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Controlling large/multiple heatbeads - relays?

Posted by pugzor 
Controlling large/multiple heatbeads - relays?
October 28, 2015 03:54PM
Hey all,

I'm starting an ambitious build which uses 4x standard MK2B heatbed PCBs to heat up a large print bed.

Obviously this is much more power/current than any of the usual controllers can handle (I'll be using a 32bit Smoothieboard or clone). I do have a dedicated power supply just for the heatbeds (720W) but the control is where I'm a bit unsure. I was just going to use some standard automotive relays but then it hit me that they'd only be a decent solution if it was kind of binary control (ie, heatbeds are either on or off) but I don't believe that's the case; MOSFETs generally vary current to the heatbeds depending on the required load, don't they? Eg, a cold bed would be heated at 100% but as it approaches the required temp, it might dial it back to 30%?

I'm not quite sure how this works or what the best solution is... Any help would be appreciated!
Re: Controlling large/multiple heatbeads - relays?
October 28, 2015 05:11PM
The best solution for heating a large bed is usually to use an AC mains voltage heater driven by a zero crossing SSR. Of course, this requires additional safety precautions to be taken.

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].
Re: Controlling large/multiple heatbeads - relays?
October 28, 2015 09:27PM
Thanks for the reply David!

I do already have the heatbed PCBs and the big DC power supply, so I guess I'm looking for a solution that matches that. I guess I'm kind of fond of using low voltage where possible too.
Re: Controlling large/multiple heatbeads - relays?
October 28, 2015 11:46PM
There's no need to use PID control for a heatbed, the old fashioned "Bang bang" control is fine.

Just stick some relays in there and be done with it. I'm using a relay to control the heatbed on my printer, it works fine.

Edited 1 time(s). Last edit at 10/28/2015 11:48PM by nebbian.
Re: Controlling large/multiple heatbeads - relays?
October 29, 2015 12:07AM
Hmm, so I can just use what would traditionally be the heatbed output as the trip to feed the relays? I'll probably still have 4x individual channels with their own relays, connected in series....
Re: Controlling large/multiple heatbeads - relays?
October 29, 2015 12:48AM
Bang-bang bed temperature control has been demonstrated to cause print quality problems. Maybe in your printer it doesn't, or maybe you don't recognize the problem because it is hidden by other print quality problems that you may or may not recognize (it looks a lot like Z wobble, commonly caused by another "good-enough" solution, threaded rods to lift the Z axis instead of lead screws), but PID control capability is provided for a reason. Why, when there's a thermistor reading the temperature, a heater on the bed, and the firmware is capable of PID control, would you use bang-bang control just to save the cost difference between a mechanical relay and an SSR? How many $1 or $2 or $5 compromises can you pile into a machine before the resulting quality problems start to bother you? Have you ever heard the expression "penny wise and pound foolish"?

Edited 1 time(s). Last edit at 10/29/2015 01:10AM by the_digital_dentist.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Controlling large/multiple heatbeads - relays?
October 29, 2015 05:11AM
I've build a small board with a mosfet to power the second hotend. It's exactly the same as in the ramps schematics, a logic level mosfet driven by a pin from the arduino board. I used the irlb8743 which should be capaple to power a heated bed too, maybe two, in parallel. You can use a couple of them driven by the same pin to make them act as one heater. The Rdson is just 3.2 milli Ohms so heat dissipation should be no problem.
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