# Negative resistance

Discussion in 'Homework Help' started by igor_quintal, Apr 26, 2014.

1. ### igor_quintal Thread Starter New Member

Apr 26, 2014
1
0
Hello,
I'm having some doubts here about the Thévenin equivalent. For an example, I had an exercise in wich I needed to calculate the Thévenin equivalent. The problem is, the Norton current is positive, but the Thévenin voltage is negative, and the resistance negative too. What does it mean having a negative Thévenin voltage and resistance ?? How can I represent and understand whats going on with the circuit ??

Thanks !!

ps: sorry for my bad english.

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,971
616
The easiest way to find Thevenin resistance is to replace the independent voltage source with a short and independent current source with open, this creates a "new" circuit, then find the equivalent resistance of this new circuit, this equivalent resistance is the Thevenin resistance. Since the "new" circuit only has resistors, the result can only be positive.

It seems to me you are doing it wrong.

Post a sample circuit. One voltage source and 3 resistors. We can walk you through it.

3. ### WBahn Moderator

Mar 31, 2012
20,230
5,755
The polarity of the Thevenin voltage (or the Norton current) is arbitrary. The equivalent resistance can be negative for active (such as opamp circuits or other circuits with dependent sources). However, when that is the case then the equivalent resistance should be negative for both the Thevenin and Norton circuits.

Please post your circuit and your work. It is impossible for us to tell what you have done right and what you have done wrong without it since we are not mind readers.