# Variable Zener Diode ~ Super Zener Diode ~

This 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 is applied to the non-inverting input of a 741 op-amp, and the output voltage is the zener voltage multiplied by the op-amp gain.

Vo=Vz×(R2+R3)/R3

Vo=5.6V×(100KΩ+220KΩ)/220

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 be used 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 is achieved by feeding the zener via R1 from the output of the op-amp. The zener current is,

1mA=V0-Vz/R1

so R1 should be chosen 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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