Powering a 24v heated bed via RAMPS

I'm no electrical engineer, but I was trying to figure out if the 11A rail of the RAMPS board could be powered with 24 volts instead of 12 so that I could use a beefier heater. I'm building a delta printer, and the heatbed is going to be a circle about 280mm in diameter. I found a kaptan heater at Trinity Labs that would be large enough, but it runs on 24v.

I looked at the RAMPS circuit diagram, and the 11A rail goes to the D10 terminal block and that's it. Obviously the current would also flow through the MOSFET and a couple of other passive components, so I'm not sure if I'd blow something by doing this.

Alternatively, I could use the RAMPS to operate a sufficiently rated relay which would then switch the bed on and off. I figure the relay would be good enough since the bed uses the band bang approach; I don't see the LED for the bed flashing like I do the one for the hot end.

Using 24V on just the 11A rail (for the heated bed only) is fine, though for safety I would remove the 11A PTC fuse (which is only rated at 16V) and either:
Replace it with a wire link, and put a 10-15A fuse on + side of the 11A input.
Replace the PTC fuse with 10-15A fuse directly (eg: buy a fuse holder and solder it in place of the PTC fuse).

If you're not comfortable doing that, then you can buy an SSR (Solid State Relay) that you can use to control the heated bed. One advantage of this is that the grounds of the two power supplies do not need to be connected.

In this case, you run everything at 12V (as normal) and you put the SSR's input on the D10 output. You then connect the output of the SSR to one side of the heated bed, and the other side to your 24V power supply. You connect the other side of the power supply to your heated bed. When D10 turns on (the heated bed output) the SSR should switch on, completing the circuit and turning on the heated bed.

Notes:
The SSR needs to be able to handle 12V on it's input, and should be capable of driving a 24V output (some are rated higher, and will either not work as well, or at all, driving a 24V device), so pay this in mind when purchasing.
If you are using two power supplies and NOT planning on using an SSR, you should test that there is no ground leakage between them. The easiest way to do this is to connect a multimeter on the volts range to both grounds and switch them both on. If the reading is 0 (or near enough), then you "should" be fine. If there is a voltage difference, then you need to change your power supplies, or you can use the SSR method instead. I include "should" in quotes as while this works in most cases, PSU manufacturers can do some really weird things to cut costs. If you're at all worried, then I suggest the SSR method.

PS: I realise you might not need all this jbernardis, but I thought some others might find it useful. FWIW: I have the same kapton heater. I'm also running my RAMPS board completely at 24V without issue, but I did check to make sure that all the components were rated to >24V first, and I've removed D1 (I power the RAMPS board directly).

Thanks for all this info and for confirming what I thought.

One other question though. I understand the ground loop issue you raise. I thought I could eliminate it by tying the grounds from the power supplies together. Even if I check with a voltmeter, that's no guarantee that the reading will be the same the next time - the two power supplies would be floating with respect to one another so the differential voltage could be anything at any time..

I don't even have power supplies yet, but you've also given me an idea that I might drive the whole thing off of 24v. I understand that I'd have to leave D1 out and power the arduino separately, but that's not a problem. I'd have to verify the components in the RAMPS kit I have are rated for 24v. The SMD components are already mounted, so I guess if they're not sufficiently rated i can't do it with this board.

Are there any other considerations? The steppers, the hot end(s), will all work OK at 24v??

Stepper motors should work fine with 24v as the drivers should cut the voltage down to match whatever current the steppers actually need. The drivers themselves absolutely must have heatsinks, and a fan pointing at them.

I will note that one of my stepper motors liked to warm up quite a lot and also vibrated a bit (not great for the extruder stepper). I noticed the same issue at 12V, where the vibration was louder but it didn't heat up as much. I've since replaced that stepper motor as it seems that it was damaged, and have not had an issue since.

The hot end will handle 24v as well, but will use a lot more current at 24v if you use the standard 5.6-6.8 Ohm hot ends. You can switch to a 22-27 Ohm resistor on your hot end to avoid this. I use 22 Ohm as it allows the hot end to heat up marginally quicker than 27 Ohms. This also means that it's drawing less current than a standard hot end at 12v. As such, I am using light gauge wiring to run my hot end with no issues.

The main issue with 24v tends to be any fans. You can get 24v fans but they're no where near as common. I use a 24v-12v converter to run my fans (just run one side of the fan off the +12v line of the converter and the other side off the RAMPS output that goes through the FET to ground).

If you have a decent 24v-12v converter that can produce enough current, you could run the steppers, fans, hot end and Arduino all off it. I believe that some of the 24v Mendel Max's used this method, while keeping the heated bed at 24v. The advantage is then you only need one power supply.

As with the PSU's, the only gotcha with a 24v-12v converter is that you need to make sure it shares a common ground between the 24v input and the 12v output. Most of the switch mode converters are like this, but not all.

Re: The board, there's a number of posts about what components that need checking for 24V operation. The main issues are F2 (it's only rated to 16 volts), C3, C4, C6, C7, C9 and C10 (depends on the supplier). You could replace the caps if you're somewhat confident with SMD soldering (by comparison to most SMD stuff, they're fairly large).

BTW: Worth checking out these threads...
[forums.reprap.org]
[forums.reprap.org]
[forums.reprap.org]

They're all about doing 24v with RAMPS and what to look out for.

Thanks - I just read through those other threads. You must get tired of saying the same thing over and over again

I have the same heated bed and was wondering if I power it at 12v will it just heat slowly? Is there any circuitry I should add to do this safely?

IronBeagle:

Running a 24V heated bed on 12V means it will put out 1/4 of the power it normally will.

This is most likely not enough even to keep the bed at temp if it was somehow already there. You will probably find it won't heat up much.

Running a 24V heatbed off your 12V supply won't require any extra circuitry, but it definitely won't be very useful.