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With a switching DC wall adapter, when I measure AC rms voltage from either of the output wires to earth ground, what am I actually measuring and is this a controllable spec of a wall adapter?

I am dealing with what must surely be an AC ground loop issue that causes 60 Hz hum on telecom equipment. I have many 9 volt switch DC wall adapters. Some work fine (meaning the 60 Hz is not measurable) while others are quite loud.

I have found that the ones that work I measure less than <10 VACrms from wall adapters output to earth ground. The ones that do not work measure >50 VACrms from either of the output wires to earth ground.

On a scope the signal is 60 Hz and a very distorted sine wave.

When trying to source a new adapter is this something I can find in the datasheet? What controls this in an isolated switching power adapter?

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Capacitance between live AC terminals and DC output wires is not zero meaning that if you measure the AC content on the DC output with respect to earth you can easily measure tens of volts AC. For a decent power supply this might cause a little tingle if you touched those outputs while being earthed. This is not unusual.

So where does the capacitance come from - the internal switching transformer has capacitance between primary and secondary and that is one route. Another route is capacitors placed between live and DC side to reduce radiated and conducted emissions due to the high frequency switching.

So you have capacitance and you'll measure a few volts to a few tens of volts.

When trying to source a new adapter is this something I can find in the datasheet? What controls this in an isolated switching power adapter?

Probably not.

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  • \$\begingroup\$ I believe adapters used in medical applications control this far more tightly than those used for other stuff (generic ITE/....) \$\endgroup\$ – ThreePhaseEel May 8 '16 at 16:02

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