Tutorial 9  Power Transfer and Load Matching 

Learning Objective 
To understand the concept of power matching. To apply the ideas to transformer matching. 

Key Question 
What are the ideas behind power matching? 

Power Transfer and MatchingIn Tutorial 7 we looked at a power supply with an internal resistance. In Tutorial 8 we saw that Thévenin’s Theorem told us that any complicated power supply can be represented as:
a simple perfect battery in series with an internal resistor, r.
We know that the graph of the voltage across the resistor, R against a current is like this:
If we plot the graph of power against resistance, we get:
From this graph, we see that:
We can measure the efficiency of the power transfer using the equation:




The graph of efficiency against resistance looks like this:
We see the following from this graph:
If we want maximum power transfer, we need to match the load so that it is equal to the internal resistance of the source. This is the case regardless of whether we have a simple source or a complex source of many EMFs and resistances. We use Thévenin’s Theorem to simplify the source into a simple battery in series with an internal resistance.




