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Contents Introduction 3 Ammeter shunts 4 Voltage multipliers 12 Digital instruments 16 Digital ammeter ‘shunts’ 16 Digital voltmeter multipliers 17 Summary 21 Answers 24 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 1 2 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Introduction The basic analogue meter movement or digital module responds to a limited range of currents or voltages. If we want to measure quantities outside this limited range, we need to incorporate additional resistors, either within a multi-range instrument, or external to the instrument. In this section, you will learn how to extend the range of an instrument using voltage multipliers and ammeter shunts. This section will help you to understand the construction of multi-range digital and analogue meters. After completing this topic, you should be able to: select an appropriate meter in terms of units to be measured, range, loading effect and accuracy for a given application explain how multi-range ammeters and voltmeters are built from a basic meter movement plus additional resistors select and use external ammeter shunts for heavy currents follow safety precautions when measuring large currents and voltages. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 3 Ammeter shunts An ammeter is connected in series with the circuit being monitored, so the circuit current flows through the ammeter. A typical analogue meter movement requires 15 mA for full-scale deflection. To measure currents in excess of this, we use an ammeter shunt, which is a resistor connected in parallel with the meter that diverts a known fraction of the current. With the shunt in place, the full-scale deflection of the meter now occurs at higher currents, and so the shunt serves to extend the range of the instrument. An ammeter shunt is a low-resistance conductor connected in parallel with the moving-coil circuit of an ammeter as shown in Figure 1. Figure 1: Ammeter shunt connection 4 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Figure 2: Arrangement of ammeter external shunt Ammeter shunts are used internally to a meter to provide the usual switchselectable current ranges of the instrument. For measurement of even larger currents, low-resistance shunts can be connected externally on some instruments. Usually the shunt is separate if the current range is greater than 25 A. An ammeter shunt must have enough capacity to prevent overheating when being used. The resistance of the shunt must be such that almost the entire current is carried by the shunt; only a small definite portion of the total current is required to operate the moving coil. Because of this definite relationship between the total current and the instrument current, the scale is usually marked to indicate the total current directly. With switchboard ammeters, the shunts are generally connected directly to the busbars at the switchboard panels and are provided with special flexible leads for connection to the instruments. As the resistance of these leads is included in that of the moving coil when the instrument and its shunt are being calibrated, it is important not to shorten or alter these leads, otherwise the accuracy of the instrument will be affected. This applies also to the leads provided with an instrument which has a set of external shunts to enable it to be used as a multi-range ammeter. In every case, when connecting these leads make sure there is effective contact. Ammeter shunts are practically always constructed of manganin, an alloy of copper, nickel and manganese. Manganin has a low resistivity, so a shunt of manganin is reasonably small. However, the main advantage is that manganin has a temperature coefficient of resistance of practically zero. Therefore, its resistance is substantially constant and unaffected by temperature variation. If an internal shunt is employed, it is very often in the form of a wire connected across the instrument terminals. An external shunt EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 5 of manganin brazed into slots cut in heavy copper terminal blocks is illustrated in Figure 2. The resistance of a shunt for a specific use may be calculated, since the moving coil and the shunt form a parallel circuit of two branches. The voltage drop across the instrument and the shunt is the same. Consider the instrument shown in Figure 3 which has an initial range of 15 milliampere and 5 ohm resistance, and suppose that it is required to operate as an ammeter capable of indicating currents up to a maximum of 5 ampere. When the pointer of the instrument in Figure 3 indicates a full-scale deflection, the p.d. across the moving coil is 75 millivolt and the current is 15 milliampere. Figure 3: Digital instrument scale reading As this full-scale position is to correspond to a current of 5 ampere in the line, then the pd across the shunt must be 75 millivolt, and have a current of (5 – 0.015) = 4.985 ampere. Therefore from Ohm’s Law the resistance of the shunt