Lab PSU not like regular PSU? - Page 1
Forum section: electronics / beginners

author: Bicurico
author_rank: Super Contributor
timestamp: January 14, 2025, 07:50:13 pm
content: Hello,I have a Uni-T lab power supply on my desk.It has now happend twice, that I try to power some device with it and the device did not work properly.One time it was a POS Windows computer that would only start with an external monitor connected - the shielding was providing the missing ground.Now it happened with a TV field meter, which would not start.The strange thing is that I only connect V+ and GND from the PSU.But a regular power supply 12V without grounding worked just fine.If both PSU's only connect through two wires to the device, if the voltage is the same, why is there a difference?What's the use of the Uni-T if devices connected to it don't work?What am I doing wrong?

author: audiotubes
author_rank: Regular Contributor
timestamp: January 14, 2025, 07:57:38 pm
content: I believe the ground is earth ground. If you want to drive a regular 12V load you use the + and - terminals, not the ground terminal.

author: BennoG
author_rank: Frequent Contributor
timestamp: January 14, 2025, 08:09:44 pm
content: All decent power supply's are floating and the + and - terminals provide the set voltage.The GND terminal is provided in case you want to ground your DUT.This way you can use your grounded soldering iron on a powered DUT without creating a short.In this way you can also use 2 PSU's to create for example +15V 0 and -15V without creating a short.Benno

author: Bicurico
author_rank: Super Contributor
timestamp: January 15, 2025, 10:08:27 am
content: Sorry for being dumb on this...Attached are two pictures:1) This is the Uni-T UTP3313TFL like the one I have. In fact, it does have a metal bar connecting the (-) and the (GND) terminals, so I just assumed it is OK to operate it like this.2) This is just an example 12V PSU, that has no GND connection.What is the difference between both?Do I understand it correctly, that I should REMOVE the metal bar between the (-) and the (GND) terminals, so that I get floating 12V instead of 12V referenced to ground?Sorry, but I am a bit unsure about this and why it matters.Case 1: I power a device that has no further connection to anything else.Case 2: I power a device that connects to i.e. my TV tab via a coaxial cable, where the outer mesh is grounded.Am I correct to assume that for case 1 I need to remove the metal bar, but need to use it for case 2?Thanks and sorry for the confusion.Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: January 15, 2025, 10:32:36 am
content: Update:I removed the plate connecting the (-) and (GND) terminals and apparently the PSU now works as expected.However, I would still appreciate an explanation for dummies (me) on when to use the plate and when to not use the plate. Also, what are the eventual security concerns on using or not using this plate?Thanks!

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 15, 2025, 03:29:41 pm
content: Perhaps the devices that you try to power has their positive grounded.

author: jwet
author_rank: Frequent Contributor
timestamp: January 15, 2025, 03:55:49 pm
content: True lab supplies look like batteries. You can stack them on other supplies, make negative supplies, positive ground them, negative ground them, etc, etc. They are "floating" with no reference to earth ground (unless you strap them with that little earth strap). There are limits to all this fun related to the supply's construction, you're usually safe to do anything up to about 100V.Usually it doesn't matter but when you start hooking up other earth grounded stuff, or other floating stuff, you get ground loops. PC's are a notorious ground loop creator. When you connect two things that are "grounded" to different ground potentials, currents will flow in the ground connections where it shouldn't. If two things are not plugged into the very same outlet, the ground potentials between their green wires could be quite different. (volts).One of the better demos of this was a video that Dave Jones did called "How to Blow Up an Oscilloscope" or similar. Its goes though the issue from a different and more expensive vantage point.

author: tautech
author_rank: Super Contributor
timestamp: January 15, 2025, 08:11:07 pm
content: Quote from: Bicurico on January 15, 2025, 10:32:36 amTypically any PSU should be floating with an option to ground reference it.You in ignorance have used yours PE referenced.Remove the link and enjoy.