Digital voltmeter

Abstract

This project mostly concerns itself with a digital voltage-measuring instrument. The usefulness of any engineering instrument can be judged by various criteria. Among the foremost of these are accuracy, versatility and ease of operation. With respect to all of these the digital instrument reigns supreme and it was because of this that we chose to build our instrument on a digital basis. It is. with a digital instrument alone that is possible to achieve a four-digit accuracy (and even more if required). The ease of turning a voltmeter into a multimeter while still retaining the full accuracy of the basic voltage instrument will be demonstrated in a later chapter. This ease is not possible with an analogue instrument and this makes the digital instrument mere versatile. And finally there is the ease with which a digital instrument could be used especially if it is auto ranging (as this one was intended to be). As mentioned previously this project concerns itself mostly with a digital, voltage-measuring instrument. Perhaps the most used electrical instrument, in both laboratory and field, is a voltmeter. And this is because most transducers (converters) are designated so that they convert the parameter of importance into a proportional voltage. Thus an electrical current is converted into a voltage if it is passed through a resistance (of known value). Conversely a resistance is converted into a proportional electrical voltage if supplied by a constant current source of known value. (This is a very common way of measuring resistance. Another way will be described later on in this report). Similarly transducers exist which are capable of converting an inductance, a capacitance, a temperature etc. into a proportional voltage. Because of this one would not be exaggerating by saying that a voltmeter is used as often to measure volts as to measure other parameters. There are various ways of realizing analogue-to-digital (A-to-D) conversion. In this project the so-called dual slope technique was considered. This perhaps is the slowest and most complicated of the possible methods. It is also the most accurate and reliable and also renders the basic voltage instrument more capable of being converted into a multimeter. In our opinion the disadvantages of the system are by far outweighed by its advantages and this fact led us to its adoption.