constantcurrent

Constant current is a mode of operation in electrical circuits in which the output current is held at or near a specified value while the supply voltage or load may vary. It is the counterpart to constant voltage, where the aim is to keep voltage constant while current changes.

The typical constant-current regulator uses feedback to adjust a pass element so the current, measured by a sense resistor, matches a reference. When the current falls below the target, the control increases the output voltage (or duty cycle); when the current rises, it reduces it. This keeps the current within tolerance over a specified range of input voltages and loads.

Linear constant-current sources employ a transistor or MOSFET in series with a load and rely on an error amplifier and reference to regulate current. These are simple and quiet but dissipate power proportional to the voltage headroom, limiting efficiency when large voltage drops exist.

Switching constant-current sources use inductors and high-frequency switching with a current-sense resistor and feedback to regulate average current. Buck, boost, and buck-boost topologies can provide constant current over wide input ranges and are common in LED drivers and battery chargers. These can achieve high efficiency but require careful layout and protection to avoid instability or current runaway.

Applications include LED driving, battery charging, electroplating, and other processes that require a stable current irrespective of voltage variations.

Key design considerations include sense-resistor accuracy, temperature dependence, power dissipation, minimum and maximum compliance voltage (the voltage range over which regulation is possible), and protection against short circuits or overload. Some designs include foldback or current-limit features to prevent damage.

Measurement typically uses a low-value resistor in series with the load, with current sensed by an amplifier. Modern drivers often integrate controller, switch, and protection into a single IC.