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    Solved Measuring heated bed resistance

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    • wilriker
      wilriker last edited by

      OK, this is actually straight forward - or should be.
      Take a multimeter, switch it to resistance measuring (set it to 200 Ohms or less), use one lead connected to positive the other to negative (I know that for a resistor like a heated bed there is not really positive or negative).

      Now, what I would expect to happen is that my multimeter takes a moment and then shows a stable reading of the resistance. Instead my multimeter starts somewhere around 0.2 Ohm and slowly but constantly increases the measured resistance by 0.1 Ohm.
      I stopped "measuring" when it got somewhere around 4-5 Ohms while still not stabilizing (~5 minutes). Reattaching the probes some moments later showed an even higher reading of 6+ Ohms from the moment of connection.

      Does this mean my multimeter is broken (I assume that actually because measuring resistance seemed never to have worked correctly) or am I just to stupid to get this right? 🤷 (It's OK to answer "yes" to the last question 😂 )

      Manuel
      Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
      with probably always latest firmware/DWC (incl. betas or self-compiled)
      My Tool Collection

      deckingman 1 Reply Last reply Reply Quote 0
      • deckingman
        deckingman @wilriker last edited by deckingman

        @wilriker Quick test - connect the leads together. It should read zero (or within 0.1 Ohm). Something at the back of my ageing brain tells me that what you describe can happen when the battery needs changing (but that could just be dementia setting in ☺ )

        Ian
        https://somei3deas.wordpress.com/
        https://www.youtube.com/@deckingman

        1 Reply Last reply Reply Quote 1
        • dc42
          dc42 administrators last edited by

          Did you measure that with the bed heater disconnected from the Duet? If yes then get a new multimeter.

          Duet WiFi hardware designer and firmware engineer
          Please do not ask me for Duet support via PM or email, use the forum
          http://www.escher3d.com, https://miscsolutions.wordpress.com

          wilriker 1 Reply Last reply Reply Quote 0
          • jamesadoty
            jamesadoty last edited by

            That is weird. I use a good quality Fluke meter and haven't run across that.

            The ohms function introduces voltage in to the circuit it's testing. Could it be high enough voltage to attempt to warm the heated bed and cause the impedance change?

            I haven't done much research on this but do know ohm meters inject voltage in to the parts being tested.

            Just thinking out loud.

            1 Reply Last reply Reply Quote 0
            • gavatron3000
              gavatron3000 last edited by

              Yeah if it's a cheapy multimeter it's likely it has a lower internal resistance than a high quality one. This affects the readings, higher quality meters are far better at reading very low resistances.

              1 Reply Last reply Reply Quote 0
              • wilriker
                wilriker @dc42 last edited by

                @dc42 said in Measuring heated bed resistance:

                Did you measure that with the bed heater disconnected from the Duet? If yes then get a new multimeter.

                I'm gonna get a new multimeter. 😂

                Manuel
                Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
                with probably always latest firmware/DWC (incl. betas or self-compiled)
                My Tool Collection

                1 Reply Last reply Reply Quote 0
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