PASSIVE COMPONENTS. Resistor values - the resistor colour code. Construction

Transcription

1

2 CHOOSING COMPONENTS This is a basic guide for choosing components for your projects. There are specialist components for different types of work which are not covered here. There are no specific details of components as these are avaliable from Manufaturers web sites and data books. In any design there is a compromise between physical size, specification, reliability and cost. It is your job as a designer to pick the components which fulfill these criteria to a given specification. PASSIVE COMPONENTS RESISTORS Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED. Construction There are four basic types of resistor, each having different characteristics and costs. Carbon, Carbon film, metalized film and wirewound. Resistors may be connected either way round. Carbon composition or carbon filmtype resistors are used in general purpose circuits where accuracy and stability with variations of temperature aren t deemed critical. Typical applications include their use as a collector or emitter load, in transistor/fet biasing networks, as a discharge path for charged capacitors, and as pullup and/or pulldown elements in digital logic circuits. Carbon type resistors are avliable from 1 ohm to 22 megohms, with tolerances from 2% (carbon film) to 5% up to 20% (carbon composition). Power dissipation ratings range from 1/8 watt up to 2 watts. The 1/4watt and 1/2watt, 5% and 10% types tend to be the most popular. Carbon resistors have a poor temperature coefficient (typically 5, 000 ppm/ C); so they are not well suited for precision applications requiring a small resistance change over temperature, but they are inexpensive. Metal film resistors are chosen for precision applications where accuracy, low temperature coefficient, and lower noise are required. Metal film resistors are generally composed of Nichrome, tin oxide or tantalum nitride, and are available in either a hermetically sealed or molded phenolic body. Typical applications include bridge circuits, RC oscillators and active filters. Initial accuracies range from 0.1 to 1.0 %, with temperature coefficients ranging between 10 and 100 ppm/ C. Standard values range from 10.0 ohms to 301 kohms in discrete increments of 2% (for 0.5% and 1% rated tolerances). Metal film resistors use a 4 digit numbering sequence to identify the resistor value instead of the color band scheme used for carbon types: Wirewound precision resistors are extremely accurate and stable (0.05%, <10 ppm/ C); they are used in demanding applications, such as tuning networks and precision attenuator circuits. Typical resistance values run from 0.1 ohms to 1.2 Mohms. High Frequency Effects: Real resistors suffer from parasitics, base material and physical size determine the extent to which the parasitic L and C affect a resistor s effective dc resistance at high frequencies. Film type resistors generally have excellent highfrequency response; they best maintain their accuracy to about 100 MHz. Carbon types are useful to about 1 MHz. Wirewound resistors have the highest inductance, and hence the poorest frequency response. Even if they are non-inductively wound, they tend to have high capacitance and are likely to be unsuitable for use above 50 khz. Resistor values - the resistor colour code Resistance is measured in ohms, the symbol for ohm is an omega Ω. 1Ω is quite small so resistor values are often given in kω and MΩ. 1 kω = MΩ = Resistor values are normally shown using coloured bands. Resistor Colour Code Colour Colour Number Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 Each colour represents a number as shown in the table. Most resistors have 4 bands: The first band gives the first digit. The second band gives the second digit. The third band indicates the number of zeros. The fourth band is used to shows the tolerance of the resistor. A resistor has red (2), violet (7), yellow (4 zeros) and gold bands. So its value is = 270 kω. On circuit diagrams the Ω is usually omitted and the value is written 270K. Small value resistors (less than 10Ω) The standard colour code cannot show values of less than 10Ω. To show these small values two special colours are used for the third band: gold which means 0.1 and silver which means The first and second bands represent the digits as normal. 2

