VOLTAGE-CURRENT CHARACTERISTICS OF
A PN JUNCTION DIODE
To
draw the voltage-current (V- I) characteristics of the PN junction diode and to
determine its knee voltage and forward resistance.
PN
junction diode (IN4007), variable DC power supply, milli-ammeter,
micro-ammeter, voltmeter, resistance and connecting wires.
where,
RF
→ Forward resistance of the diode (Ω)
∆VF
→ The change in forward voltage (volt)
∆IF
→ The change in forward current (mA)
Figure (a) PN junction diode and its symbol (Silver ring denotes the negative terminal of the diode)
Figure
(b) PN junction diode in forward bias
Figure
(c) PN junction diode in reverse bias
Care
should be taken to connect the terminals of ammeter, voltmeter, dc power supply
and the PN junction diode with right polarity.
·
In the forward bias, the P- region of
the diode is connected to the positive terminal and N-region to the negative
terminal of the DC power supply.
·
The connections are given as per the
circuit diagram.
·
The voltage across the diode can be
varied with the help of the variable DC power supply.
·
The forward voltage (VF)
across the diode is increased from 0.1 V in steps of 0.1 V up to 0.8|V and the
forward current (IF) through the diode is noted from the
milli-ammeter. The readings are tabulated.
·
The forward voltage VF and
the forward current IF are taken as positive.
·
A graph is drawn taking the forward
voltage (VF) along the x-axis and the forward current (IF)
along the y-axis.
·
The voltage corresponding to the dotted
line in the forward characteristics gives the knee voltage or threshold voltage
or turn-on voltage of the diode.
·
The slope in the linear portion of the
forward characteristics is calculated. The reciprocal of the slope gives the
forward resistance of the diode.
·
In the reverse bias, the polarity of the
DC power supply is reversed so that the P- region of the diode is connected to
the negative terminal and N-region to the positive terminal of the DC power
supply
·
The connections are made as given in the
circuit diagram.
·
The voltage across the diode can be
varied with the help of the variable DC power supply.
·
The reverse voltage (VR)
across the diode is increased from 1 V in steps of 1 V up to 5 V and the
reverse current (IR) through the diode is noted from the
micro-ammeter. The readings are tabulated.
·
The reverse voltage VR and
reverse current IR are taken as negative.
·
A graph is drawn taking the reverse bias
voltage (VR) along negative x-axis and the reverse bias current (IR)
along negative y-axis.
(i)
Forward resistance RF =
(ii)
knee voltage =
The
V-I characteristics of the PN junction diode are studied.
i)
Knee voltage of the PN junction diode =………………V
ii)
Forward resistance of the diode =………………Ω
·
The DC power supply voltage should be
increased only up to the specified range in the for-ward (0 – 2V) and reverse
(0 – 15V) directions. Forward bias offers very low resistance and hence an external
resistance of 470Ω is connected as a safety measure.
·
The voltage applied beyond this limit
may damage the resistance or the diode.
·
In the forward bias, the current flow
will be almost zero till it crosses the junction potential or knee voltage
(approximately 0.7 V). Once knee voltage is crossed, the current increases with
the applied voltage.
·
The diode voltage in the forward
direction should be increased in steps of 0.1 V to a maximum of 0.8 V after the
threshold voltage to calculate the forward resistance.
·
The diode voltage in the reverse direction
is increased in steps of 1 V to a maximum of 5 V. The current must be measured
using micro-ammeter as the strength of current in the reverse direction is very
less. This is due to the flow of the minority charge carriers called the
leakage current.
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