Chapter 9 Basic meters Core Competency Units UEENEEE003B Solve problems in extra-low voltage single path circuits UEENEEE004B Solve problems in multiple path DC Circuits Essential Knowledge and Associated Skills (EKAS) 2.8.1.2 Fundamental electrical principles 2.8.1.3 Electrotechnical principles 2.8.1.4 Circuits principles 2.8.2.1 Direct current circuit principles Chapter overview Electrical quantities such as voltage, current and resistance can sometimes be calculated, but it s often easier to measure them. This chapter describes the operating principles of an analog and digital ammeter, voltmeter and ohmmeter. It also explains meter loading, which is the effect a voltmeter has on the voltage value it is measuring. The chapter concludes by explaining the use and safety aspects of a typical analog and digital multimeter, an instrument commonly used in the field and the workshop.
Chapter outline 9.1 Introduction 9.2 Analog meter movement 9.3 Digital meter module 9.4 The ammeter 9.5 The voltmeter 9.6 Voltmeter loading 9.7 The ohmmeter 9.8 Multimeters Summing up... 9.3 Measuring instruments are specified by their accuracy, resolution and sensitivity. Accuracy depends on manufacture, calibration and maintenance; resolution is the number of decimal places a reading provides; and sensitivity determines how much current the meter takes from the circuit under test. Measuring instruments are either analog or digital. Analog meters have a meter movement with a scale and pointer, digital meters are electronic with a numerical readout. Most analog meters have a moving-coil meter movement, in which the current flowing through the coil causes it to deflect in proportion to the value of the current. The lower the full scale deflection current (I FSD ) of a meter movement, the more sensitive it is, where sensitivity is 1/I FSD, given in ohms/volt. The basic digital meter is a digital voltmeter module that has a digital to analog converter, a display and a display driver. A typical module has a maximum input of 199.9 mv.
9.6 9.4 Because an ammeter is connected in series with the circuit under test, it must have a low resistance so it does not affect the value of the current being measured. An ammeter has two main parts: a meter movement or digital voltmeter module, connected across a low value resistance called a shunt resistor. A shunt resistor is often made of manganin, a metal alloy whose resistance remains relatively constant with changes in temperature. In a multi-range ammeter, the shunt resistor for each range can be a separate component, providing the selector switch has a make-before-break action, or a tapped resistor called an Aryton or universal shunt. The resistance of a shunt is generally in the order of a few milliohms, depending on the current range it is designed for. An analog voltmeter has a moving-coil meter movement and a resistor called a multiplier in series with movement. The resistance value of the multiplier can be found with Ohm s law, or with the product of the movement s sensitivity in ohms/volt and the voltage range. The resistance of an analog voltmeter changes with each voltage range. The higher the range, the higher the resistance. A digital voltmeter has a potential divider network rather than individual multiplier resistors. The resistance of a digital voltmeter is the same on all ranges. It is usually 10 MΩ. A voltmeter can load a circuit, and therefore give misleading readings. The higher the resistance of the voltmeter, the less the loading effect. When measuring both load current and voltage, use the short shunt connection for most purposes. For small values of load current, use the long shunt connection to avoid the ammeter giving a misleading reading due to the voltmeter current.
Answers to in-chapter exercises 1. What is the value of the shunt resistor required to make a 20 A ammeter based on a moving-coil meter movement with a resistance of 100 ohms and a full scale deflection current of 2 ma? R shunt = 0.01 ohms, or 10 milliohms 2. What is the value of the multiplier resistor required to make a 100 V voltmeter based on a meter movement with a resistance of 500 ohms and a full scale deflection current of 50 μa? R multiplier = 2 MΩ 3. A voltmeter has a sensitivity of 20 k ohms per volt (Ω /V). What is its total resistance when the voltmeter is set to its (a) 20 V range and (b) 200 V range? Solution: a. 400 k Ω b. 4 M Ω Chapter summary An ammeter is connected in series with the circuit under test, and comprises a meter (analog or digital) connected across a low value shunt resistor. The resistance of a shunt is generally a few milliohms. A multirange ammeter often has a universal shunt rather than separate shunt resistors. A voltmeter is connected in parallel with the circuit under test and comprises a meter (analog or digital) connected in series with a high value of resistance. For an analog meter movement, the required resistance can be found by multiplying the movement s sensitivity in ohms/volt by the maximum voltage for that range.
The resistance of an analog voltmeter changes with each voltage range. The higher the range, the higher the resistance, and the less the loading effect the meter has on the circuit under test. The resistance of a digital voltmeter is the same on all ranges, usually 10 M Ω. Multimeters are useful for measuring resistance values over 100 Ω. For lower values, more reliable and accurate readings are obtained with specialised (high test current) ohmmeters or other measurement techniques (e.g. voltmeter/ammeter). An ohmmeter passes current through the resistance being measured. The value of this current depends on the meter, the resistance range and resistance being measured. When measuring low resistance values, a high test current (above 1 ma) gives a more reliable reading. Multimeters and their ancillaries (probes, etc.) should be rated according to the category of work being carried out. Cat III refers to 230/400 V AC mains wiring protected by a service fuse and voltage measurement should be done with a Cat III 300 V or better rated voltmeter or multimeter. Analog multimeters have a pointer and various scales that cover all the meter functions, in which a scale has major and minor divisions to give a resolution of two decimal places or less. A digital multimeter LCD display has four digits to show a reading and annunciators to show the selected meter function and multiplier (e.g. k or M for ohms, ma for amps). The maximum display value of the meter depends on its manufacture, and is typically 199.9 or 399.9. A good quality DMM will have properly rated HRC fuses to protect against overload when measuring current. Glass fuses are not suitable, and are potentially dangerous in this application. After measuring current on a DMM, replace the meter leads into the voltage terminals.
