Experiment # 1: p-n junction diode

Aim: To study the I-V characteristics of a p-n junction diode

Equipment & components required: Power Supply (0-30V), Voltmeter (0-30V), Ammeter (μA & mA range), resistors, p-n junction diode

Theory

Circuit diagrams:

220220

Fig: 1.1 Forward Biased diode Fig: 1.2 Reverse Biased diode

Procedure:

  1. Wire up the circuit shown in figure 1.1
  2. Record the voltage across the diode (V) and current (I) through it as a function of input voltage.
  3. Repeat the experiment of the reverse biased diod (ckt 1.2).
  4. Plot the relevant graphs.
  5. Find the equation of dc load line and plot it along with I-V characteristics of fig 1.1

Experiment # 2: zener diode

1. a)Aim: To study the I-V characteristics of a zener diode.

Equipment & components required: Power Supply (0-30V), Voltmeter (0-30V), Ammeter (μA & mA range), resistors, zener diode.

Theory

Circuit diagrams:

470/2W 470/2W

Fig: 2.1 Reverse Biased (zener) diode Fig: 2.2 Voltage regulation by Zener diode

Procedure:

  1. Wire up the circuit shown in figure 2.1
  2. Record the voltage across the diode (V) and current (I) through it as a function of input voltage. Find Zenar Voltage
  3. Connect the circuit shown in figure 2.2. Keep the load resistance RL at 3.3 kΏ. Vary the input voltage in short steps and record the voltage across the zener and current flowing through the zener. Repeat the above step for various RL values.
  4. Plot the relevant graphs.

Experiment # 3: Transistor characteristics BJT

Aim: 1. To study the input and output characteristics of a PNP/ NPN transistors in common base configuration

Equipment: Power Supply ( 0-15V), DMMs and potentiometer and other components.

Theory

Circuit Diagrams:

transistor used CK100

Fig: 3.1 Common Base Configuration of PNP transistor

Procedure:

A. Common Base configuration

  1. For the input characteristics of common base configuration, set-up the ckt of fig 4.1. Set VEE around 5-6 V and do not change it during the experiment (why?). Now vary VEB with the potentiometer and record IE keeping VCB at zero volt. Observations may be recorded up to a maximum of 30mA emitter current. Repeat the observations (for four sets) for various values of VCB from 0 to 10 V.
  2. For output characteristics follow the ckt of fig 4.1. Set VEE around 5-6 V and do not change it during the experiment. Set IE =0 with the pot and record the IC as a function of VCB (from 0-10V). Record at least four sets of O/p curve by varying IE from 0-20mA.
  3. Plot all the curves. Plot the load line. Find input and output resistances and current gain dc.

Experiment # 4 Photovoltaic effect: Solar cell

Objective:

To study (i) the variation of the open circuit voltage as a function of incident light intensity and (ii) the load characteristics under the illumination of a solar cell module.

Theory:

The depletion region of a p-n junction has a built-in electric field under which

negative charges drift towards the n-side and positive charges drift towards the p-side. If the p-n junction is illuminated by light with photon energy greater than the band gap energy of the material, then this will result in the creation of electron-hole pairs. Electron and hole pairs thus created in the depletion region by the incident light, get separated generating a photo voltage under open-circuit conditions (photovoltaic mode, fig. 1) or a photocurrent under some load.

If I is the intensity of light of frequency ν incident on the solar cell with a sensitive area A, and all this light is absorbed by the solar cell to create electron-hole pairs with a quantum efficiency η,then the number of electron-hole pairs created in the cell per second is

Representing the solar cell as a constant current source, the source current (photocurrent) is

given by

Thus the photocurrent is directly proportional to the intensity of the incident radiation.

Fig. 1 shows the equivalent circuit of the solar cell with the current source giving a

current iν, the ideal diode D and a shunt resistance Rp, all in parallel, together with a series resistance rs. For open circuit (Iext= 0) the voltage across the cell is nearly equal to the voltage across the diode. From the usual ideal diode equation, we can thus write

where Isois the reverse saturation current of the diode. For the usual operating conditions,

i νIso. With Iso≈ 0.01 µA, Rp ≈ 108 Ω and V ≈ 0.6V, we have the approximation relation

Thus the photovoltage Vis a logarithmic function of the intensity I. Under some load RL eqn.

3 modifies to

Fig. 2 shows a sketch of the current-voltage characteristics of a p-n junction solar cell under illumination with light. Maximum power will be delivered by the solar cell when the product iext×Vis a maximum. The optimum load is determined by the point (Vm , im) corresponding to the enclosed rectangle having largest area as shown in fig. 2.

Circuit Diagram

Procedure:

1. Illuminate the photovoltaic cell with a suitable light source and record the open circuit

voltage (toggle switch should be in “ no load “position) as a function of intensity (you

can change the intensity by placing filter plates. Transmission of each plate is given.

2. Keep the switch in “with load” position. Record the variation of voltage as a function of

load current for various intensities..

3. Plot a suitable curve to verify Eq. 4.

4. Plot variation of load current as a function ofvoltage and determine the optimum operating point for various intensities.

Experiment # 3: Rectifier and voltage regulation circuits

Aim: To construct Half wave, Full wave and Bridge Rectifier circuits using diodes study the regulation with various filter circuits.

Equipment & components required: Step down transformer with centre tap (12-0-12V) or (9-0-9V), C.R.O., diodes, capacitor and resistors, regulator chips (IC 7809 and IC 7909)..

Circuit diagrams:

Fig: 3.1 Half Wave Rectifier Circuit

Fig 3.2 Half wave rectifier with filter RC circuit

Procedure:

1.Connect the circuit as shown Fig. 3.1 for an half wave rectifier. You may assemble the circuit by using the function generator (set for 50 Hz) also instead of transformer. Check the waveform across the secondary of the transformer (or the function generator) by displaying on one of the channel of oscilloscope. After setting the oscilloscope to observe the above wave forms, connect the other channel of the oscilloscope to the output resistor of the circuit. Trace the output and input (secondary of the transformer) waveforms Measure peak to peak voltages and dc voltage if any.

  1. Connect the first filter circuits of fig 3.2.
  1. Display the input and out put voltages both on the scope. Measure the dc voltages and peak to peak ripple voltages. Also measure the time elapsed between two consecutive cycles of diode conduction. Compare the results with the theoretical estimates. (you might have to put input coupling to ac mode for some of these measurements.
  2. Vary R and C values and observe the changes.
  3. Connect the second filter circuit of fig 3.2 and measure (and trace) the voltage drop across 56 resistance. Estimate the current through the capacitor from this and explain the observations.
  4. Make the full wave rectifier circuit as shown in the fig.3.3. Measure the peak to peak input and out put voltages using the oscilloscope. Trace the output signal across the resistor using the oscilloscope.
  5. Make the bridge wave rectifier circuit as shown in fig.3.4. Trace and measure the peak to peak voltages of input and output signals.
  6. Make the ckt of fig 3.5 for full wave rectifier with filter. Trace the output both with the capacitor disconnected and connected. Measure the ac and dc voltages and compre with th ehalf wave rectifier of fig 3.2.
  7. Connect the dual power supply circuit as shown in fig. 3.6. 78xx and 79xx series IC’s are positive and negative voltage 3-pin regulators respectively. Measure the wave forms at the input and output ac and dc(both the positive and negative voltages).
  8. Assemble the voltage doubler circuit of fig 3.7 and observe th einput and output wave form.

Try to modify the circuit in fig. 3.6 to produce on output that gives variable + Ve or –Ve regulated outputs. Read the manufacturer’s manual on 3-pin regulator chips.