Heat bed temp off

3D_low

Hi. Just installed 2 duet boards on custom printers I’ve built. Cartesian types.

I’ve started running test on one of them and so far so good. I was getting poor bed adhesion with ABS when I realized that the PEI surface was way too cold. I have another printer running smoothie with a similar silicon pad and it feels a lot hotter by touch… my printer is set at 115 but feels more like 70 Celsius by hand when I compare to my other one... big scientific test is that I can leave my hand on the PEI that is supposed to be at 115 and I can’t leave it on my other machine which is set at 110. my IR handheld sensor seems to be broken so I couldn’t rely on that

I’m using a mosfet and a 24v power supply for the bed and a 12v power supply for the board (I had a bunch of 12v components). The silicon heater is a regular 24v 450w 300x300 Chinese silicon heater with embedded thermistor

M305 P0 T100000 B4138 C0 R4700

Wrong setting for a bed or is it possible that 24v heaters are using a different thermistor?

Thanks

Phaedrux

Try a B value of 3950 for that thermistor unless you know for certain 4138 is correct. 3950 is extremely common for Chinese heaters.

dc42

Does the other printer also have PEI on the bed? PEI doesn't conduct heat well.

3D_low

It does, its a silicon pad glued to a 1.6mm aluminum plate… then theres a glass with the pei glued to it.
But even if i let it warm up for a while it doesn't heat up... On my other printer i have the exact same setup and it does heat up after a while... There's a solid difference between both and i thought it was up that i could warm up the bed to 110 in about 3 min running with 400W... I'll try changing the thermistor value and see what happens

dc42

1. Does the temperature reading in DWC reach the 115C target?

2. Are you using a thermistor built-in to the silicone heater?

3. How hot does the edge of the aluminium plate feel?

3D_low

Yes it reaches the temp in dwc
It’s a built in sensor yes
Aluminum is hot but I can leave my hand on it

I’ll chevk the manufacturer specs. I feel like the reading is off. I’m heading to my workshop right now. I’ll also check if the cold temp at idle is close to the ambient. I know it’s not relevant so much but if it’s way off it may be a sign that the thermistor is not at the right setting

3D_low

Turns out the resistance at ~25C is ~43,000 ohms. I tested another silicon heater which is 12v instead of 24v and the resistance is closer to 100,000. I’ll try changing the value from 100,000 to 45,000 on my M305…

Also, at ambient, my bed was seeing 35ish Celsius while my hotend was right on at 19

3D_low

It was indeed the case. The manufacturer listed the resistance at 100k but it’s more like 50k. I changed this setting on my M305 and now I’m getting the bed up to right temp.

It does take at least a minute before the heat goes all the way into the PEI. I’ll just add a timer before start of a print.

Thanks everyone. I’m calibrating the thing as of now and i have great hopes that I’ll improve quality of my prints