UNIT - 1

UNIT 1 DIGITAL INSTRUMENTS

DIGITAL VOLTMETERSDigital voltmeters are essentially analog-to-digital converters with digital displays to indicate the measured voltage. DVM displays measurements of ac or dc voltages as discrete numerals instead of a pointer deflection on a continuous scale. The development of ICs has lead to drastic reduction of the size, power requirements and cost of DVM. Hence DVMs can actively compete with conventional analog instruments, both in portability and price.CONSIDERATIONS IN CHOOSING AN ANALOG VOLTMETERThe most appropriate instrument for a particular voltage measurement depends on the performance required in a given situation. Some important considerations are Input Impedance Voltage Ranges Decibels Sensitivity versus Bandwidth Battery Operation

Typical Operating and Performance Characteristics of DVMs Input range. From 1.000000 V to 1,000.000 V, with automatic range selection and overload indication Absolute accuracy. As high as 0.005 per cent of the reading Stability. Short-term, 0.002 per cent of the reading for a 24-hr period; long term, 0.008 per cent of the reading for a 6-month period Resolution. 1 part in 106 (1 V can be read on the 1-V input range) input characteristics. Input resistance typically 10 M_; input capacitance typically 40 pF Calibration. Internal calibration standard allows calibration independent of the measuring circuit; derived from stabilized reference source Output signals. Print command allows output to printer; BCD (binary coded- decimal) output for digital processing or recording Additional features may include additional circuitry to measure current, resistance, and voltage ratios. Other physical variables may be measured by using suitable transducers.

Classification of DVMs1. Ramp-type DVM2. Integrating DVM3. Continuous-balance DVM4. Successive-approximation DVM

Ramp-type DVMIts Operating principle is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time-interval counter, and the count is displayed as a number of digits on electronic indicating tubes.

Fig. Block diagram of a ramp-type digital voltmeterThe working principle i.e., the Conversion from a voltage to a time interval is illustrated by the waveform in Fig. 10

At the start of the measurement cycle, a ramp voltage is initiated; this voltage can be positive-going or negative-going. The negative-going ramp, (see Fig. 10) is continuously compared with the unknown input voltage. At the instant that the ramp voltage equals the unknown voltage, a coincidence circuit, or comparator, generates a pulse which opens a gate. The ramp voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second comparator generates an output pulse which closes the gate. An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units (DCUs) which totalize the number of pulses passed through the gate. The decimal number, displayed by the indicator tubes associated with the DCUs, is a measure of the magnitude of the input voltage.The sample-rate multivibrator determines the rate at which the measurement cycles are initiated. The oscillation of this multivibrator can usually be adjusted by a front-panel control, marked rate, from a few cycles per second to as high 1,000 or more. The sample-rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the same time, a reset pulse is generated which returns all the DCUs to their 0 state, removing the display momentarily from the indicator tubes.

Staircase-Ramp DVMIt is a variation of the ramp-type DVM but is simpler in overall design, resulting in a moderately priced general-purpose instrument that can be used in the laboratory, on production test-stands,

in repair shops, and at inspection stations. Stair case ramp DVM makes voltage measurements by comparing the input voltage to an internally generated staircase-ramp voltage.

