Learn Thevenin Equivalent Through Hands-On Experiments
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Time to read 6 min
Intro
If you have ever felt that Thevenin’s Theorem is just another formula to memorize, this experiment will change your mind. We will start with a quick recap of the basic calculation method for finding the Thevenin equivalent of a circuit. Then, instead of stopping at the theory, we will go through a complete hands-on experiment to measure Vth and Rth directly, and verify that the simplified model truly behaves the same as the original network.
Quick Recap: Thevenin Equivalent Theory
What Thevenin’s Theorem Means
Thevenin’s Theorem states that any linear circuit with multiple sources and resistors, as viewed from two terminals, can be replaced by a single voltage source Vth in series with a single resistance Rth. This greatly simplifies analysis by reducing a complex network to just two parameters, making it easier to predict circuit behavior under different load conditions.
Steps to obtain a Thevenin equivalent
Regarding the detailed theory of Thevenin’s Theorem, there are already many great resources that explain the full derivations step by step. Essentially they can be summarized into these general procedures while determing the Thevenin Equivalent circuit of a linear circuits.
Identify the two terminals
This is where your observation point sets, or essentially where the load conncts.Open‑circuit the load and find Vth.
Remove the load. Compute the open‑circuit voltage across the two terminals using any method you like, such as node‑voltage, mesh, or source transformation. The result is Vth.Turn off sources and find Rth
Replace each independent voltage source with a short. Replace each independent current source with an open. Look into the two terminals and compute the equivalent resistance. That is Rth.Use a test source if there are dependent sources
Keep all dependent sources active. Apply a small test voltage Vtest across the two terminals and solve for the resulting current Itest. Then Rth=Vtest/Itest. You can also inject a test current and measure voltage.Assemble the model and reconnect the load
Model is a series pair (Vth,Rth), feeding the load RL. Predict quickly:
Build and Test a Thevenin Circuit on Breadboard
Now that we have reviewed the basic theory, it’s time to put it into practice. We will use a simple circuit as example to calculate the Thevenin Equivalent circuit on paper, and then verify results with real experimental setup.
Theoretical calculations of the Thevenin Equvailent Circuit
Equipment you will need:
Here I'm grabbing equipment and some components from Lab-On-The-Go kit, or in case you don't have them, prepare the following items:
An electronics breadboard
A DC power supply (ideally with isotated ground)
A multimeter cable to measure voltage and resistance
Some resistors (200kΩ and 100kΩ)
Some jumper wires
Theoretically computed Thevenin Circuit
To give a heads-up of the theoretical results, i quickly run an LT SPICE simulation.
- The Thevenin Voltage (Vth) is calculated from the open-circuit voltage at the observation port from terminals a-b, which is Vth = 1.3V as highlighted;
- The Thevenin Resistance (Rth) can be calculated by first shorting the source and determing the equivalent resistance, which is Rth = 120kΩ, obtained by [((200kΩ || 200kΩ) + 200kΩ) || 100kΩ)].
Verify the circuit on breadboard
Part 1 of the Experiment
After calculating Vth and Rth on paper, the next step is to confirm that the real circuit behaves the same way. The breadboard build and measurements provide a direct check.
- Step 1 – Measure Vth
With the load resistor disconnected between terminals a and b, the multimeter was set to DC voltage mode. The measured open-circuit voltage was 1.310V, matching our theoretical Vth prediction within expected tolerance (using 5% resistors). - Step 2 – Measure Rth
The power supply was turned off and DISCONNECTED from the circuit. The voltage source was replaced with a short, and the multimeter set to resistance mode. The reading at terminals a and b was 120.1 kΩ , which is our measured Rth.
📌 If the source is only switched off but still connected to the circuit, the measurement may be incorrect. Internal elements of the source, such as residual capacitance or resistance, can affect the multimeter reading when determining Rth. In this setup, the meter gives a wrong measurement when power supply is not removed.
Part 2 of the Experiment
Although placing the observation port at the outermost edge of the schematic is the most common practice (since it usually represents where the load is connected and makes analysis and model replacement straightforward), from a learning perspective, the observation port can also be placed elsewhere in the circuit. For example, here i have moved the terminal to a different placement, and consequently the measured Vth changed to 3.934V as indicated on the meter.
Applications of Thevenin Equivalent Circuits
Thevenin’s Theorem is not just a neat theoretical trick—it is a practical tool that shows up across electronics, from load matching to small-signal analysis. Below are three common scenarios where it proves especially useful.
Load Analysis and Power Transfer
One of the most straightforward uses of a Thevenin model is to calculate the voltage, current, and power delivered to a load. This ties directly into the Maximum Power Transfer Theorem , which states that a load receives maximum power when its resistance equals the Thevenin resistance of the source network (RL=Rth). Under this condition, the maximum power is:
This principle is key in applications like audio amplifiers, communication systems, and energy harvesting circuits, where optimizing the load for maximum energy delivery is important.
This experiment is implemented in our Basic Electric Circuits learning kit.
BJT DC Bias and Small-Signal Impedance Analysis
In transistor amplifier design, Thevenin equivalents are often used to simplify bias networks. A typical example is a BJT DC bias circuit, where the resistor network connected to the base can be replaced with a Thevenin voltage Vth and resistance Rth Rth. This makes it easier to:
Determine the base current in DC bias calculations.
Analyze the small-signal input impedance seen at the transistor’s base.
By modeling the surrounding network with a Thevenin equivalent, the small-signal model of the transistor becomes much cleaner, making gain and impedance analysis more straightforward.
This experiment is implemented in our Fundamental Analog Circuits and Semiconductors kit
Final Notes
Whether you are designing for maximum power transfer, simplifying bias networks, or performing small-signal impedance analysis, Thevenin’s Theorem gives you a way to reduce complexity without losing accuracy. By condensing a network into a single voltage source and series resistance, it helps you focus on the part of the circuit that really matters for the problem at hand.
Key Takeaways from This Article
Reviewed Thevenin’s Theorem calculation steps: find Vth V_{th}Vth by open-circuit voltage, find Rth R_{th}Rth by turning off sources and measuring equivalent resistance.
Verified that measured values match calculated results within tolerance, confirming the model’s validity.
Demonstrated additional observation port choice to study different parts of a circuit.
Introduced typical applications: load analysis with maximum power transfer, BJT bias network simplification, and small-signal impedance analysis.
Some FAQs on Op Amp Basics
What is the basic idea of Thevenin’s Theorem?
It states that any linear circuit can be represented by a single voltage source Vth in series with a resistance Rth when viewed from two terminals.
Do I need to completely remove the source when measuring Rth ?
Yes. Simply turning it off can leave internal elements like capacitance or resistance in the path, which will affect the reading.
Why did we measure Vth with the load removed?
Because Vth is defined as the open-circuit voltage across the observation terminals, meaning no load is connected.
Where is Thevenin’s Theorem used in real circuits?
Common uses include calculating load voltage/current, maximum power transfer, and simplifying transistor bias networks for small-signal analysis.
Daniel Cao
Daniel Cao is the founder of EIM Technology, where he creates hands-on, beginner-friendly electronics education kits that blend practical hardware with clear, structured learning. With a background in engineering and a passion for teaching, he focuses on making complex concepts accessible to learners from all disciplines.
