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Measuring current and voltage Figure 3.14 A circuit which is used to show the effect of the meter’s own resistance in a circuit you apply your multi-meter across resistor R2 you find that the voltage indicated is only about 3 V. The fact is that when the multi-meter is not connected to the circuit the voltage is 4.5 V, but as soon as the multi-meter is applied, the voltage across resistor R2 falls to 3 V. Also, the voltage across resistor R1 rises to 6 V (both voltages must add up to the battery voltage, remember). Applying the multi-meter affects the operation of the circuit, because the multi-meter resistance is in parallel with resistor R2. Ohms per volt A meter’s internal resistance is stated as a number of ohms per volt. For example, the resistance of the meter we use in this book is 20,000 ohms per volt (written 20 kΩV -1 ) on d.c. voltage scales so when it’s to read 4.5 V, it’s resistance is 90 k. This 73
Starting electronics resistance, in parallel with resistor R2 forms an equivalent resistance, given by the law of parallel resistors, of: This resistance is now the new value of R2 in the circuit so, applying the voltage divider rule, the measured voltage will be: which is roughly what you measured (hopefully). Any difference between the actual measurement and this calculated value may be accounted for because this lower voltage causes a lower multi-meter resistance which, in turn, affects the voltage which, in turn, affects the resistance, and so on until a balance is reached. All of this occurs instantly, as soon as the multi-meter is connected to the circuit. What this tells us is that you must be careful when using your multi-meter to measure voltage. If the circuitunder-test’s resistance is high, the multi-meter resistance will affect the circuit operation, causing an incorrect reading. Any multi-meter will affect operation of any circuit to a greater or lesser extent — but the higher the multimeter resistance compared with the circuit resistance, the more accurate the reading. 74
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Starting Electronics i
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Starting Electronics Keith Brindley
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Contents Contents Preface vi 1. The
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The very first steps 1 The very fir
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The very first steps potential rewa
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The very first steps Photo 1.2 tool
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The very first steps that electrici
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The very first steps Going back to
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The very first steps tially exist.
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The very first steps by a scientist
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The very first steps Hint: Ohm’s
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The very first steps Electronic com
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The very first steps Larger values
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The very first steps Time out That
Page 30 and 31: On the boards 2 On the boards In th
Page 32 and 33: On the boards If you are following
Page 34 and 35: On the boards Hint: The theoretical
Page 36 and 37: On the boards Photo 2.2 Inside brea
Page 38 and 39: On the boards Down the edges of the
Page 40 and 41: On the boards Hint: The multi-meter
Page 42 and 43: On the boards of leads should be su
Page 44 and 45: On the boards it gets older and sta
Page 46 and 47: On the boards Hint: The bands group
Page 48 and 49: On the boards In practice, you migh
Page 50 and 51: On the boards called just ohmmeter)
Page 52 and 53: On the boards ends. We say resistor
Page 54 and 55: On the boards we would get: and thi
Page 56 and 57: On the boards Figure 2.9 A comparat
Page 58 and 59: Measuring current and voltage 3 Mea
Page 60 and 61: Measuring current and voltage forme
Page 62 and 63: Measuring current and voltage Netwo
Page 64 and 65: Measuring current and voltage Netwo
Page 66 and 67: Measuring current and voltage Did y
Page 68 and 69: Measuring current and voltage Figur
Page 72 and 73: Practically there Measuring current
Page 74 and 75: Measuring current and voltage Take
Page 78 and 79: Measuring current and voltage Take
Page 82 and 83: Measuring current and voltage Hint:
Page 84 and 85: Capacitors 4 Capacitors You need a
Page 86 and 87: Capacitors Capacitors We’re going
Page 88 and 89: Capacitors In our experiments in th
Page 90 and 91: Capacitors Figure 4.4 A simple resi
Page 92 and 93: Capacitors Time (seconds) 5 10 15 2
Page 94 and 95: Capacitors Time (seconds) 5 10 15 2
Page 96 and 97: Capacitors In a capacitor circuit l
Page 98 and 99: Capacitors Figure 4.11 An experimen
Page 100 and 101: Capacitors Time (seconds) Voltage (
Page 102 and 103: Capacitors plates of the capacitor
Page 104 and 105: Capacitors where ε is the permitti
Page 106 and 107: ICs oscillators and filters 5 ICs,
Page 108 and 109: ICs oscillators and filters Photo 5
Page 110 and 111: ICs oscillators and filters We’ve
Page 112 and 113: ICs oscillators and filters an osci
Page 114 and 115: ICs oscillators and filters (a) (b)
Page 116 and 117: ICs oscillators and filters Ouch, t
Page 118 and 119: ICs oscillators and filters output
Page 120 and 121: ICs oscillators and filters Value o
Page 122 and 123: ICs oscillators and filters voltage
Page 124 and 125: ICs oscillators and filters Value o
Page 126 and 127: ICs oscillators and filters Figure
Page 128 and 129: ICs oscillators and filters simple
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Diodes I 6 Diodes I We’re going t
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Diodes I Photo 6.1 A horizontal pre
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Diodes I Now, with a resistance of
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Diodes I here. We needn’t know an
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Diodes I Figure 6.9 The breadboard
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Diodes I but we never said it was a
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Diodes I Current (mA) Voltage (V) 0
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Diodes I voltages, reverse currents
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Diodes I battery voltage of 9 V sim
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Diodes I Figure 6.17 The circuit, w
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Diodes I Figure 6.19 My results fro
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Diodes II 7 Diodes II In the last c
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Diodes II the same experiment we di
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Diodes II We need to draw diode cha
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Diodes II Generally, diodes don’t
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Diodes II Diode voltage (VD) Resist
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Diodes II Diode circuits We’re no
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Diodes II Figure 7.9 Waveforms show
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Diodes II Filter tips Although we
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Diodes II We’ve already seen a de
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Diodes II as that from a 230 V a.c.
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Diodes II Figure 7.19 The same circ
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Transistors 8 Transistors In previo
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Transistors Figure 8.1 How to recog
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Transistors Black meter lead Red me
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Transistors Well, as mentioned earl
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Transistors Figure 8.8 A breadboard
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Transistors In effect, the transist
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Transistors Figure 8.10 transistors
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Transistors Hint: Yes, that’s rig
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Transistors These two operational m
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Analogue integrated circuits 9 Anal
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Analogue integrated circuits circui
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Analogue integrated circuits Figure
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Building on blocs Analogue integrat
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Analogue integrated circuits As the
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Analogue integrated circuits circui
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Analogue integrated circuits an app
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Digital integrated circuits I 10 Di
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Digital integrated circuits I Logic
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Digital integrated circuits I If we
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Digital integrated circuits I Figur
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Digital integrated circuits I It’
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Digital integrated circuits I The t
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Digital integrated circuits I Trans
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Digital integrated circuits I going
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Digital integrated circuits I OR an
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Digital integrated circuits I For t
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Digital integrated circuits II 11 D
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Digital integrated circuits II A se
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Shutting the ’stable gate Digital
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Digital integrated circuits II gate
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Digital integrated circuits II In o
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Digital integrated circuits II Figu
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Digital integrated circuits II Swit
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Digital integrated circuits II The
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Open the black box Digital integrat
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Digital integrated circuits II Ther
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Glossary Glossary 180 ° out-of-pha
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Glossary bode plots method of showi
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Glossary decibels logarithmic units
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Glossary integrated circuit a semic
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Glossary peak voltage the voltage m
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Glossary squarewave a signal which
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Glossary Components used Resistors
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Glossary Index A ampere, 9 AND gate
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