# DC power supply terminals

1. Sep 26, 2008

### fisico30

hello ,

DC (I guess AC too) power supplies have 3 terminals: plus, ground, minus.
If I set the voltage to 30 Volts, and plug one cable in the + terminal and the other in the - terminal, is the voltage across the wires 30 volts or 60 volts? When do we use the ground or the negative?

I tried with one powersupply and it gave me 60....

thanks!

2. Sep 26, 2008

### chroot

Staff Emeritus
The ground is provided for a voltage reference -- it's the same as the earth ground in the wall socket the power supply is plugged into. This is useful when you need to connect several pieces of equipment together and want them to share the same ground potential.

A typical application is to connect the - and ground terminals together, forcing the negative terminal to ground. The + terminal is then at 30V with respect to earth ground.

Alternatively, you could connect the + and ground terminals together, and the - terminal would be at -30V with respect to earth ground.

If you don't care at all what absolute voltages exist in your circuit with respect to ground (say, your circuit is battery powered), there may be no reason to use the ground terminal at all.

- Warren

3. Sep 26, 2008

### fisico30

Thank you Warren. You are very clear.
Let me make sure I get it however:

Case 1: if the power supply is set to 10V and I connected to a resistor (2 ohms) using the + and the - terminals, the voltage across the resistor should be 10V.

Case 2: if I used the + and the GRD, then the voltage on the resistor is still 10V.

Case 3: if I connect the - and the GRD together and use it as a single terminal, then the voltage across the resistor is 10V of course..

Case4: IF I had 2 resistors: one resistor connected to + and - and the other to - and GRD, the voltage across the two resistors ( between the + and -), would be 20V

Do i get it or am i still confused?
thanks!

4. Sep 28, 2008

### Phrak

You're lookin at a lab supply. Did you find an old Lambda? The guys in the
Engineering|Electronics section would have told you. It's a dual adjustable DC power supply. The center tap you're calling ground is common. It's not necessarily Earth ground. The leg measured from (+) to common gives you the voltage read on the built-in meter if there is one. If it's a dual tracking supply, if you short one side, say, the other side will follow.

The idea behind such supplies is to give you the positive and negative voltages useful for biasing operational amplifiers and other stuff like them.

Last edited: Sep 28, 2008