Zener diode is considered a special purpose diode because it can operate in a reverse biased mode. Because of this, these diodes are used in voltage regulation and waveform shaping circuits. In this tutorial, two types of voltage regulation circuits with Zener diodes are discussed. In addition to voltage regulators, the other application is waveform shaping where a Zener diode is used as a clipping element.

Zener Diode Used as a Voltage Regulator

Zener diode is mostly used in voltage regulator circuits — operating in the breakdown region can keep nearly a constant voltage across its terminal over a specified range of reverse current values. This ability makes it suitable to work as a voltage regulator and keeps a reverse voltage constant.

From the characteristic curve, I_ZK is the minimum reverse current that keeps the diode in the Zener breakdown region. Below this value, the Zener won’t be able to work as a voltage regulator. Between the current values of I_ZK and I_ZM, the curve is almost a straight line. This is the breakdown region or Zener voltage V_z. In this region, the internal diode resistance decreases rapidly, and hence the reverse current increases. V_Z is almost constant ideally, but there is a slight increase as the diode current increases. I_ZM is the maximum reverse current, beyond this value, there is an avalanche breakdown region. Between I_ZK and I_ZM the Zener voltage (V_Z) remains constant. For a particular diode, V_Z, I_ZK, Z_Z, I_ZT and I_ZM are stated in datasheets.

These diodes are used as a voltage reference element in sensitive circuits. As a result, it has a controlled reverse-biased property, which is constant over temperature and time.

In this tutorial, two types of Zener based voltage regulators are discussed.

1. Regulation with variable input voltage
2. Regulation with variable load

Regulation with Variable Input Voltage

The circuit below shows a Zener diode with an input voltage source and a resistor which is a current limiting resistor.

In the figure, the diode is driven by a voltage source which may vary within certain limits. The input voltage (V_i) may vary in a suitable range to ensure that the diode remains in its ON state. As the input voltage increases, the current flowing through the circuit increases and vice versa. Similarly, the change in input voltage and current leads to change (increase and decrease) in diode current and voltage. Keep in mind that the load resistor remains the same as well as the current flowing through the load.

For fixed values of the load resistor, the minimum input voltage (V_i(min)) is fairly larger than the nominal diode voltage (V_Z). The maximum input voltage is determined by the maximum Zener current (I_ZM). Assume Z_Z is constant from I_ZK to I_ZM.

The minimum input voltage value is determined by the Zener current (I_ZK).

To determine V_Z, using the simplified model of the diode.

V_out varies slightly because of internal resistance. V_out at I_ZK is calculated as follows:

V_out at I_ZM is calculated as follows:

The maximum value of input voltage is determined by the maximum value of Zener current (I_ZM).

Example: Determine minimum and a maximum output voltage that can be regulated with a 1N4736 diode.

Nominal Zener voltage V_Z = 6.8V

Zener test current I_ZT = 37mA

Zener impedance Z_ZT =3.5 Ω

Zener knee current I_ZK = 1mA

As is already explained, the diode voltage in the breakdown region varies slightly because of the internal resistance. This is ∆V_Z.

V_IN(max) occurs at maximum Zener current, that is I_ZM.

The minimum input voltage is 6.77V and the maximum allowable voltage is 21.88V. In this circuit, the diode works as a regulator if the input voltage varies between these voltage values.

Regulation with Variable Load

The circuit below shows a Zener diode with an input voltage source and a resistor which is a current limiting resistor. The circuit is the same as the previous circuit where Zener diode is used as limiting element. Here, the load resistor may vary but of course, the voltage across it remains the same because of the diode.

In the figure above, the diode is driven by a fixed voltage source but the load resistor is variable. The load resistor (R_L) may vary in a suitable range to ensure that the diode remains in its ON state. These regulators can maintain a constant voltage across the specific ranges of a load resistor (and hence load current). A too-small resistor will result in small voltage across the load (V_L) and hence across the diode.

When a load resistor is connected, total current divides and flows through Zener (I_Z) and the load resistor (I_L). If R_L decreases I_L increases and I_Z decreases. However, the input current (I_T) remains the same and the Zener voltage (and hence the voltage across the load) will become constant, once I_Z reaches I_ZK. At this point, the diode is in the ON state and starts working as a voltage regulator and maintains its voltage over the range of current values (that is from I_ZK to I_ZM).

Example: Determine minimum and maximum load current for which the diode will maintain regulation. Calculate minimum R_L.

The maximum current will pass through the diode if the load resistor is of infinite value. Suppose the R_L is infinite and the current flowing through it is approximately zero. In this case, maximum current will flow through the diode. Calculate it

So, it can handle this current without being damaged.

The other condition is when the minimum current will flow through the diode. But the minimum current should be greater than I_ZK, to ensure that the diode remains in the ON state. Whereas the load current is going to be maximum. Keep in mind the input voltage and current remains the same. In this case,

Calculate the maximum current through the load. When the current is maximum, the resistance is minimal. Calculate the minimum value of R_L.

The possible values of R_L = 76.7Ω to infinity. This value maintains the voltage regulation. If R_L(min) is less than 76.7Ω, in this case, more current will start flowing through R_L, and it will drop diode current from I_ZK (the minimum current to turn on the diode). And voltage regulation will not be established.

Zener Diode Used As a Waveform Clipper

The major application of these diodes is as a voltage regulator – e.g. Zener diode is mostly used in voltage regulator circuits. But they can also be used for waveform shopping or clipping purposes as well. These circuits are also called diode limiters. In these circuits, the Zener diode is used as a limiter.

According to the schematic diagram shown below, it can work as a positive peak clipper or a negative peak clipper or clips both peaks to selected Zener voltage.

Zener Diode Used as a Negative Peak Clipper

The circuit below clips the negative half cycle and sets the positive peak to the Zener diode V_Z level.

During the positive half cycle, the diode is reverse biased until input voltage is less than Zener voltage (V_i<V_Z). And hence input voltage appears across the output. When the input voltage is greater than the input voltage (V_i>V_Z), the diode starts conducting in reverse biased mode, and hence the current starts flowing through the diode. The output waveform is set to V_Z.

During the negative half cycle, the diode starts conducting at 0.7 V. The current starts flowing through the diode and the output waveform clips after 0.7 V.

Zener Diode as a Positive Peak Clipper

There is another circuit, the diode is connected in the opposite direction. It clips the positive peak and sets the negative peak to the Zener voltage level. The working of both circuits remains the same as discussed earlier.

Double Zener as a Diode Clipper

In this circuit, the sinusoidal waveform becomes the square wave after clipping through the diode. For a better square wave, the amplitude of the input signal should be large enough. In these types of circuits, one diode behaves as a limiter while the other diode behaves as a simple forward-biased diode

During the positive half cycle of the input voltage, D1 is forward biased at 0.7 V. While D2 is reversed biased until input voltage is greater than Zener reverse voltage (for D2) and forward diode voltage (for D1). Here D1 works as a limiter diode while D2 is a simple forward-biased diode. Input voltage appears across output until V_i = (V_Z + 0.7) V and then clips.

Similarly, during the negative half cycle of the input voltage, D1 works as a Zener limiter, while diode D2 is a simple forward-biased diode.