Help me understand this DSO measurement (stupid newb, or really bad wall wart?)

[nikodemus]

I was comparing a couple of wall warts with an oscilloscope, and by accident discovered this:



CH1 is the positive contact on the wall-wart output, with probe ground lead unattached. (So from positive contact to DSO ground, I think?)

CH2 is the same reading from the negative contact.

MATH is CH1-CH2. Wall-wart is rated 15V, so that matches... but that apparent 220AC between wall-wart and ground worries me quite a bit! 

I made a comparison to a second wall wart of different make (this one rated 9V), plugged into the same output, same setup with ground leads hanging, etc:



No sign of apparent mains AC in relation to scope ground.

Is this normal and harmless for some type of power supplies, or should I be about as alarmed as I am? Or what?

[madires]

Capacitor for EMI suppression between primary and secondary?

[PA0PBZ]

Since the wall wart is isolated from the mains, why would you not just use ch1 on the scope to measure the output?

[David_AVD]

The high voltage is from either a suppression capacitor or leakage across the power supply transformer.  It's a very high impedance source.

[Odysseus]

Not a huge deal.  If you really want to quantify what you're dealing with, you can measure the capacitive AC current from the output to earth ground with a meter.  It will likely be in the uA range.

[nikodemus]

Many thanks for the explanations! I'll measure the capacitative current as you suggest.

Re. why did I measure it that way? I was about to measure the wart with just CH1, when I touched the probe to one of the contacts before the ground lead was attached... The scope happened to be on fairly large vertical scale and WHAM the AC waveform jumped on the screen. Nearly pissed myself. 

...at which point I didn't quite dare attach the ground lead anymore.

A couple of followup questions:

- Is this typical/inevitable for power supplies with similar design?

- Are there situations in which such a capacitative current is highly undesirable vs. situations where is doesn't really matter at all?