31-03-2011, 04:01 PM
SUBMITTED BY
PARTHA PRATIM SAIKIA
SAMHITA PHUKAN
PRIYANKA SAHA
Md.ZIAUR REHMAN
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DESCRIPTION OF THE PROJECT
The circuit given here is of a very useful and accurate digital voltmeter with LED display using the ICL7107 from Intersil. The ICL7107 is a high performance, low power, 3.5 digit analog to digital converter. The IC includes internal circuitry for seven segment decoders, display drivers, reference voltage source and a clock. The power dissipation is less than 10mW and the display stability is very high.
The working of this electronic circuit is very simple. The voltage to be measured is converted into a digital equivalent by the ADC inside the IC and then this digital equivalent is decoded to the seven segment format and then displayed. The ADC used in ICL7107 is dual slope type ADC. The process taking place inside our ADC can be stated as follows. For a fixed period of time the voltage to be measured is integrated to obtain a ramp at the output of the integrator. Then a known reference voltage of opposite polarity is applied to the input of the integrator and allowed to ramp until the output of integrator becomes zero. The time taken for the negative slope to reach zero is measured in terms of the IC’s clock cycle and it will be proportional to the voltage under measurement. In simple words, the input voltage is compared to an internal reference voltage and the result is converted in a digital format.
The resistor R2 and C1 are used to set the frequency of IC’s internal clock. Capacitor C2 neutralizes the fluctuations in the internal reference voltage and increases the stability of the display.R4 controls the range of the voltmeter. Right most three displays are connected so that they can display all digits. The left most display is so connected that it can display only “1” and “-“.The pin5(representing the dot) is connected to ground only for the third display and its position needs to be changed when you change the range of the volt meter by altering R4. (R4=1.2K gives 0-20V range, R4=12K gives 0-200V range and R4=0 gives 0-2V range).
Notes.
• Assemble the circuit on a good quality PCB.
• The circuit can be powered from a +/_5V dual supply.
• For calibration, power up the circuit and short the input terminals. Then adjust R6 so that the display reads 0V.
• The ICL7107 is a CMOS device and it is very sensitive to static electricity. So avoid touching the IC pins with your bare hands.
• The seven segment displays must by common anode type.
• I assembled this circuit few years back and it is still working fine
DESCRIPTION OF DIGITAL VOLTMETER (DVM)
A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit.
A DIGITAL VOLTMETER(DVM) displays meaesurements of dc or ac voltages as discrete numerals instead of pointer deflections on a continuous scale as in analog instruments.
As compared to other voltmeters, a DVM offers the advantages of :
1> Greater speed
2> Higher accuracy and resolution
3> No parallax
4> Reduced human error
5> Compatibility with other digital equipment for further processing and recording
DVM are often used in dataprocessing systems or data logging systems.In such systems,a number of analog input systems are scanned sequentially by an electronic system and then each signal is converted into an equivalent digital value by the A/D converter in the DVM. The digitalvalue is then transmitted to a printer alongwith the information of about the input line from which the signal has been derived.The whole data is then printed out.In this way,a large number of input signals can be automatically scanned or processed and their values either printed or logged.
The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.
Digital voltmeters (DVMs) are usually designed around a special type of analog-to-digital converter called an integrating converter. Voltmeter accuracy is affected by many factors, including temperature and supply voltage variations. To ensure that a digital voltmeter's reading is within the manufacturer's specified tolerances, they should be periodically calibrated against a voltage standard such as the Weston cell.
Digital voltmeters necessarily have input amplifiers, and, like vacuum tube voltmeters, generally have a constant input resistance of 10 megohms regardless of set measurement range.
Digital voltmeters (DVMs) are now the preferred instruments for ac and dc measurements at all levels of accuracy and at all voltages up to 1 kV. Essentially a digital voltmeter consists of a voltage reference, usually provided by a Zener diode, an analog-to-digital converter and digital display system, and a power supply, which may be derived from either the mains or a battery. The basic range of the instrument provides measurement from zero to 10 or 20 V. Additional lower ranges may be provided by amplifiers, whose gain is stabilized by precision resistors. These electronic input amplifiers often provide a very high input impedance, perhaps exceeding 1010 &OHgr;. Since this impedance is obtained by active means, a much lower impedance may be found when the instrument is switched off. Higher voltage ranges are provided by the use of resistive attenuators, usually limited to a value of 10 M&OHgr; by economic restraints. The best accuracy is always obtained on the basic range, where it is limited to that of the analog-to-digital converter. See Amplifier, Analog-to-digital converter, Electronic power supply, Zener diode
BLOCK DIAGRAM OF DIGITAL VOLTMETER (DVM)
The block diagram of a DVM based on dual slope techniques as shown in the figure above.The dual slope analog digital (A/D) converter consists of five basic blocks:
An op-Amp used as an integrator, a level comparator, a basic clock (for generating timing Pulses), a set of decimal counters and a block of logic circuitry.
WORKING OF A DVM:
The unknown voltage Vx is applied through switch S to the integrator for a known period of time T. This period is determined by counting the clock frequency in decimal counters. During the time period T,C is charged at a rate proportional to Vx.
DESCRIPTION OF ICL 7107
FN3082.
The Intersil ICL7107 is a high performance, low
power, 3(1/2) digit A/D converters. Included are seven
segment decoders, display drivers, a reference, and a clock.
The ICL7107 is designed to interface with a liquid crystal
display (LCD) and includes a multiplexed backplane drive;
the ICL7107 will directly drive an instrument size light
emitting diode (LED) display.
The ICL7107 is high accuracy, versatility, and true economy.
It features autozero to less than 10μV, zero drift of less than 1μV/oC, input bias current of 10pA (Max), and rollover error of less than one count. True differential inputs and reference are useful in all systems, but give the designer an uncommon advantage when measuring load cells, strain gauges and other bridge type transducers.
Finally, the true economy of single power
supply operation (ICL7107), enables a high performance
panel meter to be built with the addition of only 10 passive
components and a display.
FEATURES
• Guaranteed Zero Reading for 0V Input on All Scales
• True Polarity at Zero for Precise Null Detection
• 1pA Typical Input Current
• True Differential Input & Reference,Direct Display Drive
LCD ICL7106, LED lCL7107
• Low Noise - Less Than 15μVP-P
• On Chip Clock and Reference
• Low Power Dissipation - Typically Less Than 10mW
• No Additional Active Circuits Required
• Enhanced Display Stability
• Pb-Free Plus Anneal Available (RoHS Compliant)
GENERAL DESCRIPTION
This is an easy to build, but nevertheless very accurate and useful digital voltmeter. It has been designed as a panel meter and can be used in DC power supplies
or anywhere else it is necessary to have an accurate indication of the voltage present. The circuit employs the ADC (Analogue to Digital Converter) I.C. CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal to digital and can drive a series of four seven segment LED displays directly. The circuits built into the IC are an analogue to digital converter, a comparator, a clock, a decoder and a seven segment LED display driver. The circuit as it is described here can display any DC voltage in the range of 0-1999 Volts.
Technical Specifications - Characteristics
Supply Voltage: ............. +/- 5 V (Symmetrical)
Power requirements: ..... 200 mA (maximum)
Measuring range: .......... +/- 0-1,999 VDC in four ranges
Accuracy: ....................... 0.1 %
FEATURES
> Small size
> Easy construction
> Low cost.
>Simple adjustment.
> Easy to read from a distance. > Few external components.
