Zener diode
voltage regulator
I.
Aim
To construct a zener diode voltage regulator
and measure its line and load regulation.
II.
Apparatus
Zener
diode, resistor, variable DC power supply, milliammeter, voltmeter, Rheostat and
wire.
III.
Theory
The Zener diode is like a general-purpose
signal diode. When biased in the forward direction it behaves just like a
normal signal diode, but when a reverse voltage is applied to it, the voltage
remains constant for a wide range of currents.
Fig 1: Symbol for zener diode
Avalanche
Breakdown: There is a limit for the reverse voltage. Reverse voltage can
increase until the diode breakdown voltage reaches. This point is called
Avalanche Breakdown region. At this stage maximum current will flow through the
zener diode. This breakdown point is referred as “Zener voltage”.
The
Zener Diode is used in its "reverse bias". From the I-V
Characteristics curve we can study that the zener diode has a region in its
reverse bias characteristics of almost a constant negative voltage regardless
of the value of the current flowing through the diode and remains nearly
constant even with large changes in current as long as the zener diodes current
remains between the breakdown current IZ(min) and the maximum current rating
IZ(max).
This
ability to control itself can be used to great effect to regulate or stabilise
a voltage source against supply or load variations. The fact that the voltage
across the diode in the breakdown region is almost constant turns out to be an
important application of the zener diode as a voltage regulator
i.
Characteristics
Fig 2: Zener diode
characteristic curve
Figure
2 shows the current versus voltage curve for a Zener diode. Observe the nearly
constant voltage in the breakdown region.
The
forward bias region of a Zener diode is identical to that of a regular diode.
The typical forward voltage at room temperature with a current of around 1 mA
is around 0.6 volts. In the reverse bias condition the Zener diode is an open
circuit and only a small leakage current is flowing as shown on the exaggerated
plot. As the breakdown voltage is approached the current will begin to
avalanche. The initial transition from leakage to breakdown is soft but then
the current rapidly increases as shown on the plot. The voltage across the
Zener diode in the breakdown region is very nearly constant with only a small
increase in voltage with increasing current. At some high current level the
power dissipation of the diode becomes excessive and the part is destroyed.
There is a minimum Zener current, Iz(min), that places the operating point in
the desired breakdown. There is a maximum Zener current, Iz(max), at which the
power dissipation drives the junction temperature to the maximum allowed.
Beyond that current the diode can be damaged. Zener diodes are available from
about 2.4 to 200 volts.
ii.
Zener Diode as Voltage Regulators
The function of a regulator is to
provide a constant output voltage to a load connected in parallel with it in
spite of the ripples in the supply voltage or the variation in the load current
and the zener diode will continue to regulate the voltage until the diodes
current falls below the minimum IZ(min) value in the reverse
breakdown region. It permits current to flow in the forward direction as
normal, but will also allow it to flow in the reverse direction when the
voltage is above a certain value - the breakdown voltage known as the Zener
voltage. The Zener diode specially made to have a reverse voltage breakdown at
a specific voltage. Its characteristics are otherwise very similar to common
diodes. In breakdown the voltage across the Zener diode is close to constant
over a wide range of currents thus making it useful as a shunt voltage
regulator.
The purpose of a voltage regulator is
to maintain a constant voltage across a load regardless of variations in the
applied input voltage and variations in the load current. A typical Zener diode
shunt regulator is shown in Figure 3. The resistor is selected so that when the
input voltage is at VIN(min) and the load current is at IL(max) that
the current through the Zener diode is at least Iz(min). Then for
all other combinations of input voltage and load current the Zener diode
conducts the excess current thus maintaining a constant voltage across the
load. The Zener conducts the least current when the load current is the highest
and it conducts the most current when the load current is the lowest.
Fig 3: Zener diode shunt
regulator
If there is no load resistance, shunt
regulators can be used to dissipate total power through the series resistance
and the Zener diode. Shunt regulators have an inherent current limiting
advantage under load fault conditions because the series resistor limits excess
current.
A zener diode of break down
voltage Vz is reverse connected to an input voltage source Vi across
a load resistance RL and a series resistor RS. The
voltage across the zener will remain steady at its break down voltage VZ for
all the values of zener current IZ as long as the current
remains in the break down region. Hence a regulated DC output voltage V0 =
VZ is obtained across RL, whenever the input voltage remains
within a minimum and maximum voltage.
Basically there are two type of
regulations such as:
a) Line Regulation
In this type of regulation, series
resistance and load resistance are fixed, only input voltage is changing.
Output voltage remains the same as long as the input voltage is maintained
above a minimum value.
b) Load Regulation
In this type of regulation, input
voltage is fixed and the load resistance is varying. Output volt remains
same, as long as the load resistance is maintained above a minimum value.
iii.
Design a Voltage Regulator
When selecting the zener diode, be sure
that its maximum power rating is not exceeded.
Imax
Maximum current for Zener diode
VZ
Zener Diode standard voltage
Vin
Input voltage(it is known)
Vs Voltage across series resistance
VL Voltage across the load resistance
IS Current passing through the series resistance
IZ Current passing through the Zener diode
IL Current passing through the load resistance
Vs Voltage across series resistance
VL Voltage across the load resistance
IS Current passing through the series resistance
IZ Current passing through the Zener diode
IL Current passing through the load resistance
iv.
Calculating voltage and current
The total current drawn from the source
is the same as that through the series resistor
The current through the load resistor
is
and the zener diode current
is
If the voltage source is greater than
Vz
and
If the voltage source is less
than Vz
and
IV.
Procedure
Using
the circuit diagram, identify the connections in the given platform.
Connections are made as shown in the below diagram.
i.
Line regulation
1. Choose the zener
diode to start the experiment.
2. Insert the
series resistance value.
3. Fix the load
resistance value by using Load Resistance slider.
4. Change the
Rheostat value from maximum to 0 by the interval 100.
5. Note down the
corresponding input voltage and output voltage and tabulate it.
6. Plot the graph
in which VIN at x-axis
VL at y-axis.
Percentage of
Line regulation =
ii.
Load Regulation
1. First 3 steps
are same as above.
2. Fix the Rheostat
value, to get the 12 V at voltmeter across rheostat.
3. Change the load
Resistance with the interval of 100 Ω/1000 Ω up to maximum range.
4. Note the
reading and tabulate it.
5. Plot the graph
between V0 along
x-axis and RL along
y-axis.
Percentage of Load regulation =
V.
Conclusion
The
Zener diode, with its accurate and specific reverse breakdown voltage, allows
for a simple, inexpensive voltage regulator. Combined with the right resistor,
fine control over both the voltage and the supply current can be attained.
However,
the low power ratings of standard Zener diodes and resistors make this solution
impractical for high power devices.
Percentage of
Line regulation ==
Percentage of
Load regulation = =
VI.
Precautions
Ø
All connections should be neat
,clean and tight
Ø
Key should be used in circuit and opened when the circuit is not being
used.
VII.
Uses
Ø
Zener diodes
are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits.
Ø
Another notable
application of the zener diode is the use of noise caused by its avalanche
breakdown in a random number generator
VIII.
References
Ø
Gates, Earl. Introduction to electronics. Clifton Parks, NY: Delmar,
2011.
Ø
Comprehensive Practical Physics
Ø
Harrison, Linden. Current sources & voltage references. Amsterdam
New York: Newnes, 2005.
.