ELECTRICAL

I N D I C AT I N G A N D
TEST INSTRUMENTS
INTRODUCTION

• The mode of operation of most measuring instruments is to convert the

measured quantity into an electrical signal. Usually, this output quantity is in the
form of voltage, although other forms of output, such as signal frequency or
phase changes, are sometimes found.

• We shall learn that the magnitude of voltage signals can be measured by various
electrical indicating and test instruments.
• These can be divided broadly into electrical meters (in both analogue and digital
forms) and various types of oscilloscopes.

• As well as signal level voltages, many of these instruments can also measure
higher magnitude voltages. The oscilloscope is particularly useful for interpreting
instrument outputs that exist in the form of a varying phase or frequency of an
electrical signal.
A. INTRODUCTION
• Electrical meters exist in both digital and analogue forms, although use of an
analogue form now tends to be restricted to panel meters, where the analogue
form of the output display means that abnormal conditions of monitored
systems are identified more readily than is the case with the numeric form of
output given by digital meters.

• The oscilloscope is a very versatile measuring instrument widely used for signal
measurement. It exists in both analogue and digital forms, most instruments
used professionally are now digital.

– CRT
– Digital phosphor oscilloscopes have capability of detecting and recording rapid transients in
voltage signals.
– A third type is the digital sampling oscilloscope, which is able to measure very high-frequency
signals.
– A fourth type is a personal computer (PC)-based oscilloscope, which is effectively an add-on
unit to a standard PC.
 B. DIGITAL METERS
• All types of digital meters are basically modified forms of the digital voltmeter (DVM),
irrespective of the quantity that they are designed to measure. Digital meters
designed to measure quantities other than voltage are digital voltmeters that contain
appropriate electrical circuits to convert current or resistance measurement signals
into voltage signals.

• Digital meters have been developed to satisfy a need for higher measurement
accuracies and a faster speed of response to voltage changes than can be achieved
with analogue instruments.

• They are technically superior to analogue meters in almost every respect.

• Where human operators are required to measure and record signal voltage levels,
this form of output makes an important contribution to measurement reliability and
accuracy (parallax error is eliminated and other gross errors through misreading is
reduced greatly.
• A direct output in digital form is also very useful in computer control
applications.

• Additional advantages of digital meters are their ability to measure signals of

frequency up to 1 MHz and the common inclusion of features such as
automatic ranging, which prevents overload and reverse polarity connection.
DIGITAL METERS
• The major part of a digital voltmeter is the circuitry that converts the
analogue voltage being measured into a digital quantity.
• As the instrument only measures d.c. quantities in its basic mode,
another necessary component within it is one that performs a.c.–d.c.
conversion and thereby gives it the capacity to measure a.c. signals.
• After conversion, the voltage value is displayed by means of indicating
tubes or a set of solid-state light-emitting diodes.
• Four-, five-, or even six-figure output displays are used commonly, and
although the instrument itself may not be inherently more accurate than
some analogue types, this form of display enables measurements to be
recorded with much greater accuracy than that obtainable by reading an
analogue meter scale.
DIGITAL METERS

• Digital voltmeters differ mainly in the technique used to affect the analogue-
to-digital conversion between the measured analogue voltage and the
output digital reading.

• As a general rule, the more expensive and complicated conversion methods

achieve a faster conversion speed.

• The common types of DVM include

– Voltage-to-Time Conversion Digital Voltmeter

– Potentiometric Digital Voltmeter
– Voltage-to-Frequency Conversion Digital Voltmeter
– Digital Multimeter
1. VOLTAGE-TO-TIME CONVERSION DIGITAL
VOLTMETER
• This is the simplest form of DVM and is a ramp type of instrument. When
an unknown voltage signal is applied to input terminals of the instrument, a
negative slope ramp waveform is generated internally and compared with
the input signal.
• When the two are equal, a pulse is generated that opens a gate, and at a
later point in time a second pulse closes the gate when the negative ramp
voltage reaches zero.
• The length of time between the gate opening and closing is monitored by an
electronic counter, which produces a digital display according to the level of
the input voltage signal. It is relatively inexpensive.

