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Diodes

Signal diodes | Rectifier diodes | Bridge rectifiers | Zener diodes

Also see: LEDs | AC and DC | Power Supplies

Example: Circuit symbol:

Function

Diodes allow electricity to flow in only onedirection. The arrow of the circuit symbol

shows the direction in which the current canflow. Diodes are the electrical version of avalve and early diodes were actually calledvalves.

Forward Voltage Drop

Electricity uses up a little energy pushing itsway through the diode, rather like a personpushing through a door with a spring. Thismeans that there is a small voltage across aconducting diode, it is called the forward voltage drop and is about 0.7V forall normal diodes which are made from silicon. The forward voltage drop of adiode is almost constant whatever the current passing through the diode sothey have a very steep characteristic (current-voltage graph).

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but allreal diodes leak a very tiny current of a few A or less. This can be ignored inmost circuits because it will be very much smaller than the current flowing in

the forward direction. However, all diodes have a maximum reversevoltage (usually 50V or more) and if this is exceeded the diode will fail andpass a large current in the reverse direction, this is called breakdown.

Ordinary diodes can be split into two types: Signal diodes which pass smallcurrents of 100mA or less and Rectifier diodes which can pass large currents.


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In addition there are LEDs (which have their own page) and Zener diodes (at

the bottom of this page).

Connecting and soldering

Diodes must be connected the correct way round, thediagram may be labelled a or+ for anode and k or- forcathode (yes, it really is k, not c, for cathode!). Thecathode is marked by a line painted on the body. Diodesare labelled with their code in small print, you may need amagnifying glass to read this on small signal diodes!

Small signal diodes can be damaged by heat when soldering, but the risk is

small unless you are using a germanium diode (codes beginning OA...) inwhich case you should use a heat sink clipped to the lead between the joint

and the diode body. A standard crocodile clip can be used as a heat sink.

Rectifier diodes are quite robust and no special precautions are needed forsoldering them.

Testing diodes

You can use a multimeteror a simple tester(battery, resistor and LED) tocheck that a diode conducts in one direction but not the other. A lamp may beused to test a rectifier diode, but do NOT use a lamp to testa signal diode because the large current passed by the lamp will destroy thediode!

Signal diodes (small current)

Signal diodes are used to process information (electrical signals) in circuits, sothey are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon andhave a forward voltage drop of 0.7V.


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Germanium diodes such as the OA90 have a lower forward voltage drop of0.2V and this makes them suitable to use in radio circuits as detectors whichextract 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 whensoldering, they have a lower resistance when conducting, and they have verylow leakage currents when a reverse

voltage is applied.

Protection diodes for relays

Signal diodes are also used to protecttransistors and ICs from the brief highvoltage produced when a relay coil is

switched off. The diagram shows how aprotection diode is connected 'backwards'across the relay coil.

Current flowing through a relay coil creates amagnetic field which collapses suddenly when the current is switched off. The suddencollapse of the magnetic field induces a brief high voltage across the relay coil which isvery likely to damage transistors and ICs. The protection diode allows the inducedvoltage to drive a brief current through the coil (and diode) so the magnetic field diesaway quickly rather than instantly. This prevents the induced voltage becoming high

enough to cause damage to transistors and ICs.

Rectifier diodes (large current)

Rectifier diodes are used in power supplies toconvert alternating current (AC) to direct current(DC), a process called rectification. They are alsoused elsewhere in circuits where a large currentmust pass through the diode.

All rectifier diodes are made from silicon andtherefore have a forward voltage drop of 0.7V. Thetable shows maximum current and maximum reverse voltage for somepopular rectifier diodes. The 1N4001 is suitable for most low voltage circuits

with a current of less than 1A.

DiodeMaximum

Current

Maximum

ReverseVoltage

1N4001 1A 50V

1N4002 1A 100V

1N4007 1A 1000V

1N5401 3A 100V1N5408 3A 1000V


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Also see: Power Supplies

Bridge rectifiers

There are several ways of connectingdiodes to make a rectifier to convertAC to DC. The bridge rectifier is one ofthem and it is available in specialpackages containing the four diodesrequired. Bridge rectifiers are rated bytheir maximum current and maximum reverse voltage. They have four leadsor 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.

