This document is relevant to: all Duet boards
Firmware versions: all firmware versions
This section describes how to connect, configure and calibrate thermistors and PT1000.
Note: PT100 sensors are not the same as PT1000 sensors and cannot be connected directly to thermistor inputs. See Connecting PT100 temperature sensors for how to use PT100 sensors.
Connect thermistors or PT1000 sensors to the appropriate pin headers on the Duet board.
Thermistors and PT1000 sensors are not polarised, so can be connected either way around.
Make sure that there are no short circuits between your temperature sensor wiring and any other wiring (particularly heater and fan wiring, which will be at a higher voltage), or you may damage your Duet board.
PT1000 sensors connected to thermistor inputs have lower resolution than PT100 sensors connected via the PT100 daughter board. The accuracy of PT1000 sensors should be very good on the Duet 3 and Duet 2 Maestro and generally good on the Duet 2 Wifi and Ethernet. You can calibrate the Duet's on-board analogue-to-digital converter to improve accuracy; see "Temperature calibration and ADC tuning" below.
To achieve accuracy in temperature reporting, it is essential that the firmware has an accurate model of the temperature sensor. This section covers commands needed to configure the firmware to do this.
In RepRapFirmware 3 you first create a sensor using M308 then assign it to a heater created using M950.
Thermistor and heater example:
;Duet 3
M308 S0 P"temp0" Y"thermistor" T100000 B4725 C7.06e-8 ; sensor 0
M950 H0 C"out1" T0 ; create heater 0 and map sensor 0
;Duet 3 with CAN-connected toolbard
M308 S1 P"121.temp0" Y"thermistor" T100000 B4725 C7.06e-8 ; sensor 1 (toolboard)
M950 H1 C"121.out0" T1 ; create heater 1 and map sensor 1 (toolboard)
;Duet 2
M308 S1 P"e0temp" Y"thermistor" T100000 B4092 ; sensor 1
M950 H1 C"e0heat" T1 ; create heater and map sensor 1
PT1000 and heater example:
;Duet 3 with PT1000
M308 S1 P"temp1" Y"pt1000" ; configure sensor 1 as PT1000 on pin temp1
M950 H1 C"out1" T1 ; create heater 1 and map sensor 1
;Duet 3
M308 S1 P"121.temp0" Y"pt1000" ; sensor 1 (toolboard)
M950 H1 C"121.out0" T1 ; create heater and map sensor 1 (toolboard)
;Duet 2
M308 S1 P"e0temp" Y"pt1000" ; sensor 1
M950 H1 C"e0heat" T1 ; create heater and map sensor 1
In RepRapFirmware 2.x on Duet 2, BED_TEMP, E0_TEMP and E1_TEMP map, by default, to Bed temperature, first heater temperature and second heater temperature respectively. You can use different temperature input pins for a heater, by remapping the temperature sensor input pins; see "Remapping temperature sensors to different pins" below.
The thermistor or PT1000 types are configured using the M305 commands in config.g. The heaters and temperature sensor channels are numbered 0 for the bed, then 1, 2, 3... for each extruder.
For PT1000, use the M305 command as for a thermistor, but add parameter X500+n where N is the channel number. Other parameters are not required.
Thermistor example:
M305 P0 T100000 B3950 ; set bed thermistor parameters
PT1000 example:
M305 P1 X501 ; heater 1 uses a PT1000 connected to thermistor channel 1
In RRF 3.x, a typical config.g would define the bed and hot end temperature sensor definitions with M308 and map them to the relevant heater created with M950:
M308 S0 P"bedtemp" Y"thermistor" T100000 B3950 ; sensor 0
M950 H0 C"bedheat" T0 ; create bed heater and map sensor 0
M308 S1 P"e0temp" Y"thermistor" T100000 B4725 C7.06e-8 ; sensor 1
M950 H1 C"e0heat" T1 ; create heater and map sensor 1
M308 S2 P"e1temp" Y"thermistor" T100000 B4138 ; sensor 2
M950 H2 C"e1heat" T2 ; create heater and map sensor 2
In RRF 2.x, the P parameter in M305 refers to the temperature sensor AND heater channel, where P0 is the bedtemp pins, P1 is e0temp (first hotend) and P2 is e1temp (second hotend). A typical config.g might be:
;RRF 2.x
M305 P0 T100000 B3950 ; set parameters for heater 0
M305 P1 T100000 B4725 C7.060000e-8 ; set parameters for heater 1
M305 P2 T100000 B4138 ; set parameters for heater 2
In RRF 3.x, you can simply change the pin allocation in the P parameter in the M308 command to use a different pin. For example, if you want the bed thermistor connected to e0temp (assuming it is available), use:
;RRF 3.x
M308 S0 P"e0temp" Y"thermistor" T100000 B3950
In RRF 2.x, use the X parameter in M305 to do this, where X is the desired thermistor input channel. See M305 for a list of available input channels:
;RRF 2.x
M305 P0 X1 T100000 B3950 ; set bed thermistor parameters
In some cases, you'll want a temperature sensor that is not mapped to a heater.
