Confused by power supply terminals - Page 1
Forum section: electronics / beginners

author: jgalak
author_rank: Regular Contributor
timestamp: November 30, 2017, 02:44:19 pm
content: I picked up a Tekpower TP3005P power supply (https://www.amazon.com/Tekpower-TP3005P-Programmable-Variable-Regulated/dp/B06XCQN82X) and it seems quite nice, but there's one thing that I found confusing. The power supply has 3 terminals labeled '-', 'GND' and '+'. In use, the voltage between - and + is whatever the voltage selected is, but the voltage between either - and GND or + and GND is zero.The manual (which is pretty terrible) says:"You can connect the + or - terminal to the green earth terminal of the power supply to pin the voltage to 0V (earth potential) at that terminal. If you leave the earth terminal unconnected, the output floats with respect to the ground."What exactly does this mean? It reads to me that I should literally short the - (or +) terminal to the ground terminal to have the supply ground the load to the mains ground? Or am I misunderstanding it?Thanks!

author: tszaboo
author_rank: Super Contributor
timestamp: November 30, 2017, 03:26:39 pm
content: It is not necessary to connect it.In a lot of applications, it is not allowed to connect it.It is possible, that the thing catches on fire if you connect the positive terminal to the GND.Normally, you could connect the negative to GND. But you really need to be aware of how this works.And yes, GND means there is a direct connection to the Earth terminal in the plug.

author: rstofer
author_rank: Super Contributor
timestamp: November 30, 2017, 03:45:37 pm
content: You can simply ignore the Gnd terminal for most applications.Most power supplies float and that's why you measure 0V between either + or - and the Gnd terminal. That's good! Your power supplies don't have a built-in ground reference, nor should they.If you want your power supply output to have a ground reference for some reason, you can ground either the + or - terminal. There may be some reason to do this but I haven't found one for my own projects. If you ground the - terminal then you have some + voltage referenced to earth ground. If you ground the + terminal then you have some - voltage referenced to earth ground.I haven't found a use for the Gnd terminal although there may be one. I just ignore the connection.You could also use the Gnd terminal to provide a safety ground to the enclosure of a project. Neither side of the power supply would need to be connected to Gnd but if you were doing some kind of voltage boost you might want a safety ground on the metallic parts.

author: w2aew
author_rank: Super Contributor
timestamp: November 30, 2017, 03:47:40 pm
content: Quote from: jgalak on November 30, 2017, 02:44:19 pmThis video of mine should clear this up for you:

author: jgalak
author_rank: Regular Contributor
timestamp: November 30, 2017, 04:03:26 pm
content: Thanks for the replies. Will watch video shortly.Followup question: If I'm working on a radio system, which will be connected to an antenna and ground system, is it then necessary/appropriate to tie negative into earth ground? Or is this an "it depends" kind of question?

author: rstofer
author_rank: Super Contributor
timestamp: November 30, 2017, 04:40:42 pm
content: Quote from: jgalak on November 30, 2017, 04:03:26 pmAs usual, w2aew's video gets right to the point. Excellent.Now, having said that, I wouldn't necessarily use the Gnd terminal. I could create the +- 15V arrangement by just jumpering the appropriate terminals and never make an earth ground connection. As to whether that is a good idea, yes, it depends. It's nice to have things locked down to a ground reference. But it's also a source of annoyance if the ground itself has some kind of noise. It shouldn't have, but it might.I certainly don't earth reference my battery projects so I can't see a compelling reason to earth reference any other projects. There are exceptions...Then there is USB... I am going to get a ground reference to any uC project that uses USB to communicate with a desktop (not true if I'm using a laptop running on battery). How many earth grounds do I want? Then there is my scope ground lead - it is earth grounded. All of a sudden, I have a bunch of earth grounds running around my project. Not my first choice! There are USB isolators to remove this ground connection.Earth grounding is a complicated subject. In general, the earth ground is for safety. It is intended to keep metal surfaces (enclosures and such) from getting 'hot' relative to a presumed grounded person. This would apply to any circuit operating at more than 50V (to ground). Most of my projects run at 5V and whether they are ground referenced depends entirely on whether the scope is being used or there is a USB connection.Radio is outside my pay grade. I would expect everything to be earth grounded and power supplies to be ground referenced. The last thing you would want is a transmitter output shorted to a chassis.

author: iainwhite
author_rank: Unknown
timestamp: November 30, 2017, 04:46:37 pm
content: Quote from: w2aew on November 30, 2017, 03:47:40 pmI have found that a lot of questions can be answered by watching W2AEW videos.

author: Old Don
author_rank: Regular Contributor
timestamp: December 01, 2017, 02:10:17 am
content: In the "good old days" automobiles came with either positive chassis ground or negative chassis ground so if your power supply was always a negative ground beast and use used it with a positive chassis car sparks would fly, at least until something blew. Old TV's had a floating chassis and again sparks could fly. Having a floating power supply allows the output to be used with ground defined by the device you are powering. If that device has a ground via it's power supply cord then you either match it with the P.S. or let the P.S. float and the connections to the device will create the ground path through the rest of the circuitry.OBTW, Oscilloscopes often come with their BNC shield lead grounded and testing circuits that don't want to be grounded can blow out O'scopes.

author: jgalak
author_rank: Regular Contributor
timestamp: December 01, 2017, 10:35:59 pm
content: Thank you, w2aew, that video answered all my questions. Now I'm compulsively watching the restWell, all except for the question brought up here:Quote from: Old Don on December 01, 2017, 02:10:17 amDoes this mean the circuit should or should not be grounded to protect the scope? Does anyone know if this applies to the Rigol DS1054Z?

author: imidis
author_rank: Frequent Contributor
timestamp: December 01, 2017, 10:49:21 pm
content: Negative is referenced to ground on the scope, if DUT is grounded and you put the negative from scope on the wrong part of the circuit bang!

author: jgalak
author_rank: Regular Contributor
timestamp: December 01, 2017, 10:51:35 pm
content: Quote from: imidis on December 01, 2017, 10:49:21 pmAh! Ok, yes, that makes sense. Thanks.

author: rstofer
author_rank: Super Contributor
timestamp: December 01, 2017, 11:23:20 pm
content: Quote from: jgalak on December 01, 2017, 10:35:59 pmAbsolutely it applies to the DS1054Z. Anything with a BNC connector, unless it is somehow isolated, is going to have a ground on the connector if the device is mains powered.That would include scopes, signal generators, every instrument with a metallic connector, the USB cable when connected to a desktop or some laptops under some conditions.There are isolation transformers that remove the pass-through of earth ground reference. This is a VERY advanced topic and not something I'm going to get into. I have never had a need to float my scope and, if I did, I would buy the differential probe and leave the scope alone. Or use the Math X-Y function.On just a simple breadboard project, the scope is bringing a ground, the power supply might be bringing a ground, the signal generator is bringing a ground and the USB cable is bringing a ground. It's pretty important to know where these are connected. The bad news is that they all take a different path to earth. Some paths are longer than others, some are routed near noise generating stuff, whatever. All grounds aren't the same! But somehow they all wind up on my breadboard.

author: Brumby
author_rank: Unknown
timestamp: December 02, 2017, 12:51:52 pm
content: Quote from: jgalak on December 01, 2017, 10:51:35 pmDave has done a video on exactly this. It's a good general lesson for earthing considerations as well.