MOSFETs Table
This page documents the MOSFET outputs available on Smoothieboard and their specifications.
MOSFETs are used to control high-power devices like heated beds, hotends, fans, and other accessories.
MOSFET Specifications
| MOSFET Pair |
Big MOSFETS |
Small MOSFETS |
Mixed MOSFETS |
|
|
|
| Label on diagram |
P2.7 |
P2.5 |
P2.4 |
P2.6 |
P1.23 |
P1.22 |
| Digital output pin |
2.7 |
2.5 |
2.4 |
2.6 |
1.23 |
1.22 |
| Power Input |
Between P2.7 and P2.5 |
Between P2.6 and P1.23 |
Taken from VBB |
|
|
|
| Size |
Big |
Big |
Small |
Small |
Big |
Small |
| Maximum current |
12A |
12A |
3A |
3A |
12A |
3A |
| Used by default for |
Heated bed |
|
Hotend 0 |
Fan |
Hotend 1 |
|
Understanding MOSFET Pairs
Smoothieboard has three MOSFET pairs:
Big MOSFETs Pair (P2.7 and P2.5)
- Current capacity: 12A each
- Power input: Shared between the two outputs (between P2.7 and P2.5 terminals)
- Typical use: Heated bed (high current devices)
- Note: Both outputs share the same power input
Small MOSFETs Pair (P2.4 and P2.6)
- Current capacity: 3A each
- Power input: Shared between the two outputs (between P2.6 and P1.23 terminals)
- Typical use: Hotend 0 and fan (moderate current devices)
- Note: Both outputs share the same power input
Mixed MOSFETs Pair (P1.23 and P1.22)
- Current capacity: P1.23: 12A, P1.22: 3A
- Power input: Taken directly from VBB (main power supply)
- Typical use: P1.23 for Hotend 1, P1.22 for accessories
- Note: These outputs use the main VBB power supply
| Output |
Controlling Pin |
Current Rating |
Typical Use |
| Hotend A |
PJ6 |
~5A |
Primary hotend heater |
| Hotend B |
PJ7 |
~5A |
Secondary hotend heater |
| Fan 1 |
PJ8 |
~5A |
Part cooling fan |
| Fan 2 |
PJ9 |
~5A |
Auxiliary fan |
| Bed |
PJ10 |
~10-12A |
Heated bed (dual parallel FETs) |
| SSR1 |
Logic output |
mA |
External SSR control |
| SSR2 |
Logic output |
mA |
External SSR control |
Understanding MOSFET Outputs
Smoothieboard v2 has a simplified MOSFET architecture:
Low-Current FETs (Hotend A, Hotend B, Fan 1, Fan 2)
- Current capacity: ~5A each
- Power input: Shared VFET rail (2× XT30 connectors)
- Safety feature: High-side PFET watchdog can cut power to all 4 outputs
- Note: All share the same power source, controlled by safety watchdog
Bed FET
- Current capacity: ~10-12A (dual parallel MOSFETs)
- Power input: Direct from VFET (not through PFET watchdog)
- Typical use: Heated bed
- Note: Independent of safety watchdog for bed stability
SSR Outputs
- Current capacity: Milliamps (logic level signals)
- Use: Control external solid-state relays for high-power loads
Important Safety Notes
Current Limits: Do not exceed the maximum current ratings. Big MOSFETs can handle up to 12A, small MOSFETs up to 3A. Exceeding these limits can damage the board.
Shared Power: MOSFETs in the same pair share their power input. Make sure your power supply can handle the combined load of both outputs in a pair.
Configuration
To configure a MOSFET output in your config file, use the appropriate pin number from the “Digital output pin” row.
Example for configuring a heated bed on P2.7:
temperature_control.bed.heater_pin 2.7
Example for configuring a hotend on P2.4:
temperature_control.hotend.heater_pin 2.4
Example for configuring a heated bed on P2.7:
[temperature_control.bed]
heater_pin = 2.7
Example for configuring a hotend on P2.4:
[temperature_control.hotend]
heater_pin = 2.4