04-05-2011, 03:52 PM
A voltmeter finds its importancewherever voltage is to be measured.Here we present an easyto-build and accurate digital voltmeterthat has been designed as a panel meterand can be used in DC power suppliespanels or where it is necessary to havean accurate indication of the voltage.
Circuit description
The circuit uses an analogue-todigitalconverter ICL7107 made byINTERSIL. Fig. 1 shows the pin detailof IC L7107. This IC incorporates in a40-pin case all the circuitry necessaryto convert an analogue signal intodigital and can drive a series of fourseven-segment LED displays directly.which is directly proportional tothe input voltage.At the end of the preset period,the integrator is fed withan internal reference voltage andthe output of the circuit is graduallyreduced until it reaches thelevel of the zero reference voltage.This second phase is knownas the negative slope period andits duration depends on the outputof the integrator in the firstperiod. As the duration of thefirst operation is fixed and thelength of the second is variable,it is possible to compare the twoand this way the input voltage isin fact compared to the internalreference voltage and the result iscoded and sent to the display.All this sounds quite simpleand easy but in fact it comprisesa series of complex operations,which are all made by theADC IC with the help of a fewexternal components that are used toconfigure the circuit for accurate measurement.Fig. 2 shows the circuit of thedigital voltmeter. The voltage to bemeasured is applied across points ‘INL’and ‘IN-H’ of pins 30 and 31 of IC1,respectively, through circuit resistorsR5, R6, R7 or R8 and preset VR1.Resistor R2 together with C5 formsthe circuit used to set the frequency ofthe oscillator (clock), which is set at 48kHz approximately. At this clock rate,there are about three different readingsper second.Capacitor C4, which is connectedbetween pins 33 and 34 of IC1, compensatesfor the error caused by theinternal reference voltage and alsokeeps the display steady. Capacitor C2and resistor R1 together forms the circuitthat does integration of the inputvoltage and at the same time preventsThe circuits built into the IC are ananalogue-to-digital converter (ADC),a comparator, a clock, a decoder and aseven-segment LED display driver.The circuit can display any DC voltagein the range of -1999 volts to +1999volts. It operates off ±5 volts.In order to understand the operatingprinciple of the circuit, it is necessaryto explain how the ADC IC works.An analogue-to-digital converter,better known as a dual slope converteror integrating converter, is generallypreferred over other types as it offersaccuracy, simplicity in design and arelative indifference to noise, whichmakes it very reliable.The operation of the circuit isdescribed in two stages for better understanding.During the first stage, fora given period the input voltage is integrated,and the output of the integratorat the end of this period is a voltage
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