Thévenin Equivalent

Formula

Description

Thévenin's theorem states that any linear two-terminal circuit can be replaced by an equivalent voltage source (Voc) in series with a resistance (Rth). The Thévenin voltage equals the open-circuit voltage measured at the terminals, and the Thévenin resistance equals the open-circuit voltage divided by the short-circuit current. This simplification is invaluable for analyzing complex circuits when you only care about the behavior at a specific pair of terminals, such as determining maximum power transfer or load regulation.

Variables

• Rth — Thévenin equivalent resistance (Ω)
• Voc — Open-circuit voltage at the terminals (V)
• Isc — Short-circuit current at the terminals (A)

Practical Notes

Maximum power transfer occurs when the load resistance equals Rth, delivering Voc²/(4×Rth) watts to the load. In practice, power supplies are designed with very low Rth for good voltage regulation, while signal sources may intentionally have 50Ω or 75Ω Thévenin impedance for impedance matching.