Source impedance is the internal resistance and reactance inherent to any real-world power supply, battery, or signal source that causes its output voltage to drop when current is drawn. This single concept is one of the most critical electronics fundamentals because it changes the actual voltage delivered to your load from the theoretical open-circuit value, dictating whether your components receive enough headroom to operate. Beginners commonly confuse source impedance with load resistance, assuming the power supply is a rigid, infinite voltage source and blaming the load or the code when a circuit fails, when the true culprit is the hidden resistance inside the source or its delivery path.
The Core Concept: Ideal vs. Real Voltage Sources
In textbook circuit theory, we often treat voltage sources as ideal. An ideal 5V source outputs exactly 5.00V whether it is supplying 1 milliamp or 1,000 amps. But on the workbench, ideal components do not exist. Every real-world power source—whether it is a CR2032 coin cell, a benchtop power supply, or a USB port—behaves like an ideal voltage source in series with a small resistor. This is known as the Thévenin equivalent circuit.
Think of source impedance like a kink in a garden hose right at the spigot; the water pressure (voltage) at the spigot is fine, but the kink (impedance) restricts flow (current), causing the pressure at the nozzle (load) to collapse when you open the valve. When your circuit demands a sudden burst of current, that current must push through the source impedance. According to Ohm’s Law (V = I × R), this creates a voltage drop across the internal resistance, leaving less voltage for your actual load.
Worked Numeric Example: Calculating the Sag
Let us look at a common hobbyist scenario: powering a high-draw LED strip from a standard USB power bank.
- Source: 5V USB Power Bank (Open-circuit voltage = 5.10V)
- Source Impedance: The battery's internal ESR plus the USB boost converter's output impedance totals 0.25 ohms.
- Load: A short addressable LED strip drawing 2.0 amps when all pixels are set to full white.
When the LEDs turn on, the 2.0A current flows through the 0.25-ohm source impedance. We calculate the voltage drop:
V_drop = I × R = 2.0A × 0.25Ω = 0.50V
The voltage that actually reaches the LED strip is:
V_load = V_source - V_drop = 5.10V - 0.50V = 4.60V
While 4.60V might keep the LEDs lit, it is well below the 5V nominal requirement. If the data line from your microcontroller is still swinging at a full 5.0V, you now have a signal voltage that is higher than the LED strip's power rail, which can cause data corruption, flickering, or even back-feeding current into the microcontroller's GPIO pins.
Where You Meet This in Practice
Source impedance is not just a theoretical nuisance; it is the defining characteristic that separates different types of power sources. Here is how typical sources compare on the bench:
| Power Source | Typical Open-Circuit Voltage | Typical Source Impedance (ESR + Path) | Max Continuous Current Before Severe Sag |
|---|---|---|---|
| AA Alkaline Battery | 1.5V | 0.15 - 0.30 Ω | ~500 mA |
| 18650 Li-ion (e.g., Samsung 30Q) | 4.2V | 0.015 - 0.025 Ω | ~15 A |
| Standard USB 2.0 PC Port | 5.0V | 0.40 - 1.00 Ω | 500 mA |
| Linear Bench Supply | 12.0V | < 0.05 Ω | 5 A+ |
Battery ESR and Age: As lithium cells age, their internal equivalent series resistance (ESR) increases. A brand-new 18650 cell might have an ESR of 20 milliohms, but after 300 charge cycles, it can climb to 80 milliohms. The open-circuit voltage still reads 4.2V on your multimeter, but under a 10A load, the aged cell will sag by 0.8V instead of 0.2V, triggering your BMS low-voltage cutoff prematurely.
Wire as Source Impedance: In 12V DC systems, the wire itself becomes a massive part of your source impedance. Running 10 amps through 10 feet of 18 AWG wire (which has a resistance of about 0.064 ohms per 10 feet for the round trip) adds 0.64 ohms of source impedance to your circuit, dropping nearly 6.5V before it even reaches the load. This is why automotive and solar installations demand thick, short wire runs.
