As everybody knows the voltage drop across a zener diode is dependent on the current passing th rough the diode.
Therefore, depending on the type and power of the device, there can be very noticeable deviations from the nominal zener voltage. This can be a problem, 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. ln 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 dependent on the zener voltage. In our adjustable zener circuit we use a zener diode with a zener voltage of 6 V. Other zener values could be used if resistors R1 . . . R4 are changed to suit another value. The maximum input voltage is mainly limited by the power which can be dissipated by T1 and T2. The d.c. input voltage must be at least as high as the sum of the zener voltages of Dl and D2.
The current source consisting of T1, R1 and D1 ensure that the current through D2 remains constant. Transistor T2, resistor R 2 and zener diode D2 in turn form a current source for zener D1 so that the current through this diode also stays constant. Diode D3 and the voltage divider, consisting of R3 and R4, ensure that this circuit can ’start’ (just as a thyristor made of transistors). As soon as the voltage is switched on a current flows through D3 causing T2 (and therefore T1) to conduct. The value of R3 must be selected such that diode D3 blocks as soon as the voltage across the zener dlode has stabilized. So care must be taken that the voltage at the anode of D3 ls less than the zener voltage of D2 plus the di0de’s own voltage drop of 0.6 V. This is defined by the formule: ; x U;
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Therefore, depending on the type and power of the device, there can be very noticeable deviations from the nominal zener voltage. This can be a problem, 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. ln 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 dependent on the zener voltage. In our adjustable zener circuit we use a zener diode with a zener voltage of 6 V. Other zener values could be used if resistors R1 . . . R4 are changed to suit another value. The maximum input voltage is mainly limited by the power which can be dissipated by T1 and T2. The d.c. input voltage must be at least as high as the sum of the zener voltages of Dl and D2.
The current source consisting of T1, R1 and D1 ensure that the current through D2 remains constant. Transistor T2, resistor R 2 and zener diode D2 in turn form a current source for zener D1 so that the current through this diode also stays constant. Diode D3 and the voltage divider, consisting of R3 and R4, ensure that this circuit can ’start’ (just as a thyristor made of transistors). As soon as the voltage is switched on a current flows through D3 causing T2 (and therefore T1) to conduct. The value of R3 must be selected such that diode D3 blocks as soon as the voltage across the zener dlode has stabilized. So care must be taken that the voltage at the anode of D3 ls less than the zener voltage of D2 plus the di0de’s own voltage drop of 0.6 V. This is defined by the formule: ; x U;
