Has anyone ever made a pcb heat bed with two circuits? Is it a dumb idea?

I was just wondering if it's possible or even worth it to make a heat bed with dual circuits. Bigger and bigger printers are being made and most of the time the larger print volume isn't used. So would it be possible to have two circuits on the heat bed and choose whether the whole bed is heated or not? It's essentially two different heated beds but connected by the same plastic board.

For example, if you have a 300 x 300 mm pcb heat bed, one circuit will be the inner 150 x 150mm, and the other will be the outer path. When printing something small, you can choose to just use the 150 x 150 instead of the whole bed. This would save energy and probably be easier to get an even temp.

I'm not really that knowledgeable about electronics so it might be impractical or impossible. Just thought I'd ask.

I make my own heated bed PCBs with dual circuits although this is so that they can be wired in parallel for 12V or series for 24V - both configurations giving the same power. It was pointed out to me by somebody at the TCT show last year that the same PCB layout that I use could be reconfigured to have different areas on or off.
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In this video of putting resist for etching a PCB onto copper-clad fiberglass laminate you can see how the PCB is divided into four quarters - and each can in turn be sub-divided etc.. A slight problem is that the smaller area would not be in the center of the PCB - but this could be compensated for in slicing the model. Seperate heat spreaders would of course be needed.

Interesting. So it can be done, but there would be issues with it? I didn't realize the beds with 12 or 24V were different circuits. I've never used one. Why couldn't the smaller area be in the center? And why would separate heat spreaders be necessary? Is it because a larger one would warp or carry away too much of the heat?

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Interesting. So it can be done, but there would be issues with it? I didn't realize the beds with 12 or 24V were different circuits. I've never used one.

The 24V boards basically take the 12V board and put a center tap on the trace. To run at 24V, you connect to the ends of the trace. To run at 12V, you connect + to the ends and - to the center pad (or vice versa). You could conceivably run 1/2 a 24v board by connecting just one end and the center pad, leaving the other end open.

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Why couldn't the smaller area be in the center?

I think the difficult part would be to get the traces such that only the central area is energized while the exterior areas aren't while keeping it a single sided PCB. And balancing the traces so that they have the same resistance unless you don't mind unbalanced circuits.

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And why would separate heat spreaders be necessary? Is it because a larger one would warp or carry away too much of the heat?

Yup. The heat spreader is designed to....spread the heat. If you don't have some type of thermal break, as it heats up heat will be wicked away from where you want it to where you don't necessarily want it.

While you're at it, incorporate multiple thermistors to provide per-zone temperature regulation for higher accuracy.....

Going back to the original question

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I was just wondering if it's possible or even worth it to make a heat bed with dual circuits.

and doing a lot of sucking numbers out of the end of my thumb. My 200mm X 200mm bed takes say 100W for a temperature of 110 degrees C. For your 300mm X 300mm bed this would be about 225W with the 150mm X 150mm bed taking about 68W, saving 157W. If i take a wild guess at your electricity costs at 20 US cents per kWh then on a 10 hour print would save you about 1.57kWh or about 31 cents.

Nope, I don't think that it is worth it.

Mike

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My 200mm X 200mm bed takes say 100W for a temperature of 110 degrees C. For your 300mm X 300mm bed this would be about 225W with the 150mm X 150mm bed taking about 68W, saving 157W. If i take a wild guess at your electricity costs at 20 US cents per kWh then on a 10 hour print would save you about 1.57kWh or about 31 cents.

Nope, I don't think that it is worth it.

Mike

Yeah I figured. I've done the math before and realized how inexpensive the electricity costs are, but I still can't believe it's that cheap. And when you consider how cheap it is, you're right, it wouldn't be worth going through the trouble.