Experiment-437
A
STUDY OF DIODE CHARACTERISTICS:
A NEW APPROACH INVOLVING
INTERFACING WITH COMPUTER
Jeethendra Kumar P K and Santhosh K
KamalJeeth Instrumentation & Service Unit, JRD Tata Nagar, Bengaluru-560092, INDIA
Email: labexperiments@rediffmail.com
Abstract
Forward and reverse bias characteristics of silicon diode are studied using
software and computer interfacible apparatus. The knee voltage and input DC
resistances are calculated from the characteristic curves and compared with the
corresponding data provided by the manufacturer.
Introduction
Germanium diodes are now obsolete and only silicon diodes are being used. Hence we
have selected silicon diodes for this experiment. Depending on the current rating and
peak-inverse voltage (PIV), there is variety of diodes. Among them 1N series diodes are
popular which can be used for low current (<1A) rectifier applications. For higher
current applications IN5402, 6AO5 diodes are used. Other than rectifier diodes, high
frequency (> 100Hz and <100 KHz) diodes are also available. In addition to this the so
called point contact diodes which have a linear resistance variation in the forward
characteristics used in the demodulation of amplitude modulated signals have now
become obsolete. Hence diodes are now being used for rectifying AC to DC signals.
Diode characteristics
The silicon technology improved the devices that yield characteristics close to the
corresponding theoretical values. Hence the present day diodes are more reliable than
those used about 15 years back.
A silicon diode conducts only in one direction acting like a switch which can be either
‘on’(when it conducts) or ‘off’ (when it does not conduct). A diode starts conducting
when it is forward biased and the voltage across it is at least 0.6V. This voltage is called
knee voltage. For any increase in the voltage above this knee voltage, a diode starts
conducting heavily. If it is reverse biased, it does not conduct. However, there will be a
small reverse current (in nano- ampere range) that flows in the reverse biased condition
due to holes which are minority carriers. Further, the reverse current remains constant
with reverse applied voltage and depend on the temperature of the diode. The reverse
current doubles for every 100 rise in the temperature, as shown in Figure-1 for
Germanium diode. For all practical purposes silicon diodes do not conduct in the
reverse bias condition.
There is a large reverse current (in the µA range) in Germanium diodes that is why
these are called leaky diodes. This property of doubling of the current for every 100
increase in the temperature is employed in digital thermometers for measurement of
temperature. For a few years Germanium diodes were being used as temperature
sensors. However, the Germanium technology has become obsolete now [1].
Figure-1: Reverse bias characteristics of a diode
Figure-2 shows the computer interfacible experimental set-up used in this
experiment. It consists of a regulated power supply 0-20V, digital µA/mA current
meter, and 0-20V voltmeter. The same instrument is used for study of zener diode
characteristics as well as LED characteristics.
Figure-2: Diode, Zener diode and LED characteristics
Experimental procedure
The Diode Zener diode characteristics software, developed by KamalJeeth
Instrumentation and Service Unit, is uploaded in the computer and the
Experimental set-up DZDµ-1222 is connected to the computer through USB port.
The experiment consists of two parts, namely
Part-A: Diode forward biased
Part-B: Diode Reverse biased
Part-C: Zener diode forward biased
Part-D: Zener diode reverse biased
Part-A: Forward biased
1. The DZD folder is clicked and KJISU-DZD connectivity is selected and
double clicked on the icon. The following window will open, as shown in
Figure-3.
Figure-3: Front window for study of forward bias characteristics
2. Clear the data, if any, present by clicking the ‘clear’ button of the “controls”
and choose ‘forward bias’. The electrical connections in the set-up are made
as shown in Figure-2.
Figure-4: Forward bias curve for 1N4007 diode
3. The forward voltage is set to 0V and current is noted and data is transferred
to the PC by pressing “Transmit to PC button”. The data will be recorded in
the Table appearing on the left side of the front window.
4. The experiment is repeated by increasing the voltage in steps of 0.2V initially
and in steps of 0.05V after the flow of current starts. The data is recorded in
the front window table and the curve for the forward bias case is shown in
Figure-4.
From the curve the slope of the curve after the conduction is calculated using
the Microsoft Excel work sheet as:
Forward resistance of 1N4007 diode = 380Ω
Forward knee voltage (η) = 0.52V
Part-B: Reverse biased
Figure-5: Reverse bias characteristics of silicon diode 1N4007
5. The diode is reversed in the socket and current selection switch ‘µA/mA’ is
switched to µA range for reverse bias.
6. The reverse voltage is set to 0V and data is transferred to the PC. Reverse
voltage is increased in steps of 2V up to a maximum of 20V and after each
voltage setting the data is transferred to the PC.
7. The output characteristic curve is shown in Figure-5. No current was
observed in the micro ammeter up to 20V.
Part-C: Zener diode with forward bias
8. The data in the PC is cleared by pressing the ‘Clear” button
9. The diode is replaced by the zener diode provided with the set-up and forward
bias mode is selected.
10. Forward voltage is set to 0V and data is transmitted to the PC.
11. Forward voltage is increased in steps of 0.2V up to 0.6V and then in smaller steps
of 0.05V. In each case the data is transferred to the PC and forward characteristics
of the zener diode are noted.
12. The slope of the I-V characteristic curve is calculated. This gives the forward
resistance of the zener diode when it is conducting as :
Rforward = Rsat = 202Ω
Knee voltage (η) =0.64V
Figure-6: Forward bias of 12V applied to silicon zener diode
Part-D: Zener diode reverse biased
.
Figure-7: The reverse bias characteristics of Zener diode
13. The zener diode is reversed in the socket and current selection switch
remains as before in the ‘mA’ position.
14. The reverse voltage is set to 0V and data is transferred to the PC. Reverse
voltage is increased in steps of 1V up to a maximum of 11V and thereafter in
smaller steps of 0.1V and in each case the data is transferred to the PC.
15. The output characteristic is shown in Figure-7. From the data a curve (after
conduction) is drawn using Microsoft Excel sheet, the slope gives the
forward resistance of the zener diode. The zener voltage is noted at the
center of the linear portion of the curve.
Rsat = 23.75Ω
VZ = 12.2V
Results
The results obtained are tabulated in Table-1
Table-1: Characteristic parameters of diode 1N4007 and the zener diode 1W
Zener Diode
Standard
Parameter
Diode(1N4007)
(12V)
Forward
resistance
380Ω
220
<500Ω
(Rsat)Ω
Reverse resistance (RO)Ω
∞
24
Zener Voltage (V)
12.2
Knee voltage (η) V
0.54
0.64
0.6-0.8
Discussion
The I-V signatures of diode 1N4007 and zener diode are shown in Figures- 4 and 5 for
the diode 1N4007 and Figures- 6 and 7 for the zener diode which match with the
corresponding theoretical curves. This is another classic experiment with a futuristic
outlook, making it convenient and faster. The forward resistance of the diode 1N4007
(380Ω), and of the zener diode (24Ω), reverse resistance of the diode 1N4007 (∞) and of
the zener diode (220Ω) shows that the modern versions of diodes as well as zener
diodes are more perfect compared to earlier devices, which is an indication of
improvement in the diode technology. It may, however, be noted that India does not
manufacture any semiconductor diodes.
References
[1]
Malvino A P, Electronic Principles, Edn: 3, Tata-McGraw-Hill, Page- 47, 1984