Bed heating too slow



  • Hi,
    the first days after setting up the DuetWifi everything was fine: the bed heated extremly fast.

    Now i need around 9 minutes to reach 80C or approximately 14 Minutes to 100C.
    Friends with nearly the same hardware need only 4 minutes to 80C and i am sure that it initially was a lot faster.

    Initially i used firmware 1.19, now i use 1.20.
    I do not know if the changed heating behaviour occured only after the firmware change since i used only PLA during the change time.
    Now that i use ABS again i noticed the slow heat-up.

    I have measured the resistance of the silicon heatpad: 1.7Ohms. Which gives me round about the 350W @ 24V that are promised.

    Any ideas what could cause this behaviour?

    Are there settings in the configuration that limit the current for the heatbed?

    My setup:
    300x300 heatpad 24V, 350W.
    DuetWifi, Firmware 1.20
    Meanwell 500W 24V power supply

    THX for any ideas what i could check to find the error.

    Greetings,
    Markus


  • administrators

    Have you checked that the screws in the power input and bed heater terminals on the Duet are still tight?



  • @dc42:

    Have you checked that the screws in the power input and bed heater terminals on the Duet are still tight?

    Yes, they were tight and are now tight again….
    Warm-Up time to 100C was 16 Minutes tonight 😞

    If there are no options to set the current in the configuration it can only be

    • a defective head pad?
    • power supply issue?

    Or can the duet itself be affected somehow?
    Am i correct to think of the duet as a "on/off" switch regarding the bed heater connector? So it should route through the whole current or nothing... but not a reduced current/voltage?

    Strange thing is that the system works just fine - only very slow heating....
    Voltage during print is 23.5 to 24.2V - i guess with and without heating the bed.

    Greetings,
    Markus



  • In the "first days" and the bed heated really fast, was it also nice and stable or did you get a degree of overshoot\undershoot. About the only thing that I can think of on the Duet side, is that initially the bed might have been controlled using "bang bang" mode but since those initial days, you might have tuned the heater and now it's using PWM \ PID. You could try "bang bang" mode or re-run the heater tuning and see is either of those help.



  • If you want to test the bed itself measure its heater resistance (it will be a low number so measure your leads first and subtract that from the resistance of the bed). Then use this type of calculator:

    https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html

    to determine the wattage by entering your supply voltage and the resistance. Is it the wattage you expect, you want about 0.5w/cm2. If it's not the wattage you expect or close to it, the bed is faulty. If you have a clamp ammeter you could check how many amps flow when in heating up phase and use the same calculator with volts and amps.

    Next test (carefully supervise this one) connect the bed direct to the power supply, this will heat it up as fast as it can heat, keeping reading the temp using your thermistor but there will obviously be no control of the heating. Maybe employ an ir thermometer or second thermistor to verify the temperature, in case it's a thermistor issue, any big mismatch and maybe your thermistor is faulty. If it heats to printing temp in 4 minutes then it's not your bed disconnect it at this point. Do not leave it unsupervised at any point or it will keep heating until something burns.

    If it doesn't heat up quickly then it must be either the psu, or your wiring. Use thick cables with as few joints as possible, make sure all cables are well connected.



  • THX for all your ideas.

    Measured resistance is - as reported in first post 🙂 - 1.6 to 1.7Ohms - which corresponds nicely with the 350W the pad should have at 24V.

    BUT - direct connection to the power supply showed no improvement. (good idea!)
    Heat up to 80C still takes 8:10 minutes.
    (Bed heater still uses bang-bang mode)

    So either my power supply is bad - which i dont think since the DuetWifi still reports 24.3V while the bed is heated directly connected to the power supply.
    Or the heat pad is bad, but somehow shows the correct resistance…...

    Strange.... but the Duet itself is now taken out of the equation.

    Greetings,
    Markus



  • Can you try another heat cartdrige?



  • @peirof:

    Can you try another heat cartdrige?

    Its the heat bed… not the hotend...
    Unfortunately i do not have another heatbed available at the moment.



  • Actually your heat up times aren't bad but it depends on how thick your aluminium is and whether you have anything on top of it. My bed heat times are very similar to yours but I have 10mm thick aluminium topped with 6mm thick glass but I use an 800W 240V heater so we can't really compare them.

    Do your friends all have the same thickness aluminium and do they all have the same print surface on top of that (for example, nothing at all, PEI, PrintBite, Glass etc)?