It contains a 10-M_ input attenuator, providing five input ranges from 100 mV to 1,000 V full scale. The dc amplifier with a fixed gain of 100 delivers 10 V to the comparator at any of the full-scale voltage settings of the input divider. The comparator senses coincidence between the amplified input voltage and the staircase-ramp voltage which is generated as the measurement proceeds through its cycle. A Clock (4.5 kHz oscillator) provides pulses to three DCUs in cascade. The units counter provides a carry pulse to the tens decade at every tenth input pulse. The tens decade counts the carry pulses from the units decade and provides its own carry pulse after it has counted ten carry pulses. This carry pulse is fed to the hundreds decade which provides a carry pulse to an over range circuit. The over range circuit causes a front panel indicator to light up, warning the operator that the input capacity of the instrument has been exceeded. The operator should then switch to the next higher setting on the input attenuator. Each decade counter unit is connected to a digital-to-analog (D/A) converter. The outputs of the D/A converters are connected in parallel and provide an output current proportional to the current count of the DCUs. The staircase amplifier converts the D/A current into a staircase voltage which is applied to the comparator. When the comparator senses coincidence of the input voltage and the staircase voltage, it provides a trigger pulse to stop the oscillator. The current content of the counter is then proportional to the magnitude of the input voltage. The sample rate is controlled by a simple relaxation oscillator. This oscillator triggers and resets the transfer amplifier at a rate of two samples per second. The transfer amplifier provides a pulse that transfers the information stored in the decade counters to the front panel display unit. The trailing edge of this pulse triggers the reset amplifier which sets the three decade counters to zero and initiates a new measurement cycle by starting the master oscillator or clock. The display circuits store each reading until a new reading is completed, eliminating any blinking or counting during the computation. The ramp type of A/D converter requires a precision ramp to achieve accuracy.Maintaining the quality of the ramp requires a precise, stable capacitor and resistor in the integrator. In addition, the offset voltages and currents of the operational amplifier used in the integrator are critical in the accurate ramp generator. One method of reducing the dependence of the accuracy of the conversion on the resistor, capacitor, and operational amplifier is to use a technique called the dual-slope converter.

Dual-Slope ConverterIn the dual-slope technique, an integrator is used to integrate an accurate voltage reference for a fixed period of time. The same integrator is then used to integrate with the reverse slope, the input voltage, and the time required to return to the starting voltage is measured. The Order of integrations does not matter. Consider the integration of the unknown first as shown in Fig.

The output voltage Vout is given byWhere t - elapsed time from when the integration began. The above Equation also assumes that the integrator capacitor started with no charge & thus the output of the integrator started at zero volts. If the integration were allowed to continue for a fixed period of time T1, the output voltage would be

Notice that the integrator output has gone in the opposite polarity as the input. That is, a positive input voltage produces a negative integrator output. If a reference voltage Vref, were substituted for the input voltage Vx, as shown in Fig.13, the integrator would begin to ramp toward zero at a rate of Vref/ RC assuming that the Vref was of the opposite polarity as the unknown input voltage. The integrator for this situation does not start at zero but at an output voltage of V1, and the output voltage Vout is

Setting the output voltage of the integrator to zero and solving for Vx yields

Where Tx is the time required to ramp down from the output level of V1 to zero volts. Notice that the relationship between the reference voltage and the input voltage does not include R or C of the integrator but only the relationship between the two times. Because the relationship between the two times is a ratio, an accurate clock is not required but only that the clock used for the timing be stable enough that the frequency does not change appreciably from the up ramp to the down ramp.

As the integrator responds to the average of the input, it is not necessary to provide a sample and hold, as changes in the input voltage will not cause significant errors. Although the integrator output will not be a linear ramp, the integration will represent the end value obtained by a voltage equal to the average of the unknown input voltage. Therefore, the dual-slope analog-to-digital conversion will produce a value equal to the average of the unknown input.The dual-slope type of A/D conversion is a very popular method for digital voltmeter applications. When compared to other types of ADC techniques, the dual-slope method is slow but is quite adequate for a digital voltmeter used for laboratory measurements. For data acquisition applications, where a number of measurements are required, faster techniques are recommended. Many refinements have been made to the technique and many large-scale-integration (LSI) chips are available to simplify the construction of DVMs. When a dual-slope A/D converter is used for a DVM the counters may be decade rather than binary and the segment and digit drivers may be contained in the chip. When the converter is to interface to a microprocessor, and many high performance DVMs use microprocessors for data manipulation, the counters employed are binary. One significant enhancement made to the dual-slope converter is automatic zero correction. As with any analog system, amplifier offset voltages, offset currents, and bias currents can cause errors. In addition, in the dual-slope A/D converter, the leakage current of the capacitor can cause errors in the integration and consequentially, an error. These effects, in the dual-slope AID converter, will manifest themselves as a reading of the DVM when no input voltage is present. Fig. shows a method of counteracting these effects.

