Heated Bed Power

Hi all.
Almost at the end of my quest to build a working prusa mendel but having a few problems towards the end. Primarily my heated bed and I am looking for some advice and reassurance before I do any damage to anything!

I have followed the instructions from Adrians Notes and have got nine 12ohm 15watt resistors soldered in parallel across my heated bed. However this has given me an effective resistance of 2 Ohms rather than the expect 1.3 which isn't getting my bed hot enough. My plan was to use the +3.3Volt and -12Volt rails on my computer power supply to give me 15volts running through my bed.
My maths tells me this should give me about 117Watts of heat which the bed should be able to handle.
I was wondering if anyone knew of a better way of sorting out my problem and if the STP55NF06 could actually be run with +3volts and -12volts across it.
Thanks!

I dont think it can work. The -12V is extremelly weak, it probably has something like a few hundreads miliamps, but it should be a label on the atx psu that will say exactly. Inside pc its usually unused, afaik only some video capture cards (and related) are using it to make other negative voltages (regulators), and its usually just some reference voltage or something like that helping to regulate main voltage down to zero on the positive side. Maybe slightly more consumption that ttl, but somewhere like it. So the -12V is not power, but rather only reference.

1) With atx you will have to settle for the main +12V supply. Otherwise get another psu with different voltage and that has the juice (amps) that would be required to make the power on your resistors.

2) 12/9=1.33 so it probably fits. But, about measuring the resistance, if it gives you something like that which wont fit the theory, then maybe your wiring is too thin/unsuitable, or maybe the meter can be off a little. Also maybe measure each resistor maybe one of them is slightly different and then you would have to put that one aside, otherwise branches wont be equal.

3) Keep in mind that the cables for mains, like the ones supplying the psu, have its wires rated for a max of 10 amps. For more than that you will need thicker cables, dont underestimate that as 14-15A current will turn the usual cables into heated spagetties.

Good Luck!

Edited 2 time(s). Last edit at 08/29/2011 05:32PM by NoobMan.

I use a .9 ohm bed with a uATX supply, and usign a single 12v wire from the supply will make it very very hot. I had to use 2 wires coming off the same rail to supply my ~14amps. I ended up using both pins of the 12v plug meant for the processor.

I'll try and check all the resistors tonight, could take a while to get them all unsoldered. As for my wire I'm using 15amp cable and run four leads from the power supply, 2 for ground and 2 for +12volts.

Going back to the -12volts idea, the PSU says it can supply 0.8Amps and the +3Volts can supply 28Amps. My understanding originally was that because it was grounding to the -12volts it wouldn't matter what the supply current of this rail was as long as I thickened up the wire by soldering a new one in to the PSU. Please correct me if I am wrong though.

Yes you are definitely wrong. If you use the -12V rail it needs to be rated for the full current.

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[www.hydraraptor.blogspot.com]

Well, i thought the math does fit your measurements. One 0.03 error is well due to an usual multimeter error or limit, and multimeters wont read as low (or more accurate) unless you have a conductance one like those which will read in Si (=1/r), or dunno a good RLC bridge as measuring meter, and i guess none are the case. So you know ... i was just saying that in general if things dont fit in this context, one should see why is that. But i think yours do fit with a foreseeable margin. And about the -12V, if its rated at 0.8A i dont think it can be used to sink more current than that. You want to basically sink all current into it like making it the new ground, and i seriously dont think it can manage it. Honestly, I would never attempt that myself.

Edit: Hi Mr Nophead, sry didnt seen your post in time - else i wouldnt have posted mine. Best of wishes and cheers!

Edited 1 time(s). Last edit at 08/30/2011 05:17AM by NoobMan.

Ok thanks for the help, I'll try and find a way to bring my resistance down.

Could you add some more resistors also in parallel to lower overall resistance? Whatever you do, expect the power dissipated in the bed to be lower than expected, because as it heats up, the resistance increases thus lowering the current. That's why my bed is now powered by a 16V lighting transformer

If it is made from proper resistors, rather than a PCB, the resistance doesn't change significantly when it warms up. Wire wound resistors are made from constantan which has an almost zero temperature coefficient. Copper, on the other hand, has a relatively high temperature coefficient.

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[www.hydraraptor.blogspot.com]