Practical No. 2


Aim:

To plot V-I characteristics of a semiconductor diode and to calculate its static and dynamic resistance.

Equipment Required:

S.No. Equipment
1 Power supply, 0 to 30 V d.c.
2 Meter to read d.c., 0 to 4 V
3 Meter to read d.c., 0 to 3 mA and 0 to 50 mA
4 (i) Germanium diode - OA 79
(ii) Silicon diode IN 4001
5 20 MHz oscilloscope
 

Procedure:

Part-1: Silicon junction, forward characteristics  


 
Silicon junction, forward characteristics

Circuit Connections:

Component Specification
Variable D.C. supply —
Milliammeter (30 mA)
Resistor 100 Ω
Voltmeter (10 V)
  1. Connect the circuit as shown in Figer using the silicon diode and 100 resistor with two external meters.
  2. Set the ammeter 'A' to its 30 mA Full Scale Deflection (F.S.D.).
  3. Set the voltmeter 'V' to 10 F.S.D. range.
  4. Set the d.c supply initially to 0 V.
  5. Prepare an observation table as shown in Table 2.1.
  6. Carefully turn the voltage control clockwise whilst watching the current meter.
  7. Set the current to each of these values in turn: 1 mA, 2.1 mA, 3 mA, 3.3 mA, 5 mA. For each of these values, read the voltmeter and plot the corresponding points on the graph paper with axes labelled as in Fig. 2.2.
  8. Repeat this for currents increasing by 5 mA.

Observation Table

S.No. Voltage (V) Current (mA)
1 0.5 1
2 0.75 2.1
3 1 3
4 1.2 3.3
5 1.55 5
 
graph

Result:

The graph shows that little current passes until the diode voltage has risen to about 1.5 V (cut-in voltage) but the current rises rapidly thereafter with further increase in voltage.

Part-2: Germanium Junction, Forward Characteristics

The forward characteristics of the germanium diode should be obtained in exactly the same way as for the silicon diode, except the suitable current values are 0.2mA, 0.5mA, 1mA, until the maximum supply voltage of 4V is reached.

To calculate static and dynamic resistance of diode:

Calculation 1: Static Resistance

After plotting graph, draw a vertical line at 0.7 volts.

Static Resistance (Rdc): Rdc = 0.7 / I1

Calculation 2: Dynamic Resistance

Draw the vertical line at 0.75 volts.

Dynamic Resistance (Rac): Rac = ΔVd / ΔId

Rac = (0.75 − 0.7) / (I2 − I1)

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 Frequently Asked Questions (FAQs)
To plot the V-I characteristics of Silicon and Germanium diodes, and to calculate static and dynamic resistance.
IN4001 (Silicon diode) and OA79 (Germanium diode) are used in this practical.
Static resistance (Rdc) is calculated as V/I at a specific point, e.g., at 0.7V divided by I₁ from the graph.
Dynamic resistance (Rac) = ΔV/ΔI = (0.75 - 0.7) / (I₂ - I₁).
You can explore other lab posts like Practical No.1, Full Bridge Rectifier, and more.
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