GND (PSU) vs GND (Chassis)... When does it make a difference? - Page 1
Forum section: electronics / beginners

author: Bicurico
author_rank: Super Contributor
timestamp: November 25, 2024, 04:11:38 pm
content: Hi,I had another recent question looking for a PSU connector, where I asked about GND, too:https://www.eevblog.com/forum/chat/help-what-connector-ist-this/This is a different case, but similar issue.I bought a second hand POS (Point os SALE, not Piece of Shit). It comes with a touch screen and a magnetic card strip reader, which is what I was really interested in.Anyway, it was sold as broken for 8 Euro with shipping, so I can't complain that the foot was broken (but fixable), no RAM (luckily I had a spare SDIMM) and no HDD (I had a spare suitable mini M.2).What I am misisng is the PSU: the POS specifies a simple 12V 5A, but it has 4 pins (2x 12V, 2x GND). I opened the case and it has an internal power socket (don't know the name, but has 4 pins, similar to GPU power sockets and again 2x 12V and 2x GND).I connected my lab PSU and noticed this:1) Switch on with 12V and GND pin connected: the PSO switches on and off cyclically. First I was afraid something was broken.2) When I tried to use an external VGA monitor, so exclude the TFT being broken, it just booted. Further analysis showed that the POS needs GND. The regular power socket has 4 pins, plus an extra GND all around it.So this is what I don't understand: what is the difference between the 12V/GND pins and the GND shielding around the connector?

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2024, 09:48:31 pm
content: The shielding ground is for EMC purposes. It might work without it, but it could behave erratically.

author: thm_w
author_rank: Super Contributor
timestamp: November 26, 2024, 01:18:44 am
content: Are you sure your 12V power supply has enough current (more than 5A)?Its rare to see GX12 shell used as a shield, since its not so simple to connect to, but does happen sometimes.You can measure the resistance from GND to Shell, see if there is anything, or look around inside. Can you post a photo of inside the device where the connector is?As Zero says probably shielding, but still a little unusual for it to not work at all.

author: Bicurico
author_rank: Super Contributor
timestamp: November 26, 2024, 05:26:24 am
content: Using my lab power supply with 12V and GND at 3A, the PC boots normally if I connect the shielding to ground, i.e. through a connected monitor by means of a VGA cable. The PC has it's own touch screen, so the external monitor is normally not needed.This cable is just providing ground to the chassis of the POS. I don't need to connect it to the VGA port, I just need to touch some chassis part (but it is of course easier to just connect it to the VGA port).The lab PSU never exceeds 2.3A and the POS works fine.But without this extra ground connection, the POS will not boot and restart cyclic.

author: Bicurico
author_rank: Super Contributor
timestamp: November 26, 2024, 11:21:31 am
content: It was due to my Lab Power Supply.I used a regular 12V 5A PSU, connecting just GND and 12V and it now works fine.I guess the Lab Power Supply is not grounded?

author: thm_w
author_rank: Super Contributor
timestamp: November 26, 2024, 11:17:44 pm
content: Yeah a lab supply is normally floating, so maybe it didn't like that.You can test yourself if the 12V 5A PSU is floating or not by measuring between GND and input earth on the power plug.