Powering Reprap using both 12v & 5v ATX lines

Greetings,

I'm planning to make a custom reprap, using RAMPS 1.4 for the electronics, a 300² heat bed, and all metal hot end. I've got an old ATX PSU I'm going to use to power it, which can output 18A on the 12v line, and 30A on the 5v line according to the sticker on the side. Since the 5v line needs a load, I was wondering if I could use a DC boost convertor to step up the 5v line to 12v, and use it to power the heat bed through a relay. This should reduce the current flowing through the RAMPS(and the possibility of Sir Blue Smoke) and stabilize the 12v line, while not wasting power by using a lamp or a resistor on the 5v line.

Has anyone done something like this before? Would there be any issues with wiring a system like this? Will this summon Sir Blue Smoke and his fiery minions in an instant?

Thank you very much,

You'll need a high current boost converter which sounds expensive, and when the bed cycles (which it will) your 12v line will lose regulation. I wouldn't recommend this approach. You can use a modest boost converter to generate 8V from the 5V line and use that to power the mega2560. You'll likely still need some heavier load on the 5V line to help the regulation, unless you modify the PSU internals. For the bed size you mention you will probably be better off with a silicone mat heater running off either mains or 24V.

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A_Designs
Since the 5v line needs a load

Some older PSUs needs this load, most, especially more modern ones, don't.

One way to get such a load from the heated bed would be a bed with with two heating track loops. One constantly/unregulated fed by 5 V and the other one regulated and fed by 12 V. Worth a thought if you're going to make a custom heating PCB anyways.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

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JamesK
You'll need a high current boost converter which sounds expensive, and when the bed cycles (which it will) your 12v line will lose regulation. I wouldn't recommend this approach. You can use a modest boost converter to generate 8V from the 5V line and use that to power the mega2560. You'll likely still need some heavier load on the 5V line to help the regulation, unless you modify the PSU internals. For the bed size you mention you will probably be better off with a silicone mat heater running off either mains or 24V.

Hmm, good point. So should I just power all my circuits off the 12v line and stick a resistor on the 5v line? Also, I was planning to buy this PCB heatbed, since I can't seem to find a silicone heater in my area.
[makemendel.com]

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Traumflug

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A_Designs
Since the 5v line needs a load

Some older PSUs needs this load, most, especially more modern ones, don't.

One way to get such a load from the heated bed would be a bed with with two heating track loops. One constantly/unregulated fed by 5 V and the other one regulated and fed by 12 V. Worth a thought if you're going to make a custom heating PCB anyways.

My PSU is a couple years old(Read 5+ ) with moderate use, so I'm guessing it would need the 5v loading. Also, I'm buying the above linked PCB, not building a custom one.

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A_Designs
I'm guessing it would need the 5v loading.

Instead of guessing you could simply try. Find a light bulb or something, connect it to 12 V, turn the PSU on and then you know what you have.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

It's a bit disappointing that they don't specify the power consumption on that pcb. Makes it hard to plan for.

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JamesK
It's a bit disappointing that they don't specify the power consumption on that pcb. Makes it hard to plan for.

Yes, so I mailed them about it, and they replied

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It draws approx 11 Amp currnet, when 12 Volt input is supplied. So power is approx = 131 Watt. It is made with 2mm Thick aluminium base

So I calculated that the resistance should be 1.1 ohms. How much current do 300mm heatbed PCBs normally draw?

I dont know if this helps... but if you are comfortable with ordering from china, why not try a custom silicon pad for 12v or so.
Good, i only found a 300x300mm with 24 volt rating.aliexpress link.
But i think there are plenty of options to get a 12v version. I for myself havent tried yet, but maybe this is an option for you.

Edited 1 time(s). Last edit at 05/19/2016 03:55AM by makerbro88.

I'm fine with ordering from china, they have very low prices, but it would take more than a month to ship, and I don't want to wait that long.

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A_Designs

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JamesK
It's a bit disappointing that they don't specify the power consumption on that pcb. Makes it hard to plan for.

Yes, so I mailed them about it, and they replied

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It draws approx 11 Amp currnet, when 12 Volt input is supplied. So power is approx = 131 Watt. It is made with 2mm Thick aluminium base

So I calculated that the resistance should be 1.1 ohms. How much current do 300mm heatbed PCBs normally draw?

To heat a 300mm square bed to around 100C for printing ABS, you need at least 250W power and preferably 350W. So the current draw using a 12V bed heater is more than 20A. It is far more practical to use a 24V (or higher) voltage for any bed heater larger than 200mm square.

Edited 1 time(s). Last edit at 05/19/2016 06:12AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].