Basic Electrical Engineering Series
Definition
Thevenins Theorem
"The linear network behind a pair of terminals, may be replaced by a constant voltage generator, with an emf equal to the open-circuit voltage at the terminals and an internal impedance equal to the impedance seen at the actual terminals, with all internal sources removed and replaced by their internal impedances...".
In any electric circuit we may easily calculate the current and voltages at any point using Kirchofs laws and ohms law. However as the complexity of the circuit increases the labour involved becomes tedious. Hence several electrical theorems have been developed in order to reduce the amount of work.
Thevenins is one such theorem and it is applicable to both DC and AC circuit problems. First we look at the representation of linear souces.
Linear Sources
... there are two types - constant-voltage and constant-current sources.
Constant Voltage Scource - can be AC or DC, Z is the internal impedance of the generator
Constant Current Scource - can be AC or DC, Z is the internal impedance of the generator but may be represented as an admittance.
The Theorem
The above complicated circuit is equivalent to the following :
The generator voltage is the terminal voltage 'seen' at AB in the original circuit.
The generator internal impedance is equal to the impedance 'seen' looking into AB in the original circuit with voltage generators short-circuited and current generators open-circuited.
ie in this case Zi = (((Z1+Z2)//Z3)+Z5))//Z4 (// means in parrallel with)