Electronics Made Easy: The Beginners Electronics Guide

Chapter 1

Introduction

Electronics Made Easy started as a series of lessons produced for my son in 1998 to bring him up to GCSE standard.

In general I have tried to keep mathematics to a minimum as in the real world it is rarely necessary to go above GCSE level. At this stage I must admit a certain bias in dealing with the subject, as somebody involved with electronic design in industry for most of my working life there is a tendency to over emphasise design techniques, the good bit.

Certain aspects of the 1998 GCSE Electronics Products syllabus were current technology when I started my Degree in 1968. Although I have described the operation of obsolete devices, more modern equivalents are used in the examples.

Some useful designs examples are provided at the end of the book.

Some people may find electronic theory difficult to grasp, after all you cannot see current flowing through a device. The concept of current flow from positive to negative appears to be reasonable, but in reality electrons flow from negative to positive so we describe current in semiconductors as the flow of positive holes, and forget all about electrons for the rest of the time.

When dealing with any complicated subject it pays to develop a mental model, for example let voltage be pressure, and current be water flow. Using this analogy resistance becomes a restriction or kink in a pipe, capacitance a tank and a diode becomes a non return valve. Transistors become diaphragm valves and zener diodes a tank with an overflow.

Using this mental technique should enable you to grasp the basic concepts it worked for me anyway.

PASSIVE COMPONENTS

RESISTORS

Resistors as the name suggests resist the flow of electricity. When a current is passed through a resistor a voltage is developed across it. The resistance to the flow is measured in Ohms.

Resistors are produced in all sizes and can either be of a solid material or wire wound. The material chosen depends on many factors:

1) The stability with time and temperature.

2) The required power dissipation.

3) The accuracy required (the tolerance to which it can be produced).

Common materials:

1) Copper, Low resistance meter shunts

2) Metal Alloys for high resistance wire.

3) Carbon

4) Metallic Oxides.

5) Iron (used in large high power resistors)

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Fixed Resistors With Power Ratings 0.25 Watts To 25 Watts.

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Various Potentiometers Note High Powered Potentiometer On The Left Shows Method Of Construction.

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Single Turn Open Frame, And Multiturn Trimmers.

Symbols

Resistor Variable Resistor or Potentiometer

Many resistors are identified using a colour code shown below:

1 2 3 4 Band 1 First Digit

2 Second Digit

3 Multiplier

4 Tolerance %

Band 1 First Digit

1 2 3 4 5 2 Second Digit

3 Third Digit

4 Multiplier

5 Tolerance %

Colour Code

0 Black (Blk) 4 Yellow (Yel) 8 Grey (Gry)

1 Brown (Brn) 5 Green (Grn) 9 White (Wh)

2 Red (Red) 6 Blue (Blu)

3 Orange (Or) 7 Violet (Vio)

Gold and silver bands are sometimes used to indicate the tolerance of High Stability resistors and generally indicate 1% and 2% respectively.

CAPACITORS

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Electrolytic Capacitors.

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Polyester, Polycarbonate, Ceramic And Tantalum Bead Capacitors.

In simple terms a capacitor conducts alternating current but not direct current its ac resistance or reactance is inversely proportional to frequency.

Reactance xc = 1/2pfC

Capacitors are made up of two conducting plates separated by a dielectric, as the symbol suggests, the larger the area the greater the capacitance. In some cases a thin dielectric film is coated with a layer of metal then rolled up not unlike a Swiss roll.

Electrolytic capacitors are used mainly in smoothing applications where a large capacitance is required in a small space. This is achieved due to the extreme thinness of the dielectric which is an insulating film built up by an electrolytic process on one of the electrodes. The film deposited on the electrode acts as a dielectric with a high resistance in one direction but when the polarity is reversed it presents a low resistance. Thus a voltage reversal must be avoided.

The capacitor working voltage limitation refers to the maximum peak voltage the capacitor will withstand before the dielectric breaks down.

Capacitor Types:

Electrolytic:- Polar capacitor, ie polarity sensitive. Used in applications where high values are required, at high working voltage.

Tantalum Bead:- Miniature capacitor used as a direct replacement for electrolytic capacitors in lower voltage applications (up to 35V dc).Tantalum capacitors are not available at values above 100mF.

Polyester:- These capacitors use polyester as a dielectric, they are more stable than the above capacitors but are not available above 10mF. They are not polarity sensitive and are generally used for coupling, de-coupling and less sensitive timing circuits.

Ceramic:- Ceramic capacitors have similar uses to polyester capacitors but are available at higher working voltage. Maximum value typically 0.1mF.

Polystyrene:- These capacitors use polystyrene as a dielectric, as they combine high temperature stability and high accuracy. They are used for filter and accurate timing applications but are not available above 10nF and are not used in high voltage applications.

Silvered Mica:- These capacitors have a similar specification to polystyrene but work at a higher dc voltage (500v). They are available up to 47nF. These capacitors are constructed by depositing a thin layer of silver on a thin mica slice as a dielectric.

Variable Capacitors:- These capacitors consist of two banks of blades with a sufficient gap to allow the second bank mounted on a shaft to pass through it and provide a dielectric. In some cases a dielectric sheet of polystyrene or mica is interposed. This enables the clearances to be reduced. Variable capacitors are used in tuned circuits ie transistor radio's, television etc.

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Variable Capacitors

Series and Parallel Connection

C1 C2

In Series 1/CT = 1/C1 + 1/C2

CT OR CT = C1 * C2/(C1 + C2)

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Units of capacitance Farad F

m = 1/1000,000

n = 1/1000,000,000

p = 1/1000,000,000,000

In Parallel:

CT = C1 + C2

Charge and Discharge Characteristics

When the switch S is closed current i charges capacitor C via R1. The voltage across the capacitor rises until it reaches Vout (see Fig 1 ). Vout is calculated from the equation:

Vo = Vin * R2/(R1 + R2)

(see section on voltage dividers)

The time taken to achieve full output voltage (Vout) is calculated as follows:

T (sec) = C * R1

When the switch S is opened capacitance C discharges through resistor R2, (see fig 2). The time taken to fully discharge T is calculated as follows:

T = C * R2

S i

Vin Vout

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This circuit is especially useful in generating voltage ramps and timing functions, ie delay circuits when used in conjunction with an amplifier. This enables only the near linear portion of the curve to be used. The equation assumes that the ramp is linear and in practice when this circuit is used with a CMOS buffer which will switch before the circuit has timed out, a correction factor of 0.7 is used, ie T = 0.7CR.

Capacitors in Power Supplies

Electrolytic Capacitors are used in power supplies to smooth the output DC voltage. The drawing below shows the output from a diode bridge rectifier feeding a smoothing capacitor C. The effectiveness of the smoothing capacitor depends on the amount of current fed into the circuit load R, the larger the current requirements the greater the capacitance value must be to maintain an acceptable voltage ripple on the output voltage.

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