3 e.g. red, violet, gold bands represent = 2.7 blue, green, silver bands represent = 0.56 Tolerance of resistors (fourth band of colour code) The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value between = 351 and = 429. A special colour code is used for the fourth band tolerance: silver ±10%, gold ±5%, red ±2%, brown ±1%. If no fourth band is shown the tolerance is ±20%. Tolerance may be ignored for almost all circuits because precise resistor values are rarely required. Resistor values are usually written on circuit diagrams using letters R, K and M in place of the decimal point. Replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or if the letter was M. The letter R means multiply by 1. e.g. 560R means 560Ω and 2K7 means 2.7kΩ = 2700 Real resistor values (the E6 and E12 series) Resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not! To produce a sensible range of resistor values you need to increase the size of the step as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten. The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance) 10, 15, 22, 33, 47, 68,... then it continues 100, 150, 220, 330, 470, 680, 1000 etc. Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value. The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance) 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82,... then it continues 100, 120, 150 etc. Notice how this is the E6 series with an extra value in the gaps. The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their tolerance ). For example a resistor marked 390 could vary by ±10% 390 = ±39, so it could be any value between 351 and 429. High power resistors Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this. Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300 ) or high voltages (more than 15V). The power, P, developed in a resistor is given by: P = I² R or P = V² / R where: P = power developed in the resistor in watts (W) I = current through the resistor in amps (A) R = resistance of the resistor in ohms ( ) V = voltage across the resistor in volts (V) Examples: A 470 resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W. In this case a standard 0.25W resistor would be suitable. A 27 resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W. A high power resistor with a rating of 5W would be suitable. Variable Resistors Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available. Variable resistors may be used as a rheostat the wiper and just one end of the track connected or as a potentiometer with all three connections used. Variable resistors are often called potentiometers. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm. The resistance and type of track are marked on the body: 4K7 LIN means 4.7 k linear track. 1M LOG means 1 M logarithmic track. Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board. 3

4 Linear (LIN) and Logarithmic (LOG) tracks Linear (LIN) track means that the resistance changes at a constant rate as you move the wiper. This is the standard arrangement and you should assume this type is required if a project does not specify the type of track. Presets always have linear tracks. Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and rapidly at the other end, so halfway along the track is not half the total resistance! This arrangement is used for volume (loudness) controls because the human ear has a logarithmic response to loudness so fine control (slow change) is required at low volumes and coarser control (rapid change) at high volumes. Presets These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets. Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control. CAPACITORS Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals. The capacity of an electrical capacitor is the ratio of the quantity of electricity and the electrical pressure or voltage. In other words the capacity of a capacitor depends on the amount of electricity it will hold at a certain electrical pressure or voltage. This ratio may be expressed as follows: Q = CV Q = quantity of electricity C = capacity of the capacitor V = electrical pressure or voltage The capacity may be expressed as follows: C = Q / V Where the capacity is equal to the quantity of electricity divided by the electrical pressure or voltage. The capacity of a capacitor is dependent upon the size and spacing of the conducting plates and the type of insulating or dielectric medium between the plates. The simplest form of capacitor consists of two electrodes ot conducting plates separated by air. Other dielectrics in common use are mica, paper, glass, sulphur, mineral and vegetable oils, waxes and synthetic insulating compounds such as the chlorinated groups. It is common practice to divide or identify capacitors in accordance with the type of dielectric employed in their structures. For example, there are mica capacitors, air capacitors, oil capacitors and paper capacitors. The capacity of a capacitor is dependent on the size of the plates and the space between them as well as the kind of dielectric medium employed. Knowing these facts, it becomes apparent that there must exist some fixed relationship which would allow for the predetermination of any desired capacity. The most fundamental of such a relationship is expressed as follows: C = KS / t Where C = Capacity in micro-microfarads K = dielectric constant S = area of one plate in square centimeters t = distance between plates in centimeters Capacity is proportional to the product of the area of one plate multiplied by the dielectric constant, divided by the thickness of the dielectric. Due to this increase in capacity, it is said that mica has a higher dielectric constant than air. Also in order that the mica dielectric can be said to have a certain definite dielectric constant, it has been established that air has a dielectric constant of one. Breakdown voltage When using a capacitor, you must pay attention to the maximum voltage which can be used. This is the breakdown voltage. The breakdown voltage depends on the kind of capacitor being used. You must be especially careful with electrolytic capacitors because the breakdown voltage is comparatively low. The breakdown voltage of electrolytic capacitors is displayed as Working Voltage. The breakdown voltage is the voltage that when exceeded will cause the dielectric (insulator) inside the capacitor to break down and conduct. When this happens, the failure can be catastrophic. Capacitance This is a measure of a capacitor s ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values. Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico): µ means 10-6 (millionth), so µF = 1F n means 10-9 (thousand-millionth), so 1000nF = 1µF p means (million-millionth), so 1000pF = 1nF Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems! There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol. 4

