How to use the Thevenin Equivalent Calculator
- 1. Choose the circuitDivider, two sources, Wheatstone bridge, current source, or measured Voc and Isc.
- 2. Enter the valuesSI prefixes work: 4.7k, 10m, 2m.
- 3. Get Vth and RthSee every step, a check with Norton, and the load results.
How to find a Thevenin equivalent
Any linear two-terminal network can be replaced by one voltage source Vth in series with one resistor Rth. Vth is the open-circuit voltage across a-b. Rth is the resistance seen from a-b with every independent voltage source shorted and every current source opened. If the circuit has dependent sources, find Rth = Voc/Isc instead.
Worked example: Vs = 12 V, R1 = 4 kΩ, R2 = 6 kΩ, R3 = 1.6 kΩ in series to terminal a. With a-b open no current flows in R3, so Vth = 12 × 6/(4 + 6) = 7.2 V. Shorting Vs puts R1 and R2 in parallel: 4 ∥ 6 = 2.4 kΩ, so Rth = 1.6 + 2.4 = 4 kΩ. A 2 kΩ load then draws 7.2/6 = 1.2 mA.
Exam tips for Thevenin questions
- Remove the load first and mark terminals a and b clearly.
- When finding Rth, never remove a source's internal resistance, only the ideal source.
- Check your answer: Vth/Rth must equal the short-circuit current (the Norton current).
- For a Wheatstone bridge, Vth = Va − Vb, and the bridge is balanced when R1·R4 = R2·R3.
- Maximum power goes to the load when RL = Rth, and Pmax = Vth²/(4Rth).
The tool shows these checks in the working, so you can compare each line with your answer sheet.
Frequently asked questions
Which circuits does the calculator handle?
A source with an R1-R2 divider and series R3, two voltage sources with a shunt resistor (Millman), a Wheatstone bridge, a current source with resistors, and measured Voc and Isc. It also converts a Norton equivalent to Thevenin.
How do I find Rth with dependent sources?
Find the open-circuit voltage and the short-circuit current, then choose 'From measured Voc and Isc'. Rth = Voc/Isc works even when dependent sources are present.
Can Rth be zero?
Only for an ideal voltage source connected straight across a-b. In that case a Norton equivalent does not exist.
Is this tool free?
Yes. You fill in a short form once to see your result, and the other Uncodemy tools then open without it. All the maths runs in your browser.