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Basic Components

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Sabtu, 25 Mei 2013

Integrated circuits ('chips') and holders

IC holder
NE555 timer IC, at least 3 (10 if you plan to solder projects). It is not worth ordering other ICs at this stage unless you know they are needed for some of the projects you wish to try.
555 timer ICIf you are planning to solder circuits on stripboard or PCB you will also need 8-pin, 14-pin and 16-pin DIL sockets (IC holders), at least 10 of each.


More 555IC

Crocodile clips

crocodile clipinsulated crocodile clip

Buy at least one standard crocodile clip to use as a heat sink when soldering. Miniature red and black crocodile clips (buy about 10 of each) are useful for making your own test leads using 7/0.2mm stranded wire.

Switches

Switches are not essential for breadboard circuits because you can make or break links with pieces of wire. The on/off switch from soldered projects can also be omitted if you are willing to unclip the battery instead.
If you wish to buy a few switches the most useful types are push-to-make and miniature SPDT toggle switches, 3 of each. 


SPDT toggle switch
  push-to-make switch 






 

  



 





All type off Switch

Stripboard

Buy a large sheet (or two) and cut it up as required. You can cut it neatly to size using a junior hacksaw, cutting along the lines of holes is easiest. For quickness you can break it over the edge of a workbench along the lines of holes - take care though because this needs a fairly large force and the edges will be rough. You may need to use a large pair of pliers to nibble away any jagged parts.

Avoid handling stripboard that you are not planning to use immediately because sweat from your hands will corrode the copper tracks and this will make soldering difficult unless you clean the board first.


stripboard

piezo transducer

piezo transducer
Miniature piezo transducer
Buy these if you can afford them, or wait until they are needed for a project.

Breadboard

A small breadboard (such as the Protobloc 1 shown in the picture) is suitable for simple circuits with up to two ICs, but if you intend to build more complex circuits such as counters it is best to buy a larger breadboard (such as the Protobloc 2).
Breadboards do not require soldering so the components used on them can be re-used many times. They are ideal for testing your own circuit designs and trying out ideas such as adapting a published project. 


breadboard

Storage systems for components

snap-top plastic bags
Snap-top plastic bags
drawers cabinet
Drawer cabinet
You can store all your components in a single container, such as a plastic food box, but as you accumulate more items it will become increasingly difficult to find the smaller components! A cheap solution is to organise the parts into small snap-top plastic bags which can be labelled, in fact you may find that some components are supplied like this.
Probably the best storage system is a cabinet of plastic drawers. These can be expensive, but you do not need many drawers because there is no need to have a drawer for every single component value. Many parts can be grouped together, such as decades of resistor values. For example you could organise a 15-drawer cabinet like this:
  1. Resistors 10ohm+ (third band blackonly a few, but they tend to be large
  2. Resistors 100ohm+ (third band brown)
  3. Resistors 1kohm+ (third band red)
  4. Resistors 10kohm+ (third band orange)
  5. Resistors 100kohm+ (third band yellow)
  6. Resistors 1Mohm+ (third band green) and 
  7. 10Mohm (third band blue)
  8. Presets, also variable resistors if they will fit in the drawer
  9. Capacitors low values, less than 1µF
  10. Capacitors electrolytic 1µF+
  11. Diodes 
  12. Transistors
  13. LEDs
  14. Lamps (also LED clips and lampholders)
  15. ICs (chips) and their holders (DIL sockets)
  16. Switches 
  17. Relays
  18. Connectors (crocodile clips, plugs and sockets)
  19. Other components (battery clips, piezo transducers, LDRs, thermistors)

Transistors







   

      



      

Function

transistorsTransistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.
A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).
The amount of current amplification is called the current gain, symbol hFE.
For further information please see the Transistor Circuits page. 

Types of transistor

NPN and PNP transistor symbols
Transistor circuit symbols
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).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!
Darlington pair is two transistors connected together to give a very high current gain.
In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor leads
Transistor leads for some common case styles.

Connecting

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.
If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue or website to identify the leads.
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.
Please see below for a table showing the case styles of some common transistors. 

Crocodile clip, photograph © Rapid Electronics
Crocodile clip

Soldering

Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.
Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation. 

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 are two easy ways to test it:
testing a transistor
Testing an NPN transistor

1. Testing with a multimeter

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.
The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.
testing a transistor
A simple switching circuit
to test an NPN transistor

2. Testing in a simple switching circuit

Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard
for example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it!
If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.
To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.
Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain hFE

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 first 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 identifies 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 specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable.
  • Codes beginning with TIP, for example TIP31A
    TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
  • Codes beginning with 2N, for example 2N3053
    The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.