5 POLARISED CAPACITORS (LARGE VALUES, 1ΜF +) Electrolytic Capacitors Aluminum is used for the electrodes by using a thin oxidization membrane. Large values of capacitance can be obtained in comparison with the size of the capacitor, because the dielectric used is very thin. The most important characteristic of electrolytic capacitors is that they have polarity. They have a positive and a negative electrode.[polarised] This means that it is very important which way round they are connected. If the capacitor is subjected to voltage exceeding its working voltage, or if it is connected with incorrect polarity, it may burst. It is extremely dangerous, because it can quite literally explode. Make absolutely no mistakes. Generally, in the circuit diagram, the positive side is indicated by a + (plus) symbol. Electrolytic capacitors range in value from about 1µF to thousands of µf. Mainly this type of capacitor is used as a ripple filter in a power supply circuit, or as a filter to bypass low frequency signals, etc. Because this type of capacitor is comparatively similar to the nature of a coil in construction, it isn t possible to use for high-frequency circuits. It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. It the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project s power supply voltage. 25V is a sensible minimum for most battery circuits. Tantalum Bead Capacitors Tantalum Capacitors are electrolytic capacitors that use a material called tantalum for the electrodes. Large values of capacitance similar to aluminum electrolytic capacitors can be obtained. Also, tantalum capacitors are superior to aluminum electrolytic capacitors in temperature and frequency characteristics. When tantalum powder is baked in order to solidify it, a crack forms inside. An electric charge can be stored on this crack. These capacitors have polarity as well. Usually, the + symbol is used to show the positive component lead. Tantalum capacitors are a little bit more expensive than aluminum electrolytic capacitors. Capacitance can change with temperature as well as frequency, and these types are very stable. Therefore, tantalum capacitors are used for circuits which demand high stability in the capacitance values. Tantalum capacitorsare usefull for analog signal systems, because the current-spike noise that occurs with aluminum electrolytic capacitors does not appear. Aluminum electrolytic capacitors are fine if you don t use them for circuits which need the high stability characteristics of tantalum capacitors. Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size. Modern tantalum bead capacitors are printed with their capacitance and voltage in full. However older ones use a colourcode system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µf. The standard colour code is used, but for the spot, grey is used to mean 0.01 and white means 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). e.g. blue, grey, black spot means 68µF e.g. blue, grey, grey spot means 0.68µF UNPOLARISED CAPACITORS (SMALL VALUES, UP TO 1ΜF) Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems! Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be! e.g. 0.1 means 0.1µF = 100nF. Sometimes the multiplier is used in place of the decimal point: For example: 4n7 means 4.7nF. Capacitor Number Code A number code is often used on small capacitors where printing is difficult: 1st number is the 1st digit, 2nd number is the 2nd digit, 3rd number is the number of zeros to give the capacitance in pf. Ignore any letters - they just indicate tolerance and voltage rating. e.g. 102 means 1000pF = 1nF (not 102pF!) 472J means 4700pF = 4.7nF (J means 5% tolerance). Capacitor Colour Code A colour code was used on polyester capacitors but is now obsolete. The colours should be read like the resistor code, the top three colour bands giving the value in pf. Ignore the 4th band (tolerance) and 5th band (voltage rating). e.g. brown, black, orange means 10000pF = 10nF = 0.01µF. Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band. 5

