7a. Thevenins equivalent circuit

Question 1

What are the steps to work out the Thevenin equivalent circuit?

Answer 1

1. Identify the network we want to simplify and make nodes A,B (for example) at that point.
2. Find Rth, by setting all sources to zero: Replace power sources with short circuits (just a line to represent the wire) and any current source with an open circuit (a gap in the wire).
3. Find Vth by finding the open-circuit voltage: the voltage across nodes A and B, look for simlifications
4. Solving the problem with the Thevenin equivalent circuit: drawing the final simplified circuit: for a Vth source in series with Rth and the resistor AB

Question 2

Just briefly explain what a Norton’s equivalent circuit is.

Answer 2

It is similar to Thevenin’s except it looks for the ideal current source in parallel with a resistor.

Question 3

Work out this example:

Answer 3

Answers:
* Rth = 10 ohm
* Vth = 0.5 V

Explaination:
* Set V1 to zero and then R2 and R3 become parallel and that is is series with R1, so we can find Rth.
* For Vth its trickier, there is no current being drawn between A and B, thus no current flowing through R1, so voltage across is 0. So we can take the node above R1 as being zero and a equal to B, thus we just use the potential divider equation for voltage across R3.