Accurately simulate an LED

Testing the driver circuit using LEDs, although easy, yields only typical results because the tests don’t factor in worst-case LED parameters and often generate undesirable light and heat during driver debugging. Although using a constant resistance might seem to be an appropriate approach, a resistor approximates an LED load at only one point on the current/voltage curve. An electronic load may prove to be a more useful approach. The control loops of the driver circuit and the electronic load, however, often result in system instability and oscillations.

Figure 1 illustrates a typical LED-driver circuit using a low-cost simulated-LED circuit. The simulated LED accurately mimics a real LED at a user-programmable threshold voltage. A simple Darlington current sink, Q₁, provides a wide range of LED threshold voltages. The size of the heat sink attached to Q₁ and the power capability of Q₁ are the only limits on the amount of power the simulated LED can dissipate.

You can easily tune the circuit for any LED voltage. Place a constant voltage across the simulated LED. Tune the circuit by adjusting resistor R₁ until the circuit draws the desired current. You can adjust the shape of the voltage knee by making small changes to resistor R₃, although this step is not usually necessary.