Answers to self-check questions 1. Explain the terms resolution, accuracy and sensitivity in regard to a measuring instrument such as a voltmeter. Accuracy of a measuring instrument depends on its manufacture, calibration and maintenance. Resolution refers to how many decimal points you can read the displayed value to, and sensitivity refers to how much power (current) it takes from the circuit it s connected to. 2. What two functions are performed by the hair springs in a moving-coil meter movement? The two main functions performed by the hair springs in a moving-coil meter movement are to provide a current path to the coil, and to provide a counter torque against the force between the magnetic fields. 3. A moving-coil meter movement has a full scale deflection current (I FSD ) of 50 μa. What is its sensitivity in ohms per volt? Is this meter movement more or less sensitive than one with an I FSD of 1 ma? 20 000 ohms/volt (20 kω/v). Movement is more sensitive than one with an IFSD of 1mA. 4. What are the three main components of a basic digital millivoltmeter? Main parts are an analog to digital converter, circuitry to operate the display, and the display (LCD or LED). 5. Calculate the value of the shunt resistor required to make a 25 A ammeter based on a meter movement with a resistance of 250 ohms and a full scale deflection current of 0.1 ma.
1 milliohm 6. A 200 mv four-digit digital voltmeter module is connected across a shunt resistor that is passing a current of 16 A. Determine the value of the shunt resistor to cause the display to indicate 16.00 (assuming the decimal point is manually positioned). 10 milliohms (to produce 160 mv) 7. What is the advantage of an Aryton or universal shunt compared to individually switched shunt resistors? A universal shunt remains connected at all times to the meter movement, even during range change operations, thereby protecting the meter movement from over-voltages that can otherwise occur. 8. Calculate the value of the multiplier resistor required to make a 500 V voltmeter based on a meter movement with a resistance of 1 k ohms and a full scale deflection current of 500 μa. 1 MΩ 9. Repeat the calculation in Question 8, but for a meter movement with an I FSD of 10 μa. 50 MΩ 10. What is meant by the term meter loading as applied to a voltmeter, and under what circumstances is this likely to be a problem? Meter loading refers to voltages changing within a circuit when a voltmeter is connected to measure them. The loading effect is greatest on high resistance circuits, and depends on the sensitivity of the voltmeter. The higher the meter sensitivity the less load it imposes on the circuit.
11. For the circuit in Figure 9.39, calculate the voltage across R 2 before and after the voltmeter shown as resistor R VM is connected. Figure 9.39 V R1 = 20 V before meter is connected, 14.29 V after meter is connected. 12. In Figure 9.39, calculate the voltage across R 1 if voltmeter R VM is replaced with a digital voltmeter. V R2 = 80 V as DVM is not loading the circuit. 13. Determine the reading shown on the scale in Figure 9.40 for the following function switch settings: a. all AC and DC voltage ranges b. 310 and 310 k resistance ranges c. DC current on 12A range.
Figure 9.40 a. 6 V AC range = 3.4 V, 6 V DC range = 3.3 V, 30 V AC/DC range = 16.5 V, 120 V AC/DC range = 66 V, 600 V AC/DC range = 330 V b. 310 Ω range = 240 Ω, 310 kω range = 80 kω c. 6.3 A 14. What is meant by an instrument rating of Cat III 600 V? Give an example of a Cat III electrical work environment. A Cat III 600 V rated instrument can be used when working on wiring on the load side of a service fuse (e.g. power wiring in a house) where the phase to phase voltage does not exceed 600 V AC. 15. When measuring voltage with an analog multimeter, what two things should you consider when selecting the voltage range? The loading effect and resolution of the displayed value. Higher ranges cause less loading but give less resolution (1 decimal place), low ranges cause greater loading but provide a higher resolution (2 decimal places). 16. When using a multimeter to measure current, what type of fuse should be in the multimeter, and what should you do after taking the current measurements? Briefly explain. A correctly rated HRC fuse should be fitted to the meter. After taking current measurements with a multimeter, return the probes to the voltage sockets. This avoids
the possibility of forgetting the probes are not in the correct sockets when measuring a voltage in a live Cat III power circuit. If the probes are in ammeter sockets, the resulting fault current through the ammeter shunt will quickly operate the fuse and protect the meter, the circuit and the user. 17. When measuring current and voltage in a circuit with two separate instruments, what type of connection should be used when the load current is (a) large, (b) a few milliamps? a. large load current, use short shunt connection b. small load current, use long shunt connection so the ammeter does not pass the current taken by the voltmeter. 18. What precautions should you take when measuring resistance in a circuit? Precautions include: confirm circuit is dead, touch one probe only during measurement to avoid errors, be aware there could be other components in the circuit in parallel with the resistance under test. 19. Under what conditions is an analog multimeter better for measuring resistance than a digital multimeter? An analog multimeter can provide more accurate readings in electrically noisy environments. Some analog meters provide a suitably high value of test current for testing resistances below a few ohms. A digital meter responds to electrical noise and produces a low test current, which can give misleading readings in low resistance (e.g. wiring) circuits. 20. Briefly explain how a solid state diode can be checked with a multimeter, and outline how the test is done with an analog and a digital multimeter.
Test is to see if the diode conducts with one probe polarity, but not the other. Test is done on an analog meter by setting it to a suitable resistance range, on a digital meter by selecting the diode test. Probes are connected to the diode terminals, a reading is taken, the probes are swapped over and another reading taken. Online resources For more information about the contents of this chapter, try Googling the terms voltmeter, ammeter, ohmmeter and multimeter. Wikipedia is always very informative. Also, look for websites with a.edu in the address, as these usually give uncomplicated descriptions.