Converting heatbed to 24v?
May 02, 2019 12:22AM
I have a replicate that's being powered by 24v, hotend 24v, but I made the mistake of getting a 12v heatbed by mistake off of wish. I'd prefer not switch everything over to 12v. So what's the best way of doing this and correct way to wire it up. I tried getting help on replicapes slack, but only advice I got was to use a solid state relay but no details on how on what one to buy or how to wire it up, so not sure if that's even the best way to do it. Any help please thanks

P.s. the heatbed is a mk2/ mk42 clone I think

Edited 1 time(s). Last edit at 05/02/2019 12:24AM by Repraplicape.
Re: Converting heatbed to 24v?
May 02, 2019 01:47AM
You cannot convert a fixed 12v heated bed to 24v.

Some heated bed are designed to be 12v or 24v depending on how you wire them up.

But others like the mk42 are 12v or 24v only.
(As far as I know the mk42 only comes in 12v. It wasn't until the mk52 they it came in two models 12v and 24v. NB before you rush out and get a mk52, they have a different mounting to the mk42, make sure you can use it)

To use this bed:
You would need a high current 24v to 12v converter and a external mosfet.. the cost of which exceeds buying the correct heated bed.
Or a 12v power supply and a external mosfet, somewhat cheaper.

My advise Sell the heated bed you got on ebay and get a new one.
Re: Converting heatbed to 24v?
May 02, 2019 03:14AM
You can set the firmware to use a maximum pwm to reduce the power output (at least marlin can). I've used a 12V on a 24V 15A psu without issues for some years at 75% pwm.

Just make sure your psu can handle the current. You could start at 25% and see if that works out ok, bump it up if you think your components can handle it. Doubling the voltage will put 4x more power into the bed so 25% should be about the same as running the bed at 12V.

Make sure that you have the proper safety measures in place, ie what happens if the mosfet fails with a short circuit? Thermal runaway protection and a fuse should be used.


--
Kind regards
Imqqmi

NFAN CoreXY printer:
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Re: Converting heatbed to 24v?
May 02, 2019 03:55AM
eak... wouldn't want that in my house...

firstly MOSFETS seem to like to die on....

Secondly

The average 12v heated bed is around 1.1 ohm, uses 11amp.
The same bed on 24v will use 22amps!!

even at 75 PWM of 22amps is still an average of 16.5 amps... (higher than the power supply rating...)
But it will try and pull 22 amps while on...

I suspect your setup is running on pure luck with nails for fuses.
Re: Converting heatbed to 24v?
May 02, 2019 07:47AM
Mine was 1.26 ohms so at 12V it's about 9.5A, at 24V it was 19A, and on average 14A with 75% pwm (could also have been at 50%, I don't exactly recall). In practice though, due to losses in cabling and other stuff it was well below that. When heated up the resistance goes up, current goes down too.

I guess I was lucky smiling smiley I used the same technique on a heater table I made based on an aluminium heated bed, 12V on 24V psu and still works very well, heats up to 150 degrees C under 3 minutes. The mosfet is a bit over specified so should last a long time. It doesn't even get warm. It's fused with a car fuse.

It's no more dangerous (probably less so) than mains powered silicone heater pads. Those shouldn't run without temperature sensor, mass to heat up like tooling plate and controller either.

Oh and I'm no longer running that setup, it's long since been replaced with a proper 24V alu heated bed, then replaced with a mains powered silicone heater and SSR, properly fused and all smiling smiley

Edited 1 time(s). Last edit at 05/02/2019 07:49AM by imqqmi.


--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]
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