ELECTRONICS REPAIR COURSE NOTES.

DIODES.

BY
NYERO VINCENT.

Technician and director NERS GULU.

(NYERO ELECTRONIC REPAIR SHOP GULU)

TELL. +256 775442110

NB. This book only contains the basics electronic knowledge on diodes required for repairing
electronic/electrical devices.

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 Introduction.
Electronics. Can be defined as;
1. Branch of science that deals with the study of flow and control of electrons and the study of their behaviors and effect
in Vacuum, gases, semiconductor and devices that uses such electrons.
2. Branch of physics and technology concerned with the design of circuit using electrical components/parts with the
behaviors and movement of electrons in semiconductors, conductors, vacuum etc.
ELECTRONIC DEVICES
Electronic devices are sometimes called electronics. Therefore, electronics can also be defined as device that operates
using many small electrical parts.
Examples of electronic devices.

Television (TV), phones, radios, computers/laptops, home theatres/woofers, cameras, projectors etc.

SOME OF THE COMMON TERMS, ABBREVIATIONS AND CIRCUIT SYMBOLS USED IN ELECTRONICS
(GLOSSARY).

Terms used.

Technician. Is a person who helps designs, test, manufactured, install and repair electricals and electronic equipment
such as communication equipment, medical monitoring devices, computers etc.

Multimeter. Is a measuring instrument that can measure multiple electrical properties. A typical multimeter can
measure voltage, current and resistance etc.

Electricity. Is the flow of electric chargers (electrons) within conducting matter in a complete circuit.

Electric current. Is a flow of electric charge.

Current. Is the rate of flow of electric charge, Current is represented by I and measured in Ampere.

Voltage. Voltage also known as potential difference (p.d), electromotive force (emf), electric pressure or electric tension
is defined as the electric potential difference per unit charge between two points in an electric-fields. Voltage is
measured in volt (V).

Ampere. Is a unit of measure of rate of flow of electron/electric current in an electrical conductor.

Ampere or amp is represented by letter ‘A’.

Electron (n). Is a negatively charge carrier.

Holes (p). hole is a positively charged carriers.

Anode. Positive charged electrode.

Cathode. Negative charged electrode.

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Circuit. Circuit refers to interconnection of components to provide an electrical path between two or more components.

Electrode. Is an electrical conductor used to make contact with a non-Metallic part of a circuit.

Conductor. Material that allow the passage of electricity through it.eg. Copper, iron, aluminum, gold etc.

Insulator. Material that do not allow the passage of electricity through it.

Semiconductor. Material that partially allow the passage of electricity and partially do not allow the passage of
electricity through it.ie. Its property of conductivity lies between that of a good conductor and an insulator.eg. Silicon,
germanium etc.

Voltage source. Any device that provide us with voltage.eg batteries, electric generator and wall socket. Etc. or it is a
device that convert some other form of energy.eg chemical energy, mechanical energy etc. into electrical energy.

Resistance. Is the opposition of a conducting materials to the flow of electric current. It is measured in ohm (Ω). Or it
is the ability of a resistor to limit current flow.

Capacitance. Is the ability of a capacitor to store electric charge. It is measured in FARAD (F).

Inductance. Property of a component like a coil or an inductor to oppose any change in flow of current. It is measured
in Henry (H).

Power. Product of voltage (V) and current (I). It can also be defined as the time rate of doing work. It is measured in
WATT (W).

Work. Is power used during a period of time. It is measured in joules (J).

Open circuit. A circuit connection in which there is no flow of electric current.

Short circuit. Is a connection between two points in a circuit in the potential difference is zero (0V).

Series circuit. In series connection same current flow through each and every component.

Parallel circuit. In parallel connection, same voltage is applied across each and every component.

Through – hole devices. Electronic devices with long terminals that we put in the holes of PCB.

Surface mount devices (SMD). Devices that pins/legs are soldered on the surface of the PCB without any terminal
going into the holes.

Amplifier. Is an electronic device /circuit that produces an enlarge version of a small signal fed into the circuit.

Amplitude. Magnitude or size of a signal voltage or current.

Solder. Metallic alloy used to join two or more metal surfaces.

Soldering. To put legs of components on appoint on PCB.

De-soldering. To remove legs of components from a point in PCB.

Dismantle/disassemble. To put apart parts of electrical devices.

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Assemble. To fix or put back parts of electrical devices.

