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I lost the power adapter to the module used to drive a LIDAR sensor I have. I am planning to make a new adapter but one problem I am running into is that I can't figure out if the module's PCB sources positive power from the center of the barrel jack or from the outer of the barrel jack.

What are some strategies I can use to figure out which the PCB uses?

Edit: thanks for the help everyone but I took the easy way out and emailed the manufacturer. I thought this would be a total shot in the dark but figured it was worth a try before I break something. To my surprise they actually wrote back and told me what was what.

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    \$\begingroup\$ look at polarized components, such as electrolytic capacitors and ICs \$\endgroup\$
    – jsotola
    Commented Oct 20, 2020 at 4:20
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    \$\begingroup\$ If you don't trust your best guess, put a diode in series with the voltage line (+V to anode, cathode to your device). If you picked the right pin it will work. If you picked the wrong pin it won't work but you won't have blown anything up. \$\endgroup\$
    – td127
    Commented Oct 20, 2020 at 4:40
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    \$\begingroup\$ @td127 Until you provide a schematic for that showing me otherwise, I'm sure that's not going to work. Unless the diode is the right way round, it's not going to save anything, and the point is, you don't know which way round it should go as far as the module is concerned. \$\endgroup\$
    – Neil_UK
    Commented Oct 20, 2020 at 7:24
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    \$\begingroup\$ My experience is that guitar pedals use centre negative (oops, ask me how I know) and everything else in the world is centre positive. However, I'm not going to suggest you plug it in that way round to get further evidence for that hypothesis. \$\endgroup\$
    – Neil_UK
    Commented Oct 20, 2020 at 7:27
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    \$\begingroup\$ @Neil_UK yeah, you're right. I didn't think it through. Sorry about that. \$\endgroup\$
    – td127
    Commented Oct 20, 2020 at 16:59

4 Answers 4

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Study the circuit board and the components on it. Trace along the suspected positive and negative power tracks. Look for components that have identifiable positive and negative terminals.

Electrolytic capacitors have their negative terminals clearly marked. If you can find data sheets for any integrated circuits on the PCB, see which are the power and ground pins.

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  • \$\begingroup\$ When using electrolytic capacitors to determine positive and negative, remember that silk screen annotations can be wrong and some capacitors have the polarity stripe on +ve instead of -ve: Wikipedia - Electrolytic capacitor polarity marking. \$\endgroup\$
    – Andrew Morton
    Commented Oct 21, 2020 at 10:22
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Find a signal or shield ground on the module, and check with an ohmmeter to the connector. In all likelihood the power connector ‘sleeve’ is ground also, and will be zero ohms. The pin would be positive then.

If it’s not, try the center pin to signal ground. If that is zero then the pin is GND and the sleeve is positive. You may now curse the designer roundly for being the moron that they are.

Could the box be a unicorn with an actual negative supply referenced to ground? Not if it’s a digital box that talks to other stuff, like your LIDAR unit and the upstream host, which use positive-going signals.

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    \$\begingroup\$ Just for safety, check both the sleeve and center pin. Exactly one should be close to zero ohms. If both or neither are, extra investigation is needed. \$\endgroup\$
    – MSalters
    Commented Oct 20, 2020 at 13:51
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    \$\begingroup\$ This will only give information that the the pin of the connector you are measuring is GND, it doesn't give you information about center positive vs negative. I have seen some devices that used the outer sleeve as GND, but the center pin was negative voltages \$\endgroup\$
    – Ferrybig
    Commented Oct 20, 2020 at 16:09
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    \$\begingroup\$ Not for a module like this: it’s going to use a positive supply for its digital electronics with positive signals referenced to GND. I have seen supplies where the center pin was GND, and sleeve positive however. \$\endgroup\$
    – hacktastical
    Commented Oct 20, 2020 at 16:31
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Quick note: some barrel power jacks incorporate mechanical switches ( that sometimes are used to short the center pin to ground when unplugged among other things) , so unless the part only has two terminals it is wise to insert a plug ( ideally one with nothing connected ) before making your electrically connectivity tests.

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  1. Internet search to find the manual or photos showing the AC adapter (usually the polarity is marked on the label). Older products can sometimes be found on eBay.

  2. Meter on diode test scale. If it shows a diode in one direction and higher in the other it's probably the second direction. Most digital DMMs have the red lead as positive in diode test mode. This is a bit weak to depend upon but it's not a bad confirmation.

  3. Trace out the circuit. Chances are there will be an electrolytic capacitor connected very close (electrically) to the power input. There might be a fuse, a filter or polarity protection ahead of it. Take care about interpreting diodes- it's possible they have added polarity protection as a series diode (anode goes to + in) or shunt (cathode goes to + in). Double check both leads using the continuity function on your DMM. Often it will be conclusive, but not always (for example if they have used a MOSFET for polarity or overvoltage protection).

  4. If all else fails, take some sharp tightly cropped photos of the power entry area of the PCB (both sides) and a link to whatever data you have on the model of product and ask in a new question here- someone should be able to help.

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