Basics of Electronic CircuitsExperiment 7 Characteristics of Passive Devices 2005-06/I

In this experiment, we will examine the physical features and the electrical characteristics of various passive devices used in electronic circuits. The circuit given in Fig. 7.1 will be used to obtain a display of the i-v characteristic of any “Device Under Test” (DUT) by making use of the x-y display mode available in the CRO. The two resistances used in the circuit are R = 1.00 k and RS = 1.21 k. Note that, as the inverting input of the opamp is at “virtual earth” potential, and the currents flowing into the two input terminals (inverting and non-inverting) of the opamp are negligibly small, the output voltage of the opamp is given by vo = – idR, where id is the current flowing through the DUT as shown in Fig. 7.1. As R = 1.00 k, the y axis of the x-y display on the CRO will represent the current id in mA, but with its sign inverted. The x axis, of course, represents the voltage vd, and hence a display of (–id) against vd is obtained in the given circuit arrangement. The role of RS is only to protect the DUT from accidental high voltage applied from the FG. Its value is therefore not critical.

i

Fig. 7.1 Circuit for displaying i-v characteristic, with the pin connection of the opamp1. Keep the power switch of the D-C Power Supply and the FG OFF. Connect the four sockets

in the Power Supply section of the Techlab to the four parallel bus strips (two on the top side, and two on the bottom) of the breadboard. Use an YELLOW cable for +12 V, a BLUE cable for –12 V, a RED cable for +5 V and a BLACK cable for Ground. This power supply bus arrangement will be followed for all future experiments, in order to minimise errors in connections. All power supply connections to any circuit assembled on the breadboard will be done through these four bus strips.

2. Connect the FG ground to the same ground bus in order to establish the common reference point for all voltages. Assemble the circuit given in Fig. 7.1, without any DUT, on the breadboard, and connect pin 7 and pin 4 of the opamp to the +12 V bus and the –12 V bus respectively. Be very careful about these connections, as a wrong power supply connection will almost definitely burn the opamp.

3. Set the frequency of the FG at 1 kHz. With any convenient amplitude setting for the FG, verify that vo = 0 and vd is the same as the FG output voltage.

Part A. Resistors Resistors used in electronic circuits are usually classified according to the following properties:

Composition (e.g. carbon film, metal film, wire-wound etc.),Tolerance (e.g. 10%, 5%, 1% etc), and Power rating (e.g. ¼ W, ½ W, 1W, 5 W etc.).

Of these three features, the first one has to be found out from the manufacturer's specifications. Tolerance is always indicated on the body of the resistor itself along with the value of the resistance. The value and the tolerance, unless printed on the resistor, are coded by four or five coloured bands.

For 5% and 10% resistors, the 1st and 2nd bands give the significant digits, and the 3rd band gives the number of zeros on the right of the two significant digits. For 1% and 2% resistors, the 1st, 2nd

and 3rd bands give the significant digits, and the 4th band gives the number of zeros on the right of the three significant digits The ten colours denoting the digits 0-9 are the following: Black (0), Brown (1), Red (2), Orange (3), Yellow (4), Green (5), Blue (6), Violet (7), Grey (8), White (9). The 4th or the 5th band, as the case may be, indicates the tolerance: a golden band indicating a

8 7 6 5 +VCC

741C +

VCC 1 2 3 4

id

RS + vd R +12V. CRO

id 2 7 3 + 4 6 + I(Y) II(X) + vo vd 12V.

DUT

FG

tolerance of 5%, and a brown or a red band indicating a tolerance of 1% or 2%. Absence of a tolerance band implies 10% tolerance. The tolerance band is slightly away from the other bands.

4. Identify the different types of resistors given to you from their approximate distinguishing features. Using the colour code, note down the values and tolerances of the different resistors provided. Verify the values by measuring the resistances with the multimeter.

5. Put a 1.00 k resistor as the DUT, and switch on the d-c power supply. With the peak-to-peak value of the FG output set at 10 V, observe the waveforms of the voltages vo and vd in the DUAL y-t mode. Reduce the amplitude appropriately if the waveforms are not sinusoidal. Change the display mode to x-y and sketch the i-v characteristic displayed on the CRO, with the origin and the scales for id and vd properly marked.

6. Hence verify that the resistance of the DUT matches the slope of the i-v characteristic. 7. Increase the frequency of the FG gradually, until the display becomes a loop instead of

remaining a line. Record the frequency at which the display just starts becoming a loop. Note how the shape of the loop changes as the frequency is further increased.

Part B. Capacitors and InductorsCapacitors used in electronic circuits are classified according to the dielectric material used in the fabrication of the capacitor and the range of values available varies from type to type. Some of the commonly encountered types are listed below:

Electrolytic ( 1 F) – Cylindrical body with printed value and polarity indicated by +/– sign;Polyester (0.001–10 F) – Moulded body with value either printed or colour-coded;Ceramic ( 1 F) – Tubular or disc shaped body, with value printed.

These capacitors may be assumed to have 10% tolerance unless indicated otherwise. 8. Identify the different types of capacitors from their approximate distinguishing features, and

note down their values as well as voltage ratings.9. Repeat step 5 with the given 0.047 F capacitor as well as the given coil (L 1 mH) as the

DUT, increasing the FG frequency to a suitable value so as to obtain a nearly circular display. How would you find the values of the reactances from the intercepts of these characteristics?

Part C. Temperature and Light Sensors10. Identify each of the three following sensor devices by visual

examination, and repeat step 5 with each of them used as the DUT:(i) thermistor (with negative temperature coefficient),(ii) LDR (light-dependent resistor), and(iii) photo-transistor (pin connection given in Fig. 7.2).

11. Observe the effects of (i) heating the thermistor by pressing it between two fingers, and (ii) shining light on the LDR and the phototransistor by applying a d-c voltage of 5 V to the series combination of the white LED and two resistors. Note that while the LDR has a resistive characteristic and hence the slope changes when illumination is changed, the phototransistor behaves like a light-controlled current source, and hence the flat region of the characteristic remains flat, but the value of the current changes when illumination is changed.

Part D. Different Types of Diodes 12. Identify each of the following four different types of diodes by visual examination, and repeat

step 5 with each of them used as the DUT. Connect the diode in each case so that the current id indicated in Fig. 7.1 is in the forward direction of the diode:(iv) general-purpose diode (1N4148),(v) rectifier diode (1N4007),(vi) zener diode (voltage printed on the body) and(vii) LED (light-emitting diode – red)

B left open lead E C lead

Fig. 7.2 Phototransistor Pin Connections