# Voltage and current sources?

#### Haripriyan

Joined Oct 29, 2007
1
In theorems like thevenin, it is said that a Voltage source can be replaced by a short circuit and a urrent source is replaced by an open circuit.
Why is this so?....
I feel that i know that partly....but couldnot explain that...
So could any one of you can help me by giving the reason???

#### mozikluv

Joined Jan 22, 2004
1,437

#### mozikluv

Joined Jan 22, 2004
1,437

#### bloguetronica

Joined Apr 27, 2007
1,372
In theorems like thevenin, it is said that a Voltage source can be replaced by a short circuit and a urrent source is replaced by an open circuit.
Why is this so?....
I feel that i know that partly....but couldnot explain that...
So could any one of you can help me by giving the reason???
Because an ideal voltage source has no thevelin resistance (associated series resistance), while an ideal current source has infinite thevelin resistance. The last part is true, because an aproximation to a current source would be an almost infinite voltage ideal voltage source in series with an almost infinite resistance. Such resistance would effectively limit the current by itself. It is so big, that the load has almost no effect.

#### chesart1

Joined Jan 23, 2006
269
If a power supply is off, the voltage across the output terminals is zero. The theory is that if the voltage across two terminals is zero, than no resistance exists between those two terminals. Hence the power supply would look like a short circuit when it's output is zero volts.

If a current supply is off, than the current across the output terminals is zero. The theory is that any two terminals that conduct zero current must have an infinite resistance. Hence, the current source would look like an open circuit when it's output is zero amps.