is: RS V Is 0.075 4.985 RS 0.015 The same result could be obtained from the fact that in a parallel circuit the current in the branches is inversely proportional to their resistances, that is, 6 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 I S RC I c RS RS I c Rc Is 0.015 5 4.985 0.015 Note that in each case: IS = current in shunt RS = resistance of shunt IC = current in moving coil RC = resistance of moving coil. In addition to requiring a resistance of 0.015 ohm, this shunt must be able to carry 5 ampere without overheating. For accurate indications the current must always divide between the coil and the shunt in the same proportion, that is, the ratio must remain constant. To make sure this happens despite the different temperature coefficients of the manganin shunt and the copper coil, we use a swamping resistor as shown in Figure 2. This swamping resistor consists of a manganin-wire resistor connected in series with the moving coil, and its resistance comprises almost the entire resistance of the moving-coil circuit. In this way the moving-coil circuit is the equivalent of a circuit with a negligible temperature coefficient of resistance and thus remains constant. The initial range of a meter is increased when a shunt, with the appropriate multiplying power, is connected in parallel for the required extended range. total current instrument current I IC Multiplying power Example 1 A moving-coil galvanometer has an initial range of 50 milliampere. A shunt enables it to function as an ammeter with a full-scale deflection of 10 A. Determine: (a) multiplying power of shunt (b) current in shunt: EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 7 Solution (a ) Multiplying power (b) Shunt current I S I IC 10 0.05 200 I Ic 10 0.05 9.95 A total current instrument current total current multiplying power current Multiplying power Instrument current That is, instrument current is: IC 1 ITotal multiplying power the remainder being carried by the shunt. Figure 4: Diagram for example 1 Example 2 The moving-coil movement has a resistance of 4.98 . Determine the resistance of a shunt which would have a multiplying power of 250 when used with this instrument to measure a circuit current of 1 A. Ra 4.98 I 250 Ia RS ? I 1 A 8 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Figure 5: Moving coil with swamping resistor Instrument current: total current multiplying power 0.004 A Ia Shunt current: I s I IC I 0.004 I s 0.996 A Rs I A RA Is 0.004 4.98 0.996 Rs 0.02 A single meter can be made to measure various ranges of current by the use of different shunts and a multi-position, single-pole switch as shown in Figure 6. Figure 6: Shunts and a multi-position, single-pole switch EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 9 The higher the current range, the more current the shunt has to carry, and consequently, the lower will be the shunt’s resistance in relation to the resistance of the meter. Activity 1 Work through the questions below. 1 Explain why ammeters are designed to have a very low resistance. _____________________________________________________________________ _____________________________________________________________________ 2 Explain why voltmeters are designed with a very high resistance. _____________________________________________________________________ _____________________________________________________________________ 3 Explain why a moving-coil type meter is a polarised one, that is, it is only suitable for operation on dc. _____________________________________________________________________ _____________________________________________________________________ 4 Explain why and how a moving-coil meter could be used as either an ammeter or a voltmeter. _____________________________________________________________________ _____________________________________________________________________ 5 Of what material are ammeter shunts normally made, and why? _____________________________________________________________________ _____________________________________________________________________ 6 If the resistance of a moving-coil meter movement is 3 Ω and its full-scale deflection 20 mA, determine the resistance of the shunt necessary to increase the meter range to: (a) 0.5 ampere ___________________________________________________________________ ___________________________________________________________________ 10 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 ___________________________________________________________________ ___________________________________________________________________ (b) 3 ampere. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 7 Check your answers with those given at the end of the section. