I meant a buck converter that could handle the wattage going through it. I didn’t realize they were poor in that application. But not all printer beds are PID controlled. They wouldn’t have to be at any rate. Might be simpler though to get two PSUs, a 400W 12v, a 150w or so 24v one, and supply the bed through a mosfet. Would be more efficient with 2 switched power supplies than having a linear one in the mix at any rate.
Hey there!
I would definitely recommend getting 2 power supplies, it will just be easier and possibly cheaper than any of the alternatives. Not to mention safer, buck converters are sketchy at the best of times…
Have you checked any retailers like aliexpress or banggood to see if there are any decently sized bed heaters on there?
Thanks,
Matthew
No, I hadn’t checked Aliexpress or Bangood. I maybe should. I haven’t had any problems finding the right size heater. The problem is that pretty much all of them are around three watts/square inch. I want a heater setup with no more than 1.25 watts/square inch, preferably under 1 per square inch. I don’t particularly care about the heater size, as long as I can arrange some number of them to cover a bed in the size range I want.
Hmm ok,
Yeah I would definitely check one of those marketplaces. When shopping on these sites I generally don’t ask myself if I can find the item, but rather how long it will take me to find the item 
Good luck with your search!
Matthew
Thanks for the suggestion @Matthew! Bangood didn’t seem to have much like what I needed, but Aliexpress sure did! And cheap! Actually makes me just a bit leery of it. Looks like 26 100 by 120mm 12v 12watt silicone heaters are the best fit I found. Max temperature apparently is 60c, so will probably have firmware configured max at 56 or 57c with gcode set at 55c. Does anyone think that would be a problem running that many segments in parallel with one thermistor? I was thinking of having the leads to each segment the same length so the resistance would be as close to equal as possible. I would obviously do checks with a infrared heat gun after the bed was heated to check. I don’t think it is a big deal if there is some temperature variance across the bed, as long as it isn’t too crazy. Or am I asking for trouble doing it this way?
12V/12W heaters really don’t sound like they’d do the job. Don’t forget that a typical heated bed (say 220mm x 220mm) pumps out 240W - if you’re talking about one quarter of the area (I’m going by 100m x 120mm you mention above) then they should be pumping out 60W or so.
If I look here:
https://www.aliexpress.com/item/4000454364722.html
I see something like this:
In the ad, they say that their pads output 0.4W/cm^2 - so a 100mm x 120mm pad is 120cm^2, multiplied by 0.4 is 48W which is a touch low but in the ballpark of where I think you should be.
In terms of temperature variance over a large surface, I’m guessing that you’re going to have to keep all areas of the surface you’re going to be printing on within 5C. PLA’s coefficient of expansion isn’t linear for all brands over 30C to 60C so if you don’t keep a fairly constant temperature over the bed, you may get some lifting and warping due to problems with uneven expansion/contraction.
When you say “infrared heat gun”, I’m hoping that you’re not talking about something you bought for $20 at Canadian Tire when you need something that will give you an image of an area like:
https://www.aliexpress.com/item/1005004282324329.html
I’ve had one of these for years - they’re really invaluable when working with a 3D printer. Make sure you get one with an SD Card slot so you can download the images.
@mykepredko Yes, I realize that the watt density of the heaters I am looking at are much lower than the norm. That is the intention. Most print beds are designed to heat up fast. I don’t really need that. Most of the prints will be around a week in length. An hour warm up time won’t be the worst thing. What I do need is to keep peak power draw down, even if that means the heater runs pretty much full time. The printer will be in a enclosure that will be in the 25-30C range. The underside of the bed will be insulated. I realize it will take a long time to heat up, but do you think the bed will actually fail to reach 50+C in those conditions?
As to the “infrared heat gun”, yes, that was what I was referring to, just not from Canadian Tire. For the price of the item you showed though, that would be worth considering.
I think you’re aware of this calculator:
https://jscalc.io/calc/mVSHnXdt0sjpeUFM
When I put in your numbers for the quoted heating pad (12W, 120mmx100mm, 3mm plate, 22C start, 60C end) the system basically goes tilt. It’s not until I put in 25W when you get a meaningful answer in terms of heating time and that is on the order of 10 hours (and dependent on the material used).