The input to the converter is grounded and a capacitor, the auto zero capacitor, is connected via an electronic switch to the output of the integrator. The feedback of the circuitry is such that the voltage at the integrator output is zero. This effectively places an equivalent offset voltage on the automatic zero capacitor so that there is no integration. When the conversion is made, this offset voltage is present to counteract the effects of the input circuitry offset voltages. This automatic zero function is performed before each conversion, so that changes in the offset voltages and currents will be compensated.

Fig. Block Diagram of complete dual-slope A/D converter

Successive-approximation DVM

A D/A converter is used to provide the estimates. The "equal to or greater than" or "less than" decision is made by a comparator. The D/A converter provide the estimate and is compared to the input signal. A special shift register called a successive-approximation register (SAR) is used to control the D/A converter and consequentially the estimates. At the beginning of the conversion all the outputs from the SAR are at logic zero. If the estimate is greater than the input, the comparator output is high and the first SAR output reverses state and the second output changes to a logic "one." If the comparator output is low, indicating that the estimate is lower than the input signal, the first output remains in the logic one state and the second output assumes the logic state one. This continues to all the states until the conversion is complete.

DIGITAL MULTIMETER Measurement of any quantity is a result of comparison between the quantity to be measured and a definite worldwide standard. The instruments which are used for such comparison are called measuring instruments. In electronics measurement three basic quantities current, voltage and power are important to measure. The measurement of these quantities can be used to obtain measurement of some other quantities and also can be used to analyze performance of different electronic circuits, devices and components. The measuring instruments which are used to measure current flowing in a circuit are called ammeter while the instruments are used to measure voltage across two points are called voltmeter. The instruments which are used to measure power are called power meters or wattmeters. Measuring instruments can be classified as analog instruments and digital instruments.Digital multimeters: The digital multimeter is an instrument which is capable of measuring a.c. voltages, d.c. voltages, a.c. and d.c. currents and resistances over several ranges. The basic circuit of a digital multimeter is always a d.c. voltmeter as shown in figure-5. The parameter which is to be measured has to be converted into voltage form firstly. Then if parameter in the form of voltage is analog will be converted into digital form by ADC. Digital data can now be displayed in the form of BCD, decimal or digital form. Digital multimeter can also be interfaced to other suitable device through interface terminal. To measure voltage, firstly rotate knob to a.c. or d.c. voltage terminal whatever be the required. Then voltage will be converted into digital form, and then will be displayed.

Figure- Block diagram of digital multimeterFor current, firstly rotate knob to current terminal then current will be converted into voltage form by using current to voltage converter. Then voltage will be converted into digital form and will be displayed. To measure resistance, rotate knob to resistance terminal and measure resistance. In this process resistance will be converted into voltage then it will be converted into digital form and will be displayed accordingly. From above discussion we can see that digital multimeter can be used to measure multiple type parameters and can displayed data digitally. Therefore we call it digital multimeter. So, digital multimeter can also be displayed by following figure-6. This figure is just combination of figure-

Left part of figure-6 shows conversion of multiple parameters into voltage form (from figure-5) and right part shows digital voltmeter (from figure-4) which displays parameters digitally.

