Looking for USB wall adapters with a ground pin to shield sensor wires from nois - Page 1
Forum section: electronics / projects

author: liudr
author_rank: Contributor
timestamp: August 30, 2024, 03:57:02 pm
content: I'm looking for a USB wall adapter that will power a USB device and with a ground pin. The reason is I am working on a project with micro controllers powered by 5V USB and reading analog sensors with long cables. The sensor cables have shields so I thought I should make use of it to minimize noise on the sensors. But after doing some google searches, I was unable to find USB wall adapters with ground pins. It's probably an edge case for USB wall adapters to have ground. I wonder if someone could give me a pointer. The same USB cable is also used to interact with the micro controllers. I am trying to avoid having to run a separate ground wire from inside the enclosure. I'm using metal enclosures but they are coated on the outside. So having to run a ground wire means connecting a wire between the PCB and the inside of the enclosure, then separately another wire from the outside of the enclosure to an earth ground. It's too much work considering it's going to be deployed in many places, not a one-off build. If a USB adapter has the USB shield connected to the ground pin of an outlet, that would be ideal. I understand that If I wanted to shield other electronics from EMI coming from inside the box, shielding the box would be the only way. I'm just shielding sensor cables from outside interference. Thanks.

author: Andy Chee
author_rank: Super Contributor
timestamp: August 30, 2024, 04:21:06 pm
content: USB power adapters are Class II double insulated devices, therefore it is impossible to obtain one with an electrically connected ground pin (some might have plastic ground pins for opening shutters).Also, the power point ground connection would have little to no impact on shielding sensor cables. Connecting the sensor shield to the metal enclosure should be more than adequate.Coated enclosure? Bad luck, you'll have to scrape away the powder coating and create a solid electrical connection with a star washer, solder eyelet and bolt.

author: Infraviolet
author_rank: Super Contributor
timestamp: August 30, 2024, 06:06:26 pm
content: USB wall warts are often designed as a single model for sale both to countries which have an earth pin on their plugs and countries which just have a live and a neutral. You find quite a few models let you swap the plug contacts between several they come with. Those that come with a single country's plug built in will, nonetheless, internally be just like those with swappable plugs. So nowhere is going to making a USb wall wart which comes with an earth connection.Also, floating output is a good idea for a lot of USB stuff, the manufacturer never knows what the other end of the USB cable mght lead to, and that thing might not be floating, and be referenced to a different voltage level than earth. If a USB wall wart were earthed then connecting it to a device which had a ground which was fixed relative to mains, but not at the same potential as earth, could cause a destructive current to flow betwen them through their shared "ground" even if the difference in what the wall wart and the other device considered as "earth" was only a few volts apart. It is similar to the problems you can get if you connect the ground clip of a normal oscilloscope to a non-earth location in a non-floating circuit. For low voltage stuff like USB, it is always easiest to just leave everything floating relative to mains and just ensure the ground is common between all the USB connected items.

author: liudr
author_rank: Contributor
timestamp: August 30, 2024, 07:57:16 pm
content: Thanks ANdy and Infraviolet. I did some more searching and still came empty-handed. As you can imagine, I have an edge case. Just to be clear, I am talking about the USB-A rectangular metal contact that is connected to the cable's shield, not the GND pin from the 4-pin USB layout. I've used a few USB-A male to 5-pin connector cables for my projects. Most of them appear to have 5 pins, but pin 5 is fake. The pin isn't connected to anything, not the USB ground! Only one that is really a shield is the one I custom ordered 1K qty overseas with such specificationThere is too much uncertainty, even if I find an adapter, whether the USB metal shield is separate from USB signal ground or not. I'll consider drilling the enclosure if it comes to noise vs drilling holes.

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 30, 2024, 08:28:51 pm
content: The larger notebook power supplies sometimes have a ground connector to reduce the 100/120Hz-common-mode voltage. If you take such a power supply it would be quite easy to connect GND of the output to PE of the mains connector.An off-the-shelf low power wall wart with PE will be hard to find.

author: Nominal Animal
author_rank: Super Contributor
timestamp: August 30, 2024, 10:45:16 pm
content: I am not sure, but it is possible that SkrossWorld to USA USBandWorld to Europe USBhave the USB shield earthed. Unfortunately, Skross says they'renot intended for continuous use. It's one of the very few companies that I believe might bother to do the earthing that way.Perhaps you could find properly earthed mains power strips or extension sockets, with one or two USB outputs with the shield earthed, that you could use as USB adapters?

author: Someone
author_rank: Super Contributor
timestamp: August 30, 2024, 10:58:43 pm
content: Quote from: Andy Chee on August 30, 2024, 04:21:06 pmIncorrect, while most are not all are. Many earthed options available in the market right up to fully protectively earthed Class I.

author: liudr
author_rank: Contributor
timestamp: August 31, 2024, 04:13:57 am
content: Thanks for the suggestion. I tested out a couple of Anker brand power strips with USB ports. No luck. USB shield is not connected to ground prone.