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Bizarre behavior from 24V DC power brick (solved -> grounding!)

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stoli0:
Hello friends, I'm probing a 24V DC power brick with my oscilloscope and seeing really unpredictable behavior.

The brick (a Meanwell gst60a24-p1j from mouser) measures just fine with my multimeter.. I get 23.98V, peachy..

But with my scope I get a really weird 60hz waveform - is the brick defective?

And the scope isn't broken!  Measuring a 12V DC brick does what I expect on the scope.

amyk:
How are you measuring it? In particular, how is the ground connected?

stoli0:
I've clipped the probe's ground return lead to the barrel of the DC power brick, and am probing the center with the probe tip.

The power brick is a 3 prong, as is the oscilloscope - and they're both plugged into a furman power conditioner, and the ground is common.  So I see the same result on the scope whether the scope's ground return lead is clipped to the power supply or not.

jmelson:
The power brick has no ground.  The middle pin of the power connector typically goes nowhere inside the brick.
I suspect the ground clip was not making a good contact to the barrel.  Try wrapping a few turns of solid wire around the barrel and clipping the ground to that.

Jon

GlennSprigg:
MOST 'power-bricks' these days do not make their 'DC' from the old simple methods of a rectifier & capacitor.  They often use a
'switching' power supply, using a totally different technology, (high intermediate frequencies), and with a LOT of them, I've noticed
that you do not always get meaningful results using precise & rapid instruments such as Oscilloscopes or some Digital VM's. However,
using an older style 'Analog' meter can bypass these problems.  The MAIN issue, is when the power-pack Output is totally UNLOADED!
Where-as putting an actual LOAD on it, even with a resistor, can bring the expectations back to 'normal'   :phew:

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