SIMPLE DIGITAL FREQUENCY METER Introduction: Simple digital frequency meter has many applications. It may be an experiment for beginners, laboratory equipment or a meter built into some device. Ideal wherever there is a need to measure and digitally display the frequency. Circuit DescriptionThe frequency meter is built only from common components (logic), without a microprocessor (microcontroller) that have to be programmed. The basis is 74390 double decimal counter (74HC390 - CMOS, 74LS390 - bipolar). Oscillator with IO1 (555) determines the time of counting. With the component values listed in the schematic diagram below, the counting time is 1s. Frequency meter therefore measures with a resolution of 1Hz. The displays are DIS1417 or TIL311. They have already built LATCH circuits and decoders from BCD to 7 segments. This eliminates the need to use external ones. If you would like to use the standard 7-segment display, it would be necessary to use external LATCH (eg 4-bit 7475 / 74HC75 / 74LS75 8-bit or 74373) and decoders (eg 7447 or 4543). Circuit 555 (IO1) produces rectangular signal at its output (pin 3) staying in log 1 for 1 second, followed by a short pulse of log 0. During log 1 the counters counts, during the negative pulse are the data on displays refreshed and counters are reset. This is done in two steps: At the falling edge the LATCH feature is turned off and the values in the counters are transmitter to the displays, and the rising edge of the counter then resets the counters to be prepared for next counting cycle. Frequency meter on the schematic measures in the range 0 to 9999 Hz with a resolution of 1 Hz. However, any number of digits can be chosen and also you can choose different measurement periods. If we choose as 0.1 s, it will measure up to 99.99 kHz with a resolution of 10 Hz. If we choose 0.01 s, frequency meter will measure the frequency up to 999.9 kHz with a resolution of 100Hz. When we choose a shorter counting interval it is appropriate to extend the log 0 to reduce the refresh rate. If the display refreshs 10x or even 100x per second, the value can unreadable during measurement of variating frequency. The advantage of controlling the measuring time with RC oscillator with 555 circuit is its simplicity. The disadvantage is a slightly worse accuracy. For more accurate measurements the crystal oscillator can be used.

Adjustment: The adjustment of the frequency meter is simple. Hook it to a power supply (about 5V) and connect known input frequency. Then set the trimmer P1 to display the correct value. Simple digital frequency meter schematic diagram.

Frequency meter in breadboard.

Frequency meter being tested in breadboard.

DIGITAL IC TESTERTo explain how the Digital I.C. Tester works, lets examine the humble 7400 TTLNAND gate device as an example. The datasheet says the package contains four logic gates, each one having two inputs and one output, which behave according to the truth table in Table 1.

To test the satisfactory functioning of each of the four gates in the i.c. package, each of the four input logic configurations in Table 1 must be applied to each gate and the resulting logic output levels recorded and compared against the expected results. A profile for an i.c. to be tested is first generated from the devices datasheet. Within the profile, an instruction sequence is specified that applies defined logic levels to the specified input pins, and records the results generated on the output pins. The actual results received are compared against those that are expected, and from this it is possible to ascertain if the i.c. is functioning correctly. It should be noted that some i.c.s require a great many individual logic operations to test them completely. For example, the 7430 8-input NAND gate requires 256 separate input logic level permutations to be tested.CIRCUIT DESCRIPTIONThe complete circuit diagram for the Digital I.C. Tester is shown in Fig.1. When power is supplied to the board, it first passes through bridge rectifier REC1. If the input supply input is a.c., REC1 converts it to d.c. If the input is already d.c. it ensures that the polarity is correct for IC1, which then regulates the voltage down to approximately 5V. Capacitors C1 to C4 plus C11 provide smoothing. A PIC16F877-20 microcontroller, designated as IC3, is used as the core of the circuit and is run at its maximum speed of 20MHz, as defined by crystal X1. Since this design uses RS232 protocol to interface to a serial port on a PC, a voltage level converter is employed to convert the PICs 5V logic levels to the 12V levels required by the RS232 standard (many PCs do not actually require this higher voltage for serial common input and will accept +5V/0V inputs. Ed).This is accomplished by IC2, a MAX232 line driver. Capacitors C5 to C8 are used by IC2s internal circuitry to convert the supplied voltage from 5V to 12V. Connection to the PC is via a 9-pin female D-type connector, SK2. To test the functionality of a digital logic i.c., a known set of logic levels must be presented on each pin and the resulting responses received back from the i.c. then analysed.The PIC16F877 has a total of 33 I/O (input/output) pins. Of these, 24 are used to connect the PIC to the i.c. under test. Each of the 24 I/O pins is connected to a pin on the i.c. test socket (SK3) via a 100_ resistor, within resistor modules RM1 to RM3. These resistors act as current limiters to protect the PIC and the device under test. The danger is that an output of the test i.c. could become connected to a PIC I/O pin also designated as an output. Each pin on the i.c. test socket is biased to the +5V test power rail via a 4k7_ resistor (within resistor modules RM4 to RM6). This is to force unused pins on the test socket to a known logic level, and also enables open collector TTL i.c.s that have their outputs either floating or pulled to ground to be tested. During the test cycle, the PIC sends a low logic level to the base of transistor.