    Also, where is the thermistor located? When I first built my printer, the thermistor was located between the silicon heater and the aluminium. It showed a really quick warm up time but the top of the aluminium was barely above ambient. The heater cycled off and on as the underside heated and cooled, so it took forever for the heat to transfer through to the top surface. So I drilled hole in the edge of the bed, as close as I could to the top surface and moved the temperature sensor to that position. The result was that the warm up time appeared to greatly extended but in reality, the top surface reached temperature much faster.



  • Hi Ian,
    i use a 6mm aluminum plate, with glas on top.
    The thermistor is inside the silicon heat pad and should show a temp change really quick.
    Actually, the glass itself is not lagging behind much in temperature. When the thermistor shows 100C the glass has reached 98C. (measured via infrared)

    I will use a "power-outlet energy cost measuring thing" tonight to see how much current is drawn while heating only the bed with nothing else connected.

    I will report back 🙂
    Side note: The thing is that i was absolutely happy with the new printer regarding the heat-up times. They were much faster than my old i3 clone (Anet A8). Now they are slower - so something has changed.

    Greetings,
    Markus



  • @Cumulus7:

    …............
    Side note: The thing is that i was absolutely happy with the new printer regarding the heat-up times. They were much faster than my old i3 clone (Anet A8). Now they are slower - so something has changed.

    Greetings,
    Markus

    So if the bed is physically the same, nothing has been moved or repositioned, the voltage input is unchanged then logically, the only thing left is the heater itself - or possibly the thermistor which is giving a different reading. That's a thought. Does the thermistor have the same contact with the heated surface or could that have changed? Could it have been touching the aluminium or silicon heater and now there is small air gap between the thermistor and the heated surface ?

    Also I'm assuming that the heater is stuck to the aluminium and that is is still stuck in place and has good contact?

    TBH, I'm leaning towards thinking that your current warm up time is about right for the top surface, but that when you saw a more rapid rise that was because the measurement was at the underside or the heat pad itself and the upper surface (which is what matters) may have not been fully up to temperature. If for example, the thermistor was in contact with the aluminium but now the printer has "aged" the heater might have come slightly unstuck around the thermistor so now you have a small localised air gap, it'll look like the temperature isn't rising as quickly but in reality, the heat transfer is mostly unchanged so the upper surface is getting up to temperature in the same amount of time. Not sure that I explained that very well.



  • It's a good idea to try to get your thermistor where you want the point of control. So with a multilayered bed if it was possible to place the thermistor on the bed surface you would control for the surface temp not the heater temp underneath all the aluminium and other surfaces on top. But I've not seen a scheme to do this well or safely as a disconnected thermistor is a fire risk.

    It's a shame IR thermometers are not very accurate as they depend on the surface reflectivity or that would be a good way to ensure if you want 100 deg C on top that's what you get. Maybe one of the e3d style pt100 or thermistor cartridges would work well inserted into a 3mm hole drilled into the side of an aluminium substrate near the edge, coated in thermal compound, then glued in place. If that was 100 deg C then the surface in the middle would almost certainly be 100 deg C also.

    In fact as soon as I can import some pt1000 sensors in the same cartridge format as my pt100s I have in my shop then I will try it, as pt1000 does not need a daughter board with FW 1.20.



  • I use a PT100 sensor that I got from RS that is 2mm wide and only about 4mm long which I drilled a 3mm blind hole in the bottom of my bed so that it only just stops before breaking the surface and glued it in with some Arctic Silver thermal Epoxy.

    This works extremely well and I already had the second port on my PT100 daughter board anyway so it works (the sensor was very cheap as well (£1.50 each currently) https://uk.rs-online.com/web/p/platinum-resistance-temperature-sensors/1005969/?searchTerm=100-5969&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E285C647B362C377D5B4161426250705D297C285C647B337D5B5C732D2F255C2E2C5D5C647B332C347D5B4161426250705D3F292426706F3D3126736E3D592673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D3130302D35393639267374613D3130303539363926

    and if you want the PT1000 version at 1.35 each https://uk.rs-online.com/web/p/platinum-resistance-temperature-sensors/1007530/ 2.3mmm long 2mm diameter



  • Nice will look into those Doug thanks.



  • Hi everybody,

    i measured power-draw at the wall outlet:
    Just the Duet: 9W
    Starting bed heating: 320W
    Later on, at 60C: 280W.

    So the bed heater uses around 310W at the beginning and 270W later on.
    Seems about correct.

    So i don't know what i measured or remembered from before. I guess i will live with the heat-up times for now. At least i am finished with ABS printing for now. Too much warping even on small parts (7x7x8cm) without a full enclosure. I will go back to PETG, so heating times will not be soooo bad.

    THX for all your input!

    Greetings,
    Markus


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