Loop/jumper. To connect one point to another in a circuit board.eg broken path, open circuit etc.
Direct current (DC). Unidirectional current.ie type of current that do not changes direction (flow in only one direction.eg.
batteries.
Alternating current (AC). Current which reverses direction/changes direction at regular intervals.eg. AC voltage
sources or alternators.
Impedance. Total opposition to the flow of current offered by a circuit.
Bias. A DC voltage applied to a device to control its operation.
Rectifier. Devices that converts AC into pulsating DC.
Cable. A group of two or more wires.
Polarity. Property of having a negative and positive charge/electrode.
Schematic diagram. Illustration of an electrical or electronic circuit with the components represented by their circuit
symbols and name prefixes/reference.
Troubleshooting. Procedures use for finding faults.
Terminals and connectors. Components to make electrical connection.
Network components. Components that use more than one type of passive component.
Transient voltage. A sudden high voltage spike in an alternating current system, caused by arcing or lightening.
Voltage breakdown. The voltage at which current suddenly passes in destructive amounts of dielectric.
Ripple. A small alternating current component in the output of a direct current power supply with inadequate filtering.
PWM. Pulse width modulation is a technique employed to regulate the output power by changing the pulse width. PWM
is employed in SMPS, UPS and many other power control applications.
Power surge. A momentary increase in the voltage on a utility line.
Impedance. Combination of resistance, inductance and capacitance which restricts the current through any device.
Attenuates. To reduce in amplitude.
wave form. Shape followed by any alternating current or voltage.
Bleeder resistor. A resistor or group of resistors used permanently to drain current from charged capacitors.
ESR. Equivalent series resistance.
Current limiting resistor. A series resistor inserted into circuit to limit the current to a desired value.

Abbreviations
Abbr. meaning
DC Direct current
AC Alternating current
TV Television

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 F Fuses
S Switch
T Transformer
PCB Printed circuit board
B/Bat Battery
SP/SPK Speaker
CPU Central processing unit
CRT Cathode ray tube
MIC Microphone/mouth piece
GSM Global system for mobile
BSI Battery status indicator
LED Light emitting diode
IMEI International mobile equipment identity
UEM Universal power management
P.A Power amplifier
P.F.O Power frequency oscillator
VCO Voltage control oscillator
SMD Surface mount devices
VR Voltage regulator
RF Radio frequency
VBat Positive terminal of battery/connector
Gnd/GND Ground (negative terminal)
RT Thermistor
RAM Random access memory
ROM Read only memory
Rx Receiving section
Tx Transmitting section
PWR/P.Key Power key
REC Ear piece
A Ampere
AF Audio amplifier
BGA Ball grid array
DM/d- Data minus
DP/d+ Data positive
L Inductor
D Diode
C Capacitor
R Resistor
Q Transistor
Vbus/Vchg Bus voltage/Charging voltage
MOT Motor

Circuit Symbols
Below are some of the common circuit symbols used by manufacturers when designing circuit.

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Symbol meaning

Continuity/wire

Capacitor

Power diode/rectifier diode

Zener diode

Battery (DC voltage source)

Fuse

Switch

AC voltage source

Transformer

Coil/inductor

Resistor

variable resistor

Potentiometer

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 LED

Transistor

DC voltage

AC voltage

AC current

DC current

ELECTRONIC COMPONENTS.

An electronic component is any basic discrete electronic device or physical part of an electronic system used to affect
electrons or their associated field.
Electronic components are categorized into two types;
1. passive components
2. Active components

PASSIVE COMPONENTS
Passive components are electronic components which can only receive energy which it can dissipate, absorb or store
it in an electric field or magnetic field.
Passive elements do not need any form of electrical power to operate.

Examples of passive electronic components.

Resistor, capacitor, inductor, transformer, thermistor, varistor (VDR), transducer etc.
ACTIVE COMPONENTS.
Active electronic components are components that supplies energy to a circuit.

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 Examples of active electronic components.
Transistor, generator, diodes (LED, photodiode, Zener diode, rectifier diodes), ICs, triacs, thyristor etc.
NB. All semi-conductor devices falls under passive components.

OTHER ELECTRONIC/ELECTRICAL ELEMENTS

Fuse, switch, relays, crystal and resonator, cable/wire/conductor etc.

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 DIODES.

A diode is a one-way valve. Therefore, a diode can be defined as a device that allow current to flow/pass through it in
only one direction.
When current passes through the diode, the diode blocks the reverse flow of the current as it does not conduct in
reverse direction.
Though a diode does not conduct in reverse direction, there is a point where the diode is not able to resist reverse
electrical pressure and if this limit is exceeded, the diode will break down and conduct which is called peak inverse
voltage (PIV).
Circuit symbols of diodes.

Normal diode. LED. Zener diode. Schottky diode. Photodiode.

Diode is represented by the capital letter D in circuit boards.

FUNCTIONS/USES OF DIODES.
i). Rectification.

Diodes such as rectifier/power diodes can be used for rectification.ie diodes can be used to convert AC into DC
(pulsating DC). This is because diodes restrict AC current and conduct DC current.

ii). voltage regulation.

