Zener diode is a very simple device which can conduct
in both directions.
A pn-junction diode and a zener diode are basically
the same. The only difference is that
a pn-junction diode is lightly doped whereas a zener diode is moderately doped.
That’s it ! it is the only difference.
Pn-junction diode characteristics is shown below.
Understand it well before reading further.
As evident from characteristics above, if reverse bias
voltage across a pn-junction diode is made equal or greater than PIV value, the
current (from K to A) through the diode increases rapidly, due to a phenomenon
called avalanche breakdown and the diode gets permanently damaged.
Therefore, the reverse voltage across a pn-junction
diode must be kept smaller than PIV value at all costs, to prevent diode from
damage.
A zener diode
working is exactly opposite.
Zener
diode working
A negative voltage at least equal or greater than Vz ( or also called PIV ) value
must be applied across a zener diode for it to operate, as shown in zener diode characteristics below.
Once the reverse voltage across zener is made equal to
Vz , a reverse current (from K to A) Imin
starts flowing through it. Now, even for large variations in current value
(between Imin and Imax) , the voltage across zener
remains almost constant at Vz . As shown in characteristics, zener diode has very narrow operating region shown as "zener ON " .
However, if the reverse current exceeds Imax
, the zener diode gets damaged. Therefore, reverse current must be kept below Imax
at all costs.
The reverse current builds very quickly (steep VI
curve between Imin and Imax) due to a phenomenon called
zener breakdown.
In forward biased region i.e V>V0 ,
zener diode behaves like an ordinary pn-junction diode i.e like a closed switch.
To summarise :-
1. A zener
diode behaves like an ordinary pn-junction diode i.e like a closed switch , in forward biased
region.
2. when a reverse voltage greater than Vz
is applied across zener, it starts conducting from K to A , and voltage across
it remains almost constant for any current variations between Imin
and Imax .
3. If the reverse current (from K to A) through zener becomes
greater than Imax , zener is permanently damaged.
Zener
diode symbol
A zener diode can conduct in both directions i.e from
A to K and from K to A .
Zener
diode applications
1. For current any variation between Imin
and Imax , the voltage across the zener remains almost constant at Vz
. Thus, it can be connected in parallel to the device across which voltage has
to be maintained constant.
Hence, it is commonly used as a voltage regulator.
2. As clippers and clampers.
3. As a switch. Behaves like a closed switch for V>Vo and like an open switch for VZ<V<Vo .
