Figure 6-46 is an example of a current limiter designed to limit the current from a 12 V supply to 1.5 A using a TIP31 npn transistor as the pass element. The value of R1 is calculated from the equation in Figure 6-46. The current through R1 is split almost equally in this circuit, with about 30 mA going to the TL431, and 30 mA for base drive to the TIP31. With a current load of 6 mA, and an R1 value of 128Ω a 1/2 watt resistor is sufficient. When the voltage across the current limit resistor (RCL reaches 2.5 V (TL431 reference voltage), the base drive to the TIP31 is reduced and the output current is limited to 1.5 A.