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Objective of Lecture Explain the maximum power transfer theorem.

Chapter 4.8 Fundamentals of Electric Circuits

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Maximum Power Transfer For any power source, the maximum power transferred

from the power source to the load is when theresistance of the load R 

L

is equal to the equivalent orinput resistance of the power source (R in = R Th or R N).

The process used to make R L = R in is called impedancematching.

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Power Transfer Calculation

R Th 50W

 V Th

1V 

 L

Th

Th L

 L

 L

 L L L

 R

V  R R

 R

 P 

 RV  P 

2

2

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Application When developing new circuits for a known

application, optimize the power transfer by designingthe circuit to have an input resistance close to the loadresistance.

 When selecting a source to power a circuit, one of theselection criteria is to match the input impedance to

the load resistance.

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Example Seen in ECE Labs Function generator on the Velleman oscilloscope that is used in a

number of ECE lab classes. Measurements of V, the voltage acrossthe load resistor R L, can be well below the voltage specified on the

function generator. R in = R Th = 40W

This is observed as R L approaches or

smaller than R Th.

http://www.vellemanusa.com 

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Other Examples Selection of wire – 

 Antenna wire is designed to match a 75 W load. Coaxial cable is designed to act like a 50W load.

Most electronic and electrical equipment are designed tohave an R Th of 50 W. The speakers used in some of the ECE labs are designed to act

like an an 8 W load while microphones are designed to act like a600 W load. It is important that the resistance of the wire that runs to the speaker is

 very low or the power will be dissipated in the wire rather than used tocreate sound.

For best audio quality, the input impedance of the circuit connected tothe speaker (or the microphone) should be equal to the resistance of thespeaker (or the microphone).

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Summary Maximum power transfer theorem is used frequently 

to insure that the greatest power can be transferredfrom a power source to a load.