Voltage source

A voltage source is a fundamental two-terminal device used in electronics. Its main job is to maintain a fixed voltage across its terminals.

This concept is essential for understanding how electricity powers our world. Most real-world energy sources, like batteries and generators, are modeled as voltage sources. Engineers use these models to analyze how electrical energy moves through different circuits.

To make mathematical analysis easier, scientists use a concept called an ideal voltage source. An ideal voltage source is a mathematical abstraction. It maintains a perfectly fixed voltage regardless of the load resistance or the output current.

This means the voltage stays the same even if the circuit changes. In this theoretical model, the source has zero internal resistance. Because of this, an ideal source could theoretically supply unlimited power. If a circuit has an open circuit, the current is zero. However, if the load resistance reaches zero, which is a short circuit, the current approaches infinity.

Voltage sources are categorized into two main types based on how they behave. The first type is the independent voltage source. An independent source maintains its voltage regardless of any other variables in the circuit. The second type is the dependent, or controlled, voltage source.

In a dependent source, the voltage is determined by another voltage or current elsewhere in the circuit. For instance, a mathematical model of an amplifier uses dependent sources. The magnitude of the voltage in these sources is governed by a fixed relation to an input signal.

Real-world voltage sources differ from ideal ones because they are not perfect. Every real source has a non-zero effective internal resistance. This resistance prevents the source from supplying unlimited current. Most real-world sources have a very low internal resistance, often much less than 1 ohm. To analyze these real devices, scientists use a model called a Thévenin equivalent circuit. This model combines an ideal voltage source with additional impedance elements in series.

This allows researchers to accurately predict how a real battery will behave in a complex system.

When multiple voltage sources are used together, their connections matter greatly. If you connect two ideal independent voltage sources in parallel, they must have the exact same voltage. If they do not, it creates a logical fallacy in the circuit math. If you connect them in parallel through a resistor, the source with the lower voltage actually becomes a consumer. However, if you connect an exact duplicate of a voltage source in parallel, they share the burden of the current. Each source will provide half of the electric current that the original source would have provided alone. The total voltage and current for the rest of the circuit remain unchanged.

Voltage sources are considered the duals of current sources. While a voltage source aims to keep voltage steady, a current source provides a constant current.

An ideal current source behaves very differently from a voltage source. An ideal current source provides no energy to a short circuit. It approaches infinite energy and voltage as the load resistance approaches infinity. Real-world current sources have a very high, but finite, output impedance. For example, transistor current sources often have an impedance of a few megohms at low frequencies.

Because no ideal sources actually exist in nature, these two types are closely linked. Any non-ideal current source can be viewed as a voltage source with the same source impedance. You can convert one type into the other using specific mathematical principles. These are known as Norton's theorem and Thévenin's theorem. These rules help engineers manage everything from tiny electronic components to massive electrical power systems. In large power networks, the entire system of lines and sources can even be replaced by a single equivalent impedance and an ideal AC voltage source for fault analysis.