% voltage (AC/DC), current (AC) and check continuity of a eve! ‘uit using multimeter. ARATUS AND MATERIAL Apparatus. Three carbon resistors, one standard resistance coil, a battery eliminator tapping (2 V, 4 V and 6 V), a step down transformer (6-0-6 V) with two tappings (2 V and a resistor of 100 ohm, a plug key and multimeter. 3g Multimeter. It is a single measuring device acting as an Ammeter, a Voltmeter and an eter. For this reason, it is also called AVO meter. It can measure alternating as well as direct current and alternating as well as direct in addition to resistance. For this purpose, its panel is divided into five different sections. are many ranges in each section so that it can measure from micro (10) to mega (109) its. Rotation of a knob changes the section and the range in one section. — Rotation of knob for change in ammeter range, brings shunt resistances of different values in circuit in parallel with the coil. — Rotation of knob for change in voltmeter range, brings series resistances of different values in circuit in series with the coil. — Rotation of knob for change in ohmmeter range, brings different resistances in cir- cuit in series with the multimeter cell. 1. Carbon resistors are frequently used in electrical and electronic circuits and their values vary over a very wide range. A colour code is used to indicate the value of the resistance. 2. A carbon resistance has four different concentric coloured ringe or bands on its surface. The first three bands a, 6 and c determine the value of the resistance and the fourth band d gives the percentage of accuracy called tolerance. The resistance of carbon resistor, R = (ab x 10° + T%)Q.I | 440 COMPREHENSIVE PRACTICAL PHYSICs_ LETTERS EI ORO ER RR RL ARI: EO Ny 3. To read the value of carbon resistance, the following sentence is of great helpful, B B R O Y Great Britain Very Good Wife o 1 2 8 4 5 6 7 8 9 The bold face letter B, B, R, O, Y, G, B, V, G and W, in above sentence correspond | the colours Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey and Whij respectively for a and b, while for the third c, they correspond to the multipliers 19 10!, 102, 103, 104, 105, 108, 10”, 108 and 10° respectively. The following table provides the colour code for the carbon resistors : » Gold Silver 10% No colour 20% fdodcwoxonmue PROCEDURE (a) Resistance 1. Mark the given carbon resistors as Ry, Ry, Ry. 2. Note the colour of first, second, third and fourth ring for each resistor. 3. Plug in the probes of the multimeter in appropriate terminals. Select the or range, short the other ends of probes and adjust zero. Turn the terminal marked ‘Adjust’ such that the needle of the meter shows full scale deflection and reads ze" ohm. Separate the metallic ends of the probes. Insert the resistor R, to be measured i? between the metallic ends of the two probes and read the deflection of the pointer 0” the range selected. Repeat the step 4 for other resistors R, and R, each time selecting the appropriat® range and testing the zero. = a = . Now convert using the colour code table, the values of resistors in ohms and writ” their values with tolerance.D.C. (10) nring veietance (0) Measuring D.C. vonage (c) Measuring A.C. voltage _(d) Checking continuity Using a multimeter to measure : (a) resistance, (b) D.C. voltage, (c) AC, voltage and (d) resistance and checking continuity. (6) D.C. Voltage 1. Select a D.C. source of potential difference, 6 V battery eliminator or a battery. 2. Plug the probes black in com and red in +. 3. Select D.C. volt, by turning the selector switch to range 10 V D.C. volts. 4, Touch and press other ends of probes such that red is on battery terminal marked + and black on—[Fig. (b)] and not the reading. 5. Insert red probe in terminals marked 4 V, 2 V in succession and note readings. (c) A.C. Volts 1. Turn the selector A.C. (200 Volt). 2. Touch and press the probes other ends to two terminals of A.C. source of potential drop and note the reading. 3. Use red probe in terminals 4 V and 2 V in succession and record the reading. (@) Continuity of given circuit [Fig. (d)] 1. Set the selector switch to ohm, range MQ. 2. Touch and press the ends of probes at A and B, full scale deflection indicates continu- ity. 3. Similarly check in succession the continuity between terminals B and C and termi- nals C and D. Never place the probes ends between terminals connected with the terminals of the In digital electronic multimeter for continuity test rotor is set to mark 0 and a buzzer nds when probes are connected to ends of components tested. SERVATIONS __ (a) For Measurement of Resistance :VW2 COMPREHENSIVE PRACTICAL PHYsicg_ (b) Measurement of Voltage : AC. or D.C. Voits Obs. Voltage between Voltage reading as | Difference in Voltag, S.No. | terminals V, (Volt) measured by reading and volt. multimeter V (Volt) | marked 2V~ Vz (vol t 1. 2. 3. | . 2. 3. INFERENCE 1, The measured values by multimeter match with decoded values of resistors. 2. A.C. and D.C. voltages marked on voltage sources match with voltage measure: multimeter. PRECAUTIONS 1. Instructions for handling the multimeter should be gone through thoroughly as a very handy instrument and is likely to get dam: aged if carelessly or ignorantly u 2. Select the appropriate parameter current, voltage or resistance to the measured set it on appropriate range. 3. If range of the parameter measured is not known, start with maximum. For mea ing V, never connect more than maximum 600 V.