Diodes such as Zener diodes can be used to produce a constant and common voltage (regulate voltage) when used
as a reference voltage regulator.
iii). Permit current in one direction. (Protection purposes)

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Diode being a device that conduct current in only one direction makes it to be used to block the reverse flow of current
that passes through it. In addition, diode only conduct when forward biased. This advantage makes diodes to be used
in protecting circuits from wrong polarity connection.
NB: diodes such as rectifier diodes only conducts in forward bias.ie
1. When a positive voltage is applied to the anode of the diode (forward bias), the diode conducts but when
negative voltage is applied to the anode of the diode (reverse bias), the diode does not conduct.
2. When a negative voltage is applied to the cathode of the diode (forward bias), the diode will conduct but when
positive voltage is applied to the cathode of the diode (reverse bias), the diode will NOT conduct. Therefore,
this property of a diode to conduct only in forward bias makes it useful in protecting circuits or devices
especially those that used DC voltage for their operations. This will save/protect the device from wrong polarity
connection.
(iv). indicator.
Diodes such as LEDs are also being used as indicators in electrical equipment.eg standby led for TV, traffic lights
etc.

PARTS OF DIODES.

The part with the band indicates the negative side of the diode and is called cathode.
The side which is opposite to the cathode is called the anode.
The arrow in the symbol shows the direction of conventional current.
Conventional current is the type of current that flow into the circuit from the positive potential of battery or voltage
source.
The negative (-) part/terminal of a diode is called the cathode and is usually indicated by a band or strips which
may be represented by letter K on the circuit symbol while the positive (+) part of a diode is called the anode and
it is usually indicated by alphabetical letter A on the circuit symbol.
Cathode is the negative (-) charged electrode.ie majority of charge carriers are electrons while minority of charge
carriers are holes (made of n-type material)
Anode is the positive (+) charged electrode.ie majority of charge carriers are holes while minority of charge carriers
are electrons (made of p-type material).

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 SMD diode (the bar indicates the cathode).figure below.

NB: SMD glassy Zener diode normally has a blue bar for the cathode.
All diodes are semi-conductor devices because they are constructed using semi-conductor materials such as
silicon, germanium etc.

SEMI-CONDUCTOR.
Semi-conductors are materials whose electrical conductivity lies between that of a good conductor and an
insulator.eg silicon, germanium etc.
TYPES OF SEMI-CONDUCTORS.
(i). Intrinsic semi-conductors.
Intrinsic semi-conductors are semi-conductors in their PURE form.eg a semi-conductor crystal with only silicon
atoms. Acts more like an insulator than a conductor.
(ii). extrinsic semi-conductors.
Are semi-conductors with other atoms mixed in. these atoms are called impurity atoms. The process of adding
impurity atoms is called doping. Doping alters the x-tics of the semi-conductor mainly its conductivity. Conductivity
of extrinsic semi-conductor is greater than that of an intrinsic semi-conductor. The level of conductivity is
dependent mainly on the number of impurity atoms that have been added during doping process.
Doping. Is the process that involves adding impurity atoms to an intrinsic (pure) semi-conductor. Intrinsic semi-
conductor such as silicon or germanium are almost always doped with impurity atoms to increase their conductivity.
An intrinsic semi-conductor material is therefore, material that has been doped with impurity atoms.

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 TYPES OF SEMI-CONDUCTOR MATERIALS.
There are two types of semi-conductor materials namely;
(i). N-type semi-conductor materials.
(ii). P-type semi-conductor materials.

N-type semi-conductor materials.

Are semi-conductor materials that has many more free electrons (negative charge) than holes (positive charge).
In an n-type semi-conductor material, the electron (n) are the majority charge carriers and holes (h) are the minority
charge carriers (n>h).
N-type materials are formed by adding pentavalent impurities in a pure or intrinsic semi-conductor.

P-type semi-conductor materials.

In p-type semi-conductor materials, holes are the majority charge carriers whereas electrons are the minority
charge carriers (h>n).
P-type semi-conductor materials are formed by adding trivalent impurities in pure or intrinsic semi-conductor.
When a P-type semi-conductor is combined with an N-type semi-conductor, a P-N junction is formed and it can be
called a p-n junction diode because it allows current to pass through it in only one direction.

P-N JUNCTION DIODES.

p-type and n-type semi-conductor joined to form PN-junction diode.

Actual diagram.

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 Symbol.

NB. A P-N junction cannot be formed by simply putting p-type and n-type semi-conductor. The construction of a
p-n junction diode involves special manufacturing techniques of various types.

OPERATION OF P-N JUNCTION DIODE.