• Drawbacks: Nonlinearities in the shape of the ramp waveform used, lack of

noise rejection and typical inaccuracy of ± 0.05%.
2. POTENTIOMETRIC DIGITAL VOLTMETER

• This uses a servo principle, in which the error between the unknown input voltage
level and a reference voltage is applied to a servo-driven potentiometer that adjusts
the reference voltage until it balances the unknown voltage.

• The output reading is produced by a mechanical drum-type digital display driven by

the potentiometer.

• This is also a relatively inexpensive form of DVM that gives excellent performance for
its price.
 3. VOLTAGE-TO-FREQUENCY CONVERSION DIGITAL
VOLTMETER

• In this instrument, the unknown voltage signal is fed via a range switch and an
amplifier into a converter circuit whose output is in the form of a train of
voltage pulses at a frequency proportional to the magnitude of the input signal.

• The main advantage of this type of DVM is its ability to reject a.c. noise.
4. DIGITAL MULTIMETER

• Digital multimeter is also essentially a DVM. It can measure both a.c. and d.c.
voltages over a number of ranges.

• It contains several conversion circuits (a set of switchable amplifiers and

attenuators). This allows the measurement of voltage, current, and
resistance within one instrument.

• It is used widely in circuit test applications as an alternative to the analogue

multimeter.

• It includes protection circuits that prevent damage if high voltages are

applied to the wrong range.
 C. ANALOGUE METERS
• Analogue meters are electromechanical devices that drive a pointer against a
scale. Despite the technical superiority of digital meters (esp. in accuracy and
much higher input impedance), analogue meters continue to be used in a
significant number of applications.

• First, they are often preferred as indicators in system control panels. This is
because deviations of controlled parameters away from the normal expected
range are spotted more easily by a pointer moving against a scale in an analogue
meter (rather than by variations in the numeric output display of a digital meter).
• Second, analogue instruments also tend to suffer less from noise and isolation
problem.
• In addition, because analogue instruments are usually passive instruments that do
not need a power supply, this is often very useful in measurement applications
where a suitable main power supply is not readily available.
Drawbacks:
• They are prone to measurement errors from a number of sources that include
inaccurate scale marking during manufacture, bearing friction, bent pointers, and
ambient temperature variations.
• Further human errors are introduced through parallax error and
• mistakes in interpolating between scale markings.

• Quoted inaccuracy values are between ±0.1 and ±3%.

• Types of analogue meters in use:

– Moving Coil Meter
– Moving Iron Meter
– Analogue Multimeter
 1. MOVING COIL METER

• It is a very commonly used form of analogue voltmeter because of its sensitivity,

accuracy, and linear scale, although it only responds to d.c. signals. As shown in the
Figure, it consists of a rectangular coil wound round a soft iron core that is
suspended in the field of a permanent magnet.

• The signal being measured is applied to the coil, which produces a radial magnetic
field. Interaction between this induced field and the field produced by the
permanent magnet causes torque, which results in rotation of the coil.
3. ANALOGUE MULTIMETER
• Although still widely available, the analogue multimeter is now less common
than the digital multimeter. It is a multifunction instrument that can measure
current and resistance, as well as d.c. and a.c. voltage signals.
• Basically, the instrument consists of a moving coil analogue meter with a
switchable bridge rectifier to allow it to measure a.c. signals.
• A set of rotary switches allows the selection of various series and shunt
resistors, which make the instrument capable of measuring both voltage and
current over a number of ranges.

• An internal power source is also provided to allow it to measure

resistances as well.

• While this instrument is very useful for giving an indication of voltage levels,
the compromises in its design that enable it to measure so many different
quantities necessarily mean that its accuracy is not as good as instruments
that are purposely designed to measure just one quantity over a single
measuring range.
 D. CALCULATION OF METER
OUTPUTS FOR NON STANDARD
WAVEFORMS
• Class work
• Circuit for Examples