Why not go with a 24V Mk2b PCB?

Hello again.

I'm self-sourcing an i3 (to replace an i2) with an eye toward higher quality prints, even if it costs me a little more to make. I mostly print in ABS.

This post is about the heated bed. I've already decided to go with a 24V setup, so it seems I have a couple options when it comes to the bed. Here are some things I'm considering.

1) MK2B PCB with 24V support. Kapton on glass bulldog-clipped to it. It is easily swapped out for replacing kapton or for blue tape on glass when I switch to PLA. Also it's relatively light.
2) A 24V kapton or silicon pad heater affixed to a 214mm square piece of aluminum, probably with glass on it, as above. (I don't know if the sandwich should go insulation-heater-alu-glass, or insulation-alu-heater-glass, or what. And I don't know where to find a 24V kapton square that *just* fits within the mounting screws of the squished frog. Advice welcome.)
3) A mains kapton or silicon pad heater, controlled by a SSR. This sounds like a slightly more complicated version of (2), which gets you quicker heating.
4) A MK3 aluminum PCB board, which I'd probably have to put kapton-ed glass upon to do ABS.

People on these boards who know what they are talking about seem to go the (2) or (3) route more often than not. But to me, (1) seems like a great option. Everything powered by the single supply through the (slightly modded) RAMPS. It seems like the lightest option for your Y-axis. It's cheap. (Though that's not a primary concern.) The drawback seems to be a 10-minute wait to start printing ABS. If I don't mind the wait, is this an advisable approach?

Or am I missing something?

Edited 1 time(s). Last edit at 11/14/2013 11:17PM by Karmavore.

Using a 24V heated bed PCB would work just fine, if you don't mind the wait for heating. Several of the printers I have built (until I started building over-sized ones) used PCB heated beds, and they worked great. 24V would be even better than 12V for this purpose, as it reduces the current flowing through RAMPS and your wiring, though you have to make sure everything on your RAMPS is rated for 24V. I go with option 3 on my printers lately, because I make over-sized printers and because I want fast heat up times. My last printer uses a 12"x12" 720W AC silicone heater pad under an aluminum sheet with glass on top. Works quite well, but is significantly more expensive than a PCB heated bed.

If I were buying a new heated bed, I'd go with option 3 as well. NewPerfection is right -- with an SSR, you can drive a lot more power at much lower current straight to an SSR from the wall. Also, you don't need a huge power supply to do it.

Thanks for the replies. It sounds to me like the mains-powered bed improves the speed of the bed, but has limited-to-no impact on the quality of the printed parts.

Since none of these decisions are irreversible, I think I'll stick with the MK2b plan. And my 18.8 amp 24V power supply is already purchased, so reducing the wattage of my power supply has limited benefit at this point. I also need to factor in that I'd be new to SSR use, and a 115V application might not be the best place to start.

Nonetheless, I'm curious how these beds are built. Are there photos out there that reveal the designs of the bed "sandwich" and/or SSR? Are you printing PLA on aluminum? Do your y-axes get heavy to the point of slowing your print speeds? Do you use two outlets, or create two different circuits from one plug? (If you can't tell, I'm not an EE guy.)

Using an SSR is pretty simple. The + and - wires from your electronics heated bed port connect directly to the low-voltage side of the SSR (polarity is important here). Take a plug and cord off of a defunct appliance and split the cord into hot and neutral legs. Wire one leg directly to the SSR and the other directly to the heated bed. Next you need a jumper from the remaining heated bed terminal to the unused SSR terminal. That's it.

If you're safety conscious you'd add a thermal cutoff into the circuit. Something like this is a good idea.

Personally, I use adhesive to stick a 110V silicone heating pad to the bottom of a thin aluminum heat spreader. Then I use bulldog clamps to hold the glass in place.

[i.imgur.com]

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- akhlut

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