How to make an INDUCTOR

My discussion here is very simple,unlike presenting u all the history like Wikipedia. and boring you with all its properties characteristics etc. i shall leave here a simple discussion which is very much needed practically when you work them.the main emphasis here is about HOW TO DESIGN AN INDUCTOR and its COLOR CODES!!!

Sometimes you may be unable to find a particular inductor the market. This is actually a problem faced by most of the electronic hobbyists and the problem becomes more serious if your project is RF related. The inductors required for RF circuits (antenna, tuner, amplifier etc) are almost impossible to find in the market and the only solution is nothing other than home-brewing them.

often designing inductor,we generally go with solenoid which is best suited and easy made.

With a little practice and patience you can construct almost all air cored inductors at home. The inductance of an air cored inductor can be represented using the simplified formula shown below and to calculate the inductance of an air-core inductor, the same equation may be used.

L = [d² n²] / [18d + 40l] (approximate formula)

Where� L � is the inductance in Micro Henries [�H]

• �d� is the diameter of the coil from one wire centre to another wire centre. It should be specifies in inches.
• �l� is the length of the coil specified in inches.
• �n� is the number of turns.

Notes :

• The length of the coil used in the inductor should be equal to or 0.4 times the diameter of the coil.
• As shown in the equation, inductance of the air-core inductor varies as the square of the number of turns. Thus the value �l� is multiplied four times if the value of �n� is doubled. The value of �l� is multiplied by two if the value of �n� is increased up to 40%.

Winding the coil.

• The coil must be first wounded on a plastic former of the adequate diameter (equal to the required core diameter).
• The winding must be tight and adjacent turns must be as close as possible.
• After the winding is complete, slowly withdraw the core without disturbing the coil.
• Now apply a thin layer of epoxy over the coil surface for mechanical support.
• Remove the insulation from the coil ends.

Example

Suppose you want to make an inductor which produces an inductance of 10 �H. The diameter of the coil is 1 inch and the coil length is given by 1.25 inches. You will have to find the number of turns of the coil.

Thus substituting the values in the above equation t

L = 10 inches

d = 1inch

l = 1.25 inches

n = v{L [18d * 40l]} / d = 26

Thus, the number of turns of the coil will be 26.

Number of turns/inch = 20.8

NOTE:for guys designing fm transmitters first wind 8-10 turns on a pencil (whose diameter 1/4 inches is just enough to produce required inductance for fm band).the wire may be 24 swg.for more details just mail i will provide u the details!!

those guys who cant do all these just try the softwares RFCALC or RFSIM99(links given below) this would really help you out.these softwares readily calculate and give you the dimensions you must wind,to get required inductance values.

http://electroschematics.com/835/rfsim99-download/
http://sourceforge.net/projects/rfcalc/
while using these softwares you need inches to millimeter conversion some times,when circuit needed dimensions are specified in mm for that just calculate using standard conversion

SOURCE:www.circuitstoday.com&www.elexp.com

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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 most have double throw (changeover) switch contacts as shown in the diagram.

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.

Circuit symbol for a relay

Relays Photographs � Rapid Electronics

Relay showing coil and switch contacts

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 switch.

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. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

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.

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Choosing a relay

You need to consider several features when choosing a relay:

1. 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.
2. 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. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.
3. Coil resistance
   The circuit must be able to supply the current required by the relay coil.
   

   Relay coil current =	supply voltage
   	coil resistance

   For example: 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.
4. 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, for example: "5A at 24V DC or 125V AC".
5. Switch contact arrangement (SPDT, DPDT etc)
   Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches

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Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode(eg 1N4148) is connected 'backwards' across the relay coil to provide this protection.

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

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Reed relays

Reed Relay

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.

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 dilsocket ('IC holder').

For further information about reed switches please see the page on switches

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Relays and transistors compared

Like relays, transistors can be used as an electrically operated switch. For switching small DC 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! 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 higher voltages than standard transistors.
• Relays are often 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 ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.

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Further information

For further information about relays please see the Electronics in Meccano website.