Also see: Power Supplies

Various types of Bridge RectifiersNote that some have a hole through their centre for attaching to a heat sink

Photographs Rapid Electronics

Zener diodes

Example: Circuit symbol:a = anode, k = cathode


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Zener diodes are used to maintain a fixed voltage.They are designed to 'breakdown' in a reliable andnon-destructive way so that they can be used inreverse to maintain a fixed voltage across theirterminals. The diagram shows how they are

connected, with a resistor in series to limit thecurrent.

Zener diodes can be distinguished from ordinarydiodes by their code and breakdown voltage whichare printed on them. Zener diode codes begin BZX... or BZY... Theirbreakdown 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:

y The minimum voltage available is 2.4V.

y Power ratings of 400mW and 1.3W are common.

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Transistors

This page covers practical matters such as precautions when soldering and identifyingleads. The operation and use of transistors is covered by the TransistorCircuits page.

Types | Connecting | Soldering | Heatsinks | Testing | Codes | Choosing | Darlington pair

Also see: Heat sinks | TransistorCircuits

Function

Transistors amplify current, for example they can be used toamplify the small output current from a logic IC so that it can

operate a lamp, relay or other high current device. In manycircuits a resistor is used to convert the changing current to achanging voltage, so the transistor is being used to amplifyvoltage.

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 TransistorCircuits page.

Types oftransistor

There are two types of standardtransistors, NPN and PNP, with different circuitsymbols. The letters refer to the layers ofsemiconductor material used to make the transistor.

Most transistors used today are NPN because this isthe easiest type to make from silicon. If you are new toelectronics it is best to start by learning how to useNPN transistors.

Transistor circuit symbols


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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 inunderstanding how a transistor is used, so just treat them as labels!

ADarlington pairis two transistors connected together to give a very high

current gain.

In addition to standard (bipolar junction) transistors, there are field-effecttransistors which are usually referred to as FETs. They have different circuit

symbols and properties and they are not (yet) covered by this page.

Connecting

Transistors have three leads which

must be connected the correct wayround. Please take care with thisbecause a wrongly connectedtransistor may be damaged instantlywhen you switch on.

If you are lucky the orientation of thetransistor will be clear from the PCBor stripboard layout diagram,otherwise you will need to refer to a

supplier's catalogue to identify theleads.

The drawings on the right show theleads for some of the most common

case styles.

Please note that transistor lead diagrams show the view from below with theleads towards you. This is the opposite ofIC (chip) pin diagrams which showthe view from above.

Please see below for a table showing the case styles of some commontransistors.

Transistor leads for some common case styles.


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Soldering

Transistors can be damaged by heat when solderingso if you are not an expert it is wise to use a heat sinkclipped to the lead between the joint and the

transistor body. A standard crocodile clip can be usedas 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.

Heat sinks

Waste heat is produced in transistors due to the currentflowing through them. Heat sinks are needed for powertransistors because they pass large currents. If you findthat a transistor is becoming too hot to touch it certainlyneeds a heat sink! The heat sink helps to dissipate(remove) the heat by transferring it to the surroundingair.

For further information please see the Heat sinks page.

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 totest it:

Crocodile clipPhotograph Rapid Electronics.

Heat sink

Photograph Rapid Electronics


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1.Testing with a multimeter

Use a multimeteror a simple tester(battery,resistor and LED) to check each pair of leads for conduction. Set a digitalmultimeter to diode test and an analogue multimeter to a low resistance

range.

Test each pair of leads both ways (six tests in total):

y The base-emitter (BE) junction should behave like a diode and conductone way only.

y The base-collector (BC) junction should behave like a diodeand conduct one way only.

y The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. Thediodes are reversed in a PNP transistor but the same test procedure can beused.

2.Testing in a simple switching circuit

Connect the transistor into the circuit shown onthe right which uses the transistor as a switch.The supply voltage is not critical, anythingbetween 5 and 12V is suitable. This circuit can be

quickly built on breadboard for example. Takecare to include the 10k resistor in the baseconnection or you will destroy the transistor asyou test it!

If the transistor is OK the LED should light whenthe 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 knownbase current and measures the collector current so as to display thetransistor's DC current gain hFE.

