Heated bed limited performance

[Dezdoghound]

I've got a 220 x 220 mm PCB heated bed with an aluminium plate on the top (Link) which tops out at 87 degrees.

I get 4.8 ohms measuring across the ends of the wires running to the bed (when they're disconnected from the duet), and when it's plugged in I get 24.2 V across the contacts, which should be 120 W of heating. As a check I've tried measuring the current across the contacts when heating at full power and I only get 0.4 A which doesn't seem right to me.

I've run pwm tuning and the results are here

M307 H0
Heater 0 model: gain 77.1, time constant 411.2, dead time 3.0, max PWM 1.00, calibration voltage 23.0, mode PID, inverted no, frequency default
Computed PID parameters for setpoint change: P317.3, I9.035, D666.4
Computed PID parameters for load change: P31

Any thoughts as to why I'm not getting the expected heating performance?

[dc42]

@dezdoghound it's common for PCB bed heaters to be under-powered. The resistance will rise as the bed heats up (because the heating traces are copper) so it will be about 26% higher at 87C compared to its resistance at 20C, with a corresponding reduction in heating power.

Have you tried measuring the voltage reaching the bed, in case you have excessive voltage drop in the wires?

Depending on which Duet do you have, it may be safe to turn the VIN voltage up a little.

As a rough guide, I usually recommend heating power of 0.4W per sq cm of bed, which for a 220x200 bed would be 194W. Whereas you have about 95W at 87C.

Still, 87C is more than enough for printing PLA, and enough to print PETG.

[Dezdoghound]

@dc42 Cheers for getting back to me! I'm getting 23.9 V at the bed, so close enough. Good to know that's roughly in line with what you'd expect to see. I've just got some ASA which wants more like 110, so could be time to move to a 220 V bed instead.

I've got a Duet 2 wifi for hardware.

[dc42]

@dezdoghound in that case I suggest either a 24V 200W silicone heater if your PSU can take it, or a 200W AC mains silicone heater and SSR.

[paolozampini1973]

@dc42 Good morning, however, a 220 V bed is the best system

@dc42please can you give me this information. Thank you

would like to enable fan F5 when PIN 17 is enabled and disable fan when PIN 17 is disabled ?

[Dezdoghound]

@dc42 I'll definitely move to AC bed in the future for sure. For now I've put two layers of cork on the underside of the bed which has now increased my max bed temp up to 105C. Can heartily recommend this to anybody also struggling with this issue.