# Positive and negative potential and symmetrical power supplies

#### Rufinus

Joined Apr 29, 2020
243
Hello. Warning, it is probably a silly question and my english is not the best...

I have read about that but I can´t understand it. In one text about conecting a high voltage device, it says

- Positive polarity, with the cathode earthed and the anode at high positive potential
- Negative polarity, with the anode earthed and the cathode at high negative potential

I supose you get the different potentials with a symmetrical power supply but I never understand that. If you have -10 0 +10, you can have 10 or 20 volts depending the terminals, but I don´t see the difference if you have a 20volts supply whit a 10 volts connection.

And in the example of the text, in both cases there is the same potential difference being the anode at a bigger potential difference. I mean, what is the difference connecting the anothe at +1.000V and the cathode earthed or the anode earthed and the cathode at -100V??

Thank you

Best regards

#### WBahn

Joined Mar 31, 2012
30,243
In some cases you have a preference (or perhaps no choice) as to which terminal is referred/connected to earth ground.

#### MrChips

Joined Oct 2, 2009
30,948
You are referring to two different types of power supplies.
What is the make and model of your power supply?

Here is one common style of PSU.
This is a single output supply. The -ve (BLACK) post is not actually negative. What it means is that it is at a lower potential that the +ve (RED) post. You can make this PSU into a positive or negative power source with respect to GND or any reference point of your choosing. This is known as a "floating" power supply.

This is an example of a bipolar ±15VDC PSU (picture below).
One terminal is 15V above COM and the other terminal is 15V below COM.

#### Rufinus

Joined Apr 29, 2020
243
Thanks for your answers. Yes, the second one is what I mean.

And why I would need a supply like that one? And in case I need it, I could use 2 regular 15VCD 200ma power supplies right? I mean, it would be more expensive, but I could right?

#### MrChips

Joined Oct 2, 2009
30,948
Why would you need a bipolar PSU?
They are frequently used when designing with op-amps, power amplifiers, function generators, and thousands of other circuits.

#### Rufinus

Joined Apr 29, 2020
243
Ok, but I could use 2 similar power supply instead a symmetrical power supply?

#### MrChips

Joined Oct 2, 2009
30,948
There are valid situations when symmetrical supplies are required.
These are known as tracking supplies. The positive and negative rails are always symmetrical.

#### MisterBill2

Joined Jan 23, 2018
18,986
There is a time and a reason for both types of supplies, And they are different. Also, in many instances it works very well to use two separate supplies, one providing a negative voltage relative to the common, and the other providing a similar positive voltage relative to that same common connection. Usually the case for a tracking regulation system is when the two polarities must remain similar values with the common exactly midway between .
When that is needed it should be obvious.