# Voltage Source with output of OV

1. Apr 26, 2004

### flexifirm

My friend asked me a question that totally shocked me for a second.. here was the question...

"Is a voltage source turned off the same as a voltage source outputting 0V or near 0V?"

Then he asked me if we connected the two terminals of a voltage source to two points on an already ENERGIZED dc network... and the voltage output of the voltage source matches the exact same voltage that was in the network between the two points.. what is the effect? What is the effect when we change the voltage source (variable) to an output of 0.000001V and plug it between those two points? WHat is the effect of turning off the voltage source and plugging it between those two points?

My take on it is.... a voltage source turned on but outputting 0V should be considered a short, but turned off should be considered an open (I'm thinking back to a simple power supply with a bridge rectifier a cap and a zener.... u have the bridge to provide the open - yet in one direction current can go in one terminal of the power supply... through a set of diodes and come out the other end of the supply).

Also, when using the superposition theorem.. we TURN OFF the sources... and we replace an ideal voltage source with a short.........

Any thoughts on what my inquisitive friend was blabbering about?

Last edited: Apr 27, 2004
2. Apr 27, 2004

### chroot

Staff Emeritus
A voltage source does not "output" a voltage -- it maintains a voltage between its terminals by sourcing current, if necessary. An "ideal" voltage source set for 0V would indeed act as a short. A real voltage source, however, is quite complicated inside, with transistors and capacitors and other internal components; turning it off probably won't make it into a short.

If you connect a 5V source to two wires already having a potential difference of 5V, nothing "happens," in the sense that no current flows.

- Warren

3. Apr 29, 2004

### dlgoff

If your voltage source is powered down, then the output might not be shorted.

Regards