Lesson 29

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Lesson 29 Transformers and Reflected Impedance
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Learning Objectives

• Predict the reflected impedance and derive an
  equivalent circuit using the reflected impedance.
  
• Calculate the transformation ratio to deliver
  maximum power to a load.
• Determine safe operation parameters from power
  transformer ratings.
• Explain how the transformer acts as an isolation
  device.
• List and explain several practical applications
  of transformers.

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Winding direction
REVIEW

• The polarity of ac voltages can easily be changed
  by changing the direction of the windings.

0º phase shift
180º phase shift
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Transformation ratio
REVIEW

• The transformation ratio (or turns ratio) is
  given by the symbol a.
• Because we are considering an ideal transformer,
  power in equals power out.

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Reflected Impedance

• Transformers make load impedances look bigger
  or smaller depending on their turns ratio a.
• Zpri-reflected is the impedance that appears at
  the primary windings.
• The term a2Zsec is referred to as the loads
  reflected impedance.

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Reflected Impedance
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Example Problem 1

• Use the reflected impedance concept to solve for
  the total impedance ZT. Determine Ig and ILD.

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Power Supply Transformers

• Power supply transformers are used to convert
  incoming 120 Vac to dc voltages of various levels
  necessary for different circuits.

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Impedance Matching

• Transformers are used to raise or lower the
  apparent impedance in order to achieve maximum
  power transfer.

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Example Problem 2

• An amplifier is modeled with a Thèvenin
  equivalent impedance of 36-54j ?.
• What load impedance must be chosen to ensure
  maximum power transfer occurs?
• What is the transformer load (VA) for this value
  of ZLD?

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Example Problem 3

• An amplifier is modeled with a Thèvenin
  equivalent impedance of 320-160j ?. A
  transformer is used to impedance match the load
  to the source to ensure max power transfer.
• What turns ratio should be chosen for the
  transformer, and what value of load resistance
  should be chosen to ensure max power transfer?
• What is the real and reactive power of the load?
  What is the real and reactive power delivered by
  the source?

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Impedance Matching
To ensure strongest signal, transformers are used
to impedance match each amplifier stage on this
AM radio