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Part One Fundamentals of Electricity R A Calaz C Eng, B Sc(Eng), MIET, ACGI, MSCTE Copyright Notice All rights reserved. No part of this publication may be reproduced without the express written permission of the author. All logos and trademarks are the copyright of their respective owners. The right of R A Calaz to be identified as the author of this work has been asserted by him in accordance with the Copyright Designs and Patents Act 1998.
Disclaimer This publication is intended to provide information regarding the installation of digital systems in a residential environment. Every effort has been made to make it as complete and accurate as possible, but no warranty of fitness is implied. The author shall have no responsibility with respect to loss or damage arising from the information contained in this document. Corrections and comments should be addressed to bob@calaz.com.
About the Author Bob Calaz is an acknowledged expert in the field of TV, satellite and multimedia installations having been a chartered electrical engineer for more than 40 years. For 26 of those years, he was with the Rediffusion group of companies, including a secondment to South Africa for 15 years as chief engineer of Rediffusion South Africa. He was a member of a government committee set up to define the technical standards for TV systems in multi-dwelling units, and was responsible for the design and installation of a 200 channel TV distribution system for the headquarters of the South African Broadcasting Corporation. Returning home to the UK in 1985, he founded Race Communications Ltd (based in Berkshire) to service the growing TV industry. He has since conducted many technical training courses for commercial and military personnel. Bob has presented numerous technical articles and papers throughout the world and has been regularly in demand as an advisor to a variety of commercial, industry and governmental bodies. His recent publications include two reference books on Digital TV, Satellite and Multimedia.
The complete Insider Guide set of publications This publication is one of a series covering the design, installation, operation and maintenance of digital reception and distribution systems in the home. The complete series is sub-divided as follows: Part 1 - Fundamentals of Electricity Part 2 - Digital Television Part 3 - Digital TV displays Part 4 - TV Modulation Techniques Part 5 - UHF TV Broadcasting and Reception Part 6 - Radio and TV Aerial Installations Part 7 - UHF TV Signal Distribution Part 8 - Satellite TV Reception Part 9 - Satellite TV Distribution Part 10 - Satellite IF Network Planning Part 11 - Test Equipment Part 12 - Fibre Optic Distribution Part 13 - Distribution of Voice and Data Signals Part 14 - Digital Home Technologies Part 15 - Structured Cable Networks Part 16 - VSAT Systems Part 17 - Abbreviations/Glossary of Terms
Introduction This series of publications introduces the reader to the application of digital systems in the home. It is suitable for those with no prior knowledge of the subject, whilst at the same time providing a source of reference for experienced installers who wish to know more about the subject. This part serves as an introduction to the concept of electricity flowing in a wire, and its units of measurement. The resistance to current flow is then described, leading to the interrelationship between voltage, current and power. Since voltages are used to measure the strength of a television signal, the unit of voltage measurement is then defined. The three types of basic electronic components are introduced, leading to the concept of a filter. Parts two to twelve cover radio, TV and satellite reception and distribution; parts thirteen to sixteen cover data systems and part seventeen is a reference of abbreviations and a glossary of terms.
Table of Contents Part 1 Fundamentals of Electricity 1.Electricity 2. Voltage 3. Current 4. Resistance 5. Ohms Law 6. Power 7. Prefixes 8. Voltages 9. UHF and Satellite TV Signal Levels 10. Impedance
1.Electricity Electricity can be compared to the water distribution system in your house. With everything turned off, the water pressure exists, but no water is used. When a tap is turned on, water flows in the pipe, the quantity depending on the pressure in the mains, the size (or resistance) of the pipe and the amount the tap is turned on. As more taps are opened, more water is used. If too many taps are turned on at the same time, the pressure drops and there is very little water coming from each tap. A domestic electricity supply works in much the same way. The equivalent of water pressure is the mains voltage. With all the switches turned off, no energy flows through the cables. When a switch is turned on, current flows through them, the amount depending on the voltage applied, the size of the cables and the resistance of the load. As more switches are turned on, more current is used. In a domestic environment, lighting cables are relatively thin, with a typical cross
sectional area of 1mm 2, because the current requirements are small and a circuit breaker limits the maximum current to six amps. Power cables to wall sockets are thicker (2.5mm 2 ) and the total current consumption is limited to thirty amps. A kitchen cooker utilises 4mm 2 cables to cater for currents of up to forty amps. This is why an electricity consumer unit incorporates circuit breakers of different values. All electricity cables have a resistance to current flow and the purpose of a circuit breaker is to limit the amount of current and thus the amount of heat being generated along the cable.
2. Voltage Voltage (V) can be regarded as the source of energy and is measured in Volts. The mains supply in Britain is nominally 230 Volts. Radio transmitters often work at much higher voltages whilst TV aerials receive small voltages tiny fractions of a volt. Voltage is usually the most important characteristic and all signal level meters measure voltages. A multimeter can be used to measure low voltages such as those generated by a torch battery. For higher voltages, an electrician s multimeter should be used with properly shrouded leads conforming to the relevant British Standard.
