Using a Zener diode as voltage reference in a circuit

Zener diodes serve as a reliable choice for achieving a constant reference voltage value in a circuit. It can maintain a constant voltage across it for a range of applied voltage values above the knee voltage. This feature allows it to deliver a stable voltage in circuits with variable or fluctuating power supplies. Using Zener diodes of various values enables the generation of a broad range of constant voltages without requiring an external battery supply. This is particularly advantageous since relying on a battery supply or external reference can pose practical inconveniences.

Refer: How zener diode is different from PN junction diode?

Resistor value calculation for the zener voltage reference circuit

V_R – Voltage across the Resistor,

V_Z – Zener voltage,

I_Z – Breakdown current or Current through Zener.

Calculation of resistance to obtain a reference voltage of 3V from a supply voltage of V_S = 6V using a zener diode of 3V.

The voltage difference between the V_S and V_Z drops across the resistor, which is V_R.

The V_R = I * R, where I is I_Z + I_OUT; the sum of zener current and current flow to reference.

R = V_S – V_R / I_Z + I_OUT  | I_OUT – Current driven to the reference input.

Normally, the reference inputs are given to circuits or components such as IC which has a high input impedance. So in a practical case, the current draw from a reference point would be approximately equal to zero.

Even though, let assume an output current from the reference point as 10mA and the zener current 10mA. If the input impedance of the circuit connected to the reference is low, then the appropriate value of the current should be taken for the calculation.

R = (6 – 3) /(.01 + .01) = 3/.02
= 150Ω

Therefore, a resistance of 150Ω is needed to obtain a reference voltage of 3V from a supply voltage of 6V using a Zener diode with a Zener voltage of 3V, assuming an output current of 10mA and a Zener current of 10mA.

Keep in mind that this is an example of calculation, the actual value of the Zener current may vary based on the specific Zener diode’s datasheet, the current drawn from the reference point also varies, so it’s essential to check the datasheet for accurate information.