ELECTRICAL MEASURING INSTRUMENTS are necessary because the nature of most electrical phenomena is beyond the reach of our physical senses. Measurement of electrical quantities makes possible the design, manufacture, and maintenance of the innumerable electrical devices now in use.
The main purpose of any electrical instrument is to measure and indicate the value of an electrical quantity. The measurement may be indicated by a digital numeric value or by a pointer positioned on a scale. Some instruments provide additional functionality by recording measured values over time.
This recording may be in the form of a physical indication on a moving chart, as maximum and minimum values during a time frame, or as periodic data stored in electronic memory. The devices commonly used for such measurements are voltmeters, ammeters, and wattmeters.
The field of instrumentation is extensive and includes many classifications of instruments according to portability, type of indication or record, accuracy, design features, etc. We shall briefly discuss only those instruments commonly used in meter departments. These include displaying, indicating, electronic digital, and recording measuring devices.
A digital instrument is an electronic device that measures voltage, current, and/or resistance by converting the measured analog input signal into a digital representation that is then displayed as a digital readout.
Advances in technology have led to digital instruments that are capable of high degrees of accuracy in the measurement of voltages, currents, and resistances over a wide range of values. Analog instruments indicate measured quantities by the deflection of a pointer on a scale, requiring the user to “eye up” the reading.
Today’s digital instruments display measured results as discrete numbers (digits), removing much of the interpretation error from the act of reading an instrument. Typical display technologies include liquid crystal display (LCD), light emitting diode (LED), and gas discharge.
Some instruments offer the ability to send readings to other devices such as printers or computers or to be controlled by external computers. Interfaces built into the instruments, such as RS-232-C serial communications or the IEEE-488 bus standard, provide the data transmittal and external control capabilities.
The central component of a digital instrument is the digital DC voltmeter that uses electronic circuits to sense, process, and display the measured quantities. Input quantities other than DC voltages are converted to DC by transducers.
Examples of transducers include internal shunts used to measure current and ACto- DC converters to measure AC quantities. The transformed analog quantity (now in the form of an equivalent DC voltage) is then converted to a digital signal.
Active electronic components, such as transistors, operational amplifiers, and integrated circuit modules perform this analog-to-digital (A/D) conversion.
Analog-to-Digital Conversion
Electronic instruments employ several different A/D conversion processes. These include dual-slope integration, ramp-and-counter, successive approximation, and voltage-to-frequency conversion. Each of these techniques produces a digital output equivalent to the measured analog input.
Figure 6-1 shows a simple version of an A/D converter. In this example, a binary counter increments one count with each clock pulse, until Vout equals Vin. This type of A/D converter is called a “digital ramp and counter” because the waveform at Vout ramps up step-by-step, like a staircase.
It operates as follows:
1. A positive Start pulse is applied resetting the counter to zero. It also inhibits the AND gate so no clock pulses get through to the counter while the Start pulse is High.
2.With the counter at zero, Vout 0, so the comparator output is High.
3. When the Start pulse goes Low, the AND gate is enabled, allowing pulses to enter the counter.
Figure 6-1. Analog-to-Digital Converter
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