3. Current Current (I) is the energy flowing through a cable and is measured in Amperes or Amps. All active electronic circuits use current provided by a power supply unit (PSU) and the total current requirement can be calculated by adding together all the individual current requirements. Current can also be measured using a multimeter. Note that the current consumed by a light bulb will vary, depending on its wattage.
4. Resistance Resistance (R) is defined in Ohms, and by the symbol Ω on a circuit diagram A resistor is usually the most common component in electronic circuits and its value can be determined from the coloured stripes on its body as shown. The first and second stripes indicate the first and second digits of the value in ohms, and the third stripe indicates the number of subsequent zeros. For example: Red Red Orange: The value of this resistor is 22 000 ohms.
The only resistor value likely to be encountered by installers is 75 ohms. Such a resistor would have stripes coloured violet/green/black: There is sometimes a fourth stripe to indicate its accuracy: Gold +/- 5% Silver +/- 10% No fourth stripe +/- 20% Some connectors have 75ohm resistors built into them they are known as 75ohm terminations and are used at the end of a system spur line, or on an unused high output of a distribution amplifier. Resistance values can also be measured with a multimeter.
5. Ohms Law This states the relationship between these three values: Volts = Amps x Ohms or V = I x R If two of these values are known, the third one can be calculated. The formula is often written as follows to illustrate these relationships:
Thus, V = I x R I = V/R R = V/I
For instance, if a light bulb is connected to the 230V mains power supply and consumes 0.5 amps of current, its resistance can be calculated as follows: Resistance = Voltage / Current = 230 / 0.5 = 460 ohms
6. Power Power (W) is measured in Watts. Its relationship to Volts and Amps is as follows: Watts = Amps x Volts or W =I x V or, inside a triangle: The power of the lamp in the above example can therefore be calculated as follows: Power = Current x Voltage = 0.5 x 230 = 115 watts
7. Prefixes In order to simplify the recording of high and low voltages, prefixes are used, as follows: One millionth of a volt = 1 microvolt ( V) One thousandth of a volt = 1 millivolt (mv) One volt = 1 volt (V) One thousand volts = 1 kilovolt (kv) One million volts = 1 megavolt (MV) The same prefixes are used for amps, ohms and watts (and also for other parameters, as will be seen later). For example: One thousandth of a volt = 1 millivolt (mv) One thousandth of an amp = 1 milliamp (ma) One thousandth of an ohm = 1 milliohm (mω) One thousandth of a watt = 1 milliwatt (mw)
8. Voltages Since the voltage levels at the output of a TV or satellite amplifier can be up to one thousand times higher than the input level, our industry uses a more convenient unit of measurement, called a decibel or db. Most signal meters display levels in db with reference to 1 V (abbreviated to db V) and the conversion from Volts to db V is as follows: Other meters display levels in dbmv (db with reference to 1mV) as shown below: Note that, to convert from dbmv to db V, one simply adds 60 to the reading.
9. UHF and Satellite TV Signal Levels Domestic aerial and satellite installers always measure signal levels in db V or dbmv because the mathematics is limited to simple addition and subtraction. Losses in a system are subtracted and amplifier gains are added. For instance, if the signal level from an aerial is 55dB V and the loss on the coaxial cable between the aerial and TV receiver is 10dB, the signal level at the TV is 55 10 = 45dB V: If an amplifier with a gain (or amplification) of 13dB is added at the receiver location, the signal will be increased by 13dB to 58dB V: Note: The figures inside the circles represent the signal levels in db V, a convention that will be used throughout this series of documents.
If the amplifier were fitted at the aerial location instead of being adjacent to the TV receiver, the signal levels on the cable would be different but the end result would be the same: If the signal is split to feed two outlet locations, the splitter would reduce the signal to each leg by 4dB and the levels would be as follows: These examples show that the use of db s makes the calculation of signal levels on a system very easy!
The same principles apply to satellite signals. If the signal level from the LNB is 65dBµV and the cable loss is 15dB, the level at the input to the satellite receiver is 50dBµV: It is important to be able to measure signal levels because these will determine the reliability of the system. If the levels are too high or too low, the overall performance will be adversely affected.
10. Impedance Electrical circuits are made up of three fundamental components: Resistance, measured in ohms Capacitance, measured in farads Inductance, measured in henries A capacitor and an inductor can be used together to act as a filter, to pass or reject a band of frequencies, as described in part four. Radio signals can be relayed on coaxial cables which have a centre conductor and an outer screen, separated by insulation known as the dielectric. When an alternating voltage is applied to one end of the cable with no load on the other end, a current will flow due to its inherent capacitance and inductance. Using Ohm s law (R=V/I), the resistance is known as the characteristic impedance of the cable, with the symbol z o. Virtually all cables used to relay radio, TV and satellite signals have a z o of 75 ohms. If the load resistance is also 75 ohms, the cable is then said to be matched. If the load resistance is any other value, some of the received signals can be reflected back along the cable, causing standing waves which can cause corruption of the signals being relayed.