RRF3.x handles this by allowing sensors to be defined independently of heaters, up to a max of 32 sensors. These will be shown on the 'Extras' section of the 'Tools + Extras' tab in DWC. Example:
;RRF 3.x
M308 S3 A"Chamber" P"e1_temp" Y"thermistor" T100000 B3950
In some cases, you'll want a temperature sensor that is not mapped to a heater.
In RRF 2.x, for sensors with no controllable heater, you need to create a "virtual heater" in order to be able to use the sensor. Use P parameter with number 103 or higher (101, 102 and 103 are reserved for the MCU and stepper driver temperatures see Heaters), define a sensor name with the S parameter (which will make it show up in DWC), define the "heater" ADC channel with the X parameter (ie the pins the temperature sensor is connected to), and then define temperature sensor values (T, B, C). eg:
;RRF 2.x
M305 P103 S"Chamber" X2 T100000 B4725 C7.060000e-8
There are three ways of using two thermistors to control one heater:
Note: if connecting 2 in series, the resistance of the thermistors needs to be combined in config.g. So if each thermistor is 100k at room temperature, they need to be set as 200k when declaring the sensor with M308. And if connecting in parallel, the resistances need to be half their combined average (or just half of one if both are the same).
However, when connecting thermistors in series or in parallel, although the B value remains the same, the effective C value changes. So it's easier to adjust the R parameter instead. For example, on a Duet 3 the normal R value is 2200. If you connect two identical thermistors in parallel, you can specify the usual T, B and C parameters for the thermistors, but double R to 4400. Likewise, if connecting two thermistors in series, halve the R value.
In RRF 3.x, the parameters of the M308 command define the temperature sensor characteristics. For example, to set the first hot end thermistor parameters, use a command like this:
;RRF 3.x
M308 S1 P"e0temp" Y"thermistor" T100000 B4725 C7.06e-8 ; sensor 1
The S parameter configures the sensor number the command applies to.
The P parameter defines the pins the sensor is connected to.
The Y parameter defines the sensor type, "thermistor" or "pt1000".
The T parameter (T100000) means that the resistance at 25C is 100Kohms.
The B parameter (B4725) means that the thermistor B value over the temperature range of interest is 4725. The B values quoted in thermistor datasheets are typically quoted over a range 25 to 75C, which may be OK for bed thermistor but is too low a range for a hot end thermistor. That's why the B values recommended by RRF differ from the values in the datasheet. Also, if the C parameter is used then the B value has to be adjusted.
The C parameter (C7.06e-8) is optional. Using the correct value widens the temperature ranges over which the accuracy is good. So it's not normally needed for bed or chamber thermistors, which don't operate over a large temperature range in typical 3D printers; but it's a good idea to use it for hot end thermistors if appropriate B and C values are known.
In RRF 2.x, the parameters of the M305 command define the temperature sensor characteristics. For example, to set the first hot end thermistor parameters, use a command like this:
;RRF 2.x
M305 P1 T100000 B4725 C7.060000e-8 ; set parameters for heater 1
The P parameter identifies the heater/thermistor channel that the command applies to, where P0 is bedtemp, P1 is E0TEMP, P2 is E1TEMP. P3 to P7 are the temperature inputs on the Duex expansion board.
The T parameter (T100000) means that the resistance at 25C is 100Kohms.
The B parameter (B4725) means that the thermistor B value over the temperature range of interest is 4725. The B values quoted in thermistor datasheets are typically quoted over a range 25 to 75C, which may be OK for bed thermistor but is too low a range for a hot end thermistor. That's why the B values recommended by RRF differ from the values in the datasheet. Also, if the C parameter is used then the B value has to be adjusted.
The C parameter (RRF v1.17 and later) is optional. Using the correct value widens the temperature ranges over which the accuracy is good. So it's not normally needed for bed or chamber thermistors, which don't operate over a large temperature range in typical 3D printers, but it's a good idea to use it for hot end thermistors if appropriate B and C values are known.
