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Measuring Instruments--part 1



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TOPICS:

TERMINOLOGY:

  • Ammeter
  • Ammeter shunt
  • Analog meters
  • Ayrton shunt
  • Bridge circuit
  • Clamp-on ammeter
  • Current transformer
  • D’Arsonval movement
  • Galvanometers
  • Moving-coil meter
  • Multirange ammeters
  • Multirange voltmeters
  • Ohmmeter
  • Oscilloscope
  • Voltmeter
  • Wattmeter
  • Wheatstone bridge

Measuring instruments are the eyes of the electrician. An under standing of how measuring instruments operate is very important to anyone working in the electrical field. They provide the electrician with the ability to evaluate problems on the job through the use of technical tools. They also enable an electrician to correctly determine electrical values of voltage, current, resistance, power, and many others. In this section we will cover:

  • how different types of meters are constructed. This knowledge will aid you in knowing how the meters are to be connected in a circuit and how to interpret their readings.
  • the differences between analog and digital meters, as well as how to interpret the waveforms shown on an oscilloscope. Oscilloscopes must be used to observe specific voltage patterns that would render a conventional voltmeter useless.
  • Learning goals:
  • discuss the operation of a d'Arsonval meter movement.
  • discuss the operation of a moving-iron type of movement.
  • connect a voltmeter to a circuit.
  • connect and read an analog multimeter.
  • connect an ammeter to a circuit.
  • measure resistance using an ohmmeter.
  • interpret waveforms shown on the display of an oscilloscope.
  • connect a wattmeter into a circuit.

FUNDAMENTALS:

Anyone desiring to work in the electrical and electronics field must become proficient with the common instruments used to measure electrical quantities. These instruments are the voltmeter, the ammeter, and the ohmmeter.

Without meters, it would be impossible to make meaningful interpretations of what is happening in a circuit. Meters can be divided into two general types: analog and digital.

+++++1 An analog meter.

+++++2 Basic d'Arsonval meter movement. Permanent magnet, Pointer, Spring, Moving coil

+++++3 Rectifier changes AC voltage into DC voltage. Rectifier; AC V



Analog Meters

Analog meters are characterized by the fact that they use a pointer and scale to indicate their value. There are different types of analog meter movements. One of the most common is the d'Arsonval movement. This type of movement is often referred to as a moving-coil meter. A coil of wire is suspended between the poles of a permanent magnet.

The coil is suspended either by jeweled movements similar to those used in watches or by taut bands. The taut-band type offers less turning friction than the jeweled movement. These meters can be made to operate on very small amounts of current and often are referred to as galvanometers.

Principle of Operation:

Analog meters operate on the principle that like magnetic poles repel each other. As current passes through the coil, a magnetic field is created around the coil. The direction of current flow through the meter is such that the same polarity of magnetic pole is created around the coil as that of the permanent magnet. This like polarity causes the coil to be deflected away from the pole of the magnet. A spring is used to retard the turning of the coil. The distance the coil turns against the spring is proportional to the strength of the magnetic field developed in the coil. If a pointer is added to the coil and a scale is placed behind the pointer, a meter movement is created.

Because the turning force of this meter depends on the repulsion of magnetic fields, it will operate on DC only. If AC is connected to the moving coil, the magnetic polarity will change 60 times per second and the net turning force will be zero. For this reason, a DC voltmeter will indicate zero if connected to an AC line. When this type of movement is to be used to measure AC values, the current must be rectified, or changed into DC, before it’s applied to the meter.

+++++4 A voltmeter connects directly across the power source.

+++++5 A resistor connects in series with the meter. Resistor

The Voltmeter

The voltmeter is designed to be connected directly across the source of power. +++++ a voltmeter being used to test the voltage of a battery. Notice that the leads of the meter are connected directly across the source of voltage. A voltmeter can be connected directly across the power source because it has a very high resistance connected in series with the meter movement. The industrial standard for a voltmeter is 20,000 ohms per volt for DC and 5000 ohms per volt for AC. Assume the voltmeter shown is an AC meter and has a full-scale range of 300 volts. The meter circuit (meter plus resistor) would therefore have a resistance of 1,500,000 ohms (300 ohm x 5000 ohm per volt = 1,500,000 ohm).

Calculating the Resistor Value: Before the resistor value can be calculated, the operating characteristics of the meter must be known. It will be assumed that the meter requires a current of 50 microamperes and a voltage of 1 volt to deflect the pointer full scale. These are known as the full-scale values of the meter.

When the meter and resistor are connected to a source of voltage, their combined voltage drop must be 300 volts. Because the meter has a voltage drop of 1 volt, the resistor must have a drop of 299 volts. The resistor and meter are connected in series with each other. In a series circuit, the current flow must be the same in all parts of the circuit. If 50 microamperes of cur rent flow are required to deflect the meter full scale, then the resistor must have a current of 50 microamperes flowing through it when it has a volt age drop of 299 volts. The value of resistance can now be calculated using Ohm's law: R = 5.98 M-ohm (5,980,000 ohm)

Multirange Voltmeters

Most voltmeters are multirange voltmeters, which means that they are designed to use one meter movement to measure several ranges of voltage. For example, one meter may have a selector switch that permits full-scale ranges to be selected. These ranges may be 3 volts full scale, 12 volts full scale, 30 volts full scale, 60 volts full scale, 120 volts full scale, 300 volts full scale, and 600 volts full scale. Meters are made with that many scales so that they will be as versatile as possible. If it’s necessary to check for a voltage of 480 volts, the meter can be set on the 600-volt range. It would be very difficult, however, to check a 24-volt system on the 600-volt range. If the meter is set on the 30-volt range, it’s simple to test for a voltage of 24 volts. The meter shown has multirange selection for voltage.

When the selector switch of this meter is turned, steps of resistance are inserted in the circuit to increase the range or are removed from the circuit to decrease the range. The meter has four range settings for full-scale voltage: 30 volts, 60 volts, 300 volts, and 600 volts. Notice that when the higher voltage settings are selected, more resistance is inserted in the circuit.

+++++6 Volt-ohm-milliampere meter with multirange selection.

+++++7 A rotary selector switch is used to change the full-range setting.

Calculating the Resistor Values:

The values of the four resistors shown can be determined using Ohm's law. Assume that the full-scale values of the meter are 50 microamperes and 1 volt. The first step is to determine the value for R1, which is used to provide a full-scale value of 30 volts. R1, therefore, must have a voltage drop of 29 volts when a current of 50 microamperes is flowing through it.

When the selector switch is moved to the second position, the meter circuit should have a total voltage drop of 60 volts. The meter movement and R1 have a total voltage drop of 30 volts, so R must have a voltage drop of 30 volts when 50 microamperes of current flow through it. This will provide a total voltage drop of 60 volts for the entire circuit:

When the selector switch is moved to the third position, the circuit must have a total voltage drop of 300 volts. R1 and R2, plus the meter movement, have a combined voltage drop of 60 volts at rated current. R3, therefore, must have a voltage drop of 240 volts at 50 microamperes.

When the selector switch is moved to the fourth position, the circuit must have a total voltage drop of 600 volts at rated current. Because R1, R2, and R3, plus the meter movement produce a voltage drop of 300 volts at rated current, R4 must have a voltage drop of 300 volts when 50 microamperes of current flow through it.

+++++8 A speedometer.

+++++9 A fuel gauge.


Next: part 2

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Updated: Thursday, 2013-03-07 22:18 PST