Maximum Power Transfer Theorem
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Transcript of Maximum Power Transfer Theorem
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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).