Bedheater won't turn off

Frederik561

Hello
I have a problem with my bedheater. It wont turn off and the red indicator is always on even though it detects fault and is turned off in the duet web control. I am using it with a peltier module to cool the bed down, but that is not the problem. Even if I configure it as a normal heater, it will always be on. Can anyone help with this?

/Frederik

jay_s_uk

why are you inverting the output?
If you remove the SD card, does it stay on?

Frederik561

This post is deleted!

Frederik561

This post is deleted!

Frederik561

@jay_s_uk

Inverting because I am using a peltier module to cool down. And the heater stays on even when removing the SD card. I suppose that is a bad thing?

jay_s_uk

@Frederik561 yes, that pretty much means your mosfet has died

Veti

why did you put a frequency of 50000? that puts an extreme load on the mosfet.

Frederik561

@Veti To make a more even current. A peltier module does not work well with a low frequency. The limit is 65535 Hz so I thought the MOSFET would be capable of doing that.

Frederik561

@jay_s_uk
Any way to fix it? And is the same mosfet used for all heaters or can I still use the nozzle heater?

Veti

@Frederik561
see
https://forum.duet3d.com/topic/18592/beware-of-too-high-pwm-frequency-on-out-0?_=1606733751136

Frederik561

@Veti
Ok, thank you. I have never tried to work with 3D printers before, and I don't think I had any ways of knowing that in advance. I wrote the config-file before the post was made in september and it only just gave up now, so I thought everything was fine.

Guess I have to buy a new duet 2 wifi? I don't suppose the problem can be fixed?

Veti

@Frederik561

the mosfet could be replaced.

but its not that easy
https://duet3d.dozuki.com/Wiki/Connector_and_spare_part_numbers

dc42

Are you absolutely certain that if you remove the SD card and then power up the Duet, the bed heater output turns on? Your M950 command with the output inverted will have the effect of turning the bed heater on at startup.
What temperature do you intend to cool the bed to?
What is the application?
What is the specification of the Peltier element?

Frederik561

@dc42

I am certain. I have tried removing the inversion and it still happens
I intend to cool the bed to 10 degrees C. Should be possible with 12V and 3-4 amps.
It is for a bioprinter. Basically a normal 3D printer that prints into a cool petri dish
I have tried some different peltier elements. Currently I am using a TEC1-12706 model with 2 Ohms resistance at 25 degrees C. This resistance increases with increased temperaturdifference, meaning I will always draw less than 6 amps. I have an ACR712 measuring the current at all times using an arduino

I am now using the e1 heater output with the peltier module and it works fine with bang bang. I have nothing else connected and as I understand it, the limit is 7,5A right? It will limit the amps to the peltier element with each motor drawing 1 amp and the nozzle heater drawing about 1 amp as well (only have to heat to 37 degrees C). So only about 2,5 amps max to the peltier element?

dc42

Currently, RRF is not designed to support cooling rather than heating, because an inverted output will default to on, and because RRF treats any request for a heater temperature below 40C as meaning the heater should be turned off.

I will add a feature request to support cooling as well. Meanwhile, you could try configuring the Peltier as a thermostatic fan, so that it will turn on if the temperature is above target. You can even set it to work in proportional mode, by specifying a small temperature range in the M106 command.

Note, to work efficiently in PWM mode, the PWM to a Peltier needs to be smoothed using a flyback diode, inductor and optionally a capacitor. The use of a high PWM frequency and Schottky diode allows a smaller inductor to be used.

Frederik561

@dc42
Okay, thank you. I don't have any issues making it work as intended. I have inverted in M950 and it works fine. I have set the standby temperature to 35 degrees and it does not turn on at startup. When activated to reach 10 degrees, the heater turns on and holds the required temperature perfectly. Before printing, I wait before the temperatures are reached instead of relying on the board to wait for the correct temperature to be reached, so that should not be a big issue. I am using the system you describe to smooth the PWM

In M307 the possibility for connecting a peltier module for cooling is even mentioned (inversion happens in M950 though).

It works when connected to the e1 heater and draws 3 amps when activated. 4 motors have set limits to 1 amp each and the nozzle heater draws about 1 amp as well. Is it likely that the 7.5A fuse will break?
If yes, is it okay to replace the 7.5A bladefuse with a 10A instead? Will more than 7.5A in that channel damage the board? (Duet 2 wifi)

dc42

@Frederik561 said in Bedheater won't turn off:

Is it likely that the 7.5A fuse will break?

No, because a motor set to 1A actually draws a lot less from VIN, assuming they are typical motors with only a few ohms phase resistance.

Frederik561

@dc42
Thank you! Helps a lot. Really appreciate the fast answers