A constant-current diode, also known as a current-limiting diode, or current-regulating diode, does exactly what its name implies: it regulates current through it to some maximum level. The constant current diode is a two terminal version of a JFET. If we try to force more current through a constant-current diode than its current-regulation point, it simply “fights back” by dropping more voltage. If we were to build the circuit in Figure below(a) and plot diode current against diode voltage, we’d get a graph that rises at first and then levels off at the current regulation point as in Figure below(b).
One application for a constant-current diode is to automatically limit current through an LED or laser diode over a wide range of power supply voltages as in Figure below.
Of course, the constant-current diode’s regulation point should be chosen to match the LED or laser diode’s optimum forward current. This is especially important for the laser diode, not so much for the LED, as regular LEDs tend to be more tolerant of forward current variations.
Another application is in the charging of small secondary-cell batteries, where a constant charging current leads to predictable charging times. Of course, large secondary-cell battery banks might also benefit from constant-current charging, but constant-current diodes tend to be very small devices, limited to regulating currents in the milliamp range.
Article Extracted from Tony R. Kuphaldt Lessons In Electric Circuits — Volume III Chapter 3 under the terms and conditions of the CC BY License.