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Let's just say I know nothing about electronics, so please bare with me on improper terminology.

I have a LCD screen that I have disassembled as I am trying to make a Touch screen mirror fit into a metal picture frame. The LCD included power adapter jack/plug is currently protruding from the back of the picture frame and therefor I would like to cut the cord on the power adapter end and solder the wires directly to the PCB (and the hot-glue to isolate).

It should be easy to cut the end of the power adapter and measure with a multi-meter which wire is positive, negative and ground.

However, I am having trouble finding which point on the PCB is which.

I have removed the plastic part of the jack on the PCB (image below) and was hoping this community could help me figure out which wire from the adapter should be soldered to which point on the PCB?

The large part of the image below shows the top where the plastic jack housing was. Once the jack was removed the PCB shows the markings 1,2,3 and a plus sign (the - on the large picture is the number 1 and not a minus sign). The number 3 is where the center pin from the powere adapter plug would go. Number 1 and 2 are touching.

Can anyone help me understand which is the positive, negative and ground on the PCB?

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Here is a before picture before removing the plastic jack from the PCB:

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    \$\begingroup\$ It's not "ground" that you really need to know about, but the wiring of the connector. You could go for broke, mark one side of the power cable, cut it, and then do a continuity test from the cut end (stuck in the jack) through to the points you are considering soldering to... For even less confusion, start by only cutting one conductor. However beware that one terminal on the PCB may be a presence detect could go open once you remove the connector/plug. \$\endgroup\$ – Chris Stratton Feb 20 '19 at 23:31
  • \$\begingroup\$ Do you know if the original adapter was center-ground or not? \$\endgroup\$ – evildemonic Feb 20 '19 at 23:53
  • \$\begingroup\$ I've added a picture of the original power adapter. Sadly I do not have the knowledge to know if that is enough to see if it is center-grounded or not. \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 0:20
  • \$\begingroup\$ I've added additional picture (larger picture of under the PCB and also picture where the female jack is reattached and the pins are exposed. Showing that when the male plug is not connected pin 1&2 are touching and when male plug is connected it pushes ping 1&2 apart). \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 13:51
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As I can see on the top picture, 1 and 2 pins are connected together on PCB by conductive track. They also are touching inside the connector. They are used in some schematics to cut off battery power regarding of the plug is inserted to the connector, but in your case they are shortly connected, so basically you have only two pins: 1+2 (any of them) and 3.

From the picture it's clearly seen, that 3rd pin is connected to common plane of PCB. So, the only question is, if common plane is positive or negative (modern electronics typically have negative common plane, but not always; could be opposite).

Do you have any voltmeter/multimeter? If yes, the best way for you right now would be just measure voltage on power supply plug (if you have it and it's working). You can also check the polarity of the power supply with the LED in the case that you don't have voltmeter around you.

In other case I would say, that pin 3 is negative and 1 & 2 is positive. But you have to consider, that nobody can guarantee you that this is correct without appropriate verification. Your display may be irreparably damaged if you would guess the polarity just by PCB layout!

If you have a power supply for the LCD, you can post an image with the designation label on it, so we can go further in our speculations.

EDIT:

Now it looks like @Bort said, that pin 3 is positive (+) and 1&2 is negative (-). Can you post large image with bottom side of the PCB, what you have named "Under the PCB" in top right corner of the top image? Can you see if common plates on both sides are connected? Some connecting through hole?

EDIT 2:

I can see that pins number 1 & 2 are first touching when the male plug is not connected and after the male plug is inserted the ping 1& 2 are moved apart and not touching.

Yes, this is correct, this is the way how such connectors work. But in you PCB it doesn't matter, as pins 1 & 2 are shortly connected on PCB by conductive track.

So, now I can see on larger image, that actually pin 3 is not connected to common plate on the bottom pf PCB. This indirectly confirms previous assumption that pin 3 is positive.

For now the probability of the fact, that pin 3 is positive(+) and pins 1 & 2 are negative (-) is 99%. I reserve 1% for some possible misunderstanding from photos' points of view. I hope this will help you.

