I'm currently in the process of making a DIY pen plotter using an Arduino CNC Shield and GRBL Code. Right now I'm connecting the power supply to the shield. I got a standard 12v supply and I cut off the barrel connector so I can connect the supply directly to the CNC Shield. I can't seem to figure out which wire is negative and which is positive. I don't have a multimeter on me so can't test it but I have attached a picture of the information on the power supply which I think should be able to give someone with more knowledge, insight onto the polarity. I think the left wire is negative but not one hundred percent sure. If I risk a test will this damage my circuits or power supply ? Sorry for such an easy question I just don't want to accidentally fry my circuits as have been working on this for a really long time !! Thank you so much, Araventer image description here

  • It depends on the shield. If there is a protection diode then it will be safe. If there isn't then you will end up with a smoking shield. Please link to the exact shield you are using.
    – Majenko
    Nov 28, 2020 at 12:10
  • amazon.co.uk/gp/product/B07CQRPFFW/… - Here is the link for the shield I bought. Sorry I probably should have included it in my first post! Thank you for such a quick reply
    – Arav Raja
    Nov 28, 2020 at 12:12
  • I don't see any diodes on there (or anything much at all other than some capacitors). There is no protection. Get it wrong and things will die. Maybe even exploding capacitors, which is fun....
    – Majenko
    Nov 28, 2020 at 12:20
  • You need a DMM or something else that's polarized to identify which wire is which. An LED and resistor could be used.
    – Majenko
    Nov 28, 2020 at 12:21

1 Answer 1


If you wire the power into that shield wrongly you risk killing things - and with those big electrolytics reverse polarity could get very messy indeed.

You need to identify the polarity before you wire it in, and to do that you need something that is sensitive to polarity.

If you don't have a DMM you can do it with a simple resistor and LED in series - an LED will only work if the polarity is right.

Take a normal LED of the 5mm type (they're easier to identify) and look for the flat edge on the flange around the base. That indicates which is the "negative" or cathode of the LED. Alternatively look for which of the legs is the shorter of the two, which is also an indication of which is the cathode.

Connect a resistor to this identified negative leg. You want the resistor to be at least 1kΩ, but no more than 10kΩ. You don't need the LED to be bright...

Connect your wires, one to the LED and the other to the resistor.

If the LED lights up then the wire connected to the LED is positive and the wire connected to the resistor is negative. If it doesn't light up then the wires are reversed. In this case check it by swapping the wires over - the LED should now light up.

  • Hi, Thank you so much for the answer! I currently only have a RGB LED and a ton of 220Ω resistors whichI guess I could connect to the power supply (with a very messy circuit :)). Is there no way of knowing the polarity of the wires from the info on the power supply. It says its a centre-positive supply does this not mean anything ?
    – Arav Raja
    Nov 28, 2020 at 12:55
  • That only relates to the connector at the end (which you have now cut off). The wire with the stripe might be negative, but it might not. You can only tell by testing it. For the RGB LED you need to know if it's "common anode" or "common cathode" to know how to wire it up
    – Majenko
    Nov 28, 2020 at 12:57
  • Oh yes I completely forgot about that!! Would connecting the RGB LED to a single double AA battery (as you can see my supplies are very very limited :) ) work to test if its a cathode or anode ?
    – Arav Raja
    Nov 28, 2020 at 13:10
  • 1
    You would need 2 batteries and small resistor to get the polarity of the LED. 1.5V won't be enough.
    – Majenko
    Nov 28, 2020 at 14:16
  • Thank you so much I followed all the steps and i've got the polarity now !!
    – Arav Raja
    Nov 28, 2020 at 19:00

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