The operation of a p-n junction diode can be explained by the fact that there is a movement of holes and electrons
even when no external voltage is applied across the p-n junction. Therefore, the conduction electrons in the n-
region randomly drifting in all direction will combine with holes leaving certain ions uncovered or un-neutralized.
These uncovered ions set up a field called the barrier field or barrier potential. These barrier potential drives the
remaining electrons and holes away from the junction which in turn uncovers more ions and so on.
This barrier potential is equivalent to a small battery across the junction. This barrier potential depends on the
nature of the semi-conductor material and is 0.7V in the case of silicon and 0.3V in the case of germanium.
For any current to pass through the P-N junction, the barrier potential must be over-come by application of some
external voltage (battery) across the junction to oppose the barrier potential. This is called biasing P-N junction.

BIASING P-N JUNCTION DIODE.

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Biasing. Can be defined as the application of a DC voltage to establish certain operating conditions for an electrical
device such as diodes, transistors etc. this current or voltage applied is called bias.
There are two practical bias conditions for a diode. Either of this condition is created by application of external
voltage.
(i). forward bias.
(ii). reverse bias.
Forward bias.

A p-n junction diode is said to be forward biased if an external battery (voltage) is connected across the junction
so that the polarity of the external battery is opposite to the barrier potential. This lowers the barrier potential and
allows easy flow of current through the junction (diode).
When a battery is hooked up to a diode with positive voltage (+ supply) to the positive side (anode) of the diode
and negative voltage (- supply) connected to the negative side (cathode) of the diode as shown in the diagram
above, current flow through the diode (junction). Voltage connected to the diode in this direction is called forward
bias.
At the junction, the free electrons and holes combine and are lost in the process. However, the current carriers
lost in these combinations are replenished by new current carriers resulting from the separation of electron-hole
pairs. The free electrons produced in the p-region are attracted by the positive (+) terminal of the battery (E) and
flow in the external circuit as shown in the diagram above.
In other words, when a diode is forward biased, the negative charge from negative (-) terminal of the battery (E)
repels the negative charge in the n-region forcing them into the p-region of the diode, the positive (+) force at the
positive terminal (+) of the battery (E) pulls these electrons on through the diode. This completes the circuit. Recall
the law of electrostatic.ie unlike pole attracts and like poles repels.

Reverse bias.
A P-N junction diode is said to be in reverse bias when the external voltage or battery connected across the P-n
junction aids the barrier potential. This actually raised the barrier potential and no current flow through the diode.

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When positive (+) of the battery (E) is connected to the negative side (cathode) of a diode (n-type) and negative
(-) of the battery connected to the positive of the diode (anode)/p-type, voltage connected in this direction is called
reverse bias.
in this connection, the free electrons in the n-region are attracted away from the p-n junction by the positive force
(+) of the battery (E) and the holes in the p-region are also attracted away from the p-n junction by the negative (-
) terminal of the battery E and there are practically no holes or electrons carriers left in the neighborhood of junction
and the current flow stops completely.

NB. Depletion region. Is the region adjacent to the p-n junction containing no majority carriers?
Barrier potential. Amount of voltage required to move electrons through the depletion region of a p-n junction
diode.
Zero bias. When the voltage across the diode is zero (0V).
Forward voltage. Is the voltage across a diode that occurs when it is conducting in the forward direction?
NB. DO NOT connect diode to voltage (battery) as shown in the diagrams (biasing of diodes) above as it is not
the case in practice. To connect diode to DC voltage supply line, refer to diode’s installation in circuit board
(discuss later).
NB. Diodes are connected in voltage lines with respect to Ground. Therefore, do not connect diode on ground
(GND/0V).

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 TYPES OF DIODES.
Diodes are of many types but there four common types of diodes that technicians normally come across. These
includes power diode (rectifier diodes), Zener diodes, LEDs (light emitting diodes) and signal diodes. Other types
of diodes include photodiode, Schottky diode, photovoltaic diode, varicap or varactor etc.
LIGHT EMITTING DIODE (LED).

LED also called junction diode converts electric current into light.ie when this diode is forward biased, current flow
through the junction and the diode produce light. Like any other diodes, it conducts current in only one direction.
LED have lately found a lot of uses besides being use as visual indicators on electronic equipment, they are also
being used for lighting purposes.eg the LED lamps (DC led bulb), flash lights, torch etc.
Symbol for LED.

Diagram and parts.

The shorter pin indicates the cathode (-) pin while the long pin indicates the anode (+) pin.
You can identify the cathode pin by looking at the bottom part. The side with the flat side indicates the negative
side of the LED.
You can also look carefully into the inside through the LED body, the bigger part is the cathode.

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 SMD LEDs.

SMD LEDs perform the same/similar functions like the through-hole LED but differs in their construction in that,
they use surface mount technology in their construction.
SMD LED cathode identification.

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Note that the cathode is normally marked or indicated by a color spot on the LED.
Please check circuit markings (diagram above).

LEDs CONNECTION IN CIRCUITS.

Simple LED series circuit.

To connect LED in circuit, use the formula below.

