**Zener Diode Voltage Regulator Circuit:**

**Regulator Circuit With No Load –** The most important application of Zener Diode Voltage Regulator Circuit is dc voltage regulator circuits. These can be the simple regulator circuit shown in Fig. 3-19, or the more complex regulators.

The circuit in Fig. 3-19 is usually employed as a voltage reference source that supplies only a very low current (much lower than I_{Z}) to the output. Resistor R_{1} in Fig. 3-19 limits the Zener diode current to the desired level.

The Zener current may be just greater than the diode knee current (I_{ZK}). However, for the most stable reference voltage, I_{Z} should be selected as I_{ZTÂ }(the specified test current).

**Loaded Regulator:**

When a Zener diode regulator is required to supply a load current (I_{L}), as shown in Fig. 3-21, the total supply current (flowing through resistor R_{1}) is the sum of I_{L} and I_{Z}Â Care must be taken to ensure that the minimum Zener diode current is large enough to keep the diode in reverse breakdown.

Typically, I_{Z(min)}Â = 5 mA for a Zener diode with I_{ZTÂ }= 20 mA. The circuit current equation is,

In some cases, the load current in the type of circuit shown in Fig. 3-21 may be reduced to zero. Because the voltage drop across R_{1} remains constant, the supply current remains constant at,

All of this current flows through the Zener diode when R_{L} is disconnected. The circuit design must ensure that the total current does not exceed the maximum Zener diode current.

**Regulator **Performance:

The performance of a Zener diode voltage regulator may be expressed in terms of the source and load effects, and the line and load regulations. Equations 3Â20 through 3-23 may be applied. If there is an input ripple voltage, the output ripple will be severely attenuated. The **ripple rejection ratio** is the ratio of the output to input ripple amplitudes.

To assess the performance of a Zener diode voltage regulator, the ac equivalent circuit is first drawn by replacing the diode with its dynamic impedance (Z_{z}), as shown in Fig. 3-23(a). The complete ac equivalent circuit is seen to be a simple voltage divider. When the input voltage changes by Î”E_{s}, the output voltage change is,

Equation 3-26 assumes that there Is no load connected to the regulator output. When a load is present, R_{L}Â appears in parallel with Z_{Z} in the *ac *equivalent circuit, [see Fig. 3-23(b)]. The equation for the output voltage change now becomes,

The regulator source effect can be determined from Eq. 3-26 or Eq. 3-27, as appropriate. The equations can also be used for calculating ripple rejection ratio. The input ripple amplitude (V_{ri} ) and the output ripple (V_{ro}) are substituted in place of the input and output voltages in Eq. 3-26 and Eq. 3-27. Thus, Eq. 3-26 can be modified to give a ripple rejection ratio equation,

and for a loaded regulator, Eq. 3-27 gives,

To determine the load effect of the Zener diode voltage regulator, the circuit output resistance has to be calculated.

The regulator Thevenin equivalent circuit in Fig. 3-24 shows that, assuming a zero source resistance, the circuit output resistance is,

When the load current changes by Î”I_{L},Â the output voltage change is,