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My micro USB mobile charge cable is having about 98V AC voltage on the outside surface. (PFA measurement image. The black probe is connected to the Power socket Ground port. I could not take the whole picture with one hand) The DC voltage is about 0.02V w.r.t. the Ground.

Online QA websites suggested that this may be due to a faulty charging adapter. But I have inserted another micro USB cable into the same adapter and it showed only about 1.5V AC on the surface.

Then, I took another charging adapter and the same thing happened. The first cable showed about 47V AC and the second one only 1.3V.

Thus, it seems that it is not the fault of the charging adapter. My electrical knowledge is not very good. Can someone explain why is this happening?

Edit: Just to clear the confusion I created with the example of 2 wires, actually the AC voltage is always there at the adapter output. I have put my multimeter probe on the metal just inside the adapter (without touching any of the 4 wires inside). and the inside was about 98V AC. the 2nd cable's both bare end metals were not continuous. so, the voltage didn't appear on the micro USB end.

So, my question is, why is the AC voltage appearing and does this mean that the adapter has gone bad?

enter image description here

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  • \$\begingroup\$ What is your other probe connected to, or are you just measuring vs. the air? \$\endgroup\$
    – Adam Lawrence
    Dec 4, 2018 at 16:37
  • \$\begingroup\$ @AdamLawrence The black probe is connected to the Power socket Ground port. That's why I wrote that the voltage is w.r.t. the ground. I could not take the whole picture with one hand :P \$\endgroup\$
    – Sourav Ghosh
    Dec 4, 2018 at 16:39
  • \$\begingroup\$ Sorry, missed that point. \$\endgroup\$
    – Adam Lawrence
    Dec 4, 2018 at 16:46
  • \$\begingroup\$ Do you measure continuity (0 ohms) between the exposed metal on both ends of the cable (after removing it from the charger of course)? \$\endgroup\$
    – Adam Lawrence
    Dec 4, 2018 at 16:51
  • \$\begingroup\$ @AdamLawrence The cable showing high AC voltage (50-100v) is passing continuity test (resistance is about 2.5 ohm, my multimeter buzzes continuity buzzer if resistance is less than 50 ohm). But the cable showing low AC voltage (1.5 v) is not passing the continuity test. I guess this is the reason the 2nd cable is not getting the AC voltage at the micro USB connector head because the two end metals are not continuous. But the 1st cable is getting the AC voltage from the USB end. \$\endgroup\$
    – Sourav Ghosh
    Dec 4, 2018 at 17:06

3 Answers 3

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This voltage is due to standard leakage between primary (AC side) part of AC-DC converter, and secondary part. The effective impedance of this parasitic leakage is about 100k - 1000k. The leakage is allowed to be from 75-100 uA for good medical grade PSU. Conformance to UL 60601-1 dictates that the maximum allowable leakage current is 0.3mA. This kind of current is not harmful, and easily gets grounded when the plug is inserted into device. However, high-impedance probes (oscilloscope or good DMM) will show this phantom voltage when the charger is not connected to anything.

As I understand, the coupling (which causes the leakage) is intentional, because fully isolated supplies might built up substantial DC voltage if left floating, which can be really harmful.

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  • \$\begingroup\$ The capacitive coupling primary to secondary is intentional since it would otherwise have a high frequency common mode component. An unintentional transmitter. \$\endgroup\$
    – Jeroen3
    Dec 4, 2018 at 19:20
  • \$\begingroup\$ Thank you, Ale and Jeroen3. Now I know the 'technical terms' and I can search to know more. I have just some more questions. --1. One of my adapters is leaking about 47 v and another one about 98 v. can we decide that which one is better at doing its job? --2. in my 2nd cable, I didn't detect the voltage at micro USB end because metal bare parts of the cable were not continuous (USB end and micro USB end). But my 1st cable was continuous. Which cable is better? What is the need to make this continuous surface? The current is carried by the inside wire. the end metals are just for locking. \$\endgroup\$
    – Sourav Ghosh
    Dec 4, 2018 at 19:31
  • \$\begingroup\$ I mean that my 2nd cable works fine and it doesn't give me a shock too. why did the 1st cable manufacturer make the USB end metal and micro USB end metal continuous? \$\endgroup\$
    – Sourav Ghosh
    Dec 4, 2018 at 19:41
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Just typical AC coupling from the charger primary to secondary of the flyback converter. Nothing wrong here, it's just a potential without current (very high impedance). Think of it as a very bad transformer.

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I have just read your question. This may happened due to bad grounding. Your socket has improper grounding so thats why its shows about 98 Volt Ac at the outer surface f charging cable.

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