SDG #020 Linear LED Constant Current Driver Circuits

Driving an LED properly is about controlling current, not voltage, and this build-and-measure session works through four linear approaches in increasing order of sophistication so you can see exactly what each extra component buys you.

The starting point is the humble series resistor, which is cheap and instant but leaves LED current at the mercy of supply voltage and forward voltage spread between parts. From there the LM317 is reconfigured out of its familiar voltage regulator role into a two terminal current source, where a single sense resistor sets output current against the internal 1.25 V reference. That reference voltage is also the weakness: it burns a fixed and relatively large amount of headroom in the sense resistor, which matters for battery powered work.

The discrete transistor current sink improves on that by using a base to emitter drop rather than a full 1.25 V reference, cutting sense resistor dissipation, though it inherits the temperature dependence of Vbe and the base current error that comes with a simple two transistor arrangement. The precision op-amp circuit closes the loop properly, servoing a pass element until the voltage across a low value sense resistor matches a stable reference, which gives far tighter set point accuracy, better regulation against supply changes and the option of running very small sense voltages.

Measurements are taken with Fluke 179, 289 and 87V meters fed from an Instek GPS-3303 supply, so current setpoint, dropout behaviour and regulation are compared as real numbers rather than theory. The takeaway for anyone specifying a driver: linear circuits stay simple and quiet, but every volt of headroom becomes heat, so match the topology to your supply margin and accuracy needs before adding parts.