Voltage reference stabilizes current sink

Analog circuits for long-term testing of passive components, such as 0.1 percent-tolerant resistors or high-intensity white LEDs, often require a constant current. Using two op amps and a voltage reference, you can develop a circuit that provides a constant-current sink with a variable setting of 0mA to 0.99A. The circuit in Figure 1 sinks a stable current through the load. The load current is insensitive to power-supply-voltage variations. IC₁ is a voltage reference that gives a stable 5V dc. It requires 500µA of current from the power supply. IC₂ is a National Semiconductor LM324 quad op-amp. Voltage follower IC_2A buffers the reference voltage from the rest of the circuit, which increases stability.

Resistor R₂ and potentiometer R₃ form a variable voltage divider that reduces the 5V reference voltage to a value between 0 and 3.26V. Unity-gain amplifier IC_2D drives the base of Q₁, a Darlington power transistor that has a current gain of 750, through R₄. R₄ and C₅ form a low-pass filter that prevents oscillation. You can drive Q₁ with a small base current. C₄ connects between the collector and the base of Q₁, adding further stability.

Operating as an emitter follower, Q₁ can drive an active or a passive load, such as a resistor or a high-brightness LED. Q₁’s emitter connects to R₅, a 3.3Ω, 5W grounded power resistor. The voltage at IC_2D’s Pin 14 sets the voltage across R₅, which fixes Q₁’s emitter current. Because of Q₁’s high gain, the current in the load is effectively Q₁’s emitter current.

Caption

Figure 1: A voltage reference and two op amps provide a stable voltage to R₅, which provides a stable load current.