Testing an NPN transistor

A simple switching circuitto test an NPN transistor


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Transistorcodes

There are three main series of transistor codes used in the UK:

y Codes beginning withB (or A),for example BC108,BC478

The first letter B is for silicon, A is for germanium (rarely used now). The secondletter indicates the type; for example C means low power audio frequency; Dmeans high power audio frequency; F means low power high frequency. The restof the code identifies the particular transistor. There is no obvious logic to thenumbering system. Sometimes a letter is added to the end (eg BC108C) toidentify a special version of the main type, for example a higher current gain or adifferent case style. If a project specifies a higher gain version (BC108C) it mustbe used, but if the general code is given (BC108) any transistor with that code issuitable.

y Codes beginning withTIP,for example TIP31ATIP refers to the manufacturer: Texas Instruments Power transistor. The letter at

the end identifies versions with different voltage ratings.y Codes beginning with2N,for example 2N3053

The initial '2N' identifies the part as a transistor and the rest of the code identifiesthe 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. Themost important properties to look for are the maximum collector current IC andthe current gain hFE. To make selection easier most suppliers group theirtransistors in categories determined either by theirtypical use ormaximumpowerrating.

To make a final choice you will need to consult the tables of technical datawhich are normally provided in catalogues. They contain a great deal of usefulinformation but they can be difficult to understand if you are not familiar withthe abbreviations used. The table below shows the most important technical

data for some popular transistors, tables in catalogues and reference bookswill usually show additional information but this is unlikely to be useful unlessyou are experienced. The quantities shown in the table are explained below.

NPN transistors

Code StructureCase

style

IC

max.

VCE

max.

hFE

min.

Ptot

max.

Category

(typical use)

Possible

substitutes


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BC107 NPN TO18 100mA 45V 110 300mW Audio, low power BC182 BC547

BC108 NPN TO18 100mA 20V 110 300mWGeneral purpose,

low powerBC108C BC183

BC548

BC108C NPN TO18 100mA 20V 420 600mWGeneral purpose,

low power

BC109 NPN TO18 200mA 20V 200 300mW Audio (low noise),low power

BC184 BC549

BC182 NPN TO92C 100mA 50V 100 350mWGeneral purpose,

low powerBC107 BC182L

BC182L NPN TO92A 100mA 50V 100 350mWGeneral purpose,

low powerBC107 BC182

BC547B NPN TO92C 100mA 45V 200 500mW Audio, low power BC107B

BC548B NPN TO92C 100mA 30V 220 500mWGeneral purpose,

low powerBC108B

BC549B NPN TO92C 100mA 30V 240 625mWAudio (low noise),

low powerBC109

2N3053 NPN TO39 700mA 40V 50 500mWGeneral purpose,

low powerBFY51

BFY51 NPN TO39 1A 30V 40 800mWGeneral purpose,medium power

BC639

BC639 NPN TO92A 1A 80V 40 800mWGeneral purpose,

medium powerBFY51

TIP29A NPN TO220 1A 60V 40 30WGeneral purpose,

high power


high powerTIP31C TIP41A

TIP31C NPN TO220 3A 100V 10 40W

General purpose,

high powerTIP31A TIP41A


high power

2N3055 NPN TO3 15A 60V 20 117WGeneral 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 StructureCase

styleIC

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 600mWGeneral purpose,

low powerBC478

BC179 PNP TO18 200mA 20V 180 600mWAudio (low noise),

low power

BC477 PNP TO18 150mA 80V 125 360mW Audio, low power BC177

BC478 PNP TO18 150mA 40V 125 360mW General purpose, BC178


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low power

TIP32A PNP TO220 3A 60V 25 40WGeneral purpose,

high powerTIP32C

TIP32C PNP TO220 3A 100V 10 40WGeneral purpose,

high powerTIP32A

Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of thediscrepancies 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 twotypes 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 casestyles 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.

hFE This is the current gain (strictly the DC current gain). The guaranteedminimum 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 anumber 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 ownsymbol. There are too many to explain here.

Ptot max. Maximum total power which can be developed in the transistor, note that

a heat sinkwill be required to achieve the maximum rating. This rating isimportant 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 fordifferent categories.

Possible substitutes These are transistors with similar electrical properties which will be

suitable substitutes in most circuits. However, they may have a differentcase 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 theamplified current from the first is amplified further bythe second transistor. This gives the Darlington pair a

very high current gain such as 10000. Darlington pairsare sold as complete packages containing the twotransistors. They have three leads (B, C and E) whichare equivalent to the leads of a standard individualtransistor.

You can make up your own Darlington pair from two transistors.For example:

y For TR1 use BC548B with hFE1 = 220.y 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.

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