Where, R is LED resistor (current limiting resistor), VS is the input voltage (supply voltage), VLED is LED rated
voltage and ILED is LED rated current. The LED voltage and current are usually provided by the manufacturers.
The formula will help a technician to find the right value for the resistor R (LED resistor).
Simple led circuit resistor power rating calculation.
It is necessary to calculate the power rating of the LED resistor because this value is required to ensure that
resistor will not over heat when it is connected in series with the LED. To calculate the power rating of the LED
resistor, use the formula below.
Power (P) = 𝑰𝑳𝑬𝑫𝟐 x R value (from p=IV = 𝑰𝟐 x R)
Where, R is LED resistor value, ILED is LED rated current.
NB. 1. It is recommended to double the power rating of resistor that you have calculated.
2. It is too difficult to find the exact power rating resistor that you have calculated. Therefore, you can select
a higher value of power rating.
3. Sometimes the calculated resistor value may not be available. Therefore, select the next higher value of
resistor.eg for 218Ω select 220 Ω.

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 SERIES CONNECTION.

=
When connecting LEDs in series, one resistor is used as a current limiter for all the LEDs.

To apply voltage to the combination, the technician must know the max-rated voltage for the LEDs.

Use the formula below to find the value of the resistor R (LED resistor).

𝑽𝑺−(𝑽𝑳𝑬𝑫 𝑿 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝑳𝑬𝑫𝑺)

Resistor (R) = . This is series circuit, so voltage is additive.
𝑰𝑳𝑬𝑫

Where, VS is supply voltage, VLED is LED rated voltage, ILED is LEDs rated current and R is LED resistor.

LED resistor power rating.

Power (P) = 𝑰𝑳𝑬𝑫𝟐 x R value.

Current draw = ILED. (This is series circuit, so currents are the same).

PARALLEL CONNECTION.
There are two ways of connecting LEDs in parallel.
(i). using a single (common resistor) current limiting resistor for all the LEDs.

Resistor R value for LEDs in parallel can be calculated by the formula.

𝑽𝑺−𝑽𝑳𝑬𝑫
Common resistor R = .
𝑰𝑳𝑬𝑫 𝑿 𝑵𝑶.𝒐𝒇 𝑳𝑬𝑫𝒔

Where, R is the common resistor, VS is the supply voltage, VLED is LEDs rated voltage ILED is current rating for
the LEDs.

Resistor power rating =𝑰𝑳𝑬𝑫𝟐 x R value.

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Total current draw = ILED x No. of LEDS.

(ii). each LED having its own current limiting resistor.

LEDs in parallel with separate resistor.

𝑽𝑺−𝑽𝑳𝑬𝑫
Value of resistor R = .
𝑰𝑳𝑬𝑫
This is a parallel circuit but we are finding the value of resistor for each section (LED) NOT for whole circuit. So, in
each section, the circuit becomes in series position as discussed before. Refer to simple LED series circuit. You
will find that these are the same.
Resistor power rating = 𝑰𝑳𝑬𝑫𝟐 x R value.
Total current draw = ILED x No. of LEDs. (this is parallel circuit, so currents are additive).

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 INFRARED LEDs (IR LEDs).

IR LEDs are types of LED that emits light that is invisible to the human eye. They find their application/uses in
remote control and security sensors.

When you press any key/button in your TV remote, this LED (IR LED) sends infrared signals to the IR receiver
(remote receiver) on the television board before it is transferred to the micro-processor for interpretation.

NB. You can use your mobile phone camera to see if the IR LED emits light when remote button is press.ie place
the remote IR LED facing your mobile phone back camera and see through the phone screen. If the LED emits on
pressing any button, it means that button is working. Otherwise, the remote needs servicing.
NB. These IR LEDs and the normal LEDs are similar in their appearance. They also share the same symbol.

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 IR led receiver pins configurations.

Testing infrared LED.

The IR LEDs are tested just like when testing ordinary LED but the light they emit is invisible to human eye. Actually,
you use your mobile phone camera to see if the IR LED works when forward biased. (see through the camera).

ZENER DIODES.

Zener diodes differs from other types of diodes such as power diodes in a way that, Zener diodes operates under
reverse bias condition and break down on arrival of certain voltage (voltage higher than the Zener voltage).
Zener diodes are usually connected in circuit to produce a stable reference voltage (voltage regulation). They
usually have the Zener voltage written on their body.eg 3V9 =3.9V.
Zener diodes are usually identified in circuit board by their circuit symbols, working voltage written on their body
(Zener voltage) or their circuit reference usually ZD.
Symbol.

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 Actual diagram (glassy zener diode).

The diagram below shows how you can connect Zener diode in circuit with the purpose of using it as voltage
regulator to provide constant voltage (stable voltage).