Right now I’m doing a print with a 60C bed that draws roughly 220W at full power and it is running at 25% PWM to maintain the temperature. This is in an enclosed space with an interior temperature of 33C and ambient temperature of 22C (I do monitor them). So this is similar to what you want but to maintain the temperature for a 220mm x 220mm build surface, I’m using roughly 55W continuously.
Honestly, with 12W heaters, I don’t think you’ll ever get to 60C without a lot of insulation in the chamber and even then it will probably take days to reach that temperature. Sorry.
So glad you were able to find some items on these websites!
You already seem to have done some back and forth about this. I think it will work for sure, just be prepared to wait forever for your bed to heat!
Looking forward to when the build starts!
Matthew
@mykepredko I punched in your numbers in that calculator, and came back with around six minutes for 12 watts on a 120 by 100mm bed depending on which aluminum alloy. That seems rather fast to me, but I don’t see how you got essentially an error. Did you input the dimensions in mm? The calculator takes them in meters, not millimeters. Trying that was the only way I could get such an error.
All that said, a bit higher wattage bed probably would still be a good idea. Going to around 400watts total for the entire 220 by 1400mm bed would be approximately like 60-65 watts for a 220 by 220mm bed, and, while it wouldn’t be super speedy heating up, from what you have said, and that calculator has indicated, should be very adequate.
@Matthew I am looking forward to starting the build as well! I probably will soon start ordering more components. Still a few minor things I need to track down, but nothing really significant.
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That would explain it. You have the plate set as 3 meters thick!
Good catch - sorry about that.
No problem. 
Your recommendation for some more wattage is still a good idea.
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I agree, a little more wattage might end up being better for a) heating the bed up, and b) keeping the bed at a stable temperature. The 12w one would probably suffice, a 6 min heating time isn’t too bad, especially if you ended up making some sort of enclosure. But if you can find a better one with more wattage I would agree with @mykepredko that you should get that one.
@mykepredko , Love ya buddy, You always have the best docs on hand. I swear some days you can find anything.
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You are right, they take up more room, which shouldn’t be an issue for this build, as you say.
Not sure why drag chains aren’t an option, at least for this build. The stationary end would have to be located beyond the end of the exoslide’s path, but that is no big deal.
So my plan for the frame is this, if you are looking at the printer from the end, the profile will be a 4020 lying horizontal on top of a 2020. The 20mm overhang will be to the outside. There will be a mirror image of it on the other side of the bed. The two rails will be joined by 4020 extrusions lying horizontal spaced every 230mm or so(depending on bed heater size, as I will use openbuilds 90° brackets rotated 90° to mount the bed to these extrusions via the bed mount bolts I was talking about earlier). The rails on either side of the bed will be somewhere between 1.6 and 2 meters long. The exoslides will run on them, a 40P on top, and a 20P on the outside. There will be two such carriages on each side. One will have the gantry directly attached to it, and the other will be about 100mm along the rail with a 45° brace going from it to the top of the gantry, as well as an extrusion running directly from carriage to carriage.
As to sag, the idea is that these extrusions will all be sitting either directly on the shelf surface, or with lots of printed TPU feet between the extrusions and shelf to reduce vibration transfer. If I do things right, the extrusions won’t really need to span anything.
No, the spool will definitely not be moving with the gantry. It is going to be situated on the shelf above the one the printer is on(hence my prior limited z axis space comments). It will be centered over the printer, to limit the length of ptfe tube that I will have running through the shelf to the extruder. I will probably use 2.85mm ptfe tubing to limit friction as much as possible for that guide tube.
What will be moving will be the gantry, with attached x axis stepper, z axis stepper, and extruder stepper with attached BMG. It won’t be light, but it won’t be having a 1 kg spool attached up top to throw around, that would be asking for trouble.
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So, it’s been a while with no update from me, and until now, no real progress on this project. About two months ago, I ordered and received the BTT SKR MINI E3 V3 mainboard, a BTT TFT35 V3.0, as well as the steppers and some of the cabling.
Since then, I have made no headway on this project, until about a week ago when I ordered pretty much all the remaining parts needed, and when I got home today from a trip, my Digikey order was waiting at the door. 
So now I have the PSU equipment in hand, now to wait for the rest. I’ll hopefully soon have enough parts to start chugging along on this project.
Thanks again to everyone who has helped me so far. I am sure there will be more questions coming from me once I start to dive into this project.
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It’s always an exciting time when you get to open up a bunch of new packages and create something out of your brain.
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