Electrical energy is fundamental to modern technology, powering everything from small electronic devices to massive industrial systems. The components that deliver this energy, called electrical sources, are the starting point for any circuit analysis. Electrical engineers rely on theoretical models to simplify the complex behavior of these sources and predict how a circuit will perform. The concept of a voltage source represents a device designed to create and maintain a fixed potential difference, or voltage, between two points in a circuit. Using these simplified abstractions, engineers develop circuit designs that are both predictable and reliable.
Defining the Ideal Voltage Source
An ideal voltage source is a theoretical two-terminal component that serves as the perfect model for electrical power delivery. Its defining characteristic is that the voltage across its terminals remains constant, regardless of the amount of current flowing through it. The source can theoretically supply any amount of current demanded by the connected circuit without its terminal voltage diminishing.
This independence is based on the assumption that the source has zero internal resistance (Rs = 0). In any real device, internal resistance would cause a voltage drop within the source itself, but the ideal model eliminates this loss. Circuit analysis uses specific symbols to represent this concept, typically a circle with positive and negative polarity signs, or sometimes a symbol resembling a battery. This zero-resistance property ensures that the voltage available to the external circuit is always equal to the source’s specified voltage.
Performance Under Varying Load
The performance of an ideal voltage source is consistent across all operating conditions because its terminal voltage is independent of the load current. When connected to a high-resistance load, the source delivers a small current while maintaining its fixed voltage. Conversely, if a very low-resistance load is connected, the source will deliver a massive current, yet the voltage across its terminals will not change.
This relationship is demonstrated by the current-voltage (I-V) characteristic curve, which is a graphical representation of the source’s behavior. For an ideal voltage source, this curve is a straight, horizontal line. This horizontal line illustrates that the voltage output is fixed, regardless of the current drawn from the source. A non-ideal source, in contrast, would show a downward sloping line, indicating that the terminal voltage decreases as the current increases.
Modeling Real-World Power Supplies
While the ideal voltage source is a powerful tool for theoretical analysis, it does not exist in physical reality. Real-world power sources, such as batteries, generators, and wall adapters, are non-ideal because they possess internal resistance (Rs is greater than zero). This internal resistance causes a portion of the source’s generated voltage to be lost internally as current is drawn, manifesting as a decrease in the terminal voltage.
Engineers account for this real-world limitation by modeling practical sources using the ideal voltage source concept. This is most commonly done through the Thévenin equivalent circuit, which represents a real voltage source as an ideal source connected in series with a single internal resistor. As the external circuit draws current, a voltage drop occurs across this series resistance, calculated using Ohm’s Law (V = I x R). The terminal voltage of the real device is calculated by subtracting the voltage drop across the internal resistance from the ideal source voltage. Analyzing this internal resistance helps engineers determine the maximum current a device can safely supply and ensures that the power supply can maintain a stable voltage under expected load conditions.