6 e.g. wide red, yellow means 220nF = 0.22µF. Polystyrene Capacitors This type is rarely used now. Their value (in pf) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint. In these devices, polystyrene film is used as the dielectric. This type of capacitor is not for use in high frequency circuits, because they are constructed like a coil inside. They are used well in filter circuits or timing circuits which run at several hundred KHz or less. Real capacitor values (the E3 and E6 series) Capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not! The standard capacitor values are based on a series which follows the same pattern for multiples of ten. The E3 series (3 values for each multiple of ten) 10, 22, 47,... then it continues 100, 220, 470, 1000, 2200, 4700, etc. Notice how the step size increases as the value increases (values roughly double each time). The E6 series (6 values for each multiple of ten) 10, 15, 22, 33, 47, 68,... then it continues 100, 150, 220, 330, 470, 680, 1000 etc. Notice how this is the E3 series with an extra value in the gaps. The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values. Polypropylene Capacitors This capacitor is used when a higher tolerance is necessary than polyester capacitors offer. Polypropylene film is used for the dielectric. It is said that there is almost no change of capacitance in these devices if they are used with frequencies of 100KHz or less. Variable capacitors Variable capacitors are mostly used in radio tuning circuits and they are sometimes called tuning capacitors. They have very small capacitance values, typically between 100pF and 500pF (100pF = µF). The type illustrated usually has trimmers built in (for making small adjustments) as well as the main variable capacitor. Trimmer capacitors Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer! Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF. Ceramic The term ceramic capacitors covers a large group of capacitors. Their properties may be quite different, but they all have the oxide ceramic dielectric in common. Ceramic generally means that an inorganic polycrystalline body is formed by sintering at high temperatures. By means of special production methods, extremely thin layers of ceramic materials can be obtained. These layers are stacked to construct capacitors whose electrical and mechanical properties meet stringent requirements. The multilayer capacitors consist of a monolithic ceramic block with comb-like sintered electrodes. These electrodes come to the surface at the face ends of the ceramic block where an electrical contact is made by burnt-in metallic layers. Ceramic capacitors are constructed with materials such as titanium acid barium used as the dielectric. Internally, these capacitors are not constructed as a coil, so they can be used in high frequency applications. Typically, they are used in circuits which bypass high frequency signals to ground. Polyester Metallized Polyester Film These capacitors are a kind of a polyester film capacitor. Because their electrodes are thin, they can be miniaturized. Silver Mica Mica These capacitors use Mica for the dielectric. Mica capacitors have good stability because their temperature coefficient is small. Because their frequency characteristic is excellent, they are used for resonance circuits, and high frequency filters. Also, they have good insulation, and so can be utilized in high voltage circuits. It was often used for vacuum tube style radio transmitters, etc. Mica capacitors do not have high values of capacitance, and they can be relatively expensive. Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor. 6

