Quote
dc42
A few notes:
1. If you connect your heated bed sections in parallel, they will take 14A each at 12V (perhaps a little less after voltage drop in the wiring). That's 56A total, close to the limit of your ATX PSU, and you haven't allowed for the current draw of stepper motors or fans yet. Unless your heated bed is huge, you should get enough power if you apply 12v to two sections connected in series, and the same for the other 2 sections. That brings the current down to 14A total and the bed power to 168W. That should be sufficient to heat a bed up to about 250mm square to about 120C.
2. Some ATX PSUs have a single 12V supply internally, others have 2 or more. The label on the PSU should say. If your PSU has 2 or more separate 12V supplies internally, you must not connect them in parallel.
3. If you really want to drive all 4 sections in parallel, then you would need a 56A switch to control them. Your RUMBA board won't handle anything like 56A, so you will need to use a 56A external switch. That in turn would require several mosfets wired in parallel. So instead, I suggest you use 4 external switches, each rated at 14A, all controlled from the same RUMBA output. Then you can connect each heater section to a separate output from your ATX PSU, solving tour wiring problem.
4. If you do run all 4 sections of your heated bed from 12V, that's 674W of power in total. That represents a serious fire risk, for example if the firmware freezes and leaves the heated bed at full power.
By the way, this is the new PSU and a size comparison to the RUMBA board. Next task, as I'm sure you can imagine, is to get through the rats nest...
1. I should have been clearer on this, apologies, the whole heated bed takes 14A. Each section is much less than that. On a side note to this, I ran out of Kapton tape for my heater, so I'm a bit screwed on it till I get more (and it's impossible to find in Ireland) so I'm now having a look at what power resistors I have. The original setup was 4 x 4.7R power resistors, but the temperature maxed out at around 70C. Its fine (I guess) for PLA, but I'd rather more heat. I have some 3.3R and some 2.2R, power resistors, but I'll have to hunt down their data sheets to see what their max operating temperatures are. they're just the usual 15W Aluclad type ones. Pain in the ass though.
2. This one has 2, but from each +12V contact point on the board, there's about 6 x 18AWG wires coming from it. So I'm assuming that if I'm grouping 3 x +12V and 3 x GND for the heat bed, they should all come from the same +12V and GND contact point on the board. I've tried to take a photo here, but it's quite tight in the case! You can see that at both of the 12V contacts, there's a number of wires coming out of it. The ground wires are just about visible at the side of the photo too. I figure as long as I keep them grouped from the same contact points, I'm alright?
3 + 4. If i did have a printer using 56A and 674W, I'd be fairly worried, and I wouldn't have it in my bedroom!
Oh, and a huge thanks to everyone's help with this. Isn't the old saying measure twice, cut once?