Large heated bed

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Large heated bed

Postby tmccafferty » Sun Apr 07, 2013 2:14 am

If you wanted a heated bed larger than the available circuit board units, what would be the best approach?

Maybe a matrix of power resistors mounted to the bottom of an aluminum plate?
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Re: Large heated bed

Postby BenJackson » Sun Apr 07, 2013 6:51 am

I've done that. Works great:
P1000189.JPG

Those are actually 1/8" welding filler rods, so they're not as soft as copper. They're solderable because they're covered with copper to inhibit corrosion. The two strings of resistors go to a chocolate block that's also mounted to the plate. I set it up so I could run those two strings in series or parallel depending on the voltage. My original plan was actually to put them all in series and run from wall current (with an SSR). That setup would have had more safeties (including a NC temp switch in line).

All the screws are #2 and tapped through. The divots on the face side didn't matter because it was covered with a sheet of glass.

The thermocouple went under that center washer and the leads went to another (smaller size) chocolate block in the corner. I used enameled wire for the leads and taped it to the plate with Kapton.
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Re: Large heated bed

Postby bdring » Sun Apr 07, 2013 12:29 pm

Here is my bed. I was in a hurry, so I just glued the resistors to the base using Arctic Sliver epoxy. For the wiring, I just used stripped 14 awg copper wire.

You should determine how much current you can handle and work backwards from that. You want as much current as possible because the metal can take a long time to heat up.

Most people use resistors in Parallel because resistors in the 6-10 Ohm range are easier to find than resistors around 1 ohm.

If the current is very high you might consider using a separate power source and driving them through a SS relay.

2013-01-22_21-01-36_529.jpg
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Re: Large heated bed

Postby tmccafferty » Mon Apr 08, 2013 1:04 am

Thanks to both of you.... that's pretty much what I was thinking. any idea how thick you need to go on the aluminum to get decently even heat? Anything over 1/4" would probably get too heavy. I put a IR temp gun on my Helios and it wasn't all that even.
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Re: Large heated bed

Postby vgordin » Mon Apr 08, 2013 1:29 am

At that point, why not just use the 200mm^2 heated bed with a larger piece of aluminum?
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Re: Large heated bed

Postby bdring » Mon Apr 08, 2013 1:45 am

My plate was 0.25" Thick 6061 with a piece of 0.25" glass on top. It heats evenly but about 1/2 the speed of a Helios. I don't think it needs to be super even anyway.

The @vgordin idea is not bad, but it might have trouble reaching temp if it is not well insulated. Mine is not insulated at all. I was planning on about 12 amps, but stopped at (9) 10 Ohm resistors, so it is about 11 amps (130 watts)
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Re: Large heated bed

Postby BenJackson » Mon Apr 08, 2013 2:09 am

I ran mine at about 225W. I think it's 5mm plate (not 1/4", that's a bit over 6mm). I had 1/8" glass on top. Heated to 60C pretty quick (I think 4 minutes? I graphed it somewhere). I had to replace the heater FET and bypass the polyfuse on RAMPS to hit those temps. Or I could have used an external SSR.

I have some 1/4" MIC plate (super flat) but it seems awfully thick for a moving bed.

My M2's bed is quite thin, something like 1/8" (it's not right in front of me). It's heated with a silicone heating mat.
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Re: Large heated bed

Postby vgordin » Mon Apr 08, 2013 6:41 pm

Bart, I agree about the need for insulation, I've mentioned this before, but my preferred way to get both glass and heated bed insulation is Ikea picture frames. They're cheap and you get identically cut glass for the bed and hardboard for the insulation. Has made a world of difference in heatup times for my hadron.

I also use a .125 sheet of aluminum between the circuit board and the glass with recessed m2 screws in it to allow for flat mounting the glass and to minimize hot/cold spots.
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Re: Large heated bed

Postby Wired1 » Thu Apr 25, 2013 10:30 pm

I have also been looking for a larger heater for my next printer build, something like 10"x10" or 12"x12". I see some guys are using silicon heaters now, you just stick them to the underside of an aluminium plate and they can make them any size you want. Many have in-built thermistors and leads attached. Here;s one example: http://www.aliexpress.com/item/12-Volt- ... 98230.html
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Re: Large heated bed

Postby bloomingtonmike » Thu Apr 25, 2013 10:36 pm

QUBD sells the silicone heated beds - 8X8 are $20 - $60 for 12X12 and 24X24 is $250.
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