Heated bed at 24V

Hello! I'm new to reprapping and have recently built myself a corexy currently running on a Ramps board. I'm currently switching to a duet 0.6 board with firmware version 1.9r-dc42. At the same time I'm looking to switch the heated bed to 24V to get faster heat up time. The heated bed is a mk3 aluminium bed with a resistance of 1.4 ohm.

I have a few questions:
Can the duet deal with the ~18A power that the heated bed is drawing or do I need to use an external MOSFET?
Is it better to just run the entire board on 24v (simpler wiring) or go for a dual power supply setup like dc42 has described in this thread [forums.reprap.org] with 12V for the board and 24V for the heated? The second option will help given that there are voltage spikes when the heated bed turns on and off.

If your mk3 bed heater is designed for 12V, do not attempt to run it at 24V, because that would give four times the heating power an probably burn it out in a few hot spots. Instead, use a 12V LED/CCTV PSU turned up to 14V, which will provide 36% more heating power.

If it has connections for both 12V and 24v, then wiring it for 24V and using a 24V PSU instead will give a small amount of extra heating power because of reduced losses. Again, you can turn the 24V PSU up to 28V for another 36% heating power.

The Duet 0.6 appears to be safe providing up to about 12A heater current, or more if you solder a piece of thick copper wire on the back of the board between Vin+ and bed heater + to bypass the trace on the top of the board. The Duet 0.8.5 has a better layout, so it should be good for a little more current, perhaps 14A but not as much as 18A.

My preference for the bed heater supply is:

- Small bed (up to 200mm square): single PSU for everything, either 12V or 24V.

- Medium bed (up to 300mm square): either a single 24V PSU, or a mains powered bed heater and a 12V or 24V PSU for everything else.

- Large bed: mains powered bed heater, 12V or 24V PSU for everything else.

Edited 1 time(s). Last edit at 02/29/2016 05:19AM by dc42.

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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].

Thanks for the quick reply dc42. The heated bed does indeed have both a 12V and 24V supply option. I currently run it at 14V on the 12V terminals and its still takes too long to heat up. I see that a 4 fold increase in the power being supplied to the bed is a bit extreme but I have seen various account of this being done successfully. The wiki entry for this heated bed too mentions that providing 24V on the the 12V supply will bring it to 100C in 2 minutes [reprap.org].

My plan now is to turn up a 27V power supply to 29V and apply on the 24V terminal to start with and if that goes well, apply 24V to the 12V terminal. However both of these require using external control of the supply to the heated bed. Are there some best practices/ guide on using an external mosfet with the duet. I have some IRFZ48N mosfets that I plan to use.

I guess your other option is to use a 24V supply but turn it down lower, maybe to 20V.

The IRFZ48N is not a good mosfet to drive the bed heater because it has a high maximum gate threshold voltage, also the Rds(on) even with 10V gate drive is too high for the current that the bed will draw. You might want to consider instead the Power Expander from reprap.me. I haven't used one, but the specification looks OK for this job. It is optically isolated, which is better because it makes the wiring easier and the ground wire resistance much less critical. Don't use it with fast PWM.

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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].

After fighting with several DC heaters inability to reach stated ability or taking an extremely long time to heat, I have switched to using only AC silicone heaters with SSR to control them. Haven't had any issues with time or temp since.

Thanks for the great inputs. I decided to risk it and run 24V to the bed through the mosfet. Gate signal is the bed output on the board (Ramps for now but I'll report back once I shift to Duet) so I get over 10V. I'm using bang bang if that makes a difference. Datasheet for the mosfet says 0.014 ohm corresponding Rdson. Even at 18A I figure I need to dissipate just 5W of heat. But I'm paranoid so I've used a big heatsink and fan. Temperature of the heatsink doesn't go above ambient. The bed now heats up to 100C in just over 2 minutes. Let's see how long this set up lasts.

Regarding the AC silicone heater I've got one for the delta I'm collecting parts for. Though I'm not sure whether I prefer high voltage or high current.

The problem you are going to be facing is trying to pull 18 amps through a connector rated at only 12 amps at best. The traces on the board may not be able to handle the current either. If your heater is going to need anything over 8 amps, you're better off using some type of external switching than what is on the board.

I understand your concern about the high voltage. I don't like it myself. But in this case, it provides a better solution.

You don't have to go to all the way to line voltage. If you get a GOOD DC SSR you can use the onboard FET to switch larger DC loads. The trick is finding a GOOD DC SSR. I got lucky and mhackney pointed me to one from Auber Instruments [www.auberins.com] note the 0.006 ohm on resistance which is far better than the cheap ones using BJT transistors.