Construction The Digital I.C. Tester is constructed on a single-sided p.c.b. whose component layout and tracking details are shown in Fig.2. This board is available from the EPE PCB Service, code 371. It is recommended that good quality i.c. sockets are used for IC2, IC3 and the three 100_ resistor modules, RM1 to RM3. Assemble the board in any order you prefer, but preferably making the link-wire connections first. The two link wires located to the left of SK2, the 9-pin RS232 connector (between TP7/TP8 and TP9/TP10), help determine which type of serial cable will be used, see Fig.3. If a straight-through serial cable is to be used, pin 2 to pin 2, pin 3 to pin 3, then these links should be parallel to each other as shown in Fig.3b. If a cross-over serial cable is being used(pin 2 to pin 3 and pin 3 to pin 2 as in Fig.3a) then the links should be crossed as in Fig.3c. Place a small piece of sleeving over one of the link wires so that they do not short together.

Measurement of a Time IntervalLet time interval tx is to be measured between two events A and B, occurring at times tA and tB , respectively. Considering that A comes before event B. i.e., tA < tB The time to pulse width converter for this case could be and SR Flip Flop as shown in figure below. The event occurring at time tA sets the Flip Flop and that occurring at time tB resets it. The output pulse available at Q, thus has a pulse width tx .

Errors in time interval measurement:If the time interval tx is very large compared to the time period Tc of the clock. Then the standard method is sufficiently accurate but if tx becomes comparable to Tc, then it introduces considerable error. Let

The waveform shown below shows the graphical representation of errors in time interval.

Fundamentals of Time Interval Measurements:A time interval measurement is a measurement of the elapsed time between somedesignated START phenomena and a later STOP phenomena. This is in contrast to real-time observations (time of day) used in our day-to-day living to schedule meetings or transportation, in astronomical observations and for celestial navigation among otherthings.

This led to the development of a time interval measuring electronic counter, in essence an electronic stopwatch. A time interval counter can measure electrical delays, pulse widths, and other time related electrical phenomena required in the development and maintenance of communications, navigation, television, and other present day systems. Increased measurement capability has helped bring on more and more sophistication in all of these fields until now modern electronic time interval counters are used to measure electrical events spaced as close as 0.1 nanosecond (the time required for light to travel 3 centimeters) on a one-shot basis. Time interval averaging on repetitive events gives still greater resolution than this.

IntroductionTime Interval is an important measurement frequently made with electronic counters.In this role, the counter makes an elapsed time measurement between two electrical pulses, Figure 1, just as a stopwatch is used to time physical events.

Figure 1A. In a time interval measurement, clock pulses are accumulated for the duration the main gate is open. The gate is opened by one event, START and closed by the other, STOP.

20Minimum time measurement is much less (to a nanosecond and below) than possible with a stopwatch. Also resolution and accuracy are much greater than attainable with a stopwatch.Some typical time measurements that might be made are:

Characterization of Active Components-Propagation delay of integrated circuits Radar Ranging Nuclear and Ballistic Time of Flight Pulse Measurements

Cable Measurements

Phase

o Widtho Rise Timeo Repetition Rate (Period) of pulse traino Spacing on complex pulse trains such as used byairborneo identification and navigation systems

o Propagation Timeo Cable Length

Delay Line MeasurementsThe START pulse, received at channel A of the counter in Figure 1B, opens the GATE to start the measurement, the STOP pulse occurring later in time and received at channel B closes the gate to end the measurement. Elapsed time between start and stop is measured by counting the Time Base clock frequency while the gate is open. The resolution of a conventional time interval counter (HP 5328A, HP 5345A, etc.) is determined by its clock frequency.A clock frequency of 1 MHz gives 1 msec resolution, 100 MHz gives 10 ns resolution,500 MHz gives 2 ns resolution and so on. Clearly, the elements within the time interval counter (input amplifier, main gate, DCAs) must operate at speeds consistent with the clock frequency; otherwise the instruments resolution would be meaningless. Present state-of-the- art limits resolution to about 2 nsec, although special techniques can improve on this.