7 INDUCTORS An inductor may be air-cored or have a solid core. Magnetic materials in common use for the cores of solenoids are: Soft iron: easy to magnetise and demagnetise. Used for motor pole-pieces. Silicon iron: used for transformer laminations and AC motors. Low-loss.Nickel iron alloy: also known as radio metal and mumetal, is used for high-class audio transformers and cathode-ray tube screens. Ferrites: iron oxide based materials used for a wide range of applications in radio and electronics generally. The characteristics depend on the mix of materials in the core and is extremely varied. Also known as ferroxdure and ferroxcube. Permanent magnets: tungsten steel, and alloys of iron, nickel, aluminium, cobalt, ceramic, and titanium are used. Iron oxides can also be used. TYPES OF INDUCTORS Inductors used in radio can range from a straight wire at UHF to large chokes and transformers used for filtering the ripple from the output of power supplies and in audio amplifiers. Values of inductors range from nano-henries to tens of henries. It is convenient to group them into three categories. Air core: to keep losses to a minimum it is necessary to keep the self-resistance of coils as low as possible. The only adjustment available with air core inductors is by tapping all or part of a turn, or by varying the spacing between turns. Ferrite or iron dust core: by inserting a ferrite or iron dust core in a coil it is possible to double its inductance. This means that it is also possible to halve the size of a coil for a given inductance. If the core is threaded, its position within the coil can be varied to alter the inductance. Some high-grade communications receivers have a system of cam-operated cores which are used for tuning. The type of material used for the cores or slugs is of importance and care must be taken to use the right grade for the right frequency band. This type of coil is used throughout the HF range, and into the VHF, for low-level signal circuits. Losses in the cores make them unsuitable for use in power circuits. Values range from a few microhenries to about a millihenry. Iron core: this classification includes chokes and transformers, both of which have laminated iron cores. Transformers are described in the next section. A choke is a single winding and a transformer has two or more windings. Typical values of inductance for chokes range from 0.1 of a henry to 50 henries. RELAYS A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier s catalogue should show you the relay s connections. The coil will be obvious and it may be connected either way round. The relay s switch connections are usually labelled COM, NC and NO: COM = Common, always connect to this, it is the moving part of the switch. NC = Normally Closed, COM is connected to this when the relay coil is off. NO = Normally Open, COM is connected to this when the relay coil is on. Connect to COM and NO if you want the switched circuit to be on when the relay coil is on. Connect to COM and NC if you want the switched circuit to be on when the relay coil is off. Choosing a relay You need to consider several features when choosing a relay: Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier s catalogue. Coil voltage The relay s coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. 7

8 Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm s law to calculate the current: Relay coil current = supply voltage/coil resistance A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current. Switch ratings (voltage and current) The relay s switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC. e.g. 5A at 24V DC or 125V AC. Switch contacts Most relays are SPDT or DPDT which are often described as single pole changeover (SPCO) or double pole changeover (DPCO). Reed relays Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch. The main advantages and disadvantages of relays are listed below: Advantages of relays: Relays can switch AC and DC, transistors can only switch DC. Relays can switch high voltages, transistors cannot. Relays are a better choice for switching large currents (> 5A). Relays can switch many contacts at once. Disadvantages of relays: Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. Relays use more power due to the current flowing through their coil. Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay s coil. TRANSFORMERS Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example). The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ( chip holder ). For further information about reed switches please see the page on switches. Protection diodes for relays Transistors and ICs (chips) must be protected from the brief high voltage spike produced when the relay coil is switched off. A signal diode (eg 1N4148) is connected across the relay coil to provide this protection. Note that the diode is connected backwards so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage spike in its attempt to keep the current flowing. Relays and transistors compared Like relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay s coil! Any two coils magnetically linked will act as transformers. Transformers come in as many forms as inductors, air or dust cored as well as the more familiar iron-cored type. The iron-core can take several forms. The simple transformer comprises two or more inductors (windings) sharing a common core. An alternating current is fed to one of the windings. The operation can explained by considering the magnetic field of the input winding, the primary, sweeping through the secondary winding to induce an AC current in the secondary. A common task for a transformer is to provide an AC supply at a voltage suitable for rectifying to produce a stated DC output. The number of turns on each winding determines the output voltage from the transformer. The output voltage from the secondary is proportional to the ratio of the turns on the windings. If the secondary has half as many turns as there are in the primary, and 240V AC is applied to the primary, the output from the secondary will be 120V. Transformers can be step-up or step-down (in voltage). With twice as many turns on the secondary as there are on the primary and 100 V applied, the output would be 200V. The impedance ratio is proportional to the square of the turns ratio. We can use transformers to change impedances. 8

9 This property is one of the most important properties in the use of transformers. The power output from the secondary winding cannot exceed the power fed into the primary. Ignoring losses, a step-down in voltage means that an increase in current from that lowervoltage winding is possible. Similarly, a step-up in voltage means a decrease in the current output. So the gauge of wire used for the secondary winding may be different to the wire used for the primary. Iron-cored transformers are used for audio frequencies and for power supplies. Audio frequency transformers are designed to give suitable efficiency to frequencies up to 25 khz. For speech and domestic quality radio reproduction the core material used is stalloy, while the laminations of high-fidelity transformers are made of mumetal. The construction is the same as for chokes and the same considerations of size and power rating apply. Toroidal core transformers If the core of a transformer.is of specially-selected material and is formed into a complete loop as shown in this diagram, nearly all the flux lies within the core and there is very little leakage, or flux outside the core. This results in very little unwanted coupling to adjacent magnetic circuits, and is very desirable feature in some applications. 9

