway

Bed FET Independence:

  • The heated bed FET is NOT controlled by the high-side PFET watchdog
  • This is deliberate: allows the bed to remain on even if other FETs are shut down
  • Bed thermal runaway protection is handled directly by firmware

Low-Current FETs (Hotends, Fans)

Specification Value
Outputs 4 (hotend1, hotend2, fan1, fan2)
Current Rating ~5A per output
Voltage Up to 24V
Power Source Shared +VFET rail
Safety High-side PFET watchdog
Control PWM capable (firmware-controlled frequency)
LED Indicators Yes (per output)

Bed FET

Specification Value
Configuration 2× MOSFETs in parallel
Combined Current ~10-12A
Voltage Up to 24V
Power Source Direct from VFET (not through PFET watchdog)
Control PWM capable
LED Indicator Yes
Heated Bed Power Calculation
200W bed @ 12V: 16.7A - Consider using an external SSR
200W bed @ 24V: 8.3A - Within bed FET capacity

SSR Outputs

Specification Value
Outputs SSR1, SSR2
Type Logic-level (3.3V or 5V signal)
Current Milliamps (for SSR coil drive)
Use Cases Solid-state relay control, auxiliary outputs

Use SSR outputs to control external solid-state relays for:

  • High-power heated beds (>12A)
  • AC-powered heaters
  • Other high-current loads

Power LED Indicators

Each MOSFET output has its own LED indicator showing when it’s active:

  • Hotend1 LED: Hotend 1 heater active
  • Hotend2 LED: Hotend 2 heater active
  • Fan1 LED: Fan 1 active
  • Fan2 LED: Fan 2 active
  • Bed LED: Heated bed active

  • Big MOSFET pair: Their outputs are labeled P2.7 and P2.5 on the schematic, the input connector for them is found between them. They are found on the 4X and 5X boards. To power those MOSFETs, you need to provide them with power by wiring their power input to the power supply.

  • Small MOSFET pair: Their outputs are labeled P2.6 and P2.4 on the schematic, the input connector for them is found by their side, between P2.6 and P1.23. They are found on all of the boards. To power those MOSFETs, you need to provide them with power by wiring their power input to the power supply.

  • Mixed MOSFET pair: Their outputs are labeled P1.22 and P1.23 on the schematic. The pair is called “mixed” because it consists of one big MOSFET and one small MOSFET. They do not have a specific input, they take power directly from VBB (the Stepper motors power input described in the Stepper Motors chapter). To power those MOSFETs, you need to provide them with power by wiring their power input (which is the same as the one for the stepper motors) to the power supply.

Multiple Power Inputs: Contrary to other boards, Smoothieboard does not have a single power input, but multiple power inputs.

This allows you to use different voltages for different things if you want, and makes it easier to use more current as the current is shared between more connectors. It does mean wiring one or two more connectors though.

If you are trying to control MOSFETs and they are not turning on, make sure you provided power to their power input.

MOSFETs list:

MOSFET group MOSFET name Controlling pin Output connector Input method Voltage Current
Big MOSFETs First big MOSFET 2.7 X15 Big MOSFETs power input X13 12-24V 12.5A max
Big MOSFETs Second big MOSFET 2.5 X10 Big MOSFETs power input X13 12-24V 12.5A max
Small MOSFETs First small MOSFET 2.4 X7 Small MOSFETs power input X6 12-24V 3A max
Small MOSFETs Second small MOSFET 2.6 X8 Small MOSFETs power input X6 12-24V 3A max
Mixed MOSFETs Third big MOSFET 1.23 X16 VBB (motor) input 12-24V 12.5A max
Mixed MOSFETs Third small MOSFET 1.22 X9 VBB (motor) input 12-24V 3A max

MOSFETs diagram

Polarity Important: MOSFET power inputs have a polarity, make sure you connect + on that connector to + on your PSU, and - to - on the PSU.

Heater elements, however, do not have a polarity, so you do not have to worry about polarity on the outputs.

If you are using another output element like a Peltier or a Spindle, you need to be careful to respect the polarity for the outputs too.

Never use the big MOSFETS for more than 12.5A (and monitor connector and MOSFET temperatures at that current use, too much heating can be a sign of a bad wire connection), and the small MOSFETS should never be used for more than 3A.

Trying to power a 40W (or more) hotend cartridge heater at 12V with the small FETs will destroy them, usually locking (melting) them to the “ON” state (shorted) and possibly destroying the circuitry driving the MOSFET gate.

If you need to control more than 12 Amps, you cannot do it with one of the MOSFETS on board, however, you can use a Solid State Relay.

For information see the Solid State Relay Appendix on this page.

Alternative Power Method: In the case of both the Big MOSFETS pair and the Small MOSFETS pair, you take power from the PSU (Power Supply Unit) to them via their respective power input connectors.

There is an alternative, however (for currents up to 2 Amps or 4 Amps). For each pair, you can use jumpers (one jumper for the small MOSFETS pair (JP28), two parallel jumpers for the two big MOSFETS pair (JP11 and JP27)).

If you solder the pins for those OR connect a jumper to those pins, closing the circuit to VBB (the stepper motors power input), allowing you to take the power from those MOSFETS from the same place as the stepper motors do.

In the case of the big MOSFETS, you have to solder and put in place two jumpers, in parallel, in order to handle more current. Jumper Current Limitation: However, WARNING, each jumper is rated for only 2A of current.

This means you cannot use this way of powering your MOSFETS if you are going to use more than 2A (for the small MOSFETS) or 4A (for the big MOSFETS, with both jumpers used, for 2 x 2A).

Do not use the jumpers to power a heated bed, for example, as it uses much more than 4A.

Wiring MOSFET Outputs

Power Connection

All MOSFET outputs on Smoothieboard v2 are powered from the VFET power input:

Simplified Wiring
Unlike v1, Smoothieboard v2 has a single VFET power input (2× XT30 connectors) that powers ALL MOSFET outputs. No separate power inputs for different MOSFET groups!

Wiring steps:

  1. Connect your 12-24V power supply to the VFET XT30 connectors
  2. Connect your heaters/fans between the MOSFET output terminals and ground
  3. That’s it - no jumpers or multiple power connections needed!

MOSFET Output List

Output Type Current Typical Use
Hotend1 Low-current FET ~5A First hotend heater
Hotend2 Low-current FET ~5A Second hotend heater
Fan1 Low-current FET ~5A Part cooling fan
Fan2 Low-current FET ~5A Auxiliary fan
Bed Dual parallel FETs ~10-12A Heated bed
SSR1 Logic output mA External SSR control
SSR2 Logic output mA External SSR control

Polarity and Safety

Polarity Important: MOSFET power inputs have a polarity, make sure you connect + on your PSU to the + terminal on the XT30 connectors.

Heater elements (resistive) do not have a polarity, so you do not have to worry about polarity on the outputs.

If you are using a Peltier element or a DC motor, you need to respect the polarity for the outputs.

High-Current Loads (>12A)

If you need to control more than 10-12A (the bed FET limit), use the SSR outputs to control an external Solid State Relay:

  • SSR1/SSR2 provide logic-level signals to drive SSR coils
  • Use a properly rated SSR for your load (e.g., 40A SSR for AC-powered heated beds)
  • SSRs are recommended for AC mains-powered heaters

For information see the Solid State Relay Appendix.
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