AC adapter issues

I have recently discovered some issues with some ac adapters, USB hub 5VDC adapters, external hard drive enclosure 12VDC adapters from no-name or small-name manufacturers.

Problem?

The ground pin carries a large and persistent 60Hz AC voltage against earth ground. My scope maxed out at about +-38V when measuring the ground of these adapters. It looks like sine wave with some signs of rectification. Where the voltage crosses ground is not a single point, but a small range, similar to that of a diode's turn-on curve. The voltage is AC. I grabbed the negative of my scope with my right hand and positive (connected to the adapter ground) with left hand to provide a poor ground path but the signal didn't disappear.

Out of curiosity, I tested a few other adapters, from netgear etc. The ground carries a small random voltage of a few volts as I expected and grabbing connections the same way I described the voltage vanishes into a volt or less, as I expected.

I didn't notice these voltages until yesterday. I've been having issues with my HDMI output. Occasionally the output will black out for a second or so. Sometimes it's more frequent and annoying. I thought maybe it was the old monitor. I bought a new monitor and the issue still showed up. I thought my laptop has issues, until yesterday. I was unplugging my powered USB hub to take my laptop from my desk (very rarely I do that). As I was unplugging the USB cable that connects to my USB hub, my monitor blacked out again with some popping sound on the monitor's integrated speaker. Out of curiosity, I tried to plug and unplug the USB cable, and same thing happened again! I then tried not to fully insert the USB plug, but only touching the shield to the shield of the port, and it happened again!

So I tested its output. Relative voltage between power and ground is about 5V, that is if I ground its ground with my scope ground. If I test the adapter's ground or power, I get overload: 40V+ AC at 60Hz. Several cheap adapters 5V and 12V have the same issues.

Then I tested a few brand-name adapters for routers and found they had good results.

So what the heck? Are these manufacturers too cheap to produce adapters out of the right designs? My computer has been plagued by this random occasional black out for over a year. I think that matches with how long I have owned that powered USB hub. This sucks. I wonder if any USB devices have been damaged by this voltage.

I had a similar issue a cheap LED strip driver 12V power supply. There was leakage through the earth terminal which periodically caused the house central RCD protection to trip. It took me ages to locate the problem on this power supply.

Assuming these power supplies are two prong and don't have a ground pin there outputs are isolated from the AC Power. Seeing floating voltage like this is not uncommon, what would be more relevant is if there is a large amount of current flow from the AC ground to the output ground. One way to test this is to start with something like a 1m or 100k Resister and connect it between the two grounds. If the voltage disappears then there's not much current there and this floating voltage will likely never do any damage.

If you think the USB Hub Power Supply is causing a problem then remove it and connect each device to you computer one at a time. If one of the units causes the same problem then it's not the Hub causing it. Another thing to try is to disconnect the Hubs Power Supply and plug your computer to it, if the problem persists then once again it's not the Hub PS. Side note: most hubs will work without a Power Supply unless your powering Pocket Drives or other higher current USB Devices.

I have in a couple of cases ran a permanent wire from the AC Ground to the USB Hubs ground to solve issues in very electrically noisy environments. This could help fix your glitches but I cannot be held responsible if it causes any damage to your equipment even though it's highly unlikely it will. Again try a resister from one ground to the other and lower the value until the issue goes away if there's very little current flowing switch it out with a wire.

liudr: Problem?

The ground pin carries a large and persistent 60Hz AC voltage against earth ground.

So your AC adapters are all 'three pin' on the mains (wall) side?

I attached a 1Mohm resistor across ground and earth. Voltage didn’t disappear for those trouble makers. There is a leak, over ±30V.

The adapters don’t have the third prongs.

I attached a 1Mohm resistor across ground and earth. Voltage didn't disappear for those trouble makers. There is a leak, over +-30V.

Its all about the current. This may or may not be a problem (that's only 30µA). Depends on the product and application. Most all classifications for safety allow much more leakage current, except for some medical products required to be within 10µA leakage. For household wiring, Class A GFCIs must trip within 6000µA (6mA).

Try testing with 10K from DC GND to earth.

http://www.psma.com/ul_files/forums/safety/estguide2.pdf

dlloyd: Its all about the current. This may or may not be a problem (that's only 30µA). Depends on the product and application. Most all classifications for safety allow much more leakage current, except for some medical products required to be within 10µA leakage. For household wiring, Class A GFCIs must trip within 6000µA (6mA).

Try testing with 10K from DC GND to earth.

http://www.psma.com/ul_files/forums/safety/estguide2.pdf

I totally agree with this post and in a round about way and not properly to the point was what I was trying to say in my previous post.

As I pointed out I have connected the output ground to the AC ground a couple of times in very electrically noisy environments. That cleaned up glitches that where happening because of the problems in these environments and may also solve your problems.

I don’t think the small current is an issue. The issue is the transient current due to poor contact. If the ground loses contact with the USB ground, which happens with cheap usb cables, it will create some transient current that probably disturbed my HDMI signal.