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A constant current source is an electronic circuit that delivers a fixed amount of electrical current to a load, regardless of variations in the load's resistance or the voltage supplied to the source. This is in contrast to a voltage source, which aims to maintain a constant voltage across its terminals.

The ideal constant current source would provide an unchanging current irrespective of the voltage across its terminals. In reality, practical current sources have limitations. They often have a maximum voltage they can sustain across the load and a minimum voltage below which they cannot operate correctly. The behavior of a real current source can be modeled as an ideal current source in parallel with a large output resistance.

Constant current sources are essential in various applications where a precise and stable current is required. These include powering LEDs, charging batteries, operating sensors, and in certain types of test equipment. For example, LEDs require a specific current to operate at their intended brightness and to prevent damage from overcurrent. By using a constant current source, this precise current is delivered to the LED, ensuring consistent performance and longevity.

Different circuit topologies can be employed to create constant current sources, often involving transistors, operational amplifiers, and other passive components. The specific design depends on factors such as the desired current level, voltage range, accuracy, and cost.