Lab grade power supply outputs

Hi !
sorry for the trivial question.
I see on a power supply i have 3 outs: -, GND, +.
I have a unit needing +12VDC, just two wires.
I guess the positive must be connected to + terminal.
But the other ? to the - or GND terminal ?
I would say GND ... maybe ?
Does this mean that is a dual power supply maybe ?
Thanks a lot indeed, gino
 

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The owner's manual for my lab grade power supply tells me to connect "-" to "GND".
Then connect the load between "+" and "-".
Maybe the owner's manual for your power supply, discusses this

Hi ! thanks you a lot !
I am completely stupid ... :eek:. I made a mistake.
Mine is at home and it is this one here (only + and -) :eek:

http://mascot.blob.core.windows.net/media/1010/719.jpg

i cannot find the user manual.
As i see always 3 outputs i was convinced that mine was the same.
But my guess is that in case of 3 outs the instruction for your unit should be valid in general.
However i am bidding for another one that has indeed the 3 outs.
Thanks a lot again, gino
 
- + and GND is ambiguous. It could be a dual polarity supply. It could be a single supply, but isolated.

Hi ! thanks a lot for the valuable advice.
I will check with the tester of course but what you say it is exactly what I thought.
When i see -/+/GND i think of dual power supply.
I will check with the tester before doing big mistakes.
Thanks again, gino
 
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Set output voltage = 5 volts
Connect 1K resistor from "-" to "GND".
Measure voltage across 1K resistor.
If measured voltage = 5V then it's a dual output "bipolar" power supply.
If measured voltage = 0V then it's a single ended, isolated, "floating" power supply

Hi and thanks a lot indeed.
I will do that.
To be precise the one i already have has only red and black terminals (i guess + and -), but not the yellow one.
Another one i have ordered has indeed the 3 terminals, red/black/yellow.
I will do what you recommend.
Thanks again, gino
 
It would be best if you post a photo of the entire front panel, including the model number.
The odds are that the +/- are the supply terminals, and the ground terminal is the chassis.
Most dual supplies have two output terminals per voltage, plus the chassis ground

Hi and thanks a lot for the very helpful reply.
But i do not understand the reason of the GND terminal.
For safety reason i guess the chassis should be already grounded at the mains input.
Usually the devices to be supplied with DC have only two contacts.
There is a dc socket with just + and -, not GND.
Strange.
However i will check with the tester before connecting the device.

The all issue of power supplies is very very interesting to me.
But also very very complicated.
I have declared a personal fight to noise/ripple.
When i see extremely low noise floors i am amazed and admired.
A low noise floor is an evidence of very good design and execution always.
This also after watching an educational video for beginners explaining noise with a simple example.
The noise floor is like a carpet. A carpet can have short fibers (low noise floor) or long fibers (high noise floor).
If a small object falls on the carpet you can spot it easily when the fibers are short, but it is very difficult when the fibers are longer (higher noise floor).
Distortion products instead are other objects different from the one fallen.
They are something not present in the original signal. Very very bad.
So my dream is low noise and low distortion as much as possible.
I will post some pictures this evening of the one i already have at hand.
Thanks a lot again, gino
 
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ginetto61 said:
To be precise the one i already have has only red and black terminals (i guess + and -), but not the yellow one.
Another one i have ordered has indeed the 3 terminals, red/black/yellow.
So we are now talking about two quite different PSUs - this could get confusing!

But i do not understand the reason of the GND terminal.
For safety reason i guess the chassis should be already grounded at the mains input.
Usually the devices to be supplied with DC have only two contacts.
There is a dc socket with just + and -, not GND.
The GND terminal will be connected to ground. It may enable you to ground one side of the DC output.

When there are only - and + outputs the - may be connected to ground internally, or may not, or there may be a switch or internal wire link to make or break this connection.

Why are you buying power supplies if you don't understand how to use them?
 
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So we are now talking about two quite different PSUs - this could get confusing!

