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

[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'   

[artag]

Check your scope's ground lead. They break internally near the end - usually the probe end, not the crocodile.

It may be that, as one reply said, the Meanwell has no earth connected to DC but the other that you tried does.

[BeBuLamar]

I don't know but the waveform looks purely AC and how the Fluke 87 measures it as DC is beyond me.

[JustMeHere]

When there is no load on a switching power supply there can be a "weird" waveform due to "pulse skipping"

Touch the probe to your finger tip.  You should see the 60hz.  Then touch the ground lead.  If it goes down/away, your ground lead is working.

[JustMeHere]

I don't know but the waveform looks purely AC and how the Fluke 87 measures it as DC is beyond me.

Dave recently did a video on how DMMs will filter put 60hz noise.

[stoli0]

Mystery (mostly) solved!

The power brick has no ground.  The middle pin of the power connector typically goes nowhere inside the brick.

You're exactly right, Jon.  The power brick has no ground.  I tested continuity between the ground pin of the AC cord and the barrel of the DC adaptor and nada.

Check your scope's ground lead. They break internally near the end - usually the probe end, not the crocodile.

It may be that, as one reply said, the Meanwell has no earth connected to DC but the other that you tried does.

Also correct.  My scope's ground lead was in fact broken.  On BOTH probes.  Sigh.  Time to buy some new probes.

By securely fastening a wire around the DC barrel, connecting it to the chassis ground lug on the scope, then probing the center, I got the 24V I was expecting (which jives with the multimeter.)

Any thoughts about what exactly I was measuring with the ground wire disconnected?

[stoli0]

For others in a similar situation, replacement ground lead plugs are available at a very reasonable price by searching amazon for "oscilloscope ground lead".  I found four for $10, vs. $25 for 2 in keysight's probe accessory kit.

[amyk]

Any thoughts about what exactly I was measuring with the ground wire disconnected?

Capacitively coupled mains voltage...

That's what I was alluding to in my first reply.

[stoli0]

I have to admit my Google searches haven't told me enough to connect my observations (in my limited knowledge) to the theory regarding capacitive coupling of mains voltage.

In the spirit of the beginners forum, would someone be able to go a bit deeper on how that explains these observations?

[tooki]

For others in a similar situation, replacement ground lead plugs are available at a very reasonable price by searching amazon for "oscilloscope ground lead".  I found four for $10, vs. $25 for 2 in keysight's probe accessory kit.

Or cut open the old ground lead and remake it with fresh wire. Alligator clips are easy to find, but the clip that snaps onto the probe side is unobtainium. (There was actually a thread about those just a few days ago, LOL!)

[artag]

Yes, you can usually just pull back the clip cover or slit away some heatshrink to expose the joint. Pull the stub of old wire away from the break and cut back the ground lead to good copper.
If that's too short, a supply of extra-flexible test lead wire (or better still, unmeltable silicone-insulated wire)  will be needed to replace it. Standard 7/24 or similar will break again rather quickly.

[BrokenYugo]

I have to admit my Google searches haven't told me enough to connect my observations (in my limited knowledge) to the theory regarding capacitive coupling of mains voltage.

In the spirit of the beginners forum, would someone be able to go a bit deeper on how that explains these observations?

A capacitor blocks DC current entirely (ideally) but also exhibits reactance, that is, it passes AC current to some extent, this reactance (measured in ohms, just like a resistor) varies based on the capacitance and frequency per X_c=1/(2×pi×f×C).

Switching power supplies typically include a capacitor between the "hot" mains referenced side to the "cold" isolated side, you'll often hear it called the Y cap, referring to it's safety ratings for this service (if it failed shorted it could kill, so it's designed not to). The primary purpose of this capacitor, as I understand, is to shunt high frequency RF noise from the switching action, on account of the formula this capacitor will be of low enough reactance at those high frequencies to do that job, but high enough reactance (probably in the megaohm range) at mains frequency to only let a harmless current through. That trickle of mains current is still enough to detect with the high input impedance of an oscilloscope or DMM though.

[artag]

That trickle of mains current is still enough to detect with the high input impedance of an oscilloscope or DMM though.

If you have several such supplies all run off the same RCD or RCBO as UK wiring is inclined to do, they also tend to add up and get a bit close to the breaker's limit, causing nuisance trips.

[ledtester]

Try the following measurements:

With your multimeter in DC volts mode, measure the voltage across the terminals of the power supply You should get what you expect from the power supply.

In AC volts modes, measure the AC voltage from one power supply terminal and earth ground. You might see a large AC voltage -- perhaps even as high as 50VAC.

What's going on is something like this:



In the first case you are measuring DC voltage across A and B and you get the expected voltage of the power supply. In the second case you are measuring the AC voltage across B and C (or A and C) and you're seeing the coupled mains voltage.

For a third experiment, put like a 1K resistor across B and C and measure the AC voltage across it. You'll likely find that the voltage decreases a lot -- perhaps down to just millivolts. This says that the coupling (the capacitor) is rather weak.

In the case of your oscilloscope measurements, note that a probe's ground is implicitly tied to earth ground. So if it is not connected you will be measuring between B and C (or A and C). However, if the tips of your probe are connected to A and B you will be shunting out the AC source and only be measuring your power supply's DC output.

Corrections to this explanation are appreciated.