Heatbed MK2a - Can it run on 24V?

My Reprap-Kit came with a Mk2a Heatbed, it is suggested everywhere to run it with 12V, but... Its soo slooooow to heat up. So I thought I may be able to run it on 24V.

The Mk2b and Mk3 models all have "dual" 12/24V design, but I don't see any real reason why the Mk2a can't run on 24V, unless its about the circuits on the board, which I know nothing about. But I was unable to find any information about the Mk2a with 24V...

So, does anyone know; can the Mk2a Heatbed be run on 24V?
(I don't have a LED on it)


thank you!

A typical Mk2a heated bed has 1.0 to 1.2 Ohms of resistance (if yours has more, that may be the problem). At 12V this gives 120 to 144W of power. At 24V, this gives 480 to 576W of power, which is enough to get the PCB very hot and possibly burn out traces if you aren't careful. P=I^2/R

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Ah yes thank you! Though the question was whether the circuits can handle the increased power
(Though I had not calculated it, and did not know the resistance)

But anyway, I realized that the Mosfet on my RAMPS won't take that much, apparently it can only handle 140W (It's an IRLB8743) - if I'm reading the datasheet right. So it would probably burn first. Also I did a quick calculation and even 14V would be too much for it already

Also I just checked and under strain my power supply gives up and only supplies only 10.5V which boils down to only ~92W, I'll try another lane on the ATX PSU tomorrow...

PS: *cough* Formula Typo!

If you're using Marlin, you can set BED_MAX_POWER in your configuration.h to something less than 256 (full power). I've never tried it before, but I think it would work just fine with 24v as long as you reduce BED_MAX_POWER to something reasonable. If, as Eddu mentioned, the mosfet can only handle 140W, then you'd have to set the BED_MAX_POWER to 70 which should give you an effective power dissipation of about 131.25W assuming 1.2ohm resistance.

Also, now that I read your initial post, it sounds like your printbed resistance is higher than 1.2 ohm so you're not getting as much power as you should. In that case, increasing BED_MAX_POWER from 70 (at 24V) would help, as long as you keep the effective dissipated power under about 135W. Math is your friend.

I measured the Heatbed with a Multimeter and it has "exactly" (as far as a cheap Multimeter can measure) 1.2 Ohm

Like I wrote above, I also measured the voltage of the 12V line to the Heatbed and it only had 10.5V with the Heatbed turned on (12.5 when off), that's 28W missing, so it is my current aim to see if I can't get those out of my current PSU before buying anything new.

Thanks for the tip though, it may come in handy later

Where did you measure the 12V line? Was it at the heatbed, or at the power supply? What gauge of wire are you using to your heatbed?

You can modify your Mk2a heatbed to run on 24V at the same power level as at 12V. It requires cutting one of the power traces, and attaching your wires across the cut. The bed is basically 4 resistors in parallel, and if you cut after 2 of them in parallel, then you end up with 2 in parallel, which are in series with another 2 in parallel. When run on 24V, this leaves each bank of 2 in parallel running on 12V, which is the same as it was unmodified. The LED current works out ok as well.

or run a 12 volt fan in serials to lower the voltage

I measured at the first open wire after the PSU -- at the RAMPS power terminal. Also after the MOSFET and directly at the heatbed contacts. After some optimization and playing around with the wiring, I measured 10.5 V directly at the heatbed as well as at the RAMPS MOSFET terminal (approximately). At the power terminal I measured about 11V (so it seems the MOSFET and fuse burn another .5V). I also measured at the power terminal with the circuit open, which showed about 12.3V

Yes, the wires from the PSU are way too thin. No Idea what gauge they are - it's pretty hard to measure at this scale with stranded wires, but they're certainly not designed for 10A.

I *am* tempted to rip the PSU apart and replace the wires with decent ones, but I don't think it will do me any good, as I think the problem is the PSU itself and the thin wires just make things worse.

*sigh* this reprap is turning into way more work and/or expense than I had accounted for. And thank you for all your help again.

Eddu Wrote:
-------------------------------------------------------
>
> But anyway, I realized that the Mosfet on my RAMPS
> won't take that much, apparently it can only
> handle 140W (It's an IRLB8743) - if I'm reading
> the datasheet right. So it would probably burn
> first. Also I did a quick calculation and even 14V
> would be too much for it already

You are reading the datasheet wrong, in this case. The 140W that is referred to is the allowed power dissipation of the MOSFET. It is rated for a continuous current of 78A, so that is plenty for this application. One way to find out if you have much power going into the MOSFET is to see if it is warm.

If you are not seeing a significant voltage drop across your heatbed wires, then the wire gauge is probably fine.

If you are seeing a droop right at the power supply, likely the internal wiring would have little to do with it. More likely, is that it is not working correctly, or is not capable of driving the heatbed load.

nechaus Wrote:
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> or run a 12 volt fan in serials to lower the
> voltage

That's not going to do much, unless the fan also draws 10A. You can't just run any two devices that need 12V in series, unless they also need the same current.

Related to this, I would like to use a 200 x 300mm heated bed which I can only find in 12v guise, but power it from its own dedicated 24v PSU via a mechanical relay with bang bang control Is this a risk for damaging the heat bed? Will it rapidly overheat when current is applied at this voltage or will it be fine as long as the thermistor reads correctly and the ramps board shuts it off via the relay at the desired temperature ie 110 deg C?

Would I be better using a DC-DC solid state relay and limiting the bed power in firmware?

Or just running it on 12v via the relay with a dedicated PSU?