Duet WiFI with closed loop temperature sensing and SCR

  • Hello,

    I am experimenting with an infrared emitter to heat the build area. The emitters require the use of a silicon controlled rectifier (SCR). The input for the SCR is a 4-20mA @ 5 volts DC, 250 ohms, optically isolated signal. The closed loop system will evaluate the temperature sensor inputs and output a value between 4-20mA to drive the variable output of the power, which drives the IR emitter.

    1. does the Duet WiFi support a variable output to drive a SCR?

    2. If so, what is the best method to integrate with the Duet?

    I plan to use either a thermistor or thermocoupler for sensing the bed temperature. I will use an optical pyrometer for sensing the build material.

    Thanks for your advice.


  • administrators

    Is this ia 1-off requirement, or is there a plan to manufacture 3D printers incorporating these devices?

    Regarding the current loop output for the IR emitter, we have met a similar requirement in the past but I will need to check on the rights to the design that we did.

    Regarding the pyrometer output, there are different options depending on whether the pyrometer is powered by the loop, or has power input terminals separate from the current loop output terminals.

  • Hi David,

    The current requirement is for only a single machine. If it works, we may build 2-3 additional machines. There is no intention to become a mass manufacturer of machines 🙂

    The optical pyrometer provides traditional thermocouple (J or K) outputs.



  • administrators

    I see, the pyrometer is not a 4-20mA output device. It's just the heater that requires a 4-20mA input.

    The 4-20mA driver we built for someone else provided optical isolation. As you don't need that, you can use a simpler solution. For example, take one of the heater outputs on the expansion channel, smooth it using an RC filter, then use an op amp and a small mosfet to convert the 0-3.3V output to 4-20mA. You should also invert the signal either in a logic gate or in the op amp so that the heater is off at 3.3V average input and on at 0V, because the expansion heater outputs are inverted.

  • The pyrometer we are using supports both analog and digital outputs. Are you able to share the 4-20mA driver with the optical isolation? Maybe this will be a better solution for me long-term. I can provide more details on the pyrometer if needed.

  • Hi David,
    any luck with the closed loop 4-20mA isolated input/output circuit? are you able to share that circuit design?

  • administrators

    I have pushed the KiCAD project to Github:


    Hope that helps!

  • Thanks T3P3Tony!

  • Hi Tony/David,
    Is this circuit capable of being used bi-directional? For example, if I use the pyrometer output signal (4-20mA) will it convert to a 3.3v signal? and then use another board for the heater outputs taking the 3.3v signal to a 4-20mA signal to the SCR? or is it designed only to take the heater output to the SCR input? (3.3 to 4-20mA)

    Regarding the KiCAD files, thanks for the work on the PCB layout. Do you have any of these boards already fabricated and available for sale? If not, is it ok to have it fabricated along with your silk prints front and back? any restrictions?


  • administrators

    It's not bidirectional, it only converts one way from a PWM signal to 4-20mA.

    I don't think we have any spare boards. Tony should be able to answer your other questions. I suggest you load the .csv file into a spreadsheet and look at it, because we reviewed the design and came up with some suggested component changes.

  • ok, great advice! much appreciated.

    Is it best to use the expansion header I/Os? Are all the heater pins PMW signals? HEATER3….HEATER7 (pins 8, 13, 18, 23, 31)?

    So....., grab any one of the heater pins and a ground to the interface board and then out of the interface to the SCR 4-20mA inputs? Then...., feed the pyrometer signal back to the Duet through one of the thermocouple inputs? Seems straight forward 🙂

    I have reviewed the spreadsheet and have noted your findings.

    Thanks again for the help.

  • administrators

    The heater control pins on the expansion board are active low, so you need to connect the input of the converter board between the chosen heater control pin and +3.3V. You probably also need to reduce the input series resistor, because that design was intended to be driven from the 12V/24V E0 or E1 heater output.

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