10 ACTIVE COMPONENTS DIODES Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves. Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph). Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µa or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown. Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. Testing diodes You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. A lamp may be used to test a rectifier diode, but do NOT use a lamp to test a signal diode because the large current passed by the lamp will destroy the diode! Signal diodes (small current) Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA. General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V. Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal. For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied. Rectifier diodes (large current) Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode. All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. Bridge rectifiers There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled. The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct. Note that some have a hole through their centre for attaching to a heat sink Photo Diodes A photodiode consists of an active p-n junction which is operated in reverse bias. When light falls upon the junction, a reverse current flows which is proportional to the illuminance. The linear responce to light makes it usefull for photodetector applications. It is also useful for light activated switches. Zener diodes Zener diodes are used to maintain a fixed voltage. They are designed to breakdown in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current. Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example. Zener diodes are rated by their breakdown voltage and maximum power: The minimum voltage available is 2.7V. Power ratings of 400mW and 1.3W are common. LIGHT EMITTING DIODES (LEDS) LEDs emit light when an electric current passes through them. LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is usually the larger electrode. LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. 10

11 The colour of an LED is determined by the semiconductor material, not by the colouring of the package (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as water clear ). The coloured packages are also available as diffused (the standard type) or transparent. Tri-colour LEDs The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on. Bi-colour LEDs A bi-colour LED has two LEDs wired in inverse parallel (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above. LEDs are available in a wide variety of sizes and shapes. The standard LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular. Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other crosssection shapes include square, rectangular and triangular. As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60 but others have a narrow beam of 30 or less. An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly. The resistor value, R is given by: R = (VS - VL) / I VS = supply voltage VL = LED voltage (usually 2V, but 4V for blue and white LEDs) I = LED current (e.g. 20mA), this must be less than the maximum permitted If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. Flashing LEDs Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC fl ashes the LED at a low frequency, typically 3Hz (3 fl ashes per second). They are designed to be connected directly to a supply, usually 9-12V, and no series resistor is required. Their fl ash frequency is fi xed so their use is limited and you may prefer to build your own circuit to fl ash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit. LED Displays LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). TRANSISTORS A transistor may be used as a switch or as an amplifi er. There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors. The leads are labelled base (B), collector (C) and emitter (E). In addition to standard (bipolar junction) transistors, there are fi eld-effect transistors which are usually referred to as FETs Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on. The drawings on the right show the leads for some of the most common case styles. Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above. Transistor codes There are three main series of transistor codes used in the UK: Codes beginning with B (or A), for example BC108, BC478 The fi rst letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifi es the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifi es a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable. 11

12 Codes beginning with TIP, for example TIP31A TIP refers to the manufacturer: Texas Instruments Power transistor. Odd numbers are NPN, even numbers are PNP. The letter at the end identifi es versions with different voltage ratings. Codes beginning with 2N, for example 2N3053 The initial 2N identifi es the part as a transistor and the rest of the code identifi es the particular transistor. There is no obvious logic to the numbering system. Choosing a transistor The most important properties to look for are the maximum collector current IC and the current gain hfe. Structure This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type. Case style There is a diagram showing the leads for some of the most common case styles in the Connecting section above. This information is also available in suppliers catalogues. IC max. Maximum collector current. VCE max. Maximum voltage across the collector-emitter junction. hfe This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. The gain is often quoted at a particular collector current IC which is usually in the middle of the transistor s range, for example 100@20mA means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Darlington pair This is two transistors connected together so that the amplifi ed current from the fi rst is amplifi ed further by the second transistor. This gives the Darlington pair a very high current gain such as Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. You can make up your own Darlington pair from two transistors. Testing a transistor Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there is aneasy ways to test it: Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range. Test each pair of leads both ways (six tests in total): The base-emitter (BE) junction should behave like a diode and conduct one way only. The base-collector (BC) junction should behave like a diode and conduct one way only. The collector-emitter (CE) should not conduct either way. These are reversed in a PNP transistor but the same test procedure can be used. Heat sinks Waste heat is produced in transistors due to the current fl owing through them. Heat sinks are needed for power transistors because they pass large currents. If you fi nd that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air. Ptot max. Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifi ers, the power is roughly IC VCE. For transistors operating as switches the maximum collector current (IC max.) is more important. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating. To make a fi nal choice you will need to consult the tables of technical data which are normally provided in catalogues. Case style 12

