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My problem is I've bought a 3d printer from china. As usual with consumer electronics the psu is a heap of shit.

I've read on forums you can connect two atx supplies in series to generate 24v instead of 12. HOwever they say you must cut the earth connection. why?

Am guessing the gnd is connected to the earth hence it's then no longer in series

Ageis
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  • Why not to use one ATX power supply connecting to -12 and +12 which form 0+24 V (but you need to mind current)? However in this case printer's GND will become -12V power rail, bringing complexity in making proper grounding. I think it would be best for you to purchase new 24 V power supply with required current and IP ratings. – Anonymous Nov 01 '16 at 10:50
  • well the original was 15a lol – Ageis Nov 01 '16 at 10:55
  • Oh, can you post a picture of the printer? Looking at specs for 400W ATX PSU I see 15 A for +12 V line, and 0.8 A for -12 V line. That's why using single PSU was not considered :) – Anonymous Nov 01 '16 at 10:59
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    btw i was laughing at the fact that am even considering doing this. Anyone with an idea of what the earth connection is for knows it's bad idea to cut it off. – Ageis Nov 01 '16 at 11:16

4 Answers4

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As you can guess, you cannot connect two separate supplies in series if at least one point of them is common.

You know, ATX PSUs' outputs are isolated from mains:

schematic

simulate this circuit – Schematic created using CircuitLab

But you don't know that whether the output return path (i.e. GND) of any ATX PSU is connected to earth (e.g. via a series capacitor) or not. If return path is connected to EARTH, you cannot connect these two supplies in series. To avoid the risk of a failure, you should cut EARTH connection. But note that EARTH connection is of vital importance.

Rohat Kılıç
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Don't jump to conclusions. Check the continuity between the Earth line on the line cord and the "ground" ( DC Common ) on the DC output connector with an Ohmmeter to see if they are actually connected. If they are indeed isolated you should see a minimum of 10KOhms, probably much more. If they are isolated, no need to cut.

FiddyOhm
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  • can I ask why they would want to connect gnd to earth? I do plan on getting a 24v supply if mine isn't isolated. – Ageis Nov 01 '16 at 11:19
  • There are many good reasons to isolate DC Common from Earth in some situations, and many good reasons to connect them in other situations. As far as an ATX supply goes, I think here in the USA they are isolated, but I'm not sure about other countries. When in doubt, Ohm it out! – FiddyOhm Nov 01 '16 at 17:01
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The best way is to locate the point in the power supply where the black 0v cables are connected to the metal case of the ATX supply. Disconnect the 0v connection from the case thus floating the power supply but leaving the metal case with its safety earth still connected. Do the same with the other P/S and you can now connect the P/S's in series in whatever configuration you like, ie 12v plus 12 for 24v or 12v plus 3v for 15v etc . don't use the -12v etc as they are very low current.

TerenceJ
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One atx must be ground disconnect and you'll have 24 volts make sure have that atx isolated and that's it,put it without the case and in a place where you can use it safe,ground in the 110 vac is connected to neutral in the main panel anyway,if you use the case, don't touch each other ,a soft start when you connect the load will keep both atx working if not,on will turn off,a 5 volt load in the atx will rise the output to 12volts,a 12 volts bulb will do the job , i have 2 atx connected in series the main case is grounded both psu are isolated from the case ,I need positive and negative not a ground ,atx coming with the negative connected with ground ,I isolated both atx from ground everything works fine,exagerate things about to die I red in this post,atx has positive and negative,ground in atx is for the case...that's all

Juan
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