Project Description
A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit.
A DIGITAL VOLTMETER (DVM) displays measurements 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 data processing 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 digital value 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.
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.
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. 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.
In order to understand the principle of operation of the circuit it is necessary to explain how the ADC IC works. This IC has the following very important
> Great accuracy. > It is not affected by noise. > No need for a sample and hold circuit. > It has a built-in clock. > It has no need for high accuracy external components
Figure 2.2 7-segment display pinout MAN6960
An Analogue to Digital converter, (ADC from now on) is better known as a dual slope converter or integrating converter. This type of converter is generally preferred over other types as it offers accuracy, simplicity in design and a relative indifference to noise which makes it very reliable. The operation of the circuit is better understood if it is described in two stages. During the first stage and for a given period the input voltage is integrated, and in the output of the integrator at the end of this period, there is a voltage which is directly proportional to the input voltage.
At the end of the preset period the integrator is fed with an internal reference voltage and the output of the circuit is gradually reduced until it reaches the level of the zero reference voltage. This second phase is known as the negative slope period and its duration depends on the output of the integrator in the first period. As the duration of the first operation is fixed and the length of the second is variable it is possible to compare the two and this way the input voltage is in fact compared to the internal reference voltage and the result is coded and is send to the display. All this sounds quite easy but it is in fact a series of very complex operations which are all made by the ADC IC with the help of a few external components which are used to configure the circuit for the job. In detail the circuit works as follows. The voltage to be measured is applied across points 1 and 2 of the circuit and through the circuit R3, R4 and C4 is finally applied to pins 30 and 31 of the IC. These are the input of the IC as you can see from its diagram. (IN HIGH & IN LOW respectively). The resistor R1 together with C1 are used to set the frequency of the internal oscillator (clock) which is set at about 48 Hz. At this clock rate there are about three different readings per second. The capacitor C2 which is connected between pins 33 and 34 of the IC has been selected to compensate for the error caused by the internal reference voltage and also keeps the display steady. The capacitor C3 and the resistor R5 are together the circuit that does the integration of the input voltage and at the same time prevent any division of the input voltage making the circuit faster and more reliable as the possibility of error is greatly reduced. The capacitor C5 forces the instrument to display zero when there is no voltage at its input. The resistor R2 together with P1 are used to adjust the instrument during set-up so that it displays zero when the input is

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1 comment:

  1. Hello, good to know about this project report about voltmeter as well as its advantages. The electrical potential difference between two points in an electric circuit of this voltmeter is perfectly explained. thanks for the post.