EASTERN MEDITERRANEAN
UNIVERSITY
DEPARTMENT OF ELECTRICAL &
ELECTRONICS ENGINEERING
EENG341
EEE 341LAB
LAB
ELECTRONICS
ELECTRONIC II
EXPERIMENT 3
The Zener Diode
Std. No. Name &Surname:
1________ _______________
2________ _______________
3________ _______________
Group No :_____________
Submitted to:_____________
Date
:_____________
Suat SELIM
1
Objectives:
¾ To recognize zener diodes in various physical forms and to distinguish them from
rectifying diodes.
¾ To understand the constant-voltage characteristic of a reverse-biased zener diode.
¾ To understand the use of a zener diode in a simple voltage regulator circuit.
The Zener Diode Reverse Characteristics
Construct the circuit of Fig. 3.1.
Vr
0-20V d.c.
variable
1K0
Id
V
V
VS
high-resistance
meter
10V dc
Vd
BZY88
C7V5
0V
•
•
Fig 3.1.
The method obtaining the voltage-current characteristics is the similar to that of
Assignment 1 but notice that the zener diode is reverse biased. U=sing the power
supply variable control, set VS to the values given in Table 3.1.
For each value record Vr, then calculate:
V
V d = V s − V r and I d = r = V r mA
1000
Copy the results table as shown in Table 3.1.
Table 3.1.
Vr
Vd=Vs-Vr
VS
(V)
(V)
(V)
0
2
4
6
6.5
7.0
7.5
8.0
8.5
9.0
10.0
15.0
20.0
™ Plot Vd against Id and also plot Vs against Pd in Fig. 3.2.
Suat SELIM
Id=Vr
(mA)
Pd= Vd * Id
(mW)
2
Pd (mW) Id (mA)
100
20
90
18
80
16
70
14
60
12
50
10
40
8
30
6
20
4
10
2
0
0
2
4
6
8
10
12
14
16
18
20
Vd & Vs (V)
Fig 3.2. The Zener Diode Characteristic
Questions
1. From Fig 3.2 describe the characteristics in your own words.
2. The nominal voltage of the BZY887V5 is 7.5V. Does your graph agree with this
exactly? If not, can you suggest a reason for any difference?
3. Why is the series resistor in Fig 3.1 necessary?
4. The power dissipated in the diode for each value of Vd and Id is calculated and entered
into the last column of your Table 3.1. The maximum allowable power dissipation of
type BZY88 is 400 mW. Does your maximum value of Pd approach this limit?
Suat SELIM
3
A Simple Zener Diode Voltage Regulator
•
•
The Zener diode has a region in its reverse characteristics of almost constant voltage
regardless of the current through the diode. This can be used to regulate or stabilize a
voltage source against supply or load variations.
A Zener diode whose breakdown voltage is the desired constant voltage is connected
across the load as shown in Fig 3.3.
33 R
0-20V dc
variable
IL
Id
A
100mA d.c.
27 R
20V dc V
A
10 mA d.c.
or 1A d.c.
RL
220 R
0V
Fig 3.3. A Simple Zener Diode Regulator
•
•
Construct the circuit of Fig 3.3.
Copy the results table as shown in Table 3.2.
Vs
(V)
12
14
16
18
20
•
•
•
Table 3.2
IL
(mA)
0
All readings are for Vd just
equal to the Zener voltage.
Now set the potentiometer temporarily to make IL=0, and then slowly increase VS
until the diode just begins to conduct current, say 1mA. Record VS and IL=0 in the
first row of the table.
Now set the potentiometer to maximum (clockwise). The extra current drawn by RL
will reduce the diode current to below 1mA.
Increase VS to 12V; the diode current will increase above 1mA. Then adjust RL until
the diode current just returns to approximately 1mA. Again record VS and IL
¾ Repeat this for VS=14, 16, 18 and 20V.
Suat SELIM
4
™ Plot IL against VS.
IL (mA)
400
350
300
250
200
150
100
50
0
2
4
6
8
10
12
14
16
18
20
VS (V)
Fig 3.4.
•
Every point on your graph represents a condition where the Zener diode has only just
reached its breakdown voltage. Thus, for a given IL, a lower value of VS will take the
diode out of breakdown and, for a given VS, a higher value of IL will do the same.
• Therefore the whole of one side of your graph is an area where the diode is not in
breakdown and thus is not holding VL constant. Mark this area in your graph.
¾ What sets a limit to the minimum load current and the maximum supply voltage?
•
•
Set the Id meter to the 1A d.c. range.
Copy the results table as shown in Table 3.3.
Table 3.3
Vs
(V)
20
19
18
17
16
15
•
•
IL
(mA)
All readings are for Id just
equal to 150mA.
Now start by reducing RL to minimum (anti-clockwise) and then set VS to 20V and
increase RL until the diode current Id reads 150mA.
Read the load current IL and record it against VS =20V in Table 3.3.
Suat SELIM
5
•
Reduce VS in steps of 1V, each time resetting RL to give Id=150mA approximately
and record IL. Continue until it is no longer possible to set Id to 150mA.
¾ Plot VS against IL on the same axes used for your previous graph (Fig 3.4).
¾ Shade on your graph the area that now represents the useable range of VS and IL
CONCLUSIONS:
Suat SELIM
6