Know about USB power delivery and weird AC? (1 Viewer)

[e9999]

Sort of a power question, indirectly.
So, yesterday, I was playing with a NCVT-3P (non-contact voltage tester) toy from Klein. (Way more sensitive than my old Fluke.)
Anyway, by accident I saw something odd: I have a USB cable coming off a PD power supply. It was disconnected at the other end. To my surprise, the NCVT reacted strongly from the cable. Now, to be clear, this tester is labeled as an AC device, there is no indication it should do anything with DC. So my question is why would there be some noticeable AC voltage in an open USB cable given that it was only connected to a power supply, not a data source? I can sort of imagine there may some carrier wave or something like it when transferring data but that is not the case here. Could that be an indication that the power supply is defective or poorly-designed and some AC is leaking into the USB port? Or could that just be the cable picking up 60Hz from nearby wiring, I would think that would be a tiny signal?
Thoughts?

 

[LandLocked93]

Do the readings from the tester correlate to a setting on the PS in terms of voltage or frequency?
I'll guess it's picking up on spurious waves that the cable is picking up. But have no idea really.
I feel like I've seen similar behaviour on my mm when it's set to AC.

 

[george_tlc]

PD will have a switching supply to provide the various DC voltages that PD supports. Switching supplies are typically quite RF noisy. A cable will be a great antenna, both to receive and to radiate.

I wouldn't be overly concerned. You could try the Klein around other USB cables - if you have USB PD in your vehicle for instance... or even a battery USB brick etc.

I wouldn't be surprised if those non-contact AC testers can be fooled by many RF noisy sources.

cheers,
george.

 

[e9999]

I tried a bunch of USB chargers, some with no PD feature. They all did the same thing, with a signal detected on the cable. Thought it could be the location since I have a biggish (100W) PD USB charger there, so tried far away with a non-PD brick on a wall outlet. Same thing. So not limited to PD chargers, it seems. The only time there was no signal detected was when I put the cable directly on a USB outlet on a LiFeP battery power box, so from a DC source. So maybe in that instance the voltage reduction is done some other way than with the AC systems. One of these days, if I get the ole scope out, I may try to see what frequency is in that cable.

 

[george_tlc]

Yep, not surprised. Many offline switchers will have an X or Y rated capacitor from the mains side (behind the high voltage bridge) across the 'transformer' to the ground DC side. It's part of the safety path and likely trips up the Klein style tester.

Time to look for another windmill to chase

cheers,
george.