Manufacturers of thermistors typically quote the B value over quite a small temperature range. This is fine for the bed thermistor, but the effective B value over the temperature range of the extruder may be somewhat different.
Here are some suitable T, B and C values for popular thermistors:
| Thermistor type | Used by | T parameter (resistance @ 25C) | B parameter | C parameter |
|---|---|---|---|---|
| Semitec 104GT2 | E3D hot ends | 100000 | 4725 | 7.06e-8 1 |
| Epcos B57863S0103F040 | RepRapPro Ormerod heated bed | 10000 | 3988 | |
| Honeywell 135-104QAD-J01 | RepRapPro Ormerod/Huxley/Mendel hot end | 100000 | 4138 | |
| Typical Chinese bed heater thermistor | Various bed heaters | 100000 | 3950 |
1 The C parameter can be written in Scientific notation as shown here or in normal decimal notation (in this example C0.0000000706, just be careful to use the right number of zeros); see below for an explanation of the C parameter.
If you have a datasheet for the thermistor you are using, and if it provides a resistance-versus-temperature table, you can calculate the correct value for the B and C parameters. The simplest way is to use the calculator on the heater tab of https://configurator.reprapfirmware.org/.
For the technically minded: firmware 1.17 and later uses the Steinhart-Hart thermistor model. The M305/M308 C parameter is the Steinhart-Hart c parameter, the M305/M308 B parameter is the reciprocal of the Steinhart-Hart b parameter, and the Steinhart-Hart a parameter is calculated from the resistance at 25C. If you don't provide a C parameter, then C defaults to zero and the Steinhart-Hart equation reduces to the beta-value equation.
The R parameter is the value (in ohms) of the series resistor, which is part of the temperature sensing circuit on the controller electronics. You do not need to specify this value as it is set by the firmware which detects the hardware used and sets the value accordingly.
The default values for Duet boards are:
| Duet version | R value | Notes |
|---|---|---|
| Duet 3 | 2200 | |
| Duet 2 WiFi / Ethernet | 4700 | |
| DueX2 and DueX5 expansion boards (up to v0.10) | 4700 | |
| DueX5 expansion board (from v0.11) | 2200 | See PCB revision notes. |
| Duet 2 Maestro | 2200 |
To test that the temperature sensors are working at room temperature:
Don't worry if your heaters show temperatures that differ by a few degrees - this is quite common, especially when the sensors are thermistors (which are not very accurate at room temperature).
Once the thermistors have been checked for proper operation when cold, check they report correctly when the associated heater is turned on. Mains power will need to be connected and turned on.
WARNING: If you enable the heater but do not observe an increase in the temperature reading, turn off the controller immediately and check your wiring.
It is recommended that you tune your heaters after ensuring their functionality; see Tuning heater control
The analog-to-digital converters (ADCs) in microcontrollers have gain and offset errors that vary between one chip and another. These errors in turn introduce errors in the temperature readings taken from thermistors and PT1000 sensors that are connected to the thermistor input pins.
Duets may need their analog-to-digital converters (ADCs) calibrated to report accurate temperatures. Using M305 (RRF 2.x) or M308 (RRF 3.x) H and L parameters, it is possible to tune the Duet's ADCs high and low gain offset to improve readings from thermistors and/or PT1000 temperature sensors.
Thermistors with very high resistance when cold (typically sold as "high temperature thermistors") are particuarly difficult to read at low temperatures, so when they are at room temperature it is not uncommon for them to read inaccurately or even to show as open circuit. There has been discussion of a number of workarounds; a search of the forum may yield some good suggestions. For really satisfactory results you may need to switch to a thermocouple, PT100 or PT1000 sensor. The accuracy of readings taken from these thermistors improves substantially at normal printing temperatures.
Duets provide automatic, semi-automatic (in some cases) and manual methods of calibration. Under some conditions the residual errors may be high enough to warrant correction. In particular:
All Duets have some degree of self-calibration to measure and cancel out gain and offset errors, either in the ADC itself or in external hardware.
If you find the auto-calibration is not accurate enough for your requirements, we suggest:
This facility is supported in RepRapFirmware 3.2 and later for Duet 3 and Duet Maestro main boards, and for Duet 3 EXP3HC and TOOL1LC boards. The procedure is:
The following is for RepRapFirmware 3.x for all boards except the Duet Maestro, but should also work in RepRapFirmware 2.x, by changing M308 command to the appropriate M305 commands.