13 INTEGRATED CIRCUITS Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon). The chip is packaged in a plastic holder with pins spaced on a 0.1 (2.54mm) grid which will fi t the holes on stripboard and breadboards. Very fi ne wires inside the package link the chip to the pins. Pin numbers The pins are numbered anticlockwise around the IC (chip) starting near the notch or dot. The diagram shows the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes. Sinking and sourcing current Chip outputs are often said to sink or source current. The terms refer to the direction of the current at the chip s output. If the chip is sinking current it is fl owing into the output. This means that a device connected between the positive supply (+Vs) and the chip output will be switched on when the output is low (0V). If the chip is sourcing current it is fl owing out of the output. This means that a device connected between the chip output and the negative supply (0V) will be switched on when the output is high (+Vs). It is possible to connect two devices to a chip output so that one is on when the output is low and the other is on when the output is high. The maximum sinking and sourcing currents for a chip output are usually the same but there are some exceptions, for example 74 series TTL logic chips can sink up to 16mA but only source 2mA. The 555 Timer This 8-pin chip is used in many projects, a popular version is the NE555. Most circuits will just specify 555 timer IC and the NE555 is suitable for these. The 555 output (pin 3) can sink and source up to 200mA. This is more than most chips and it is suffi cient to supply LEDs, relay coils and low current lamps. To switch larger currents you can connect a transistor. The 556 is a dual version of the 555 housed in a 14-pin package. The two timers (A and B) share the same power supply pins. Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specifi ed (to increase battery life) because their maximum output current of about 20mA (with 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555. The 555 timer can be used to make monostable, bistable and astable circuits. For further information on these circuits and the 555 timer itself please see the Electronics in Meccano website, the relevant articles are in the Digital Electronics section. LOGIC CHIPS Logic chips process digital signals and there are many devices, including logic gates, fl ip-fl ops, shift registers, counters and display drivers. They can be split into two groups according to the type of circuit used inside the device: 4000 Series CMOS and 74 series TTL Series CMOS This family of logic chips is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced see-moss and stands for Complementary Metal Oxide Semiconductor. However the CMOS circuitry also means that they are static sensitive. Earthed wrist straps and earthed work surfaces should be used when handling these devices. Further information can be found on the Schools web site at: For information about specifi c ICs in this range, including pin connections, see the following web page: series CMOS characteristics: Supply: 3 to 15V, small fl uctuations are tolerated. Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the chip behave erratically and it will signifi cantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the chip is not being used in the circuit! Outputs can sink and source only about 1mA if you wish to maintain the correct output voltage to drive CMOS inputs. If there is no need to drive any inputs the maximum current is about 5mA with a 6V supply, or 10mA with a 9V supply (just enough to light an LED). To switch larger currents you can connect a transistor. Fan-out: one output can drive up to 50 inputs. Gate propagation time: typically 30ns for a signal to travel through a gate with a 9V supply, it takes a longer time at lower supply voltages. Frequency: up to 1MHz, above that the 74 series is a better choice. Power consumption (of the chip itself) is very low, a few µw. It is much greater at high frequencies, a few mw at 1MHz for example. 13