In the circuit shown above, the Zener diode is connected in reverse bias.ie cathode is connected to the positive
voltage rail through the current limiting resistor R and anode is connected to the negative rail.
The resistor R has the function of providing a voltage drop that varies with the load current.
If the Zener diode (ZD) is rated 12VDC (break down voltage) with a maximum current of 150mA and 10W power
dissipation, there must be at least 12V DC across the Zener diode to provide current through ZD so that it can
operate in its break down mode.
NB. Voltage in excess of the Zener voltage of the Zener diode will be shorted to ground and not appear on the
positive rail (+V). In other words, if the voltage from the supply increases above the Zener voltage, the Zener diode
will conduct and voltage (+V) will be shorted to GND (short circuit). This will cause the device/circuit to shut down.
NB. When replacing a Zener diode, be sure to use the exact Zener in terms of voltage rating and power rating.
NB. If you see a diode in circuit and you are not sure if it is a Zener diode, confirm by checking if the anode is on
the GND/0V and the cathode is on the supply rail (+V). If this is the case, then it is a Zener diode. Otherwise it is
not.
When troubleshooting a circuit and you measure a short circuit between the supply line and the GND/0V, besides
suspecting an IC, transistors, capacitors, MOV, rectifiers etc. first scan around the circuit, if there is a Zener diode
in the circuit because a dead Zener diode usually reads short/continuity when faulty (dead).

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 Examples of Zener diodes.
1W Zener diodes.

RECTIFIER DIODES (POWER DIODES).

Diodes possesses unidirectional x-tics and presents a high resistance in one direction and low resistance in the
other direction. This makes a diode to work as rectifier irrespective of its type.
Symbol.

Actual diagram.

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They are sometimes represented by D or VD in circuit boards.
Power diodes also known as rectifier diodes do not behave in the same way when connected to AC as when
connected in DC supply line.
Diode (power diode) conduct current in only one direction but only when it is in forward bias this advantage makes
it useful in reverse polarity protection.ie protecting circuit that uses DC voltage source from wrong polarity
connection.eg if you have a woofer that uses DCV for its operation, you can protect it from wrong polarity
connection that may kill your woofer by connecting a power diode in series with the supply line (+V) as shown
below.

The diode (D1) will conduct when its anode is connected to the positive potential of the voltage source and the
other wire connected to negative potential of the voltage source.
When the connection is reversed/switched, the diode D1 will be in reverse bias hence it will NOT conduct current
thus protecting the woofer from wrong polarity connection.
NB. Diode only function as a rectifier when connected in series to AC voltage supply line.
When one end of a diode is connected to AC supply line in forward bias, the diode will block/restrict the AC but
will produce pulsating DC at its other end.
This process/behavior of a diode to block AC but produce pulsating DC is called rectification and the diode is
called the rectifier.
Therefore, we can define rectification as the process of converting AC into pulsating DC.
Rectifier will now be defined as device that converts AC into pulsating DC. A rectifier has to be a device with
unidirectional x-tics.

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In the diagram above, the AC (L) is connected to the anode of diode D1. Therefore, the diode will only conduct
during the positive half cycle of the AC (forward bias) but will restrict the AC.ie it will convert the AC into pulsating
DC. The pulsating DC will appear at the cathode of the diode D1.
In the diagram above, the anode of the diode is connected to the AC supply line L meaning a positive pulsating
DC voltage will be produced at the cathode with respect to the neutral line N which acts line the ground/GND (0V).
To get a negative voltage (-V), the cathode of the diode D1 should be connected to the AC supply line as shown
below. In this case the diode will only conduct during negative half cycle of the AC.

The pulsating DC can be smoothened by connecting a filter element (capacitor or LC circuit) across the pulsating
output line and the GND/0V.
One or more p-n junction diodes can be used as rectifier for rectification.
When one diode is used, it is called half wave rectification.
When two diodes are used, it is called full-wave rectification.
When four diodes are used, it is called full-wave bridge rectification.

Half wave rectification.

Positive half wave.

Diode D1 conducts during the positive half-cycle (alternation) of the AC signal.

Negative half-wave.

Diode D1 conduct only during negative half-cycle of the AC signal.

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NB.in half-wave rectification, only one rectifier diode is used to convert AC into pulsating DC.
In half-wave rectification, output frequency equals the input frequency.
There will be a gap in the wave form for half-wave rectification. To bridge this gap, two diodes are used as shown
below and it will be called full-wave rectification.
Full-wave rectification.

Converts during both positive and negative half cycles of the AC signal.
Objective is to fill the gap in half wave rectification output wave form.
Output frequency of full-wave is twice the input frequency.

Full-wave bridge rectification.

Also converts during both positive and negative half cycle of the AC signal
NB. For complete description on rectification refer to power supplies notes.

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 BRIDGED RECTIFIER.

= = = =
Bridge rectifier consists of four diodes joined together. Other bridge rectifier comes in a single package and has
four leads.

