Heated printbed. Not enough heat.

So I have the 4x 2.2ohm resistors from mendelparts. Currently I have 12V going to them in series. I would like to increase that to 17-18V but all I have is a PC power supply. Would it be ok to only use two of the resistors? I would still have 12V. But the wattage would go from 16.35w to 32.7w from what I have seen on mendelparts.com, 32 watts produces the right temp. But I would only be using two resistors. would this still work? I'm a bit worried about the heat distribution. If i put the resistors on opposite corners of the heat bed, do you think the middle might have an ok heat distribution?

That's odd as my heated bed is exactly the same and at 12V on a 60W laptop charger I can hit 55C easily. In fact I copied the heated bed and made another one from scratch fro a Prusa I'm building and that too easily gets up to temp.

I tried 18V initially and mine got too hot for PLA. You do need to get an even heat and I'm not sure if just 2 resistors will work. I guess the only way to find out is to try

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Experimenting in 3D in New Zealand

4 2.2R in series will give you 8.8ohm ideally you want 1 ohm so the best you can do is put them in a series parallel combintaion giving you 2.2ohm

NelsonRap: Do you have the 1.0Ohm resistors? that would produce 36watts at 12V. From what I understand I am trying to get somewhere between 32-36 watts to obtain the right temp.

rcs: having 2.2ohm total would produce 65watts at 12v....right? Wouldn't that be way too hot? And what do you mean by iddeally you want 1 ohm? If you mean per resistor then I would need a total of about 4ohms. which two 2.2ohm resistors in series would get me 4.4, am I correct in my calculations?

I use 10 2.2R in a series parallel combination which is about .85R on a 12V atx 550W power supply with a 5 R on the 5V rail. I am using ramps1.2 and control the bed through an auto relay to take the load of the mofffet. when it get up to temp it switches on/off. my bed take aprox 13 min to get to 110C.
what I mean by 1R is the total R of the heated bed to be able to heat up in an acceptablr time.

Edited 1 time(s). Last edit at 12/10/2011 04:45PM by rcs.

Nope I definitely have 2.2ohm ones as I buy them in at that value. On my original they are in series but on my Prusa which runs an ATX PSU they are wired individually with all wires going back to the PSU

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Experimenting in 3D in New Zealand

I'm trying the two resistors in series at 12v now. The whole bed is definitely warmer. The heat is almost evenly distributed, there is a small area at opposite corners (where the resistors are) that is a little bit warmer. I might have to avoid printing at those corners, but I could live with that.

By the way. How hot is 55C? Can you touch it or is it too hot? Right now I can touch the spot above either resistor for a couple seconds before It feels too hot. The print is about 1/4 done, so far no lifting! =)

You can touch it briefly but I think anything above 50C is too hot to hold. I use 60C for PLA.

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[www.hydraraptor.blogspot.com]

Yes I can only hold my hand on for a few seconds, changing glass has to be done quick as I have no control on my heat bed due to running Gen6 so its on or off only

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Experimenting in 3D in New Zealand

I am surprsed that you can heat your bed up to 55C with 2 2.2R in series and get even heat over the bed. I think that 55C would be a bit to hot to hold. But if its working OK thats good.

I would guess that the majority of the bed is maybe 50C while just the two corners are hotter. I do have the resistors on opposite corners, and at the very edge of the corners. Either way, my prints are no longer lifting. =)