Real-World Scenario Walkthrough: The ESP32 Brownout
To see how ignoring this electronics fundamental causes real headaches, let us walk through a classic embedded systems failure.
The Setup: You are prototyping an IoT sensor using an ESP32 DevKit V1. You power it using a generic, unbranded 5V/1A USB wall adapter connected via a 1.5-meter, thin 28 AWG USB cable. You write the code, plug it in, and the ESP32 boots perfectly. Then, you add the code to connect to WiFi and transmit data to an MQTT broker.
The Numbers: When the ESP32's WiFi radio transmits, it draws a sudden current spike of roughly 500mA. The cheap wall adapter has a high internal source impedance of about 1.6 ohms due to poor regulation and undersized filter capacitors. The long, thin USB cable adds another 0.8 ohms of resistance. Your total source impedance from the wall to the DevKit's Vin pin is 2.4 ohms.
The Outcome: During the WiFi TX spike, the voltage drop is V = 0.5A × 2.4Ω = 1.2V. The 5V rail at the ESP32's Vin pin sags to 3.8V. The DevKit's onboard AMS1117-3.3 linear voltage regulator requires a dropout voltage of about 0.9V at this current level to maintain a stable 3.3V output. With only 3.8V coming in, the regulator output sags to 2.9V. The ESP32's CPU and RF circuits become unstable at 2.9V, throwing a guru meditation error and resetting. The board boots, tries to connect to WiFi, sags, and resets again in an infinite boot loop.
The Fix: Swap the 28 AWG cable for a short, 20 AWG USB cable (reducing cable impedance to ~0.1 ohms) and use a high-quality 5V/2A name-brand switching power supply with a source impedance under 0.1 ohms. Alternatively, solder a 470µF low-ESR electrolytic capacitor directly across the 5V and GND pins on the DevKit to supply the transient current spikes locally, bypassing the high-impedance cable.
Common Confusions and Troubleshooting
Is lower source impedance always better?
For power delivery, absolutely. You want your power supply's source impedance to be as close to zero as possible so voltage remains stable regardless of load. However, in RF engineering and high-speed signal transmission (like audio lines or Ethernet), impedance matching is required. A 50-ohm coaxial cable expects a 50-ohm source and a 50-ohm load to prevent signal reflections. In those specific signal domains, a near-zero source impedance would actually cause data corruption.
How do I measure the source impedance of my power supply?
You cannot measure it directly with an ohmmeter while the supply is live. Instead, use this practical load-test method:
- Measure the open-circuit voltage of the power supply with your multimeter (e.g., 12.00V).
- Connect a known resistive load that draws a significant but safe amount of current (e.g., a power resistor that draws 1.0A).
- Measure the voltage again while the load is connected (e.g., it drops to 11.80V).
- Calculate the difference in voltage (12.00V - 11.80V = 0.20V).
- Divide the voltage drop by the current drawn (0.20V / 1.0A = 0.20 ohms). That is your source impedance.
Why does my multimeter show 5V, but my oscilloscope shows 4.2V?
Digital multimeters (DMMs) typically average voltage over a period of a few hundred milliseconds. If your microcontroller draws a 500mA spike for only 5 milliseconds during a WiFi transmission, the DMM will average that brief sag with the long periods of low idle current, displaying a deceptively healthy 4.9V. An oscilloscope, set to a fast timebase, will capture the transient millisecond-level voltage droop, revealing the true dynamic source impedance of your circuit. For debugging power sags, always trust the scope over the DMM.
Mastering source impedance shifts your mindset from hoping a circuit works to knowing it will work. By treating every power source as a Thévenin equivalent and calculating the voltage drop before you solder a single header, you eliminate the most common class of 'ghost in the machine' bugs in hobbyist electronics.
References:
1. All About Circuits: Thévenin Equivalent Circuits
2. HyperPhysics: Thévenin's Theorem
3. Espressif ESP32 Hardware Design Guidelines