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 11 Voltage multipliers A voltmeter is connected in parallel with the circuit component whose voltage is being monitored, and uses the same moving coil or moving iron or digital meter movement as does an ammeter. These movements will reach full scale deflection at a relatively low voltage. Additional components are required to allow these basic meters to measure larger voltages. A voltage multiplier is a resistor connected in series with the basic meter movement, so that a known fraction of the total measured voltage is dropped across this resistance. With a voltage multiplier in place, the full-scale deflection of the movement now represents a higher measured voltage, so the multiplier extends the range of the basic instrument movement. We can convert a moving-coil meter into a voltmeter by connecting a voltmeter multiplier of high resistance in series with the moving coil, as shown in Figure 7. The resistance of a voltmeter multiplier must be such that with full-scale voltage applied, the current in the moving coil is limited to the small value required for full-scale deflection. Figure 7: Converting a moving-coil meter into a voltmeter The value of the series resistor (the multiplier) may be determined by applying Ohm’s law as shown in the following example. Example 1 A moving-coil instrument has an initial range of 75 millivolt and a resistance of 5 ohm. Determine the resistance of a voltmeter multiplier to be connected in series with the coil to allow this instrument to operate as a voltmeter capable of indicating to 150 volt. 12 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Figure 8: A moving coil instrument with a multiplier Full-scale deflection current: I initial voltage range RV 0.075 5 0.015 A = 15 mA 0.015 ampere is the maximum current which the moving coil should carry. Hence total resistance of circuit to limit the current to 0.015 ampere when a potential difference of 150 volt is applied, is expressed as: R V Ic 150 0.015 10 000 The instrument coil has a resistance of 5 ohm and the resistance of the multiplier: = 10 000 – 5 = 9995 Ω Note from Figure 8 that the potential difference across the multiplier: = 150 – 0.075 = 149.925 volt Voltmeter multipliers are usually wound with fine-gauge insulated manganin or similar wire of low resistance temperature coefficient. The resistance of the moving coil is such a small portion of the total resistance that the resistance variation with temperature is negligible. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 13 A voltmeter multiplier may be either internal or external. A single instrument may be a multi-range voltmeter if it is provided with a set of multipliers. If the multiplier is internal, it may be tapped to provide various ranges. The ranges may be selected by switching, or in some cases a separate terminal is provided for each range. Sometimes a multi-range voltmeter has a separately marked scale for every range and sometimes the scale is only marked for one range. In the latter case, it is necessary to use a conversion factor to obtain the correct measured value. For example, if a multi-range voltmeter has a scale graduated from 0 to 150 V and it is connected to the 0 to 15 V range, any apparent value indicated by the instrument pointer will have to be divided by 10 to obtain the correct value. With the calculation of suitable size series resistors, any instrument can be designed to measure any desired voltage. A typical circuit is shown in Figure 9. Figure 9: Multi range voltmeter with multipliers You will observe from this diagram that the multipliers are simply a tapped series resistor. The higher the voltage to be measured, the higher the resistance needed. 14 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Activity 2 Work through the questions below. 1 A moving-coil meter has a full-scale deflection of 50 mV and a resistance of 4 Ω. Determine the multiplier resistance needed to increase the range of the meter to: (a) 30 V (b) 150 V ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 2 Describe what is meant by voltmeter sensitivity. How is it expressed? _____________________________________________________________________ _____________________________________________________________________ 3 Why is it necessary that a voltmeter not ‘load-up’ a circuit? _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Check your answers with those given at the end of the section. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 15 Digital instruments Digital meter modules also have a limited range, and again, we use external resistors to increase the range of the instrument. The characteristic of the digital module are however quite different from the analogue movement. In particular: The digital meter module responds to voltage, rather than current. The digital meter module typically has very high input resistance. Digital ammeter ‘shunts’ A digital ammeter can be built from the basic digital voltage module by including a shunt resistor. The value of the shunt resistor is chosen so the maximum current for that range causes a voltage drop across the shunt resistor equal to the full-scale input voltage of the digital module. The digital module takes almost no current, so the calculation is a simply based upon Ohm’s Law. The circuit arrangement is