Where, ~ is ac input pins.

DC+/dc+ is positive dc output voltage pin.
DC-/dc- is negative dc output voltage pin.
Examples of rectifier diodes.
Part number Current rating Voltage rating
1N4001 1A 50V
1N4004 1A 400V
1N4007 1A 1000V
1N5408 3A 1000V
1N5406 3A 600V
1N5404 3A 400V
RL207 2A 1000V

Examples SMD rectifier diodes.

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 SIGNAL DIODE.

Another type of diodes is the signal diode which are usually used in circuit for small signal processing of up to
100mA.eg 1N4148 (signal switching diode), 1N914 etc.
Symbol for signal diode.

Signal diodes looks similar to glassy Zener diodes. Therefore, you can differentiate them from the glassy Zener
diodes by confirming on the circuit board, they have reference that begins with letter D like ordinary diodes. Zener
diodes usually have their Zener voltage written on their body.eg 3V9 = 3.9V, 5C2 = 5.2V etc.
You can also check if the anode of the diode is connected to ground/GND (0V). If so it is a Zener diode. Otherwise
it is a signal diode.
Most of the glassy Zener diode has a blue ring/bar for the cathode indication.
You can also differentiate Zener diode from signal diode by checking the circuit symbols.
Circuit Symbol for Zener diode.

NB. When replacing signal diode in circuit, make sure to use the exact replacement.

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 OTHER TYPES OF DIODES.
FAST RECOVERY DIODE (FR DIODE).

The FR diodes also known as fast recovery rectifier diodes.eg FR157, FR107 etc. are special diodes used in high
frequency circuit like the switch mode power supply (SMPS) to convert AC signal into pulsating DC which is then
smoothen to pure DC by connecting filter elements like capacitors, LC circuit etc.
FR diodes share the same circuit symbol and reference as the ordinary (normal) diodes.
Symbol for FR diodes.

Circuit reference is VD or D.
NB. In CRT TV SMPS, when you find FR rectifier diode is bad, never replace with an ordinary diode used in
rectifying the AC because it will over heats and blow off.
Examples of FR diodes.
Part number Current rating Voltage rating
FR107 1A 1000V
UF4007 1A 1000V
FR153 1.5A 200V
FR154 1.5A 400V
FR207 2A 1000V
FR301 3A 50V
FR307 3A 1000V
FR302 3A 100V
FR303 3A 200V
FR304 3A 400V

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 SCHOTTKY DIODES.

Schottky diodes also known as hot carrier diodes are semi-conductor diodes with a lower forward voltage drops
than a standard diode and very fast switching action. The voltage for Schottky diodes is between 0.15V and 0.45V.
The lower the voltage drop means higher circuit efficiency. They often come in a dual package with the two diodes
cathode being common (joined together).
Parts.

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 Circuit symbol.

Schottky diodes can also be used in rectification application besides switching circuit.

Commonly encountered Schottky diodes includes the 1N58xx series rectifiers such as the 1N581x (1A) and
1N582x (3A) through-hole parts and the SS1x (1A) and SS3x (3A) surface mount parts.

Examples of through-hole Schottky diodes.

Examples of SMD Schottky diodes.

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 PHOTODIODE.

Photodiode also called light sensor/light detector/photo detector is a type of diode that receives light energy and
then converts it into electrical energy (voltage or current). They are p-n junction diodes that operates in reverse
bias condition.
Symbol.

Photo diode has a small transparent window that allows light to strike the p-n junction. This ionizes the covalent bonds
in the p-n junction and the hole and electron pair is generated. Photo current (reverse bias current) is then produced
due to thermal generation of electron-hole pairs in the depletion region. The reverse current increases with the light
intensity at the exposed p-n junction. When there is no incident light, reverse current is almost negligible and is called
the DARK CURRENT. An increase amount in light intensity produces an increase in the reverse current.
One of photo diode application is solar cell panel.

DIODES COMMON FAILURE MODES IN CIRCUITS.

The most common reasons for diode failures are excessive forward current and a large reverse voltage.
Diodes failure modes are; breakdown, leaky, open, short etc. but the two common failure modes that technicians
usually detect/find when troubleshooting circuits (electronic devices) are;
(i). open diode.
(ii). shorted diode.
Open diode.
Open diode always reads infinity both ways (either way) when testing with multi-meter set in diode test mode or
continuity test mode.
Diode usually fail ‘’open’’ due to over current (excessive forward current) that flow to the diode. An open diode does
not allow current to pass through it (no output voltage). Therefore, when you detect/test and find no output voltage
(zero voltage) from the diode, it is an evidence of an open diode.
NB. When a diode is connected to a supply (voltage source) with higher current than the diode rated current, the diode
heats too fast (too hot) and fail open. Therefore, make sure the diode current and voltage rating match or is higher
than the supply.

