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Using 2 13A ATX PSUs instead of a single one

Posted by Thomas V 
Using 2 13A ATX PSUs instead of a single one
August 07, 2017 09:00AM
well, as you know, you need quite a beefy PSU if you are going for an ATX approach, the system draws 20 Amps plus there should be reasonable margin so you don't overload the supply. i found 2 ATX PSUs around at home but both of them deliver 13 amps on the 12V rails. Can i wire them to a single input cable and into the RAMPS?

Oh, and this is my first post here at the RepRap forums, Hi everyone!
Re: Using 2 13A ATX PSUs instead of a single one
August 07, 2017 09:10AM
It's generally a bad idea to connect power supplies in parallel. Slight voltage differences between them will cause all sorts of problems for the regulators and there's no guarantee they'll share current equally. If one gets overloaded, it will fail and then the other will quickly fail, too.

If you're really concerned about operating margins, get a 25A power supply. But, 12V is a bad way to power a bed heater. It's probably cheaper to buy a line powered heater and SSR and just use one of your 13A supplies to run the rest of the printer.


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Re: Using 2 13A ATX PSUs instead of a single one
August 07, 2017 09:59AM
This is one of the reasons is switched to 24V. 12V and high amperage are a good combination to get overheating terminals and possible fire situations.
Either use mains (110/230) or 24 Volt. If you use mains make sure someone who is proficient with this checks your setup before you use it the first time, mains votage can be deadly!


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Re: Using 2 13A ATX PSUs instead of a single one
August 10, 2017 02:22PM
As stated, you really can't connect two different PSU's in parallel to add to their current. The PSU uses its output as an input to its circuitry to handle regulation so putting two in parallel will confuse both PSU's and mess up their voltage output regulation.

Proper wire gauge selection will allow you to use a 30 Amp 12V supply for your bed heater with no fear of fires or any other problem. Looking around I see that the average bed heater pulls about 10Amps at 12V.

So, to select wire that will handle 10Amps DC, 15Amps if we apply the "add 50% safety margin" rule, and it is usually stranded wire so that it will bend, look for a wire gauge/current calculator or table. I found one here:
[www.rowand.net] . This table is for stranded wire, which is what we would be using because it bends easily. This table says 16-18 gauge wire will do the trick for short runs. This is about 1.1 to 1.3mm diameter. Which is common AC power plug wire - or high quality speaker wire, which is what I use on my 3D printers. With this the heater gets hot, the wire doesn't even get warm.

The universal wisdom is that you want 50% overhead so you don't stress your power supply. This means you need a 15Amp supply for your bed heater. All other electronics use this supply as well, especially your hot end heater, which at the typical 40W-12V means it needs about 3.5Amps. Misc. other fans in your system will be about .1Amps each, assume at least 3, so that is another .3 Amps. Now add in your controller board, LCD display (which is usually a pig) should add another .5Amp, just to be conservative. This means that your system will need 14.3Amps, round that to 15 Amps. Now, give yourself a 50% margin and you should be hunting for a 22.5Amp 12V supply, at the minimum. There are plenty of 30Amp, 12V supplies out there for $20 or less.

Sorry if that was a bit long winded. But this is what you are going to want to use if you want to be able to run "both" of your heaters and all your other electronics at the same time, safely and reliably on 12V.

DLC
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