3.1 Voltage Source

We have seen in Chapter 2 that an LED behaves like a constant voltage load with low equivalent series resistance (ESR). This behavior is like a Zener diode in fact Zener diodes make a good test load, rather than using expensive high power LEDs!

Driving a constant voltage load from a constant voltage supply is very difficult, because it is only the difference between the supply voltage and the load voltage that is dropped across the ESR. But the ESR is very low value, so the voltage drop will also be low. A slight variation in the supply voltage, or the load voltage, will cause a very large change in current; see curve A in Figure 3.1.

Figure 3.1: LED Current Versus Supply Voltage

If the variation in supply voltage and forward knee voltage ( V _f) is known, the variation is current can be calculated. Remember that there are variations in LED voltage drop due to manufacturing tolerances and operating temperature. Most supply voltages from a regulated supply have a 5% tolerance, but from unregulated supplies like automotive power, the tolerance is far greater.

These equations assume that ESR is constant. In practice, the V _f and voltage drop across ESR are combined, since manufacturers quote the voltage drop at a certain forward current. The actual V _f can be determined from graphs, or measured.

If there is a large difference between the source and load voltage, and a high ESR, there...