Can't get bed to stay @ 110 and having both bed and hotend faults



  • I am having good success with my bed up to 80 for PeTG but when I tried to get it up to 110, it gets there (slowly) and then after a bit of the first layer is printed it faults. I did notice a temp drop as soon as it started printing on the bed. I have read the posts and blogs about bed tuning and suspect that it might be something with that?

    Any thoughts? Here is the bed settings from the config file.

    [[language]]
    
    ;***Choose if you want to use the M301 or the autotune results and M307 command. Don't use both.
    M301 H1 P10 I0.10 D100 T0.50 S1.0  		    ;PID settings for extruder 0 (no need for M301 if using M307)
    ;M301 H2 P10 I0.10 D100 T0.50 S1.0   		;PID settings for extruder 1
    
    M570 S120                					;Increase to allow extra heating time if needed
    
    ;***Use for autotune. Comment out M301 if you do.
    ;M307 H0 A142 C293.5 D5.4 B0 				;Autotune result for bed
    ;M307 H1 A588.5 C242.9 D6.7 B0				;Autotuen result for first nozzle
    
    


  • Power supply too small?



  • 24v/15a… should be sufficient?

    Also, why would the outer area of my bed heat faster than the center? The edges were just at 88c and the center was 60c?



  • What size heated bed (wattage)?



  • What is your bed size shape and thickness? Are you using a mains bed heater? If not, what is the bed heater voltage and wattage? What is power supply rated at? Why is M307 commented out? Do you use a print cooling fan and if so, is the temperature drop coincidental with that turn in on?


  • administrators

    1. What firmware version are you running?

    2. Don't use M301, use auto tuning to generate a M307 command instead.

    3. The format of your M570 command is wrong, unless you are using very old firmware.

    4. You say that your 24V 15A PSU "should be sufficient", but that is impossible to say without knowing the size of the bed and the power rating of the bed heater.

    5. Have you put thermal insulation under the bed?



  • 1. FVV Version 1.18.1 (2017-04-09)
    2. I was trying to auto tune but took quite a while and decided to stop it.
    3. I will look at the docs again for this
    4. The bed is 200mmx300mm. / Resistance between 4.5 and 5 ohm for the 24V (115 - 128 Watts)
    5. I used cork for insulation, not sure if this is considered to have a similar result

    Thanks


  • administrators

    Good information. Cork is Ok for under bed insulation, it's what I use. However, 128W is ok (just) for a 200x200 mm bed but too little for a 200x300mm bed. I recommend 0.4W per sq cm of bed, and the minimum that works if you want to achieve 110C is about 0.25W to 0.3W per sq cm.



  • So by that math (I could be wrong). A 200x300mm bed (600SqCM) would require a minimum of 240w. The power supply I have is 360w. That means there should be a good PSU or am I looking at it incorrectly.



  • The heater itself has insufficient power, due to its resistance.



  • @interwebsdesigner:

    So by that math (I could be wrong). A 200x300mm bed (600SqCM) would require a minimum of 240w. The power supply I have is 360w. That means there should be a good PSU or am I looking at it incorrectly.

    Don't forget that you will need also some power to run the steppers and the hotend(s)..

    I ran a 310x310 bed with around 280W of power to heat it. I had 2 layers of cork under the bed and had to put additional cork on the upper side around the object to keep up at 90°C Couldn't get it to higher temps and it took an eternity to get there.

    Now I have a Keenovo 750W heater@230V driven by an SSR running at a 400W, Still with insulation at the underside.
    Bed heats in 2 minutes to over 100°C. could achieve easily 130-140°C.
    Higher powered beds are potentially unsafe when powered from a 12V source due to the high currents. 24V is better and 230V is in my opinion the best solution. Avoiding high currents through the controller and especially all connectors that add extra resistance and may become hotspots starting a fire.

    People tend to think 12V or 24V is safer then 230V (and that is the case when touching), but forget the inherent higher danger from high currents.



  • Yes the bed is just underpowered or under insulated, although underpowered will have more of an effect than under insulated.



  • Thank you for the information all. A couple more questions. Sorry for the newb questions, I am pretty new to electrical…but getting there

    So right now, I have a 24v/15a (360w) psu running the board. I have an DC/DC SSR on the bed at the moment.

    If you are using 230v via relay what is your power source for the 230v?



  • In the US I use a 110 volt heated bed and pull the power from the same feed to my 24 volt power supply. If you have access to 220 volt then you would use that.



  • Ok. So 110 (In the US also) can come directly from the wall to power the bed.

    Basically instead of the PSU to the SSR just splice the wall feed that goes to the PSU?

    If that is the case then I will need an AC/DC SSR instead of DC/DC correct?


  • administrators

    @interwebsdesigner:

    Ok. So 110 (In the US also) can come directly from the wall to power the bed.

    Basically instead of the PSU to the SSR just splice the wall feed that goes to the PSU?

    If that is the case then I will need an AC/DC SSR instead of DC/DC correct?

    Yes.



  • Are you using a dc-dc ssr now? Some these have really big inefficiency (does the ssr get hot?), try running the bed directly off the duet instead of using an ssr, it can handle these sort of currents.

    You need one for mains bed as there is no mains voltage switching capability on the duet. AC dc ssrs are quite efficient so don't get that hot.



  • Yes, DC - DC right now. Not hot at all (currently printing @ 50c though).

    I will try going directly to the bed from duet and see what results that has.

    So to talk it through. The SSR basically takes a DC current input, once it goes active it trips the power straight from the PSU which is 24v. The Duet can handle 24v so the inefficiency of the SSR could be slowing the heating process?

    This is a DBOT and I used the BOM from that, I would make the assumption that because they used RAMPS that the SSR was required?


  • administrators

    If your DC-DC SSR is a SSR-40DD or similar, it will be dropping about 1.5V-2.0V and will get very hot when the bed heater is running at full power, unless you have a huge heatsink on it, and will reduce the power getting to the bed heater significantly. If the SSR is the Auber Instruments one that we recommend in the wiki, it will run cool and it won't cause a significant reduction in bed heater power.



  • The duet (wifi/ethernet) should be able to handle 18amps (12-24v) for the bed heater, if your bed heater has 4.5ohms resistance then at 24v it will draw 5.3amps so you're well within that limit.

    I'd only use accessory DC switching devices (SSR, or better for DC an external Mosfet board) if you are close to or over the 18A for the bed, or if you want to use a second PSU (to essentially isolate the two PSU's).

    With AC mains/line you have to use an SSR (or mechanical relay but this isn't the ideal option).

    It still isn't going to perform spectacularly as its still only putting 128watts into 600cm2 which is 0.21w/cm2 so around about half the power that is recommended for reasonably quick heating and good maintenance of temperature, but if you're losing power through the SSR this is going to be lower still.


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