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MAKE UR OWN SIMPLE COMPONENT TESTER

This simple project may be used for testing components, as well as checking circuit board tracks, wires and connections for continuity (conduction). It tries to pass a small current through the item being tested and the LED will light brightly, dimly or not at all according to the resistance of the item:

• LED bright means the resistance is low, less than about 1k
• LED dim means the resistance is medium, a few k
• LED off means the resistance is high, more than about 10k

When not in use the 9V PP3 battery should be unclipped or the crocodile clips attached to a piece of card or plastic to prevent them touching. You could add an on-off switch in the red wire from the battery clip and this may be the best option if you mount the simple tester in a box.

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Parts Required

• resistor: 390
• red LED 5mm diameter, standard type
• battery clip for 9V PP3
• crocodile clips: miniature red and black
• stripboard: 5 rows � 7 hol

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Stripboard Layout

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Testing stripboard, PCB tracks, wires and connections

Circuit diagram

Connect a crocodile clip on each side of the suspected fault:

• LED bright means there is a connection.
• LED off means there is no connection.

If you are testing a stripboard or PCB which has components soldered in place, beware of possible connections via the components and allow for this when interpreting the results.

Stripboard circuits can suffer from two common problems: solder bridging between adjacent tracks making a connection where there should be none, and tracks broken with a track cutter which have an almost invisible thread of copper conducting across the break.

If a PCB has etched poorly the tracks may be very thin in places or there may be traces of copper bridging between adjacent tracks.

Wires and connections may be checked for continuity (conduction).

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Testing components

Connect a crocodile clip on each side of the component. They can be connected either way round unless stated otherwise in the table below.

Component	Test results for a component in good condition
Resistor	LED bright for low resistance, less than about 1k. LED dim for medium resistance, a few k. LED off for high resistance, more than about 10k.
Variable Resistor	Across the two ends of the track the LED brightness will depend on the resistance value (see above). Between one end of the track and the wiper you should see the LED brightness vary as you adjust the variable resistor. However, for high resistances (>10k) the LED will only light near one end of the track.
Diode	LED bright with red lead to anode and black lead to cathode (stripe). LED off with black lead to anode and red lead to cathode (stripe). a = anode, k = cathode (the end with a stripe)
Zener Diode	LED bright with red lead to anode and black lead to cathode (stripe). LED dim with black lead to anode and red lead to cathode (stripe) if the zener diode voltage is less than about 7V. LED off with black lead to anode and red lead to cathode (stripe) if the zener diode voltage is greater than about 7V. a = anode, k = cathode (the end with a stripe)
LED Light Emitting Diode	LED bright with red lead to anode and black lead to cathode (short lead) - the LED being tested will also light. LED off with black lead to anode and red lead to cathode (short lead). a = anode (long lead), k = cathode (short lead, flat on body)
Transistor B = base, C = collector, E = emitter Please refer to a supplier's catalogue to identify the leads.	For each pair of transistor leads connect the tester leads first one way, then the other way. These are the results for an NPN transistor in good condition: CE pair: LED off both ways. BC pair: LED bright with red lead on B, LED off the other way. BE pair: LED bright with red lead on B, LED off the other way. These are the results for a PNP transistor in good condition: CE pair: LED off both ways. BC pair: LED bright with black lead on B, LED off the other way. BE pair: LED bright with black lead on B, LED off the other way. Note that you can use the tester to identify the B lead (the one which always conducts one way) and to distinguish NPN and PNP transistors (by the tester lead colour when B conducts). However, the tester cannot distinguish the C and E leads.
Capacitor less than 1�F	LED off. Please bear in mind that a broken connection will give the same result.
Capacitor 1�F and greater	If the capacitor is polarised (most will be) connect the red lead to positive (+) and the black lead to negative (-). The LED will flash briefly when first connected. Reverse the connections: the LED will give another brief flash. With low values like 1�F the flash will be almost too brief to see, but larger values such as 100�F will give longer flashes. Electrolytic capacitors may leak a little when connected the wrong way round, making the LED light dimly continuously.
LDR Light Dependent Resistor	LED bright when the LDR is in bright light. LED dim when the LDR is in normal room light. LED off when the LDR is in darkness.
Thermistor	LED dim when the thermistor is warm. LED off when the thermistor is cold. These are typical results, the exact results depend on the thermistor's resistance.
Lamp	LED bright. Note that the lamp itself will NOT light because the test current is too small.
Switch	LED bright when switch contacts are closed (on). LED off when switch contacts are open (off). Note that you can use the tester to identify the switch contacts if necessary.
Fuse, Motor, Loudspeaker, Inductor, Relay coil, Wire	LED bright.

