New User - Planning Power - 24V system 12V Bed

McAdam

New Here, I had a search on the forum but didnt really find any clear answers.

I am upgrading my current machine which runs on 12V. I will be adding a 2nd extruder and so need to upgrade the PSU anyway.

My plan is to go with 24V for all of the benefits, but my bed is only a 12v unit.

I read about some people limiting the power through a bed PWM value, but I don't think that's a good way to go for a couple of reasons I've seen on here.

So what would the best way be?

The current plan is 2 PSU's, one 12v for the bed and a 24v for everything else.
So just send the Bed output of the board through an SSR?
Is there another way I'm not realizing?

Thanks for the input!

DjDemonD

I'd say you have three options:

get a 24v bed heater, not usually a massive investment but the faster heat up times, smaller (safer) cabling required and the lower overall complexity (1 PSU only required if bed is less than 18A) is surely a big plus. Some PCB bed heaters can be rewired for 24v in which case you are there.
Stick with the bed heater and power it with a second PSU @ 12v, use an external mosfet board (https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313.TR0.TRC0.H0.Xexternal+mosfet+board.TRS0&_nkw=external+mosfet+board&_sacat=0) to run it, exactly like an SSR but cheaper/better. You connect the bed heater output from the duet to the control pins on the mosfet board and route your power from your second PSU through the mosfet board to your bed. These things work and are cheap/easy to use.
Run your 12v bed on 24v - sounds bad right? If you do this with 1 PSU @24v then you might run into trouble, the reason being that a 12v bed generates 4 times the power at 24v! So even if you set 25% PWM, each pulse is still 4x the power there are just fewer pulses. The way you achieve this is with a second 24v PSU that only outputs the max power you want your bed to generate. So if your bed was 150w (12v x 12.5A) before then you would use a 150W 24v PSU (or go slightly higher to 200w), you still need a mosfet board as the two PSU's are not going to be connected to each other. You will need cooling on the PSU for the bed (if it doesnt have it built in add a fan) as you'll be running it at its maximum output whenever the bed is heating up (100% PWM). I did this with a 12v system, chose to run the bed on 24v but limiting it with a second PSU 250W @ 24v via a mosfet board as it was painfully slow to heat up on 12v or when rewired properly for 24v, this has worked fine for over 18 months now.

dc42

I really can't advise #3 above, you would be running the PSU with a load that it is not designed for.

There is 4th option, which is to use a separate 12V supply for the existing bed heater, and use the Duet to drive it directly. You need to connect the +ve bed heater wire to +12V instead of to BedHeater+. And connect the negative outputs of both PSUs to VIN-.

AdiGital

Just out of curiosity, looking at the schematics, wouldn’t the 4th option cause the led (D6) to be constantly on?

dc42

Yes, but it will be brighter when you turn the bed heater on.