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  • \$\begingroup\$ Thanks for the feedback. I've added a picture of the original LCD power adapter. I do have a multi-meter but would need directions on what to measure. Would gladly post pictures or videos. \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 0:25
  • \$\begingroup\$ Hi, I've added a larger image from bottom side of the PCB to the original post. I also added a picture where I retro-fitted the female jack back on the PCB. The plastic is broken and exposed the pins on the inside. I can see that pins number 1 & 2 are first touching when the male plug is not connected and after the male plug is inserted the ping 1& 2 are moved apart and not touching. \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 13:49
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    \$\begingroup\$ @SkuliAxelson I updated the answer replying to new photos. See EDIT2. \$\endgroup\$ – cyclone125 Feb 21 '19 at 14:56
  • \$\begingroup\$ Thank you @cyclone125. I've uploaded another image (last one hopefully) showing the stripped male plug. The silver wire that was wrapped around the center wire was soldered to the side of the male plug and should therefor be the negative (-) wire that would be connected to PCB pin 1? The center wire would then be the positive (+) connected to PCB pin 3? (while nothing connected to pin 2 and pin 1 and 2 should not be touching) I'm asking to be sure as from the bottom side of the PCB the pin3 does not lead anywhere, while Pin1&2 lead to the copper woven piece. \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 18:03
  • \$\begingroup\$ @SkuliAxelson Yes, center tip is positive, outer sleeve is negative. Pin 3 can go somewhere on the top of the PCB or may be this is multilayered PCB, and there are some invisible layers in between top and bottom layers. I would measure polarity on PCB before disassemble the connector, but now it's too late, so anyway you don't have any other choice except trying it and hoping that polarity is correct. Connect positive (center) wire to pin 3 on PCB, and negative (outer) to pin 1 or 2 and that's it, everything what you can do in this situation. Nothing more. \$\endgroup\$ – cyclone125 Feb 21 '19 at 18:21
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Assuming that power supply is the original, the symbol to the right of the OUTPUT current shows that the outer pin is - and the center pin is +. The diagram is shown looking head-on with the barrel jack. The + is shown touching the center. This is the most common configuration, but should never be assumed!

Power Supply Polarity

Zoomed in and color-coded:
Power supply polarity color coded

Looking at the PCB after the plastic female jack was removed, it looks like pin 1 is - (ground), as it has the shape of the arm that pinches the outside of the male barrel jack. Pin 3 is +, and pin 2 is a switch that is physically disconnected once the barrel jack is inserted. The switch is used to detect if the plug is in, but that feature may or may not be used by the device.

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  • \$\begingroup\$ Thank you for the feedback. I added a picture with multi-meter measurements of the power adapter plug. Not sure if that would help at all. \$\endgroup\$ – Skuli Axelson Feb 21 '19 at 1:03
  • \$\begingroup\$ @SkuliAxelson - The multimeter reading confirms that indeed, the center-pin is +, and the outside is -. But that only confirms the power supply, not the PCB. My assumptions are based on the shape of the jack on the PCB \$\endgroup\$ – Bort Feb 21 '19 at 1:12
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At first glance it would seem the positive one is the one marked "+" on the silk screen. However on closer inspection, that may just be marking the centre of the connector jack.

On the top side the other two pins appear to be connected to the ground plane but this may just be an optical illusion as your close up of the reverse shows them connected to a trace.

I would try measuring continuity between each pin and the mount hole (which should be ground). One of the pins will most likely connect to ground, and that will be the negative terminal of the jack.

Additionally, if the power adapter works, you can simply measure the voltage using your multi-meter. It will read positive when the ground of your meter is connected to the negative terminal of the supply. That will tell you whether the jack is centre-positive or centre-negative.

As a final point, it's a two-pin barrel jack, so the "negative" terminal will be the same as ground.


Based on your new images and measurements, the power supply is indeed centre positive.

That means that pin 3 is indeed positive, and pins 1 & 2 are negative/ground.

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