For an ideal transformer:
[
\frac{Z_p}{Z_s}=\left(\frac{N_p}{N_s}\right)^2
]
Where:
- (Z_p) = impedance seen at the primary
- (Z_s) = impedance connected at the secondary
- (N_p), (N_s) = primary and secondary turns
So, the load impedance reflected to the primary is:
[
Z_{\text{in}}=\left(\frac{N_p}{N_s}\right)^2 Z_L
]
For impedance matching a source resistance (R_S) to a load (R_L):
[
\frac{N_p}{N_s}=\sqrt{\frac{R_S}{R_L}}
]
Example: Match an (8,\Omega) speaker to an amplifier requiring a (32,\Omega) load:
[
\frac{N_p}{N_s}=\sqrt{\frac{32}{8}}=2
]
Use a transformer with turns ratio
primary : secondary = 2 : 1. The 8-ohm speaker then appears as 32 ohms at the primary. This follows the turns-ratio-squared relation described in this
transformer reference.For an
ideal transformer in Electrical Network Theory (ENT):
1. Impedance matching
Transformer is used for impedance matching using:
[
\frac{Z_p}{Z_s}=\left(\frac{N_p}{N_s}\right)^2
]
For maximum power transfer:
[
Z_{\text{input seen at primary}}=Z_{\text{source}}
]
Hence,
[
\frac{N_p}{N_s}=\sqrt{\frac{Z_{\text{source}}}{Z_L}}
]
2. Phase difference
- The induced primary and secondary emfs are in phase or 180° out of phase depending on the dot convention and chosen voltage polarities.
- With both voltages measured from dotted terminal to undotted terminal, (V_p) and (V_s) are in phase.
- Primary and secondary currents are in opposite directions with reference to the dot markings, so they are considered 180° phase-opposed for an ideal transformer.
So: impedance changes by turns ratio squared; phase relation is determined by dot convention.