PT100 Daughter Board Failing?

  • I am having an issue with the PT100 daughter board failing. I am running a Duet WiFi with the PT100 daughter board and an E3D PT100 sensor. Shortly after getting my printer running it started to show spikes in the indicated temp and shortly failed to read anything other than 2000c.

    I switched the sensor to the second channel on the board and it worked for a few days before it started reading 2000c. I checked the sensor and got a reading of appx. 108 ohm at room temperature. The wiring is passing a continuity test as well.

    Does anyone know how to verify the daughter boards operation or heard of this issue coming up?

  • administrators

    Can you check that you don't have an intermittent short between your heater cartridge and the PT100 sensor. Check continuity between the sensor case and the pt100 wires, between the heater cartridge case and the heater cartridge wires. It is possible that there was an issue with the chip on the PT100 board however it is odd that both failed.

  • Really confused now. Wire continuity is good all the way to the daughter board. However, when I run a jumper from the PT100 sensor to the board, I get a reading of 30c on Channel 201. It is too high though, should be around 22c. When I run the jumper to channel 200, I get 19c. Checking resistance in the wiring I get 108.6 at sensor, 109.6 at termination to the board and 108.7 on the test jumper.

    Is there an auto calibration causing this difference between channels?

    Is 1.0 too much resistance?

  • administrators

    30C on one channel and 19C on the other is way more variation than is acceptable. A 1 ohm difference at room temperature is about 2 degrees of temperature error. 108.6 is about 22C as you say.

    This is not the same issue that you reported though with 2000C being reported. (2000C is what is displayed when there is no PT100 connected or the daughterboard is not present).

    It is possible that you have a intermittent connection on the wiring or a source of noise that is causing these spikes.

    One idea is to use 4 wires rather than 2. this will help with both the resistance loss in your cables and noise. David shows how to do this in his blog post here:

  • That is the confusing part. I am still getting 2000c with the original wiring while it is pushing 109.6, triple checked.

    The difference between channels is consistent and repeatable using fresh wire.

    I will give the 4 wire option a try.

  • administrators

    is it a solid 2000C or spikes? if spikes then I suspect noise.

  • Solid. Channel 200 gave me spikes before it went out.

    This setup has been working and printing just fine on channel 201. It stopped reading in between prints. I contributed channel 200 going out due to firmware tweaking but know I am having second thoughts.

  • administrators

    My suspicion is that you have a short between the PT100 wiring and the heater cartridge, or between the PT100 wiring and ground, or that your wiring is picking up too much interference from something else such as a stepper motor cable. Another user found that he had to route the PT100 wires away from the extruder stepper motor wires, or use twisted pair (I forget which he did in the end).

    To test the board, you could disconnect the PT100 wiring from the terminal block and connect a resistor there instead (and of course you need the wire links). A 100 ohm resistor should give a reading of 0C.

  • Using a quality multimeter (Fluke 77 IV),

    With jumper wire direct from the sensor, I set the temperature to 180c. The sensor was reporting 382 ohms when the board said it was at 180c.

    I have ruled out anything in my wiring as far as shorts.

    To eliminate the noise issue, I routed shielded and twisted pair wire separate of all wires with and without the motors enabled.

    I don't have a spare PT100 sensor to compare against so all I could do is test what it was sending. With probes hooked up, I moved wires around to see if there were any drops or rises. There were non and it stayed consistent at 108.6.

    I currently don't have a 100 ohm resistor to test the board. I did verify that the wires going into the board are reading 108.7. I rewired all jumpers for a 2 wire setup and verified continuity across contacts.

    If the board is good, I am at a loss.

  • Finally got the new twisted pair wire ran and while the ohm readings are the same, the temp readings are now correct. Looks like noise was the issue, thanks for pushing the wires. Next step is building a oscilloscope.

Log in to reply