Cable runs - general advice please.



  • Just started thinking about wiring details for the printer I'm building. The bed heater is a 240V AC silicone heater which came with a thermistor but I've also fitted a pt100 so I have the option to use either. Obviously the bed heater will be controlled via a SSR but I'll have 240V AC running to the bed and the thermistor and pt100 wires coming from the bed. It'll be neater and tidier if I can bundle these wires all together but just wanted to check if that is likely to give me any interference problems.

    Also, is it advisable or not to screen the stepper motor cables (and any other cables come to that). I seem to find conflicting advice when I "Google it".

    Thanks


  • administrators

    The thermistor inside the silicone heater will measure the temperature of the heater, not the bed, so it will tend to read higher than the bed temperature. But it may be easier to get a stable bed temperature, because the thermistor will not suffer the temperature lag of the bed.

    I presume you have a single hot end, also with a PT100, and the dual PT100 daughter board. In which case, you have a spareb PT100 channel anyway. Why not connect both the thermistor and the PT100? You will be able to switch between them in firmware.

    The thermistor is not likely to pick up any interference from the mains cable. It's possible that the PT100 might, however the chip that reads it has a 50 or 60Hz interference reject feature, which at present is set in firmware to reject 50Hz.

    It's not normally necessary to shield any of the cables unless you are trying to achieve CE certification in respect of EMI emissions. However it's best to use twisted pair wiring for endstop connections, and to keep all four wires to a stepper motor tightly bundled together, e.g. as 4 conductors of the same multicore cable. Otherwise the varying stepper motor current can induce voltages in the endstop or other circuits.



  • Just on the temperature readings using the thermistor in the silicon heatpad…
    I have found mine in the heatpad that is directly attached to the 8mm plate to be just about spot on measured with both IR thermometer and thermal imaging camera.

    My instinct (not backed by facts 🙂 ) says that a high powered pad directly attached to the aluminium plate will generally be close enough it doesn't matter and will tend not to read high and based on the thermal imaging will actually read fractional low initially as there are less heat elements at the point and the thermistor is more reading the plate than the pad.



  • @dc42:

    I presume you have a single hot end, also with a PT100, and the dual PT100 daughter board. In which case, you have a spareb PT100 channel anyway. Why not connect both the thermistor and the PT100? You will be able to switch between them in firmware.

    @ DC42. Yes, that's what I was planning to do.

    @All. Thanks for the info and tips guys.


Locked
 

Looks like your connection to Duet3D was lost, please wait while we try to reconnect.