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I have a MASSFIDELITY relay, wireless Bluetooth receiver for which the power supply is missing. I found a manual for the device that states it requires DC 5V 1A power, but says nothing about polarity. I ordered a so-called replacement power supply, but the one I received was 12V so I no longer trust power supply retailers to know what this device needs. Is there a way to determine what the polarity of a power supply should be by looking inside the device? The device can be opened easily so as to see the back of the power input lug etc..

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  • \$\begingroup\$ Does the device have any labeling? \$\endgroup\$
    – Eugene Sh.
    Sep 12 at 18:53
  • \$\begingroup\$ Yes, probably, by tracing out a bit of the circuit. Not so much by looking at the connector unless it's marked (and it probably isn't, connectors are not inherently one polarity or the other, that's up to the designer who uses the connector). \$\endgroup\$ Sep 12 at 18:53
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    \$\begingroup\$ Yes, provide some pictures. \$\endgroup\$
    – Jens
    Sep 12 at 19:00
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    \$\begingroup\$ Crank out your lab supply. Set your current limit low and turn up the voltage slowly. Does it start to pull a lot of current in the 0.7 V range? Reverse the polarity and try again. \$\endgroup\$
    – winny
    Sep 12 at 19:18
  • \$\begingroup\$ Needed neither additional details or clarity. (More always helps but ...). \$\endgroup\$
    – Russell McMahon
    Sep 19 at 2:52

2 Answers 2

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Here is a photo (from here) of the official power supply.

It sort of looks to me like center positive (the lower right symbol), but I would not risk a $250 device based on that blurry guess.

enter image description here

Unfortunately the construction inside is not that easy to follow from a simple single photo. Try to get a photo of the back of the connector board looking in the direction of the arrows (photo from here:

enter image description here

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Sphero's answer indicates that it's probably center-positive. I found a couple more pictures on Google Images for replacement adapters, which are also blurry but look more like center-positive than center-negative. Center-positive is generally the standard for most devices these days.

Looking at the board, it seems like the ground plane is likely connected to the chassis via the mounting screws, so it's possible that the polarity could be found by measuring the resistance from the DC barrel jack connector to the case.

Get a multimeter, put it in resistance mode, and put one probe on the metal chassis or a screw. Then touch the other probe to each part of the barrel jack.

  • If the outer part of the barrel jack to chassis is near 0Ω, then it's almost certainly center-positive (which we suspect it is).
  • If the center pin to chassis is near 0Ω, then it's center-negative. This is more unusual and you might want to do further investigation to confirm.
  • If neither show low resistance to the chassis then you'll need to investigate further, e.g. by taking the cover off and probing inside.

You might be tempted to use continuity testing mode on the multimeter to do this kind of connectivity check, since the beep is seen as more convenient than reading the display, but I would avoid it here because you can get misleading results while the input capacitors charge up. With the multimeter in resistance mode you can tell this is happening because the apparent resistance will start low and slowly increase over time, whereas with a direct connection (ground to ground) it will stay low.

Note that this test isn't a 100% guarantee and there's always some risk of damage if you guess wrong. If you're not willing to take a risk, I recommend contacting the manufacturer to confirm the polarity.

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