Figure 1B.Block Diagram for Basic Time Interval Counter

1. START and STOP trigger circuits open or close a gate which in its elementary form is merely an ON- OFF switch connected between the clock pulse source and the electronic pulse counter.

2. The counted clock frequency is typically 0.1 Hz to 100 MHz in decade steps or 500 MHz each derived from a crystal oscillator.

2.Digital IC Tester

An Integrated Circuit tester (IC tester) is used to test Integrated Circuits (ICs). We can easily test any digital IC using this kind of an IC tester. For testing an IC, we need to use different hardware circuits for different ICs; like we need a particular kind of tester for testing a logic gate and another for testing flip flops or shift registers which involves more complication and time involved will also be more.This IC tester is constructed using 8951 microcontroller along with a keyboard and a display unit. It can test digital ICs having a maximum of 24 pins. Since it is programmable, any number of ICs can be tested within the constraint of the memory available. This IC tester can be used to test a wide variety of ICs which includes simple logic gates and also sequential and combinational ICs like flip-flops, counters, shift registers etc. It is portable and easy touse.The block diagram of the programmable digital IC tester is as shown in below. It consists of two 8951 microcontroller ICs, a 24-pin IC socket, a keyboard unit, a display unit and indicators.To test a particular digital IC, one needs to insert the IC into the IC socket and enter the IC number using the keyboard and then press the ENTER key. The IC number gets displayed in the 7-segment display unit.

Four LEDs are provided as indicators. If the IC being tested is a logic gate, then each of the 4 indicator LEDs correspond to the 4 gates of the IC. In any other case wherein theinserted IC is not a logic gate, all the 4 LEDs work as a single indicator.

Start

Initialise LCD

Accept IC number fromKeyboard

Set pins high or low as per the property of IC

Check the conditions of ICoutput pins

NoIC not ok

Is IC ok?

Yes

IC ok

End

Figure 1C: Flowchart of Programmable Digital IC Tester

Figure 1C: Block Diagram of programmable digital IC TesterThe programmable digital IC tester consists of 2 8951 microcontroller. One is calledmaster and the other slave. The master controls the slave and the working of the display and keyboard units. The slave IC is used for testing the IC inserted into the IC socket. Out of the four ports available in the 8951, three ports port 0, port 1(Not all pins), port 2 are connected to the 24 pins of the IC socket. Port 3 is used for parallel communication between the slave and the master.The keyboard and display units are interfaced with the master IC. Four indicators are also connected to the master IC. When we feed the number of the IC being tested, the number gets stored in the memory of the master IC and also gets displayed in the in the 7-segment display. The display unit is connected to the lower pins of port 0 of the master IC. The keyboard is connected to port 1 and the four indicators are connected to the port 1 of SlaveIC.Parallel communication is used between the master and the slave. There are four data lines and three control lines for transfer and control of data between the master and the slave. Port 3 of the slave IC and port 2 of the master IC are used for parallel communication.To test a particular digital IC, one needs to insert the IC into the IC socket and enter the IC number using the keyboard and then press the ENTER key. The IC number gets displayed in the 7-segment display unit. The IC number gets transferred to the slave using parallel communication.The process of parallel communication is as follows:1. The slave sends a ready bit to the master IC when it is ready to receive data.2. If the slave is ready, the master places the data on the data lines.3. A control bit gets set as soon the master transmits the data.4. The slave checks whether the bit is set, and then receives the data.5. Another control bit gets set as soon as the slave receives the data.6. A third control bit gets set whenever the slave is ready to accept another set of data.

