1. • The basic purpose of a power supply is to provide one or more
fixed voltages to the working circuit, with sufficient current-
handling capacity to maintain the operating conditions of the
circuit. These can be of two types:
• UNREGULATED: The output of such supply changes due to :
a) change in the load
b) change in the source voltage
• REGULATED: The output of such supply does not change due to:
a) change in the load
b) change in the source voltage
REGULATED POWER SUPPLY IS REUIRED FOR CRITICAL ELECTRONIC
APPLICATIONS.

2.  A voltage regulator circuit automatically maintains the output
voltage of a power supply constant, regardless of
i) a change in the load
ii) a change in the source voltage
+
Transformer
(V)S Filter Regulator VDC LOAD
and rectifier
-

3. ZENER DIODE SYMBOL:
ANODE CATHODE

4. OUTPUT CHARECTERISTICS OF
ZENER DIODE
FORWARD
ZENER BIAS
BREAKDOWN
I
V
REVERSE
BIAS
ZENER DIODE WORK AS A VOLTAGE REGULATOR
IN REVERSE BIAS

5. ZENER DIODE CHARCETERISTICS
 Voltage after breakdown is also called zener voltage VFZ
 VZ remains nearly a constant, even though current IZ varies
considerably.

6.  The zener diode is typically connected reverse
biased, in parallel with the load.
 Resistor Rs limits current to zener.
I RS IL
+ IZ +
vDC (IN) DZ VL RL
- -

7. ZENER DIODE CHARACTERISTICS
 In the reverse bias region, a reverse leakage current flows
until the breakdown voltage is reached.
 At this point, the reverse current, called zener current IZ
, increases sharply.
VR VBR
IZ
IR

8. ZENER DIODE CHARACTERISTICS
 In the forward region the zener diode acts like a regular silicon
diode, with a 0.7 volt drop when it conducts.
IF
FORWARD BIAS
REGION
0.7V VF

9.  The width of depletion region is very less at high
impurity concentration resulting in high electric field
intensity at depletion region sufficient to break the
covalent bonds and free the electrons. A large
number of covalent bonds break resulting in
formation of a large number of electron –hole pairs
and sudden increase in the reverse current. This is
known as zener effect.