Limiting current on heated bed output using duet

Heya,
I was wondering if there is a way of limiting the current / duty cycle of the heated bed output in the config of the duet.

Not the current, but you might be able to limit the maximum pwm value. Marlin supports this, i don't know about others.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

That would work.
Could you tell me how to?

In the current Marlin version check configuration.h for this entry

#define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current

setting it to 127 will half the maximum power.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

In RepRapFirmware the bed is bang-bang by default. You can change it to PID mode using the appropriate M305 command. In PID mode there is no limit on the max PWM as such, however there is a configurable limit on the I-term accumulator, which coupled with the P term can put an upper limit on the PWM.

I could add a facility to limit the bed PWM in the firmware, but I am not yet convinced that it would be both useful and safe. If what you are tryin to do is to run a 12V bed heater from 24V then I strongly advise against that. Better to use a 12V PSU turned up to 14V.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Heh,
I'm trying to run a 12V bed off a 12V supply, but it draws too much current and the power supply switches off.
The bed is way overpowered for the purpose anyways, so I wanted to turn its max power down a notch.
I found the M301 command and tried using the S value to scale everything down without success.
I'll have a go at the I limit tomorrow, not quite sure what it does though at the moment.

Edited 1 time(s). Last edit at 01/27/2016 05:53PM by floppah.

It sounds to me that your PSU is underpowered. What is its power rating?

If it has a voltage adjustment pot, try turning the voltage down to about 11V.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Well yes, there are two sides to a coin.
Bed is rated 120W which is a lot for a 150mm diagonal. PSU is rated 120W as well, but it also has to power the rest of my printer.
I don't really want to go for a higher powered PSU, as that would involve changing power connectors and switches as well.

I agree, 120W is a lot of power for a bed of that size. However, even if you use PWM to reduce the average bed current, that won't necessarily solve the problem. The PWM frequency is about 1kHz, which is far lower than the 30kHz+ switching frequency of the PSU. So even if you run the bed heater at 50% PWM to get half the average power, half the time you will still be overloading the PSU.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

1 kHz should be fine. For these supplies its the average power that counts, not the momentary current.