I am trying to use an 12V OEM type power supply to supply 12V to stepper motors through an L293 h bridge in an Arduino powered circuit but my current setup starts burning all the negative wires and almost killed my pc when I plugged the Arduino in the USB port. I am not sure what could be causing this. I have connected my Live, Neutral and Ground wires correctly and I am then using the "adj. +V" pins as my motor voltage supply and I have connected the "-V" to the common rail/ground of my circuit. The power supply does seem to work fine and if I put an LED with a resistor between "adj. +V" and "-V" nothing strange happens and the LED goes on just as normal. I should also mentioned that the circuit where the ground wires fried has been used hundreds of times with a PC power supply and gave no trouble at all.

I am guessing that the OEM setup is probably the problem here. I have found this page http://reprap.org/wiki/RepRapPro_Mendel_power_supply indicating that I should connect the ground on mains to the -V as to earth my circuit. This could possibly explain my trouble as it means that once I plug in the Arduino in it seeks earth through my PC, or is that just absurd? Even if that is the case, following the above tutorial I have come across an issue, in the testing segment they state that there should be no resistance between ground and -V and this is indeed the case once I have done as stated in the beginning of the tutorial and connected ground and -V. The troubling thing is though that there is also almost no resistance between +V and the ground pin, even without me making the aforementioned connection. I know they don't state this as something to test or watch out for but my head tells me this means that +V and -V are then connected and making a short and that that is something very dangerous, I am therefore to afraid to test the power supply after connecting ground and -V.

Do any of you have experience with these power supplies and know if this is normal or have had the issue before and might know of a possible solution?

Here is photo of the terminals without a connection between GND and -V: enter image description here

EDIT: I have now removed all the wires and there is still a connection between +V and earth.

EDIT: The circuit basically entails that firstly all the ground pins meaning the ground on the Arduino and the -V on the power supply and the ground pins of the h bridge are all connected on a common rail. The VCC on the h bridges are then connected to the 5V on the Arduino and the +V on the PSU is only connected to the motor voltage pin on the h bridge. But as I previously said, this circuit works fine with a pc power supply. Also, for some reason only the black wires connecting the ground pins were burned, the 12V and 5V wires are all fine.

  • \$\begingroup\$ Does your power supply have a "ground" connection? \$\endgroup\$ Commented Nov 19, 2013 at 12:12
  • \$\begingroup\$ Please provide a link to the power supply datasheet. \$\endgroup\$
    – Joe Hass
    Commented Nov 19, 2013 at 12:12
  • \$\begingroup\$ Yes, it does have a ground connection, I cannot seem to find a data sheet for it, but seems almost exactly the same as the one in the reprap link on it I can only find the name WW-1230A \$\endgroup\$
    – Gerharddc
    Commented Nov 19, 2013 at 12:20
  • \$\begingroup\$ I have now added a picture, but the one RedGrittyBrick's post seems exactly the same \$\endgroup\$
    – Gerharddc
    Commented Nov 19, 2013 at 12:55
  • \$\begingroup\$ Are you sure you're not doing something silly like shorting 12V to 5V? A diagram of how you're wiring everything would be a huge help in figuring out what's wrong. \$\endgroup\$ Commented Nov 19, 2013 at 13:22

3 Answers 3


If your PSU looks like this 12 Volt (single voltage output), 30 amp power supply:

enter image description here

  • The adj label probably refers to an adjustment screw, not to any of the terminals.
  • The -V label does not mean -12V, it is equivalent to COM on the PSU in your reprap link.
  • I would not expect the terminal to have a low resistance to the +V terminal with all cables disconnected.
  • \$\begingroup\$ Ok so -V would be the common rail or ground in a normal DC circuit? Because that is how I am using it. But my question still stands if connecting -V to ground/earth will fix my problem or will create a fire considering the low resistance between +V and earth (could this mean my PSU is broken? Or will this connection disappear once it is on?) \$\endgroup\$
    – Gerharddc
    Commented Nov 19, 2013 at 12:58
  • \$\begingroup\$ Does the resistance change over time? If it does your meter is charging the internal capacitors.. \$\endgroup\$
    – Spoon
    Commented Nov 19, 2013 at 13:28
  • \$\begingroup\$ @Spoon To some extent yes, it starts at about 25 and then slowly goes down to 16 where it seems to stay, would that correlate to it indeed being a capacitor? \$\endgroup\$
    – Gerharddc
    Commented Nov 19, 2013 at 13:42
  • \$\begingroup\$ 25 ohms or volts ? \$\endgroup\$
    – Spoon
    Commented Nov 19, 2013 at 13:50
  • \$\begingroup\$ @Spoon it was ohms but you were right, I took a change and powered it with the connecting wire and it was fine. \$\endgroup\$
    – Gerharddc
    Commented Nov 19, 2013 at 14:05

The following earthing configurations are quite common in DC power supplies:

  • Positive output (i.e. +12V): low-side connected to earth
  • Negative output (i.e -54V): high-side connected to earth

Note that in both of these cases it's the reference rail that's earthed. A +12V supply is +12V with regard to the earthed rail; a -54V supply is -54V with regard to the earth rail. In both cases, the 'zero' rail is the earthed rail.

Most(if not all) ATX PC power supplies earth the reference rail, so when you connect your PC peripheral, if the positive rail of the OEM power supply is indeed earthed, you are shorting the OEM power supply rail via earth, and the connections going back to the PC are the ones that smoke (the wires aren't rated for heavy current and burn).

You shouldn't fear a short circuit on a safety-approved power supply. Part of the safety testing done on the design will prove that it will protect itself without a hazard - units will either latch off safely or hiccup in a low-energy mode indefinitely, or if something does fail, it will not cause a shock hazard or spew debris everywhere.

You really should test the power supply per the wiki entry (with V- earthed) since the worst that could happen is the power supply will protect itself, and if the power supply does deliver output, your observation about earthing is incorrect.


Ok so the reason that there is low resistance between +V and ground is merely because there is a capacitor between them and my multimeter was charging it. The reason for only the negative wires frying then was probably because the ground on the arduino/pc had a much lower potential than of the PSU and a very large amount of current then then went from PSU GND to PC/Arduino gnd which was at earth level.


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