shown in Figure 10. Figure 10: Digital ammeter with shunt resistor 16 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Example 3 Calculate the value of a shunt resistor for a current range of 2 A for a digital module that has a full-scale input voltage of 200 mV. I in 200 mV I FSD 2 A Rshunt ? Rshunt Vin I FSD 200 10 3 2 100 10 3 0.1 Digital voltmeter multipliers Unlike the analogue voltmeter which uses a single resistor in series with the movement, the digital voltmeter uses a voltage divider network to reduce the voltage being monitored to the level required by the basic digital module (usually 200 mV). The concept is shown in Figure 11. Figure 11: Digital voltmeter with voltage divider Because of the way in which the voltage reduction network is connected, the input resistance of a digital voltmeter is the same for all ranges. It is usually around 10 megohm. By comparison the input resistance of an analogue voltmeter depends on the voltage range that the meter is set to. The typical value of input resistance for an analogue voltmeter is around 20 000 ohms per volt. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 17 Example 4 Calculate the value of R1 in the circuit shown below to allow the digital module to reach its full-scale input voltage of 200 mV if an input of 20 V is being measured. Figure 12: Circuit for example 4 Current flowing in R2 is: 200 mV 10 k 2 105 V = 20 μV I Therefore current flowing in R1 must also be 2 × 10–5 A. The voltage drop across R1 is: 20 VR2 20 0.2 19.8 V Therefore: 19.8 2 105 990 000 R1 18 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Activity 3 1 For the circuit shown in Figure 13, calculate the resistance and power rating of resistors R1, R2, R3 and R4. Assume the digital module has a full scale input voltage of 200 mV. Figure 13 2 For the circuit shown in Figure 14, calculate the voltage at the digital module terminals (ignore any current flowing to the digital module). Assume the digital module has a full scale input voltage of 200 mV. Figure 14 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 19 ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Check your answers with those given at the end of the section. If you have Hampson, check the hazards and precautions with instruments on pages 40 and 41. If you have Jenneson, refer to Sections 7.4.4 and 7.4.5 on pages 134 and 135. 20 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Summary This section covered single and multi-range digital and analogue meters and the effects of analogue meters in circuits. Measuring current An ammeter is connected in series with the circuit being monitored, so the entire circuit current has to flow through the ammeter. Ammeter shunts An ammeter shunt is a low-resistance conductor connected in parallel with the moving-coil circuit of an ammeter. The resistance of the shunt must be such that almost the entire current is carried by the shunt; only a small definite portion of the total current is required to operate the moving coil. A digital ammeter has shunt resistors like those in an analogue ammeter. Measuring voltage A voltmeter is connected in parallel with the circuit component whose voltage is being monitored. A voltmeter uses the same moving coil or moving iron or digital meter movement as does an ammeter. In order to convert these meter modules to usable voltmeters, additional components are required. Voltmeter multipliers The high resistance of a voltmeter multiplier must be such that with full-scale voltage applied, the current in the moving coil is limited to the small value required for full-scale deflection. Analogue voltmeter The resistance of the voltmeter circuit for each volt of full scale deflection is expressed in ‘ohm per volt’ Voltmeter multiplier connections Voltmeter multipliers are usually wound with fine-gauge insulated manganin or similar wire of low temperature coefficient. Any instrument can be designed to measure any desired voltage. Digital voltmeter The digital voltmeter uses a voltage divider network to reduce the voltage being monitored to the level required by the basic digital module. Loading effect of voltmeters in circuits The value obtained for the resistance of the circuit whose voltage is being measured will be much lower than the true value calculated. The ‘loading effect’ of a voltmeter, can be reduced by using a voltmeter with a high sensitivity. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 21 Check your progress 1 The basic moving coil meter movement can be used to measure larger currents if used in conjunction with a: (a) series resistor (b) multiplier resistor (c) shunt resistor (d) loading resistor. 2 ( ) The basic moving coil meter movement can be used to measure higher voltages, if used in conjunction with a: (a) shunt resistor (b) multiplier resistor (c) parallel resistor (d) loading resistor. 3 ( ) A 2 mA meter movement has a resistance of 70 Ω. Calculate the value of a shunt resistor to be used if the meter is required to measure 500 mA. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 4 A 1.5 mA meter movement has a resistance of 150 Ω. Determine the value of a multiplier resistance to be used to read 5 V. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ 22 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 5 Referring to the circuit shown in Figure 15 determine the voltage across R2 if the voltmeter has: (a) an infinite resistance (b) a resistance of 20 k. Figure 15 _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Check your answers with those given at the end of the section. EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 23 Answers Activity 1 1 A particular load needs a particular current o make it operate efficiently, a lamp or a motor. If an ammeter connected in series with it has a high resistance, it will reduce the circuit current and thus the load will not operate successfully. If, however, it has a very low resistance, the circuit current will change minimally and the load will operate efficiently. 2 By having a very high resistance voltmeters take very little current and thus consume almost no power. Since they are placed in circuit not to use up power, but only to give a voltage reading, it is imperative then that they have this very high resistance. Voltmeters are connected between points of different potential, so they must have a high resistance in order to limit the current to just enough to make the meter indicate. 3 The magnetic field produced by current flowing in the moving coil reacts with the magnetic field from the surrounding permanent magnet producing a torque and turning the moving coil. A pointer attached to the coil likewise moves past a graduated scale giving a particular reading depending on the amount of current in the moving coil. If there was AC in the moving coil, the field produced by it would be continuously changing in direction such that its reaction with the permanent magnet field would alternate each one hundredth of a second, by trying to move the pointer first upscale then downscale. The result of this would see the pointer wavering around zero. The meter then is only suitable for DC, and to produce an upscale or readable deflection must have current flowing into its + terminal and out of its—terminal. 24 4 The moving coil moves its pointer across to full-scale deflection when a very small applied voltage (usually millivolts) causes a very small current (usually milliamperes) to flow in it. The meter can thus be graduated in millivolts to read this voltage or in milliamperes to read the current. For higher values of voltage or current, multipliers or shunts could be used to extend its range. 5 Ammeter shunts are normally made of manganin. It is a material with an almost zero temperature co-efficient of resistance which means that if it gets hot with large currents flowing through it, its resistance hardly changes. We are confident then that the currents flowing through it and EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 its associated meter don’t vary in ratio, and the meter will not have its accuracy of reading affected. 6 (a) Figure 16 VFSD I FSD Rmeter 0.02 3 0.06 V I sh I - I meter 0.5 0.02 0.48 A Vsh Vmeter 0.06 V Rshunt Vsh I sh 0.06 0.48 0.125 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 25 (b) Figure 17 I sh I I meter 3 0.02 2.98 A Vsh Vmeter 0.06 V Rshunt Vshunt I shunt 0.06 2.98 0.0201 Activity 2 1 (a) Figure 18 26 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 I meter Vmeter Rmeter I multiplier 0.05 4 0.0125 A Vmultiplier V Vmeter 30 0.05 29.95 V Rmultiplier Vmultiplier I multiplier 29.95 0.0125 2396 (b) Figure 19 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 27 I meter (part A) = 0.0125A Vmultiplier V Vmeter 150 0.05 149.95 V Rmultiplier Vmultiplier I multiplier 149.95 0.0125 11 996 2 Voltmeter sensitivity is a term which nominates the resistance of the voltmeter load itself for each volt of full-scale deflection. It is expressed in ‘Ohms per volt.’ It may be calculated from the simple equation: Ohm/volt 3 1 Full-scale deflection in amperes Since a voltmeter is connected in parallel with a load the circuit current splits up between them. The break-up of current depends on the resistance of each. If RV is equal to the load resistance then the current splits up, half in each. This we certainly don’t want as we would like all, if possible, of the current to flow through the load. What we want then is a voltmeter with a resistance at least 20 times bigger than the load resistance, in order that only a small percentage of the current flows in the voltmeter with most of it going through the load. A large current through the voltmeter causes a loading effect in the circuit, an effect that is undesirable. Activity 3 1 R1 = 1 k R2 = 1 R3 = 0.1 R4 = 0.01 2 R R4 Vter min als Vinput 3 Rtotal 100 k 16 10 M 160 mV 28 EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 Check your progress 1 (c) 2 (b) 3 0.281 4 3183.3 5 (a) 50 V (b) 44.4 V EEE042A: 15 Employ ammeter shunts and voltage multipliers NSW DET 2017 2006/060/05/2017 LRR 3670 29