i have a TPLINK wall charger from my old router. It's almost new and i would like to use it for my 5V regulator project. The prboem is that i dont know which of the 2 wires is positive and which negative. If i connect it wrong the 3 polorized caps on the circuit will blow up.

Charger specs:

  • Model: T090085-2C1
  • Input: 100-240V~
  • Out: 9VDC .85 Amps

I know tha using a LM7805 with this adaptor is not very efficient but i'm doing it for the science :)

  • \$\begingroup\$ Your DMM says ? \$\endgroup\$ Jan 25, 2015 at 19:52
  • \$\begingroup\$ DMM? Im noob :P \$\endgroup\$ Jan 25, 2015 at 19:54
  • \$\begingroup\$ Digital multimeter. \$\endgroup\$ Jan 25, 2015 at 19:55
  • 4
    \$\begingroup\$ It doesn't say -9V either way? Are you sure it's measuring DCV? \$\endgroup\$ Jan 25, 2015 at 20:00
  • 2
    \$\begingroup\$ Your meter should have a red lead and a black one. Make sure they're plugged into the sockets of the same color on the meter. Get a battery. Put the red on the plus terminal of the battery and the black on the minus and read the voltage, with the selector set to "DC volts". For a digital meter there should be a "+" symbol displayed somewhere on the screen, in addition to the voltage. Reverse the leads. You should see a "-" symbol displayed somewhere. \$\endgroup\$
    – Hot Licks
    Jan 25, 2015 at 22:09

3 Answers 3


The icon at the bottom of the first picture shows that the barrel connector on that wire is center positive. If you cut off and don't have the connector, or you don't have a multimeter then take any led and a 500 to 1000 ohm resistor, and test the wires to see which is positive.

  • 1
    \$\begingroup\$ It worked with the led and the resitor. The positive was the wire with the patern \$\endgroup\$ Jan 25, 2015 at 20:19

If you don't have a multimeter or LED you may consider an alternative method:

  • Take a glas of water and add a teaspoon of salt
  • Insert the two wires into charger's 9V output plug
  • Dip the other sides in the glas and keep them at least 1cm apart
  • Bubbles will appear

=> The wire where you observe the densest bubble stream is the minus pole.

You way also want to watch this or this.

  • \$\begingroup\$ That's an awesome solution. \$\endgroup\$
    – HilarieAK
    Feb 14, 2016 at 9:59

Next to both methods described by Passerby, you can also try to use your Multimeter. When it says 9V, the red wire from your multimeter is connected to the positive wire of the power supply. If it shows the negative sign (9V- or -9V) the red wire is connected to the negative wire of your supply and the black one is connected to the positive wire.

You'll most likely find that the wire with the white dashes on it is the negative one.

  • \$\begingroup\$ @Ignacio Vazquez-Abrams said the same but my multimeter doesnt support negative sign :P 10$ DMM FTW! \$\endgroup\$ Jan 25, 2015 at 20:21

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