Multiple heatbeds on same 12V PSU

I have four MK2 heatbeds that I connected to separate thermo-relay.
My PSu is 900W ATX unit, and I want to stay with 12V DC.

Problem is following-if we take into account that 1 heatbed requires 10A 12V, which means 120W-four in parallel would be 40A 12V. 40A requires really thich wires that I can't put inside my printer.

So, how to connect this?

If everything is serial connection, all four would take 10A (as one heatbed), but voltage would be divided to only 3V per heatbed, which means 30W per unit-that's too low and means too slow heating.

What do you think about following solution:

Two heatbeds in serial connection and that two pairs in parallel connection-that means two sections with each hatbed having 10A 6V which is 60W of power-total 12V 20A from PSU-pair od AWG18/16 could handle this.

Quote
3dMistery
Problem is following-if we take into account that 1 heatbed requires 10A 12V, which means 120W-four in parallel would be 40A 12V. 40A requires really thich wires that I can't put inside my printer.

You'd only need 40 amps up to a common point where all heat beds are joined. Individually, they'd still just pull 10amps. You can also run multiple conductors to spread the load between them all, the load doesn't have to be carried by a single conductor.

It seem that I made a mistake.

I measured resistance of heatbeds and it shows 3,5 to 3,9 ohm. That means that current per heatbed is 3 to 3,4 Amps. And it changes the things drasticaly. It means that four heatbed in series would draw about 12 A which is ok.

But still, specs on wiki indicate 10A draw which means 1,2 ohm resistance. Could specs be wrong or I have non-standard heatbeds?

Your measured resistance is more what a 12V/24V dual power bed sees when wired for 24V operation. If you have such a board, you need to measure between I believe pads 1 and 2 and 2 and 3 if you're going to use it at 12V. If it's not a dual voltage board and is designed for 12V, then you have a very weak board and it probably will have trouble warming up for PLA temps, let alone ABS.

It has only two pads, it's MK2a, not Mk2b. They are all Geeetech branded and looks like any other MK2a bed.

Edited 1 time(s). Last edit at 05/05/2015 06:21PM by 3dMistery.

If you measured each bed individually (not wired together) and their resistance was the 3.5 to 3.9 ohm then I would say you have a bad batch of boards. Usually the boards are designed for around 120 watts (12 volts, 1.2 ohms, 10 amps). Manufacturing differences can play with the resistance but 3.5+ ohms is way out of spec for a 12V board. You're only getting about 40 watts out of those boards.

Yes, I measured all heatbeds individually.

I found what was causing those results-heatbeds are fine, but my multimeter probes are not-they have about 2 ohms of resistance-time for change.

So beds are within specs.