What's the 'ground' terminal for on all these DC power supplies? - Page 1
Forum section: electronics / beginners

author: icon
author_rank: Regular Contributor
timestamp: April 01, 2012, 10:01:13 pm
content: HiSorry if this is a dopey question. On all these Atten/GW Instek/Wun Hung Lo bench power supplies (typically 0V-30V, 0A-3A~5A) that I'm looking at, there's a positive and negative terminal, and ground. What's ground connected to?Ages ago I built a simple 317/337 +/-20V power supply (hasn't everyone?) which has a 'real' ground - you can clip a scope earth connection to it, power a sine wave generator with it and see a waveform symmetrical around 0V. I haven't seen anything to suggest that these DC bench power supplies can be used like that - that if the supply is set to 30V then the negative terminal is at -15V relative to ground. Maybe it ain't so? Maybe it's too obvious to need stating? - but equally there are supplies out there that *don't* have a ground terminal. What are they doing differently?Help relieve my ignorance!John

author: Unknown
author_rank: Unknown
timestamp: April 01, 2012, 10:20:51 pm
content: It's connected to the ground pin in the mains plug. The power supply is floating, so if you want a grounded supply, you connect either the positive or the negative output terminal to this terminal, depending on whether you want the voltage to be positive and negative relative to ground. This is much more flexible than a power supply with a 'real ground', since you can freely put floating supplies in parallel/series. If you need +/- 15 V, you can take two 15 V supplies, connect the positive terminal from supply 1 to the negative terminal of supply 2, and to ground.I usually tie at least one of the supply rails to ground if I don't have any other ground connections. This prevents the rails from floating to undesirable potentials when accidentally connected to some other voltage source (eg. mains). I only disconnect (one of) the connection(s) to ground when putting multiple supplies in series or when ground loops may be an issue.

author: Unknown
author_rank: Unknown
timestamp: April 01, 2012, 10:25:41 pm
content: QuoteIt's for connecting stuff to ground.

author: icon
author_rank: Regular Contributor
timestamp: April 01, 2012, 10:45:22 pm
content: Quote from: almThanks; that explains it perfectly.Quote from: Uncle Vernon08:25:41 > 08:20:51Sadly, you lose the Internet race. But thanks for playing along.RegardsJohn

author: Unknown
author_rank: Unknown
timestamp: April 01, 2012, 11:09:19 pm
content: Quote from: icon on April 01, 2012, 10:45:22 pmRaces are generally a contest of getting from here to there or there to here as quickly as possible. Generally racing is a poor tool in the attainment of knowledge.Ask an obvious question and you'll get an obvious answer! Add some attitude and it becomes sport.Enjoy!

author: george graves
author_rank: Super Contributor
timestamp: April 01, 2012, 11:55:50 pm
content: Quote from: Uncle Vernon on April 01, 2012, 11:09:19 pmOh please continue, I find your post fascinating.

author: SgtRock
author_rank: Unknown
timestamp: April 02, 2012, 12:35:42 am
content: Dear Alm:--I thought you gave a very good answer to the question. I had no idea what the ground lug was for. But, I am confused on one minor point. You stated:"I usually tie at least one of the supply rails to ground if I don't have any other ground connections. This prevents the rails from floating to undesirable potentials when accidentally connected to some other voltage source (eg. mains). I only disconnect (one of) the connection(s) to ground when putting multiple supplies in series or when ground loops may be an issue."--I restate below to see if I understand you correctly.--When using a single supply, one may connect one or all of the negative rails to the ground lug. Or one may connect one or all of the positive rails to the ground lug. [But one should never connect both positive and negative rails to the ground lug.] Obviously.--When using multiple supplies in series, connections to the ground lug should be made on only one of the supplies to prevent ground loops.--I am pretty much of a beginner myself, so please correct me if I am wrong. And, I still have very little idea when or when not to float a supply."Thunder is good, thunder is impressive; but it is lightning that does the work."Mark Twain (Samuel Langhorne Clemens ) 1835 1910Best RegardsClear Ether

author: RCMR
author_rank: Frequent Contributor
timestamp: April 02, 2012, 01:14:30 am
content: I have to laugh -- my el-cheapo wun hung lo PSU came with a two-pin mains plug -- ie: no earth!How brave do you have to be to use one of those eh?

author: Unknown
author_rank: Unknown
timestamp: April 02, 2012, 01:30:58 am
content: Quote from: SgtRock on April 02, 2012, 12:35:42 amWhen putting multiple supplies in series, it only makes sense to connect one of the terminals on one of the supplies to ground, otherwise you would be shorting some of the power supplies.Quote from: SgtRock on April 02, 2012, 12:35:42 amGrounding is often fine and safer in some cases. If you connect a floating +5 VDC supply to mains, you will have a nice 115 V / 230 V AC signal superimposed on your power supply, as opposed to a tripped breaker as you might expect. Grounding at multiple points (supposedly at the same potential) creates loops, which can induce noise, so floating supplies may make life easier when working with low level signals. Floating supplies can also be nice when working with a scope, since you can connect the ground clip anywhere in the circuit. Keep in mind that the ground clip of the other channel is connected to the first one, however.In many cases, it doesn't really matter, but to ground it by default is slightly safer in my opinion.Quote from: RCMR on April 02, 2012, 01:14:30 amDoes it have a ground output terminal? That would be funny. Apart from that I don't see why it would be unsafe provided they designed it as double insulated, something that they might have skipped.

author: aluck
author_rank: Regular Contributor
timestamp: April 05, 2012, 07:13:56 am
content: Quote from: alm on April 01, 2012, 10:20:51 pmI don't see how it could happen. Usually your project is not connected to PSUandmains simultaneously, doesn't it?

author: Unknown
author_rank: Unknown
timestamp: April 05, 2012, 07:46:11 am
content: The key word isaccidentally. Why wouldn't it be connected to both at the same time? Say you are working on a power supply for the motors, but are still using a lab supply to power the logic. It doesn't have to be mains, but may be another power supply.It's never happened to me, but it's just common sense. Similar to grounding large metal objects like work surfaces. If it can be at any potential, I prefer this potential to be ground. Especially when connections are long and numerous, like for complex projects.

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2012, 10:43:56 pm
content: Quote from: alm on April 05, 2012, 07:46:11 amIt depends on what you're doing. I generally prefer to use a floating supply because there's less of a risk of problems involving ground loops or short circuits. I do see why it's sometimes necessary to connect one side of the supply to earth i.e. when the PSU is used to power a high voltage circuit such as a neon lamp power supply.