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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 ). You have to connect your PSU to the power input connector for those FETs, and connect your power consuming element ( be it heating element, spindle, etc… ) to the power outputs of those same FETs. Essentially Smoothie connects/disconnects the element ( connected to the mosfet connector ) from the PSU ( connected to the power input connector ) as needed to maintain temperature or as requested by G-codes.

There are three main pairs of Mosfets on the board :

  • Big MOSFETS pair : Their outputs are labelled 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 MOSFETS pair : Their outputs are labelled 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 MOSFETS pair : Their outputs are labelled 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.

Power input

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

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, locking them to the “ON” state. Less powerful

If you need to control more than 12Amps, you can not 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.

Jumpers

In the case of both the Big MOSFETS pair, and the Small MOSFETS pair, you take power from the PSU to them via their respective power input connectors.

There is an alternative however. For each pair, you can use jumpers ( one jumper for the small MOSFETS pair ( JP28 ), two parralel 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.

Current rating

However, WARNING, each jumper is rated for only 2A of current. This means you can not 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.