Thevenin’s theorem simplifies the process of solving undetermined values of voltage and current in a network. As you can see in the diagram, there’s a network connected to the left side of terminals A and B and a load to the right side of it. Thevenin’s theorem states that the network can be replaced by a voltage source with a series resistance. To calculate the Thevenin equivalent voltage and series resistance, you can use the following equations.
Imagine you have two or more parallel branches with different voltage sources, it won’t be easy to find the common voltage across them. Millman's theorem provides an easy way to solve the common voltage for this kind of circuit. The common voltage is calculated by converting first the voltage sources to current sources and adding them all. Then the total current source is divided by the sum of the parallel conductances to get the common voltage.
Δ (Delta) and Y (Wye) Network Conversion
When you study circuits that involve three-phase AC power, expect that you will deal delta and wye networks. You might need to convert a delta network to wye or wye to delta in order to solve the unknown values of a circuit. These equations will help you do that.