12v Heated Bed using 24v on Smoothie

I am building a 400 x 400 printer that is using an MK2 heated bed. I'd like to run the printer at 24v, but the print bed is only 12v.
This will be run via Smoothie.

Looking for suggestions as to how I could rig this up. It's possible I could install a dedicated 12v PSU, but I'm not sure how to regulate the temperature from Smoothie.

Otherwise, I can use 12v for the whole system, but would prefer 24v.

TIA

Will you be printing ABS?

In general, 12V heaters are woefully inadequate for any but the smallest printers. If you plan to print ABS, you want about 0.5W/cm^2 of bed area. That means your 40 x 40 cm bed should have about 800W of heat, otherwise it will take a loooong time to get up to the 105C or so that ABS needs, if it manages to get there at all. As a point of reference, my 300 x 300 x 8mm aluminum bed in UMMD has a 750W, line powered heater and gets up to 105C in about 4.5 minutes.

If you're not planning to print ABS you can use a bit less power, but you're still not going to use 12V for a 400x400 bed.

Using a line powered heater can actually be lower cost than a low voltage heater because you don't need to include the heater power in your low voltage supply that powers the rest of the printer. You use a DC-AC SSR, which won't need a heatsink or fan, to switch power to the heater. You will need an electrical fuse, and should include a thermal cutoff to protect against thermal runaway in case the controller goes crazy or the SSR fails, but they only cost about $1. Here's how I wired the line powered bed heater in UMMD (scroll down the page a bit to get to the wiring details).

I have a 24V 450W heater in SoM and regret using a 24V heater- I had to buy an industrial surplus MeanWell power supply that has a cooling fan that is just short of a vacuum cleaner noise level. I should have used a line powered heater and a smaller, much cheaper, fanless power supply like the MeanWell LRS-200-24. You can learn from my mistake.

Yes, use 24V to run everything else. You may have some trouble locating small fans, LEDs, etc., for 24V operation. That's no big deal. Get a handful of small buck converters to drop the 24V to 12V or whatever is needed. I used some of these in two printers. They are cheap, small, and reliable.

Edited 1 time(s). Last edit at 11/24/2017 10:11AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Thanks tons for the reply.

I should clarify. I already purchased a 400 x 400 12v aluminum heated bed. Already have 24v components sourced (fans, lights, etc.) and that was easier than I expected. I also already have a 24v 350 watt PS.

For reference, I use 12v heaters on all my 200 x 200 printers and it takes less than 2 min for 100c.

If I use a secondary PS for the bed, how do I control that from my controller (MKS Sbase 1.5)?

You can use a mosfet hooked up to the motherboard, but powered by the 12V PSU.

What is the power rating of the 400x400 12V heater? And how thick is the aluminum?

Edited 1 time(s). Last edit at 11/24/2017 12:47PM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

i ve done it mate using duet board. i installed a 12v mk2b to 24v ( lowered it down to 21v after some days of running) and i am pretty happy with it . my bed climbs up to 105c from 15c-20c in a couple of minutes.

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Delta Printer
Duet 0.8.5 firmware 1.19

nominal 1.0 ohms (~1.2).
~10 amps which would be 120watts. No documentation on what the 'power rating' is but given those values that sounds reasonable.
With 24volts, that would be 20 amps. I'm pretty sure the MKS board would be stretching it to support that much current. Is there any way to add resistance to the heatbed?

After more research, it looks like a DC-DC SSR is the way to go. $15 on Amazon is reasonable for a noob like me.

Perhaps in the future I will try a MOSFET (about $2)

I just installed a DC-DC converter from Amazon (24v-12v, 20A). Works like a charm. Running it from a 24V meanwell PSU.

Why not just limit the bed PWM output to 50% 25% in firmware? That'll effectively give you 12V max to the bed.

Edited 1 time(s). Last edit at 01/08/2020 10:23AM by NewPerfection.

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Quote
NewPerfection
Why not just limit the bed PWM output to 50% in firmware? That'll effectively give you 12V max to the bed.

That would give you four times the power for half the time, so still twice the power. Plus a risk of meltdown if the PWM fails and the bed heater mosfet gets stuck on.

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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].

50% PWM is incorrect. you need to look at power

For example most 12v heated beds are 1.1ohm (ish)

For 12v
P = (12^2)/1.1
= 131 watts

For 24v
P=(24^2)/1..1
= 524.wats (4 times that of 12v)

You need PWM of 25% for the same power

And even then your pulsing 24v at 21 amps to a bed that was designed for 12v at 11amps. It probably not going to last long

Edited 2 time(s). Last edit at 01/08/2020 03:52AM by Dust.

You're right, 25% would be the correct PWM value. However, as long as the frequency is high enough, the bed should have enough thermal mass that the pulsing at higher current won't hurt it. What will hurt it is getting too hot. If the frequency is at even a low 1kHz, the temperature fluctuation compared to running a constant 12V will be negligible. A bang-bang type controller would likely have significant issue with temperature fluctuations at the higher power level though.

The concern about the PWM failing may be valid, but failing PWM would cause most printers to catch fire (hotend), has this ever actually happened?

Edited 1 time(s). Last edit at 01/08/2020 10:29AM by NewPerfection.

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