As soon as the slave gets the IC number, it compares the IC number with the stored

list and then goes to the corresponding service routine. The service routine checks the particular IC. Depending to the IC, it assigns some ports as input ports and some as output ports. It gives the corresponding inputs and checks for the output according to the IC logic. If the output is according to the IC logic, the slave sends the data to the master IC. If the IC is a logic gate, then the data will consist of 4 bits, where each bit corresponds to each gate of the IC. In case the IC is not a logic gate, the data consists of a single bit, which may be a 0 or a1. Corresponding to these four bits, the master sets or resets the lower port of P3, which is connected to the indicators.Four LEDs are interfaced to the master IC as indicators. If the IC being tested is a logicgate, then each of the 4 indicator LEDs correspond to the 4 gates of the IC. In any other case wherein the inserted IC is not a logic gate, all the 4 LEDs work as a single indicator.Using this IC tester, any digital IC can be tested provided its program is written. For a particular IC, the corresponding program must be written in the slave. The total number of ICs that can be tested using this IC tester depends on the memory available in the slave. The8951 microcontroller consists of a 4k ROM, using which around 150 ICs can be tested. To test more number of ICs, a microcontroller with an 8k ROM can be used.The keyboard is provided with a RESET button, which when pressed resets both themicrocontrollers and the 4 shift registers.

3.MultimetersA multimeter is an instrument which measures electrical parameters such as AC or DC voltage, current, and resistance. Rather than having separate meters, a multimeter combines a voltmeter, an ammeter, and an ohmmeter. The two main kinds of a multimeter are analog and digital.A digital multimeter has an LCD screen that displays the value of the parameter being measured. while in an analog multimeter display, a needle moves through a graduated scale. Topmost scale is usually for resistance and the readings increases from right to left while other scales readings increase from left to right. Another name for an analog multimeter is Volt-Ohm-Milliammeter (VOM).Each type of meter has its advantages and disadvantaged. When used as a voltmeter, a digital meter is usually better because its resistance is much higher, 1 M or 10 M, compared to 200 for an analogue multimeter for a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an analogue display. Most modern multimeters are digital and traditional analogue types are becoming obsolete.Multimeter is a device used for the measurement of AC or DC voltages, currents and also resistances. Some meters are also capable of measuring temperatures, conductivities and frequencies. Multimeters can be analog or digital, or combination of these (the most modern meters).

i) Voltage measurement by multimeter:

For the case of a VOM, a zero adjustment has to be made every time the multimeter is to be used. To do the zero adjustment, set the mode selection knob in resistance mode. Connect the two leads to positive and common terminals respectively and short the leads. The needle should move to extreme right to the last reading on the ohms scale. If it stops before or goes beyond then the zero adjustment knob has to be rotated (clockwise or anticlockwise) such that the needle rests at the last reading on the right end of the bar on the ohms scale. Subsequently, to measure voltage, the multimeter has to be first set in AC or DC mode.

After selecting a suitable range defined by the uppermost limit of the expected value, the range knob has to be set. Next connect the common (gnd) terminal through a lead (black) to the gnd of the circuit and the red lead to the point where voltage is to be measured. For the case of an analogue multimeter, if the needle goes the wrong way the leads have to be reversed or if the needle doesnt move at all the range has to be changed.

(ii) Resistance Measurement:

To measure resistance in a circuit, first the power supply is to be turned off (or disconnected) otherwise the multimeter might get damaged. Next, select a range on the multimeter and touch two metal points in the circuit. If the needle doesnt move or goes all the way to the end of the scale, select another range. One can not use this method to measure the resistance of a resistor in the circuit because there may be other paths between the nodes of a resistor.

One leg of a resistor must be disconnected from the circuit to make sure that the only path between the two probes is through that resistor. To measure the resistance of a resistor, select the range on the meter that might be closest to the right value and use the probes to touch either side of the resistor. If the right range is selected then the needle will be somewhere between the left and the right end of the scale.

Sem/Sub: 04- Electronics Instrumentation20