Transformers and
Impedance
The Inductor (Review)
• Coil of wire.
– Stores energy in a
magnetic field
• Value of inductance
depends on:
–
–
–
–
Number of turns.
Spacing of turns.
Size of the wire.
Material of the core.
The Transformer
•
Two inductors
–
Parallel and close
together.
– Iron core facilitates
transfer.
•
Two basic functions
1. Voltage transformation.
2. Impedance
transformation.
How a Transformer Works
Transforming Voltage
• The ratio of the number
of turns in the primary
and secondary
determine the voltage
change.
– Step up (increase)
– Step down (decrease)
Impedance Defined
• Resistance is the opposition to the flow of current.
– Capacitors and inductors react differently in opposing the
flow of ac current than they do in opposing the flow of dc
current.
– Resistance to the flow of ac is called reactance.
– Resistors react the same for ac or dc.
• Impedance is the opposition to the flow of ac current.
– Includes the both components of opposition: resistance and
reactance
– Measured in Ohms
– Symbolized by the letter Z
Importance of Impedance
• If there is an impedance mismatch between the
source and the load, there will not be
maximum transfer of power or energy.
– i.e., speakers (generally 8-ohms impedance)
operate most efficiently if connected to an 8-ohm
impedance amplifier.
• Transformers serve to transform or match the
impedance between the source and the load.
Transforming Impedance
• The ratio of the number
of turns in the primary
and secondary
determine the
impedance change
– Step up (increase)
– Step down (decrease)
Your Turn
• 2250 turns in the primary, 500 turns in the
secondary, 120 volts ac applied to primary.
• What is the output voltage?
• 600 ohms output impedance of an amplifier, 4
ohm speakers.
• What is the turns ratio of the impedance
matching transformer required?