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Heatbed mosfet @30A, 12V

Posted by LarsK 
Re: Heatbed mosfet @30A, 12V
July 20, 2016 08:09AM
A question for the electrical engineers out there : is it safe to drive a power mosfet like IRFP4004 from the heated bed output on the Duet ? Basically using the integrated mosfet on the duet to drive the external more powerful mosfet.

Everything is going to be powered from only one 12V power supply.
Unfortunately, I am not that knowledgeable when it comes to MOSFETs... don't know what is better.. to switch the high side.. the low side ? .. What would be the right resistor value to drive the Gate in my case.. The heated bed will have a resistance of around 250 mΩ, so the MOSFET will be switching around 50Amps of current.

Sorry for the "MS Paint" type of drawing, I had nothing else on hand at this moment.


Re: Heatbed mosfet @30A, 12V
July 20, 2016 10:09AM
The problem with driving a mosfet like that is that it will work in the wrong sense. When the bed mosfet is on, your external mosfet will be off, and vice versa. You could use a small signal.mosfet to invert the signal.

Alternatively you could use a P channel mosfet to switch the high side, then the signal will already be the right way round. But you can't get P channel mosfets with such a low on-resistance as N channel, so you would have to use several in parallel anyway.

I've lost track of where this thread started. If your bed heater is dual voltage, you could switch the system over to 24V. The bed heater would then draw 15A, which the Duet 0.8.5 should be able to switch directly if you either add a wire to the back of the board or run the bed heater positive wire directly to the power supply.

Edited 1 time(s). Last edit at 07/20/2016 10:10AM by dc42.



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

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heatbed mosfet @30A, 12V
July 20, 2016 01:23PM
Oh, I just hijacked this thread to ask a question, so I won't have to open a new one. I am building my own heater and was thinking of ways to control it with the duet...
Re: Heatbed mosfet @30A, 12V
July 20, 2016 04:41PM
For the record, mine is with the 0.6 Duet which has a pin out for the Mosfet gate driver so it is just pluging a cable to that and running it to my external MOSFET gates.

I do understand the idea about using a transistor and controlling 12V instead. If I make an updated circuit someday, then I will maybe do that.


One thing I do not understand with Duet is how to redefine pins. For example how to hijack one of the pins in the expansion connector to act as gates for the heating control (even as such will only output 3.3V).
Re: Heatbed mosfet @30A, 12V
July 21, 2016 12:44PM
You can redefine which heater controls the bed using the H parameter of the M140 command. You can control spare pins on the expansion connector using the M42 command. See [reprap.org] for details.



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

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heatbed mosfet @30A, 12V
July 21, 2016 04:04PM
Quote
dc42
You can redefine which heater controls the bed using the H parameter of the M140 command. You can control spare pins on the expansion connector using the M42 command. See [reprap.org] for details.

Ah, that actually makes perfect sense.


Extrapolating on that;


unromeo21, what you need to do is look at [reprap.org] then you identify pin 22 on the header as heat[4]. This will give you a gate driving signal that you put to a transistor that then drives your Mosfet array.

Then in your slicer you ask it to always run the following two lines in the the beginning of the print:

M140 S55 H4
M140 S0

This will select and hold H4 as your heater.


Totally untested but pretty sure that should work straight out the box.
Re: Heatbed mosfet @30A, 12V
July 21, 2016 05:07PM
Or just put M140 H4 in config.g.

Btw the heater output pins on the expansion connector are active low, so 0V means heat on and 3.3V means heat off.



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

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heatbed mosfet @30A, 12V
July 22, 2016 01:10PM
Quote
dc42
For 30A I would use a good quality SSR. Definely not a SSR-40DD. There is an SSR that has been mentioned on these forums before that would be suitable, I think the brand name starts with A. There are also some Crydom SSRS that would be suitable, but they cost more. The Power Expander from reprap.me is another possibility but I suspect that the connector and PCB traces may not be rated for 30A.

Mine hasn't arrived yet, so I can't comment on how well it works, but you must mean Auber Instrument's MGR-1DD80D100. If it actually does what it claims, it's a pretty good choice.
Re: Heatbed mosfet @30A, 12V
July 22, 2016 03:14PM
Quote
CKTofu
Quote
dc42
For 30A I would use a good quality SSR. Definely not a SSR-40DD. There is an SSR that has been mentioned on these forums before that would be suitable, I think the brand name starts with A. There are also some Crydom SSRS that would be suitable, but they cost more. The Power Expander from reprap.me is another possibility but I suspect that the connector and PCB traces may not be rated for 30A.

Mine hasn't arrived yet, so I can't comment on how well it works, but you must mean Auber Instrument's MGR-1DD80D100. If it actually does what it claims, it's a pretty good choice.

Thanks, that's the one I meant.



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

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