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TRANSISTORS

Function

Transistors 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 h_FE.

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Types of transistor

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!

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 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.

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 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 h_FE.

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.

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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 I_C and the current gain h_FE. 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 explained below

NPN transistors
Code	Structure	Case style	IC max.	VCE max.	hFE min.	Ptot max.	Category (typical use)	Possible substitutes
BC107	NPN	TO18	100mA	45V	110	300mW	Audio, low power	BC182 BC547
BC108	NPN	TO18	100mA	20V	110	300mW	General purpose, low power	BC108C BC183 BC548
BC108C	NPN	TO18	100mA	20V	420	600mW	General purpose, low power
BC109	NPN	TO18	200mA	20V	200	300mW	Audio (low noise), low power	BC184 BC549
BC182	NPN	TO92C	100mA	50V	100	350mW	General purpose, low power	BC107 BC182L
BC182L	NPN	TO92A	100mA	50V	100	350mW	General purpose, low power	BC107 BC182
BC547B	NPN	TO92C	100mA	45V	200	500mW	Audio, low power	BC107B
BC548B	NPN	TO92C	100mA	30V	220	500mW	General purpose, low power	BC108B
BC549B	NPN	TO92C	100mA	30V	240	625mW	Audio (low noise), low power	BC109
2N3053	NPN	TO39	700mA	40V	50	500mW	General purpose, low power	BFY51
BFY51	NPN	TO39	1A	30V	40	800mW	General purpose, medium power	BC639
BC639	NPN	TO92A	1A	80V	40	800mW	General purpose, medium power	BFY51
TIP29A	NPN	TO220	1A	60V	40	30W	General purpose, high power
TIP31A	NPN	TO220	3A	60V	10	40W	General purpose, high power	TIP31C TIP41A
TIP31C	NPN	TO220	3A	100V	10	40W	General purpose, high power	TIP31A TIP41A
TIP41A	NPN	TO220	6A	60V	15	65W	General purpose, high power
2N3055	NPN	TO3	15A	60V	20	117W	General 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
Code	Structure	Case style	IC max.	VCE max.	hFE min.	Ptot max.	Category (typical use)	Possible substitutes
BC177	PNP	TO18	100mA	45V	125	300mW	Audio, low power	BC477
BC178	PNP	TO18	200mA	25V	120	600mW	General purpose, low power	BC478
BC179	PNP	TO18	200mA	20V	180	600mW	Audio (low noise), low power
BC477	PNP	TO18	150mA	80V	125	360mW	Audio, low power	BC177
BC478	PNP	TO18	150mA	40V	125	360mW	General purpose, low power	BC178
TIP32A	PNP	TO220	3A	60V	25	40W	General purpose, high power	TIP32C
TIP32C	PNP	TO220	3A	100V	10	40W	General purpose, high power	TIP32A
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.

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.
IC max.	Maximum collector current.
VCE max.	Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits.
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. 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 a heatsink 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.
Category	This 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 substitutes	These 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.

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Darlington pair

This 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 h_FE1 = 220.
• For TR2 use BC639 with h_FE2 = 40.

The overall gain of this pair is h_FE1 � h_FE2 = 220 � 40 = 8800.
The pair's maximum collector current I_C(max) is the same as TR2.

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SOLDERING GUIDE

How to Solder

First a few safety precautions:

• Never touch the element or tip of the soldering iron.
  They are very hot (about 400�C) and will give you a nasty burn.
• Take great care to avoid touching the mains flex with the tip of the iron.
  The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock.
• Always return the soldering iron to its stand when not in use.
  Never put it down on your workbench, even for a moment!
• Work in a well-ventilated area.
  The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work.
• Wash your hands after using solder.
  Solder contains lead which is a poisonous metal.

Preparing the soldering iron:

• Place the soldering iron in its stand and plug in.
  The iron will take a few minutes to reach its operating temperature of about 400�C.
• Dampen the sponge in the stand.
  The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet.
• Wait a few minutes for the soldering iron to warm up.
  You can check if it is ready by trying to melt a little solder on the tip.
• Wipe the tip of the iron on the damp sponge.
  This will clean the tip.
• Melt a little solder on the tip of the iron.
  This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge.

