Variable Zener Diode ~Super Zener~

Variable Zener Diode – SUPER ZENER

This variable zener circuit is intended primarily to produce a stable reference voltage in battery operated equipment designed for minimum current consumption. Despite the fact that only 1mA flows through the zener the output voltage showed a fluctuation of less than 1mV for supply voltage variations of 10 to 30 volts.

PARTS LIST
R1	2.2kΩ
R2	100kΩ
R3	220kΩ
D1	5.6V 400mW zener
IC1	741  op amp

The reference voltage from the zener diode apply to the non-inverting input of a 741 op-amp and the output voltage of the zener voltage multiply by using the op-amp gain.

V_o=V_z×(R₂+R₃)/R₃

V_o=5.6V×(100KΩ+220KΩ)/220KΩ

Vo=5.6V×1.45=8.14V

 

Vo=OUTPUT VOLTAGE

Vz=ZENER VOLTAGE

 

This approach has two advantages. Firstly, a low temperature coefficient zener (5.6V) can use to provide any desired reference voltage simply by altering the op-amp gain. Secondly, since no since no significant current is ‘robbed’ from the zener by the op-amp input, the need only be fed by a small current. So that the resistance the of  zener does not affect the output voltage the zener current must be fairly constant. This achieve by feeding the zener via R1 from the output of the op-amp. The zener current is,

1mA=V₀-V_z/R₁

So R1 select to give a zener current of about 1mA. The reference voltage obtained from the op-amp output can supply currents of up to 15mA. One point to note when using  this circuit is that the supply voltage must be at least 2V greater than the output voltage of the circuit.

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