Equivalent Circuit of Zener Diode

The analysis of circuits using zener diodes can be made quite easily by replacing the
zener diode by its equivalent circuit.

(i) “On” state. When reverse voltage across a zener diode is equal to or
more than break down voltage V z the current increases very sharply, In
this region, the curve is almost vertical. It means that voltage across zener
diode is constant at V z even though the current through it changes.
Therefore, in the breakdown region, an **ideal zener diode can be
represented by a battery of voltage V z as shown in Fig.1(ii).

Under such conditions, the zener diode is said to be in the “ON” state.
(ii) “OFF state.

Went the reverse voltage across the zener diode is less than
V z but greater than 0 V, the zener diode is in the “OFF” state. Under such
conditions, the zener diode can be represented by an open-circuit as
shown in Fig. 2(ii).

…………………………………………………………………………………………………………………………..
 The current is limited only by both external resistance and the power dissipation of
zener diode.
 This assumption is fairly reasonable as the impedance of zener diode is quite small in
the breakdown region.

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