Hi ! yes they are two different PSUs. Sorry for the confusion.
Initially i thought that they both had 3 outs. Not the case.
I have still to take some pictures
The one I already have in home is this one:

http://www.distrelec.biz/Web/WebShopImages/portrait_medium/_t/if/labornetzgerate-mascot-719.jpg

the other one I should receive soon is instead this one

http://i.ebayimg.com/images/g/7EAAAOSwyjBW6P3g/s-l1600.jpg

The GND terminal will be connected to ground. It may enable you to ground one side of the DC output

you mean the same GND of the mains in ? the center pin ?
Because the interface have a dc socket with only +/- . There is a two contacts socket on the rear panel. A GND contact is missing.

http://www.cliffinc.com/products/dcconnectors/dc13large.jpg

When there are only - and + outputs the - may be connected to ground internally, or may not, or there may be a switch or internal wire link to make or break this connection.
Why are you buying power supplies if you don't understand how to use them?

well because i want to see if a better power supply has some kind of impact on sound of a unit.
It is a two months project. I have an usb interface that needs 12VDC two contacts, + and -. So i do not know really where to connect the GND cable.
However this interface has some spikes in the noise.
Until now changing ps (all cheap ones) has little to no effect at all. The spikes are always there at the same Hz.
If a lab grade supply will give me the exact same spikes it will mean that the issue is inside the interface. And of course i will have to live with that.
Or change the interface.
I will start trying the Mascot 719 power supply (i do not know if it is still working by the way. It has been left unused for some years).
Hope so.
Thanks a lot again, gino
 
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ginetto61 said:
you mean the same GND of the mains in ? the center pin ?
Because the interface have a dc socket with only +/- . There is a two contacts socket on the rear panel. A GND contact is missing.
Are you saying that there is no mains ground? What do you mean by "dc socket"? AC supply? A GND connection where there is no ground is meaningless. Maybe you should ask the supplier?
 

rayma

Member
2011-04-29 8:37 pm
But i do not understand the reason of the GND terminal. For safety reason
i guess the chassis should be already grounded at the mains input.

Usually in lab supplies the voltage output terminals are all floating wrt ground.
Also there is usually an optional chassis ground socket available,
which is permanently connected to the mains ground.
If you wish to ground one end of the voltage supply, a jumper is connected to it.
 
Are you saying that there is no mains ground? What do you mean by "dc socket"? AC supply? A GND connection where there is no ground is meaningless. Maybe you should ask the supplier?

Hi ! i am attaching a picture of the dc socket, the 12VDC/1.5 A input on the left side of the picture.
It has only two contacts + and - . But this is quite standard for this kind of interfaces, when they are not powered from usb directly.
They usually need 12VDC power supply and usually a wall power adapter comes with them. The idea is to try something more serious.
It could very well be that even a better ps will have no effect.
However i will feed the interface with 12VDC coming from these lab grade power supplies and see for the noise level.
This is the issue.
Kind regards, gino
 

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Usually in lab supplies the voltage output terminals are all floating wrt ground.
Also there is usually an optional chassis ground socket available,
which is permanently connected to the mains ground.
If you wish to ground one end of the voltage supply, a jumper is connected to it.

Hi and thanks a lot for the kind and helpful reply.
To be sure i will check with the tester the voltage between the red and black terminals and use those ones.
I understand that for safety reasons what matters is that the metallic chassis is grounded through mains ground. I will try to check this as well.
Kind regards, gino
 
The ground is a connection to the case / chassis of the power supply. Regardless of whether the input is AC or DC it is often required that the case of all devices including whatever you power with this supply be 'grounded' to the same potential.

This will almost always be the same potential as the mains ground and in most cases but not all the B- will be connected to the same potential.

To power a device use B+ and B- only. If it is a device with a metal, grounded case, use the ground terminal to ground it as long as there is a connection from the power supply ground to mains ground otherwise it's irrelevant.
 
ginetto61 said:
Hi ! i am attaching a picture of the dc socket, the 12VDC/1.5 A input on the left side of the picture.
It has only two contacts + and - . But this is quite standard for this kind of interfaces, when they are not powered from usb directly.
They usually need 12VDC power supply and usually a wall power adapter comes with them. The idea is to try something more serious.
OK. I thought we were talking about the output DC arrangements from two different PSUs. Now we are talking about the input DC arrangements of the load. It is easy to confuse me!

So we have two supplies, which may or may not be grounded or floating, single or dual polarity. We have a load, which may or may not ground one side of its supply.

You do realise that the cables may pick up more noise/interference than the supplies put out? Hence the final stage of filtering, when necessary, has to be at the device end not the PSU end. If a device needs a very clean supply then it should arrange this itself, not rely on something else to do it.