The 78XX series voltage regulator integrated circuits do not support an input voltage greater than 28 V for some types and 35 V for others, depending on the series and the output voltage value. Thus, if we have a higher input voltage and we need to adjust a low output voltage, a pre-reduction will be necessary. This can be done with the circuit shown in the figure, which admits an input of up to approximately 70 volts. The maximum output current of this circuit is 1 ampere and the 2N3055 power transistor must be mounted on a good heat radiator. The 25 V zener diode reduces the input voltage of the 78XX to approximately 25.7 V, while resistor R1 determines the current in the zener. The zener must be 2 W and the dissipation of resistor R must be calculated as a function of the input voltage, a typical value being 2 W.
