9
\$\begingroup\$

This is probably a very simple question but.. I've been trying to find replacement AC->DC adapters for various electronics around the house, but I ran into one which doesn't list its polarity. It simply has "12V 𝌂" printed next to the power socket, without either of the polarity symbols! I found the owner's manual online and checked it, and it wasn't any more specific..

.. So, what does it mean when a device doesn't list its polarity? Is there a default assumption of positive or negative? Or does it mean that the polarity of the power doesn't matter??

\$\endgroup\$
6
  • 2
    \$\begingroup\$ There is no default, but if there was, it would be center positive. That said, you could look up the fcc id for the product, or google for a replacement and look at the images for the replacement power adaptor. Also, polarity almost always matters for DC devices. \$\endgroup\$
    – Passerby
    Oct 15, 2013 at 21:49
  • 1
    \$\begingroup\$ It might be ac output - please provide a link to the owner's manual if you want a better answer (or is it some embarrassing adapter for a "household" item you'd rather not mention LOL). \$\endgroup\$
    – Andy aka
    Oct 15, 2013 at 21:52
  • 1
    \$\begingroup\$ Probably the simplest way is to put a known-polarity LED and 1K resistor in and see which way works. Therefore if you ruin something its just a cheap LED \$\endgroup\$
    – skyler
    Oct 15, 2013 at 22:14
  • \$\begingroup\$ To be clear, the device in question is not a power supply but something requiring a power supply, correct? \$\endgroup\$
    – Alfred Centauri
    Oct 15, 2013 at 22:31
  • 1
    \$\begingroup\$ @skyler I got the feeling OP meant the device, not the supply. \$\endgroup\$
    – Passerby
    Oct 15, 2013 at 22:37

5 Answers 5

Reset to default
5
\$\begingroup\$

If you have a device that is not marked, you can't necessarily assume one polarity or the other. I have, for example, electronic music keyboards and MIDI controllers which sometimes are swapped (Casio is center negative on one, Korg and Yamaha are center positive).

If you have the original adapter, check what its polarity is, either by its label or use a voltmeter to determine its polarity.

If you don't have the original adapter, you could try assuming that the device is center positive, but you risk causing damage to it if it does not have polarity protection. Instead, consult its manual, look online, or as a last resort, take it apart!

If you have an adapter that isn't marked, determining polarity is simple with a voltmeter. If you get a positive voltage then the side connected to the black/common terminal of the meter is negative/ground.

\$\endgroup\$
6
\$\begingroup\$

If the device has any metal part or other known ground point you can use the continuity test function of the multimeter ("the beep") to check if the center contact or the outer contact of the power supply socket is connected to ground.

\$\endgroup\$
5
\$\begingroup\$

As JYelton mentioned in his post, you should not assume the polarity.

12V 𝌂 means that the adapter outputs 12V and the symbol next to it means "DC".

You can confirm the polarity by using a digital multimeter. One of the quality of a good digital multimeter is that it can display negative values. This feature is needed as you don't know the polarity.

Connect your adapter as pictured below. In the picture below, adapter's center conductor is being probed by positive (red) lead of the multimeter and the outer of the barrel is being probed by negative (black) lead.

If you get a reading with no minus sign on the meter then your inner conductor is positive and outer is negative. If your multimeter gets a negative reading or displays a minus, then the inner conductor is negative and outer is positive.

Make sure that the read lead goes to positive terminal and black to the negative terminal at the multimeter end.

polarity testing

If looking to buy a multimeter here is another post to refer to.

Another way to test the polarity is by using LED and resistor if you don't have a multimeter.

Wire up a LED + 1K ohm resistor like the image below:

LED Test

Note the polarity of the LED; the flat side of the LED is negative. Polarity of resistor doesn't matter.

Now connect the other end of the resistor (top of the diagram) to the center conductor and the LED flat side wire to the outer conductor of your adapter.

If the LED glows then your center conductor is positive and outer is negative.

If LED doesn't glow reverse the wires. If LED glows in this case, then your center conductor is negative and outer is positive.

If LED still doesn't glow, check your wiring and your adapter.

If you have LED + resistor and no multimeter, you should get one.

LED Polarity

\$\endgroup\$
2
  • 2
    \$\begingroup\$ Neat answer but I think the OP is talking about the appliance not the adapter - I got confused about this too last night! \$\endgroup\$
    – Andy aka
    Oct 16, 2013 at 7:17
  • \$\begingroup\$ @Andyaka you are right, I'll revise my answer. \$\endgroup\$
    – Chetan Bhargava
    Oct 17, 2013 at 4:47
1
\$\begingroup\$

I have suggestions for anyone with this problem. Take the device apart and trace either of the input paths to any polarized component and then verify with a meter. It is best to see if you can do this multiple times or on each line.

If this proves to be too difficult you can also try detecting a DEAD short from a polarized component to an input terminal. This is usually easiest with negative. Put the meter probe on a negative terminal of a polarized component and see if it is a dead short to either of the input terminals. If it is not a dead short (almost infinite resistance) keep looking. A TINY bit of resistance on your highest scale may indicate the resistance of the wire and connections between the component and the input but be wary of much more than double the resistance of your meter's leads touched together.

Electrolytic capacitors, LED's, PCB markings, red and black power input wires, and outputs to polarized dc motors or linked PCB's can all be used for this purpose.

I have the same problem with a 12v cooler/heater by Mobicool. The input just indicates 12v dc with no polarity. The manuals I found online have nothing to say about it either. I have not taken it apart yet but with the above techniques it will be easy I am sure. Well, as easy as they make taking it apart and putting it back together in the first place.

Gary, lifelong hardware hacker.

\$\endgroup\$
1
\$\begingroup\$

I give you a simple methode to determining polarity for your DC or ac power source not marked 1st circuit just 02 LED enter image description here

if the LED1 glow ==> 1 negative pole, 2 positive pole

if the LED2 glow ==> 2 negative pole, 1 positive pole

2nd circuit just by using a bridge rectifier enter image description here No matter what is the input (dc or ac power source) you got a positive voltage with marked polarity, just all

enter image description here

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.