You are now ready to start soldering:

• Hold the soldering iron like a pen, near the base of the handle.
  Imagine you are going to write your name! Remember to never touch the hot element or tip.
• Touch the soldering iron onto the joint to be made.
  Make sure it touches both the component lead and the track. Hold the tip there for a few seconds and...
• Feed a little solder onto the joint.
  It should flow smoothly onto the lead and track to form a volcano shape as shown in the diagram. Apply the solder to the joint, not the iron.
• Remove the solder, then the iron, while keeping the joint still.
  Allow the joint a few seconds to cool before you move the circuit board.
• Inspect the joint closely.
  It should look shiny and have a 'volcano' shape. If not, you will need to reheat it and feed in a little more solder. This time ensure that both the lead and track are heated fully before applying solder.

Crocodile clip Photograph � Rapid Electronics

Using a heat sink

Some components, such as 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 component body. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper.

Further information

For a much more detailed guide to soldering, including troubleshooting, please see the Basic Soldering Guide on the Everyday Practical Electronics Magazine website.

Soldering Advice for Components

It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this!

1. Stick all the components onto a sheet of paper using sticky tape.
2. Identify each component and write its name or value beside it.
3. Add the code (R1, R2, C1 etc.) if necessary.
   Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given.
4. Resistor values can be found using the resistor colour code which is explained on our RESISTORS post.
   
5. and make your own RESISTANCE COLOR CODE CALCULATOR to help you.
6. Capacitor values can be difficult to find because there are many types with different labelling systems!
   

Some components require special care when soldering. Many must be placed the correct way round and a few are easily damaged by the heat from soldering. Appropriate warnings are given in the table below, together with other advice which may be useful when soldering.

For most projects it is best to put the components onto the board in the order given below:

	Components	Pictures	Reminders and Warnings
1	IC Holders (DIL sockets)		Connect the correct way round by making sure the notch is at the correct end. Do NOT put the ICs (chips) in yet.
2	Resistors		No special precautions are needed with resistors.
3	Small value capacitors (usually less than 1�F)		These may be connected either way round. Take care with polystyrene capacitors because they are easily damaged by heat.
4	Electrolytic capacitors (1�F and greater)		Connect the correct way round. They will be marked with a + or - near one lead.
5	Diodes		Connect the correct way round. Take care with germanium diodes (e.g. OA91) because they are easily damaged by heat.
6	LEDs		Connect 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.
7	Transistors		Connect the correct way round. Transistors have 3 'legs' (leads) so extra care is needed to ensure the connections are correct. Easily damaged by heat.
8	Wire Links between points on the circuit board.	single core wire	Use single core wire, this is one solid wire which is plastic-coated. If there is no danger of touching other parts you can use tinned copper wire, this has no plastic coating and looks just like solder but it is stiffer.
9	Battery clips, buzzers and other parts with their own wires		Connect the correct way round.
10	Wires to parts off the circuit board, including switches, relays, variable resistors and loudspeakers.	stranded wire	You should use stranded wire which is flexible and plastic-coated. Do not use single core wire because this will break when it is repeatedly flexed.
11	ICs (chips)		Connect the correct way round. Many ICs are static sensitive. Leave ICs in their antistatic packaging until you need them, then earth your hands by touching a metal water pipe or window frame before touching the ICs. Carefully insert ICs in their holders: make sure all the pins are lined up with the socket then push down firmly with your thumb.

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What is solder?

Reels of solder Photograph � Rapid Electronics

Solder is an alloy (mixture) of tin and lead, typically 60% tin and 40% lead. It melts at a temperature of about 200�C. Coating a surface with solder is called 'tinning' because of the tin content of solder. Lead is poisonous and you should always wash your hands after using solder.

Solder for electronics use contains tiny cores of flux, like the wires inside a mains flex. The flux is corrosive, like an acid, and it cleans the metal surfaces as the solder melts. This is why you must melt the solder actually on the joint, not on the iron tip. Without flux most joints would fail because metals quickly oxidise and the solder itself will not flow properly onto a dirty, oxidised, metal surface.

The best size of solder for electronics is 22swg (swg = standard wire gauge).

