Author Topic: What's the 'ground' terminal for on all these DC power supplies?  (Read 14387 times)

0 Members and 1 Guest are viewing this topic.

Offline icon

  • Regular Contributor
  • *
  • Posts: 246
Hi

Sorry 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
 

alm

  • Guest
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #1 on: April 01, 2012, 10:20:51 pm »
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.
 

Uncle Vernon

  • Guest
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #2 on: April 01, 2012, 10:25:41 pm »
Quote
What's the 'ground' terminal for on all these DC power supplies?
It's for connecting stuff to ground.
 

Offline icon

  • Regular Contributor
  • *
  • Posts: 246
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #3 on: April 01, 2012, 10:45:22 pm »
Quote from: alm
{Helpful stuff}

Thanks; that explains it perfectly.

Quote from: Uncle Vernon
It's for connecting stuff to ground.
08:25:41 > 08:20:51
Sadly, you lose the Internet race. But thanks for playing along.

Regards
John
 

Uncle Vernon

  • Guest
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #4 on: April 01, 2012, 11:09:19 pm »
Sadly, you lose the Internet race. But thanks for playing along.
Races 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!  ;D
 

Offline george graves

  • Super Contributor
  • ***
  • Posts: 1259
  • Country: us
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #5 on: April 01, 2012, 11:55:50 pm »
Races are generally a contest of getting from here to there or there to here as quickly as possible.

Oh please continue, I find your post fascinating.   8)

Offline SgtRock

  • Supporter
  • ****
  • Posts: 1200
  • Country: us
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #6 on: April 02, 2012, 12:35:42 am »
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 1910

Best Regards
Clear Ether
 

Offline RCMR

  • Frequent Contributor
  • **
  • Posts: 405
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #7 on: April 02, 2012, 01:14:30 am »
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?
 

alm

  • Guest
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #8 on: April 02, 2012, 01:30:58 am »
--When using multiple supplies in series, connections to the ground lug should be made on only one of the supplies to prevent ground loops.
When 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.

--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.
Grounding 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.

I have to laugh -- my el-cheapo wun hung lo PSU came with a two-pin mains plug -- ie: no earth!

Does 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.
 

Offline aluck

  • Regular Contributor
  • *
  • Posts: 246
  • Country: ru
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #9 on: April 05, 2012, 07:13:56 am »
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 don't see how it could happen. Usually your project is not connected to PSU and mains simultaneously, doesn't it?
 

alm

  • Guest
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #10 on: April 05, 2012, 07:46:11 am »
The key word is accidentally. 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.
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 13017
  • Country: gb
  • 0999
Re: What's the 'ground' terminal for on all these DC power supplies?
« Reply #11 on: April 05, 2012, 10:43:56 pm »
The key word is accidentally. 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.
It 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.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf