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Safety Thermistor

Hot-ends have a heating element (controlled by a MOSFET) and a thermistor to read its temperature.

When the thermistor is read, the Smoothie calculates the temperature, and then turns the heater on or off depending on whether heat is needed or not.

A few bad things can happen:

• The thermistor can get disconnected from the board, in this case, Smoothie detects it and turns everything off.
• The thermistor can fall off the heater, in which case Smoothie will think the hot-end is cold, and heat it too much.
• The MOSFET can fail, and be always on, resulting in the hot-end being over-heated.

One way to prevent the last two bad things is to add a second thermistor to the hot-end. The second thermistor also reads the temperature on the hot-end, and if something goes bad, Smoothie turns the PSU off, preventing damage.

To implement this, you need three things:

• A switch module that turns the PSU on and OFF. (See the Switch documentation.)
• A temperaturecontrol module that doesn’t control any MOSFET but reads just the safety thermistor.
• A temperatureswitch module that turns a switch off if a temperaturecontrol module goes above a given temperature.

Here is an example configuration:

temperatureswitch.psu.enable               true             # Enable temperatureswitch module for PSU
temperatureswitch.psu.switch               psuswitch        # Designate switch module to use
temperatureswitch.psu.designator           F                # Designator for the safety thermistor
temperatureswitch.psu.threshold_temp       45               # Turn the PSU OFF above this temperature, and ON below this temperature. In °C.

switch.psuswitch.enable                    true             # Enable switch module for PSU
switch.psuswitch.input_on_command          M80              # Command to turn PSU on
switch.psuswitch.input_off_command         M81              # Command to turn PSU off
switch.psuswitch.output_pin                1.22!            # Pin for the switch control, 3rd small FET, or pin on header
switch.psuswitch.output_type               digital          # Output type, on/off only

# Define a read-only temperaturecontrol for a PSU cutoff
temperature_control.psu.enable             true             # Enable temperaturecontrol module for PSU
temperature_control.psu.thermistor_pin     0.25             # Pin for the safety thermistor to read
temperature_control.psu.heater_pin         nc               # Set to 'nc' for read-only temperature control
temperature_control.psu.thermistor         Semitec          # Thermistor type
temperature_control.psu.designator         F                # Designator for the safety thermistor

A few notes:

• If you have a voltage regulator as your only 5V source on Smoothieboard, this will not work, as turning the PSU off will also turn the 5V off.
• If you use an ATX PSU, you can wire the 5VSB supply to the 5V input on the Smoothieboard, in which case the Smoothieboard will still get its 5V even if the PSU is shut down by the switch module. Wire the ATX PS_ON to the ground of a small MOSFET. This will allow it to turn on/off with M80/M81, and will also shut off in case of a crash.

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