stable zener

15 stable zener As everybody knows the voltage drop across a zener diode is depen- dent on the current passing through the diode. Therefore, depending on the type and power of the device, there can be very noticeable devi- ations from the nominal zener voltage. This can be a problem, 12 ... 25 V 7-38 especially in circuits where a stable d.c. voltage is essential. The most logical way of solving the problem is to keep the current through the diode constant so that the zener voltage can not change. In order that the load connected to the zener diode draws a constant current, the zener can be supplied by means of a current source. Then the current through the current source is made dependant on the zener voltage. In our circuit we use a zener diode with a zener voltage of 6 V. Other zener values could be used if re- sistors R 1 ... R4 are changed to suit another value. The maximum input voltage is mainly limited by the power which can be dissipated by Ti and T2. The d.c. input volta...