Desoldering

At some stage you will probably need to desolder a joint to remove or re-position a wire or component. There are two ways to remove the solder:

Using a desoldering pump (solder sucker)

1. With a desoldering pump (solder sucker)

• Set the pump by pushing the spring-loaded plunger down until it locks.
• Apply both the pump nozzle and the tip of your soldering iron to the joint.
• Wait a second or two for the solder to melt.
• Then press the button on the pump to release the plunger and suck the molten solder into the tool.
• Repeat if necessary to remove as much solder as possible.
• The pump will need emptying occasionally by unscrewing the nozzle.

Solder remover wick Photograph � Rapid Electronics

2. With solder remover wick (copper braid)

• Apply both the end of the wick and the tip of your soldering iron to the joint.
• As the solder melts most of it will flow onto the wick, away from the joint.
• Remove the wick first, then the soldering iron.
• Cut off and discard the end of the wick coated with solder.

After removing most of the solder from the joint(s) you may be able to remove the wire or component lead straight away (allow a few seconds for it to cool). If the joint will not come apart easily apply your soldering iron to melt the remaining traces of solder at the same time as pulling the joint apart, taking care to avoid burning yourself.

First Aid for SOLDERING Burns

Most burns from soldering are likely to be minor and treatment is simple:

• Immediately cool the affected area under gently running cold water.
  Keep the burn in the cold water for at least 5 minutes (15 minutes is recommended). If ice is readily available this can be helpful too, but do not delay the initial cooling with cold water.
• Do not apply any creams or ointments.
  The burn will heal better without them. A dry dressing, such as a clean handkerchief, may be applied if you wish to protect the area from dirt.
• Seek medical attention if the burn covers an area bigger than your hand.

To reduce the risk of burns:

• Always return your soldering iron to its stand immediately after use.
• Allow joints and components a minute or so to cool down before you touch them.
• Never touch the element or tip of a soldering iron unless you are certain it is cold.

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CAPACITORS

we are studying from our schooling what is a capacitor and its properties,so i dont want to bore you starting from its defination,the article here is just an update for your basic theoritical knowledge,

There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.

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Polarised capacitors (large values, 1�F +)

Examples: Circuit symbol:

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220�F in picture) and radial where both leads are at the same end (10�F in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

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. If 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 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, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in �F. The standard colourcode is used, but for the spot, grey is used to mean � 0.01 and white means � 0.1 so that values of less than 10�F 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). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.

For example: blue, grey, black spot means 68�F
For example: blue, grey, white spot means 6.8�F
For example: blue, grey, grey spot means 0.68�F

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Unpolarised capacitors (small values, up to 1�F)

Examples: Circuit symbol:

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!

For example 0.1 means 0.1�F = 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:

• the 1st number is the 1st digit,
• the 2nd number is the 2nd digit,
• the 3rd number is the number of zeros to give the capacitance in pF.
• Ignore any letters - they just indicate tolerance and voltage rating.

For example: 102 means 1000pF = 1nF (not 102pF!)

For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).

Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Capacitor Colour Code:

similar to resistor color code is also being used for capacitors !!!!
A colour code was used on polyester capacitors for many years. It is now , but of course there are many still around. 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).

For example:

brown, black, orange means 10000pF = 10nF = 0.01�F.

Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.

For example:

wide red, yellow means 220nF = 0.22�F.

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.

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Real capacitor values (the E3 and E6 series)

You may have noticed that capacitors are not available with every possible value, for example 22�F and 47�F are readily available, but 25�F and 50�F are not!

Why is this? Imagine that you decided to make capacitors every 10�F giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.

To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 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.

Variable capacitors

Variable Capacitor Symbol

Variable Capacitor Photograph � Rapid Electronics

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 = 0.0001�F). The type illustrated usually has trimmers built in (for making small adjustments - see below) as well as the main variable capacitor.

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.

Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

Trimmer capacitors

Trimmer Capacitor Symbol

Trimmer Capacitor Photograph � Rapid Electronics

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.

Trimmers are the capacitor equivalent of presets which are miniature variable resistors.

Uses of Capacitors

Capacitors are used for several purposes:

• Timing - for example with a 555 timer ic controlling the charging and discharging
• Smoothing - for example in a power supply
• coupling - for example between stages of an audiosystem and to connect a loudspeaker
• Filtering - for example in the tone control of an audiosystem
• Tuning - for example in a radiosystem
• Storing energy - for example in a camera flash circuit.

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