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CENTER FOR ENERGY ENGINEERING

NAME : SAMEER KUMAR MAURYA

Reg. No. : CUJ / I / 2012 / IEE / 025

SUBJECT : MEASUREMENT and INSTRUMENTATION

Construction of a Digital Voltmeter

Definition-A voltmeter is an instrument used for measuring electrical potential difference between two points of an electrical circuit. Why we use Digital voltmeter? And Difference between Analog and Digital voltmeter? Because it gives fast response than analog voltmeter. Analog voltmeter consist inertial moving parts which takes time to show the reading through pointer.

Analog meters use a needle and calibrated scale to indicate values.

Digital meters display those values on a digital display.

Voltmeter finds its importance wherever voltage is to be measured. It calculate accurate values.

Digital multimeter Measuring the voltage though digital multimeter

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Construction part

The construction includes following parts :

A) A pulse train generator B) Control and Gating C) Counting section D) Analog input/ Transducer E) Latching and Display section F) Completing the DVM

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.EXPLAINATION: A)Pulse train generator : In electronics, this is called a clock. A Clock generation is a circuit that produes a clock signal (which can simply be square signal or may be other complex signal) and behaves as such for use in synchronizing a circuit operation.

Here a pulse generatot is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulse.

Here we using the 555 timer chip to generate the pulse. 555 timer IC is an integrated circuit used in variety of timer, pulse generation and oscillator applications.The 555 timer can be used to provide tume delays,as an oscillator,and as a flip flop element.

B) Basic Control and Gating : A way to monitor the voltage across the capacitor and generate signals when the charging starts at zero volts and when the voltage becomes equal to the voltage to be measured, Vin. This section called control and gating.To carry out the function of Part B we use a solution based on an integrated circuit called a comparator. The entire purpose of this IC is to measure the larger voltages from the input signal Vin and the voltage across the capacitor. The comparator has to do its job without drawing any appreciable current, otherwise it will interfere with the constant current charging. To achieve near zero input current we will use a LF355 FET input 1 opamp as our comparator. The opamp is a chip with two inputs labeled + and – (with the voltages at these points called V+ and V) and called the noninverting and inverting inputs. It has just one output. Of course like any IC it requires power and ground connections, in this case two power supplies (Vcc = +10V and Vee =-10V these are just conventional names for the power voltages).If the voltage at the + input is greater (more positive) than the voltage at the – input (V+> V), the 355 will drive its output high, near to Vcc. If the opposite becomes true, (V> V+), the output will go low, near to Vee.

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C)Counting section : A way to “count” the number of clock pulses that occur between the start and stop charging signals.This is the counting section. Counting Pulses with the 74LS192 Chip-Digital electronic chips of the type known as “TTL” use 0V for an off signal (0) and 5V for an on signal (1).This is the defining characteristic of digital electronics: any input or output can only be in one of two possible states. The TTL output of the breadboard's function generator generates a pulse train for use with the TTL “family”. So does an appropriate 555 circuit. The TTL chips used in this class are all designated by the letters “LS” in its

JEDEC number (Example: 74LS00 is a quad NAND gate).

Switching the input to 74LS192 On and Off with a NAND Gate The voltmeter will work by counting the pulses from the 555 between the time the capacitor charging starts, and the time the 355 comparator detects that the capacitor voltage has surpassed Vin and therefore changes its output state.To accomplish this we will use a simple NAND gate in the circuit shown above.

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D)Analog input/ transducer : here a constant current capacitor charging circuit that can be turned ‘ON’ and ‘OFF’ and ‘reset’ to zero volts by the start and stop of charging signals. There is transducer which take the signals and turns into another electrical signal.Now this signal goes to next level called latching and display. E)Latching and Display section : Here we need to create a human readable display of our pulse counting.This will involve a seven segment display, a display driver,and a latch. Latching term is used to store the information to display it. This section includes a human readable way to display the number of pulse counted during the interval. The MAN71A is a seven segment LED display used for displaying a single human readable number. It contains seven barshaped LEDs arranged in the shape of a squared off ‘8’. The IC, as mentioned earlier, is capable of driving four common-anode, seven-segment displays. Displays that is shown on screen are connected such that they can display all the numbers from ‘0’ to ‘9,’ while display 1( black squre) can display only digit ‘1’ (and ‘-’ sign when the voltage is negative). When TEST switch is pressed, test pin is pulled high (to V+) and all segments turn on to show ‘1888’ on the display. F)Completing the DVM : To complete the DVM we finally connect the all above 5 section i.e, analog input/transduction section , the clock generator,control and gating section,the counting section, and the latch and display section. See the circuit daigram below-

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Block diagram of Digital voltmeter

A.Input is done through the input probes, the point 2,3,4, and 5.

B. Selecting AC or DC voltageC. In case of AC voltage signal is sent through point 2(ACV),now it goes through the calibrated attenuator.Here it attenuator reduses the intensity of signal.In case of DC voltage the signal goes through point 5(DCV)C.In case of AC voltage rectifier D.Analog to digital convertorthis signal sent to display section.

Block diagram of Digital voltmeter

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Figure 2

A.Input is done through the input probes,sw is moving switch to select

the point 2,3,4, and 5. AC or DC voltage and AC or DC current

In case of AC voltage signal is sent through point 2(ACV),now it goes through the calibrated attenuator.Here it attenuator reduses the intensity of signal.In case of

goes through point 5(DCV). C.In case of AC voltage rectifier circuit changes the ac signal into dc signal.

nalog to digital convertor receive the signal and turn it into digital form,now this signal sent to display section.

is moving switch to select

In case of AC voltage signal is sent through point 2(ACV),now it goes through the calibrated attenuator.Here it attenuator reduses the intensity of signal.In case of

changes the ac signal into dc signal. receive the signal and turn it into digital form,now

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E.Display section show the electrical singal in the form of digit.

Analogue-to-digital converter- It is the heart of the digital voltmeter. ADC process is done by the using IC PIC16F877.It consist 40-pin necessary to convert an analog signal to digital and can drive a series of 4 seven-segment LED display.

TERMS: Attenuation- It is also called extinction.it is the gradual loss in intensity of any kind of flux through a medium. Example; sunlight is attenuated by dark glass. Here in electrical attenuated affects the propogation of waves and signal in circuits. Buffer amplifier - Buffer amplifier is one that provides electrical impedance transformation from one circuit to another ciruit. Transducer – Which convert one form of energy to another form of energy i.e, mechanical force into electrical signal. Example : pressure sensor