Electrical Measuring Instruments & Measurements

• Chapter 43 covers electrical measuring instruments and measurements

Introduction

• Tests and measurements are crucial for designing, evaluating, maintaining, and servicing electrical circuits and equipment
• Electrical quantities like current, voltage, resistance, and power require transformation into visible indications for detection
• Instruments aid this process by indicating the magnitude of quantities through a pointer on a graduated scale (analogue) or a decimal number (digital)
• Digital instruments, especially computer-based ones like the digital storage oscilloscope, are increasingly favored over conventional equipment

Electronic Instruments

• Ammeters measure current and connect in series within a circuit
• Voltmeters measure potential difference (p.d.) and connect in parallel with the part of the circuit requiring the p.d measurement
• Ohmmeters measure resistance

Multimeters

• Digital Multimeters (DMMs), with Fluke being a top brand, are almost universally preferred for their accuracy, resolution, and ruggedness
• DMMs measure DC and AC (RMS) currents and voltages, resistance, continuity, and temperature

Wattmeters

• Wattmeters measure electrical power in a circuit
• They connect to measure power supplied to a load
• Wattmeters contain a current coil (in series with the load, like an ammeter) and a voltage coil (in parallel with the load, like a voltmeter)

Instrument Loading Effect

• Measuring instruments can draw power from the circuit, affecting measurements
• The instrument's "loading" effect relates to the current it draws, altering circuit conditions
• Voltmeter resistance can be calculated using sensitivity, often stated in 'kΩ per volt' of full-scale deflection (f.s.d.)
• Voltmeters should have high resistance, ideally infinite

Example problem 1

• Calculate the power dissipated by the voltmeter and by resistor R when (a) R = 250Ω, (b) R = 2M Ω, given a voltmeter sensitivity of 10 kΩ/V
• Because power dissipated in the voltmeter can be insignificant compared to the load, this is often considered

Example problem 2

• An ammeter with a full-scale deflection (f.s.d.) of 100mA and resistance of 50Ω measures current in a 500Ω load with a 10V supply
• Calculate the ammeter reading expected, the actual current, power dissipated in the ammeter, and power dissipated in the load

The Oscilloscope

• Oscilloscopes are graph-displaying devices charting electrical signal changes
• They display how signals change over time
• From the graph it's possible to determine time and voltage values, frequency, circuit behavior, DC/AC signal components, and noise levels

Analogue Oscilloscopes

• Direct voltage measurements use the 'volts/cm' switch on the cathode ray oscilloscope (c.r.o.)
• Trace position on screen, with and without voltage, indicates magnitude
• Alternating voltage measurements display sinusoidal waveforms
• The 'time/cm' switch gives the periodic time T of the sine wave
• The 'volts/cm' switch enables amplitude or peak value determination

Digital Oscilloscopes

• Digital oscilloscopes collect waveform data for display using additional data processing systems
• Digital oscilloscopes adjust signal amplitude with a probe attached similarly to analogue ones

Example problem 4

• For an oscilloscope square voltage waveform with the 'time/cm' switch at 100μs/cm and 'volts/cm' at 20V/cm
• One can determine periodic time, frequency, and peak-to-peak voltage

Example problem 5

• For a pulse waveform with the 'time/cm' switch at 50ms/cm and the 'volts/cm' switch at 0.2V/cm
• One can determine periodic time, frequency, and pulse voltage magnitude

Wheatstone Bridge

• Wheatstone bridges compare an unknown resistance with known values
• The variable resistor is adjusted for zero galvanometer deflection
• With no current flowing, the bridge is "balanced" when voltage A equals voltage B

Example problem 9

• In a balanced Wheatstone bridge with known resistances between points
• The unknown resistance calculation can be completed

D.C. Potentiometer

• D.C. potentiometers are null-balance instruments determining EMF and PD via comparison with known values
• The potentiometer can be used to create a variable d.c supply

Example problem 10

• In a D.C. potentiometer example, find the dry cell EMF if balance is at 650mm
• The balance is already known using a standard cell of 1.0186 volts with balance existing at 400mm