Wiring multiable bed heaters as a single unit?

achrn

@infiniteloop said in Wiring multiable bed heaters as a single unit?:

@mendenmh said in Wiring multiable bed heaters as a single unit?:

you could wire two strings of heaters, each string of two in series, with the strings in parallel. This would give you a single 300W heater

Do you use the Windows calculator? I better scratch my head, because that tells me twice as much for the seriell-parallel config you propose

24V 300W, so they are about 2 ohm each.
So 2 in series will be 4 ohm, and each such string will dissipate 150W (24 x 24 / 4) or (2 x 12 x 12 / 2).
So two such strings will be 300W.
I'm not sure what's astonishing at that.

However, 300W seems a bit too low to me for a 400 square print bed. I might go for 1200W in preference to 300W, but it is a lot of current. For the OP, in that case you'd connect the four heaters in parallel, and make sure your power supply wiring and switching can handle the current (50A!) You could connect in parallel and drive at a different voltage to get the power that you need, but really for a bed this powerful I think a mains voltage heater and SSR is the way to go.

https://docs.duet3d.com/en/User_manual/Connecting_hardware/Temperature_connecting_thermistors_PT1000 has a discussion about the ways to connect multiple thermistors to one input - it's under the heading 'Using multiple temperature sensors for a single heater'. Take not of what it says about behaviour when something fails.

You are potentially prone to irregular heating - if the tolerance on your heaters is not precise and one heats more than the others, you'll get irregular heating. In principle it's no worse than the fact that a heater manufacturer might not produce an exactly uniform distribution of heat in one mat, but I think there's potential for things to be difficult with the proposed arrangement.

infiniteloop

@achrn Obviously, I was driven by Murphy’s- instead of Ohm’s law. Thanks for putting this right. However, I can’t overemphasize my fundamental concerns: too much power, no reasonable control (due to the 4 thermistors). Not to mention the inevitable losses in the power supply and a cabling with AWG 7 or better …

Nvigr8

I am interested in this as well. I've got a printer with a 440x413 bed and a single large heatpad. It does not heat evenly at all. Theres a 20 degree difference between the middle of the plate and edges as well as significant bowing of the bed in the middle where the heat is more concentrated.

The consensus seems to be that using four pads is overall a bad idea.

In that case what is a good idea? What would be the best way to evenly head such a large bed?

With Prusa's new XL printer, he uses a modular approach with numberous tiny heatbeds. Would something like this be possible and if so, how? Using, for instance, a Duet 2 Wifi and a 24V, 25 Amp, 600W power supply?

jens55

For what it's worth, here is my thought on this ...
While 1200W is a bit over the top, it is not unrealistic.
Unless I am mis-understanding, OP wanted a simple approach which would be a single thermistor (and a thermal fuse !!!!). While one could use any and all thermistors built into the heat pads, I would use a single thermistor attached at the center of the bed.
Running 1200W on 24V is IMHO a mistake. As mentioned by others, the appropriate tool here is a mains powered heater.
As far as even heat distribution is concerned, increasing the mass (thickness) of the aluminum bed will go a long way of evening things out. Insulation on the underside of the heat pad/ build plate assembly also helps to some degree. While totally even heat is nice, I do not think it is the end of the world if things are not totally even. In fact you might introduce more uneven-ness depending on what material you put on top of the build plate to actually print on. I print on glass and would not be surprised at all at a 20C temperature difference at various points on the build plate.
The build plate is heated to facilitate adhesion of the plastic being laid down on the build plate. Material such as PLA can stick easily, other materials are a bit more finicky but you can use adhesion promoters and/or different material on top of the build plate. In other words, IMHO it isn't a big deal if you have 10 or even 20C difference in temperature in different areas of the build plate. Assuming you actually require the first layer to go all the way to the edge of the build plate (in my case this is very rare indeed), most material can use a range of build plate temperatures and a compromise that gives adequate temperatures everywhere is usually possible.
400 mm square heat pads are readily available for 120 or 240V.
Think thermal fuse !!!! (can't stress this too much)

jens55

@nvigr8 said in Wiring multiable bed heaters as a single unit?:

significant bowing of the bed in the middle where the heat is more concentrated.

Consensus is that using tooling plate aluminum should reduce the issue of warping. If you use standard rolled 6061 you have all kinds of internal stresses. Tool plate aluminum is cast and relatively stress free. The material used also does not expand/contrtact as much as normal 6061.
WIth that size bed, I would use 3/8" thick material and 1/2" would not be unreasonable either.

Nvigr8

@jens55 I totally agree that a precision milled aluminum plate of that thickness would be a much better choice than the cheap, thin sheet that comes stock. However, you are also talking about piece of metal that costs several hundred dollars to procure unfortunately. Personally I was hoping to explore a less expensive alternative.

jens55

@nvigr8, try https://www.midweststeelsupply.com/store/index.php

The plate, as long as it is rectangular, will probably be between $50 and $75 USD ... I am assuming you are i the US. I had a couple plates shipped to Canada and it was not bad at all. I am pretty sure that was the supplier I used but if I am leading you astray then I am sure somebody will point you in the right direction.

jens55

17.7 in *17.7 in by 1/2" thick tooling plate is just shy of $83

Nvigr8

@jens55 Wow! That is so much cheaper than anything else I've found! Thanks for the info! I'm gonna try that route first. Now to peel that giant heat mat off...

Nvigr8

@nvigr8 Question: If I'm going to switch to a 3/8" 15.75x16.25 aluminum plate will I need to upgrade the Y stepper motor? Right now I'm using a single Nema 17 to drive the Y axis. Will that be powerful enough? Or should I go to two Nema 17's or a single larger stepper? Thanks.