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I have HP 1514 printer. I noticed that the usb type A plug coming from printer has as buzzing feeling when touching skin. I connected one multimeter probe to the USB plug and the other one to ground pin in my power outlet as shown in the sketch below. The multimeter shows 18V. Shows 0 when measuring current with same setup.

Test Setup

I believe that this is not okay? What could be the cause? Could I damage my Macbook by using this printer (I have used it for a year with no problems, though)

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  • \$\begingroup\$ My printer is not grounded, so the USB "GND" often has a potential against earth. Has your USBs "GND" continuity to earth (or should it have?) \$\endgroup\$ – PlasmaHH Nov 2 '16 at 15:02
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    \$\begingroup\$ I'd expect a higher voltage actually :P \$\endgroup\$ – marcelm Nov 2 '16 at 15:06
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    \$\begingroup\$ "Shows 0 when measuring current with same setup" you shouldn't be measuring current the same way you measure voltage, a meter set to current will have ideally no resistance thus you would be shorting power and ground \$\endgroup\$ – Gorloth Nov 2 '16 at 15:07
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    \$\begingroup\$ @GerbyfromDerby there's no correct way to measure current in this case. Luckily for you, there was no fault, so you didn't blow anything up or hurt yourself. \$\endgroup\$ – hobbs Nov 2 '16 at 17:23
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    \$\begingroup\$ "I connected one multimeter probe to the USB plug and the other one to ground pin in my power outlet" Are you connecting the multimeter probe to the metal frame of the USB plug? Or, which pin on the USB plug are you connecting the multimeter probe to? \$\endgroup\$ – Kevin Fegan Nov 2 '16 at 19:14
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Yes, this is normal, almost.

Your printer has a power supply, which is supposed to be "isolated" from mains AC. It does this by converting AC to DC, then converting the high-voltage DC into high-frequency AC, then it uses a transformer to reduce the voltage, and then rectifies it back to DC, to the level of digital logic and motor controls. The transformer preforms the isolation, but not a full isolation, because there is still some parasitic capacitive coupling, mostly between primary and secondary winding. It is this parasitic coupling that creates some discomfort when you make a contact between ungrounded printer and grounded computer or AC outlet ground. If you would try to use a high-impedance oscilloscope probe, you will likely see some ugly waveform at mains frequency (50 or 60 Hz or 100 or 120), with amplitude up to 55V (on a 110V AC). This "leakage" exists on all AC-DC adapters, to one degree or another.

However, this parasitic current is usually small, about 100uA, but on a bad PSU it can be up to 500uA, in accord with UL safety standard. When you connect a USB cable, this current will be shorted by system shield, and will do no harm.

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    \$\begingroup\$ That only explains the AC part, not where 13 V ofDC comes from. \$\endgroup\$ – winny Nov 2 '16 at 17:30
  • \$\begingroup\$ @winny, not 13V, but 18V :-). And I am not sure if it was really any DC. Or the particular AC-DC PSU has some coupling asymmetry, due to some inexpensive half-way secondary rectifier, or else. \$\endgroup\$ – Ale..chenski Nov 2 '16 at 18:07
  • \$\begingroup\$ Not impossible, but even the tiniest of auxiliary power supplies I've taken apart have all been full bridge. \$\endgroup\$ – winny Nov 2 '16 at 21:22
  • \$\begingroup\$ @winny -- the power supply's ground is 18v above the wall socket ground, that's all. (It could be that the power supplies ground is too high, or the wall socket's ground is too low). -- Either way, it shouldn't be a huge deal, since the wall socket ground is not the ground that the USB is using for logic. -- Voltage is just the "potential" difference between two points, it's relative, not absolute. \$\endgroup\$ – BrainSlugs83 Nov 3 '16 at 0:49
  • \$\begingroup\$ @BrainSlugs83, Again, that's for AC. There is no DC voltage at all on the grid side or the transformer would saturate immediately. Something needs to drop or inject DC voltage before you can measure it. \$\endgroup\$ – winny Nov 3 '16 at 7:01

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