Choosing a transistor

Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current IC and the current gain hFE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating.
To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced. The quantities shown in the table are explainedbelow.
NPN transistors
CodeStructureCase
style
IC
max.
VCE
max.
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
BC107NPNTO18100mA45V110300mWAudio, low powerBC182 BC547
BC108NPNTO18100mA20V110300mWGeneral purpose, low powerBC108C BC183 BC548
BC108CNPNTO18100mA20V420600mWGeneral purpose, low power
BC109NPNTO18200mA20V200300mWAudio (low noise), low powerBC184 BC549
BC182NPNTO92C100mA50V100350mWGeneral purpose, low powerBC107 BC182L
BC182LNPNTO92A100mA50V100350mWGeneral purpose, low powerBC107 BC182
BC547BNPNTO92C100mA45V200500mWAudio, low powerBC107B
BC548BNPNTO92C100mA30V220500mWGeneral purpose, low powerBC108B
BC549BNPNTO92C100mA30V240625mWAudio (low noise), low powerBC109
2N3053NPNTO39700mA40V50500mWGeneral purpose, low powerBFY51
BFY51NPNTO391A30V40800mWGeneral purpose, medium powerBC639
BC639NPNTO92A1A80V40800mWGeneral purpose, medium powerBFY51
TIP29ANPNTO2201A60V4030WGeneral purpose, high power
TIP31ANPNTO2203A60V1040WGeneral purpose, high powerTIP31C TIP41A
TIP31CNPNTO2203A100V1040WGeneral purpose, high powerTIP31A TIP41A
TIP41ANPNTO2206A60V1565WGeneral purpose, high power
2N3055NPNTO315A60V20117WGeneral purpose, high power
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
CodeStructureCase
style
IC
max.
VCE
max.
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
BC177PNPTO18100mA45V125300mWAudio, low powerBC477
BC178PNPTO18200mA25V120600mWGeneral purpose, low powerBC478
BC179PNPTO18200mA20V180600mWAudio (low noise), low power
BC477PNPTO18150mA80V125360mWAudio, low powerBC177
BC478PNPTO18150mA40V125360mWGeneral purpose, low powerBC178
TIP32APNPTO2203A60V2540WGeneral purpose, high powerTIP32C
TIP32CPNPTO2203A100V1040WGeneral purpose, high powerTIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
StructureThis 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 styleThere 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.
You can ignore this rating in low voltage circuits.
hFEThis 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. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts.
Why hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here.
Ptot max.Maximum total power which can be developed in the transistor, note that aheat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly IC × VCE. For transistors operating as switches the maximum collector current (IC max.) is more important.
CategoryThis shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories.
Possible substitutesThese are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board.

Darlington pair

Darlington pairThis is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. 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.
For example:
  • For TR1 use BC548B with hFE1 = 220.
  • For TR2 use BC639 with hFE2 = 40.


The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800.

The pair's maximum collector current IC(max) is the same as TR2.

Heat sinks for transistors

Heat sinks are needed for transistors passing large currents.
Heat sink
Heat sinkPhotograph © Rapid Electronics
Waste heat is produced in transistors due to the current flowing through them. If you find 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.
The rate of producing waste heat is called the thermal power, P. Usually the base current IB is too small to contribute much heat, so the thermal power is determined by the collector current IC and the voltage VCE across the transistor:
P = IC × VCE   (see diagram below) 
Insulation kit
Insulation kit
Heat-conducting paste
Heat-conducting pastePhotographs © Rapid Electronics
The heat is not a problem if IC is small or if the transistor is used as a switch because when 'full on' VCE is almost zero. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly on most of the time and VCEmay be about half the supply voltage. These power transistors will almost certainly need a heat sink to prevent them overheating.
Power transistors usually have bolt holes for attaching heat sinks, but clip-on heat sinks are also available. Make sure you use the right type for your transistor. Many transistors have metal cases which are connected to one of their leads so it may be necessary to insulate the heat sink from the transistor. Insulating kits are available with a mica sheet and a plastic sleeve for the bolt. Heat-conducting paste can be used to improve heat flow from the transistor to the heat sink, this is especially important if an insulation kit is used. 

Heat sink ratings

Heat sinks are rated by their thermal resistance (Rth) in °C/W. For example 2°C/W means the heat sink (and therefore the component attached to it) will be 2°C hotter than the surrounding air for every 1W of heat it is dissipating. Note that a lower thermal resistance means a better heat sink.

This is how you work out the required heat sink rating:NPN transistor with load
  1. Work out thermal power to be dissipated, P = IC × VCE
    If in doubt use the largest likely value for IC and assume that VCEis half the supply voltage.
    For example if a power transistor is passing 1A and connected to a 12V supply, the power P is about 1 × ½ × 12 = 6W.
  2. Find the maximum operating temperature (Tmax) for the transistor if you can, otherwise assume Tmax = 100°C.
  3. Estimate the maximum ambient (surrounding air) temperature (Tair). If the heat sink is going to be outside the case Tair = 25°C is reasonable, but inside it will be higher (perhaps 40°C) allowing for everything to warm up in operation.
  4. Work out the maximum thermal resistance (Rth) for the heat sink using: Rth = (Tmax - Tair) / P
    With the example values given above: Rth = (100-25)/6 = 12.5°C/W.
  5. Choose a heat sink with a thermal resistance which is less than the value calculated above (remember lower value means better heat sinking!) for example 5°C/W would be a sensible choice to allow a safety margin. A 5°C/W heat sink dissipating 6W will have a temperature difference of 5 × 6 = 30°C so the transistor temperature will rise to 25 + 30 = 55°C (safely less than the 100°C maximum).
  6. All the above assumes the transistor is at the same temperature as the heat sink. This is a reasonable assumption if they are firmly bolted or clipped together. However, you may have to put a mica sheet or similar between them to provide electrical insulation, then the transistor will be hotter than the heat sink and the calculation becomes more difficult. For typical mica sheets you should subtract 2°C/W from the thermal resistance (Rth) value calculated in step 4 above.
If this all seems too complex you can try attaching a moderately large heat sink and hope for the best. Cautiously monitor the transistor temperature with your finger, if it becomes painfully hot switch off immediately and use a larger heat sink!

Why thermal resistance?

The term 'thermal resistance' is used because it is analagous to electrical resistance:
  • The temperature difference across the heat sink (between the transistor and air) is like voltage (potential difference) across a resistor.
  • The thermal power (rate of heat) flowing through the heat sink from transistor to air is like current flowing through a resistor.
  • So R = V/I becomes Rth = (Tmax - Tair)/P
  • Just as you need a voltage difference to make current flow, you need a temperature difference to make heat flow.