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I have two non contact voltage detectors (Klein and Greenlee) ..both are rated to detect AC voltage with a sensitivity range from 50 to 1000 V. I used these detectors on 9 different cell phone power adapters with only the USB cable attached to the adapter (so no current is flowing). On 5 of the power adapters it states that the output is DC at about 5 V with a range from 0.3 to 1.0 amps. Everything I have researched says that these non contact detectors will only detect AC voltage; not DC. On four of these adapters I detect voltage and on one I do not. The results are the same for both detectors and are repeatable. Can anyone explain why these detectors that are rated for AC use at 50 to 1000V apparently can detect voltage on a DC supply at about 5 V?? ...and why voltage is not detected on one (1) of the power adapters??

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    \$\begingroup\$ Conducted emissions would be my guess. One might be galvanically isolated whereas the othersare not. Not really my area of expertise, so just commenting. \$\endgroup\$
    – vicatcu
    Commented Sep 6, 2018 at 3:07
  • \$\begingroup\$ And you may be able to get some of them to behave differently if you plug them in upside down into the wall socket and so switch the live and neutral. \$\endgroup\$
    – KalleMP
    Commented Sep 6, 2018 at 6:30

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They are not detecting the DC voltage of the power supply. They are detecting the AC that is present due to leakage current in the power supply.

Every power supply will have leakage, and when the power supply is not grounded, this leakage current has no low resistance return path, and will create a voltage.
This can be detected by non contact AC voltage detectors.

Switch mode power supplies have this by design (C1), or by parasitic effect of the transformer inside.

smps leakage model

The leakage current is very low, below a few hundred uA or even less. You might have noticed a "buzz" feeling touching some appliance, that is the result of this leakge.
Large appliances can leak a higher current, and this might give a tingle.

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They may be rated for 50V AC, with some degree of accuracy. But they can pick up lower voltages. And they aren't picking up 5V DC, they are picking up 5V AC! Because the way that these power adapters work is by using transformers, rectifiers and switching regulators, which cut in and out at a few thousand cycles. The 5V DC you see is really rapidly alternating current with a reference of 2.5V above 0, instead of your typical 60V to -60V 50Hz mains AC voltage.

It's like the illusion of persistence of vision. Flash a light fast enough and your eyes can't tell the difference between the light always on or flashing.

Secondly, there could be some AC coupling on the DC side, and that can often be picked up by both non-contact and contact voltage meters. Enough that my DMM with auto detection gets confused when I measure some DC power supplies.

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