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 Shorted diode.
A shorted diode usually reads low resistance, zero ohm or continuity (buzzer) either way when testing with digital multi-
meter set in diode test mode or continuity mode.
In other words, it reads the same voltage drops in both direction (either way) when testing with digital multi-meter set
in diode test mode.
A diode usually fails ‘’short’’ due to large reverse voltage that exceeds the diode reverse voltage. When a diode in
power supply goes short circuit, large current can flow and obvious damage occurs such as blown fuses.
NB. Diodes can be damaged by high voltage/over voltage or current.
A shorted diode neither block the reverse flow of current nor convert AC into DC because it becomes a conductor and
behave like a wire when it shorted.
A shorted diode may also burn open if no other failure occurs.

EFFECTS OF A SHORTED DIODE IN CIRCUIT.

In this chapter, we are going to look at effects of a shorted diode in full-wave bridge rectifier circuit as it is the most
common type of rectifier being used in building electronic power supply.

In the diagram above, when point D is positive with respect to point B, assuming diode D1 is shorted.
Diode D3 conducts and current (-) flow to the load.

Diode D1 being shorted will behave like a wire, therefore, D1 will not convert the AC current into DC current. This AC
current will move to the load hence causing component which requires DC for its operation to fail in that line.eg filter
capacitor C1, load etc.

When D is negative with respect to point B. diode D4 and D2 will both conducts.

Diode D1 being shorted, the negative (-) AC current flow to the junction C. diode D2 being in forward bias with this
negative AC current will turn on and current will flow back to the transformer through the shorted diode rather than the
load. Hence VDC2 will be zero (0V) and large voltage will be directly applied to D2.

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The excessive forward current may force D2 to fail open. The transformer and the shorted diode are likely to burn/go
‘’open’’.

In circuit without a transformer, this will cause the fuse to blow.

NB. When you find a shorted diode in circuit, make sure you check every other component before replacing the shorted
diode.eg driver IC, transistors, coil, other diodes etc.

NB. Rectifier diode can fail in one of the following ways;

(i). become open circuit.

(ii). short circuit.

(iii). Leaky.

(iv). break down when under full load.

NB. Always replace a diode with the same or higher rating than the original specification.

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 DIODE TESTING.

Diode can be checked/tested for open circuit, short circuit and leaky except its breakdown in full operating voltage.
Diode break down when under full load means diode test okay with your digital multimeter but failed when a high
voltage flows through it.

LED TESTING WITH DMM AND VOLTAGE.

LED is very easier to test as it will glow/emit light when forward biased.

PROCEDURES FOR TESTING LED USING A DIGITAL MULTIMETER.

(i). Remove/disconnect the LED from circuit board.

(ii). connect the red test lead to VΩmA and black test lead to COM jack and then Set/switch the multimeter dial to
diode test mode. As shown below.

(iii). Connect the test leads of the multimeter across the diode terminals/electrodes.

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 Conclusions.

1. With red test lead connected to the anode of the LED and black test lead connected to the cathode of the
LED, the LED is said to be in forward bias. Therefore, the LED will glow (emit light). This means the LED is
OK (working). Otherwise it is dead (faulty/defective).
2. When the test leads of the multimeter are switch/reversed.ie red test lead connected to the cathode of the
LED and black test lead to the anode of the LED, the LED do not glow because it is said to be in reverse bias.

NB. When the diode does not glow/emit light when connected as in conclusion (1), the LED is said to be faulty and
must be replaced.

LED TESTING WITH VOLTAGE.

Things needed are. Voltage source (battery), current limiting resistor, connecting wires and LED.

LEDs are sometimes tested by connecting a voltage from a DC voltage source across the
terminals/leads/electrodes of the LED in forward bias condition.ie positive supply to the Anode and negative (0V)
supply to the cathode.

In this case, the voltage from the DC voltage source must NOT exceed the voltage rating of the LED.

NB. When testing LED with voltage, make sure even if the voltage of the DC supply source matches the voltage
rating of the LED, you DO NOT reverse the polarity of the connection. Otherwise the LED will short or go open
(damaged).

It is advisable that a current limiting resistor should be connected in series with the LED as shown above.

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TESTING ORDINARY/NORMAL DIODES (POWER DIODE/RECTIFIER DIODE, SIGNAL DIODE) using DMM.

(i). connect red test lead to VΩmA jack and black test lead to the COM jack.

(ii). Switch the multimeter dial to diode test mode position.

(iii). Identify the cathode and the anode part of the diode.

(iv). Connect the multimeter test leads across the terminals of the diode.

Conclusions.

1. A forward voltage drops in millivolt (mV) is displayed with the multimeter red test lead is connected to the
anode of the diode and the black test lead connected to the cathode of the diode (forward bias).

2. When the test leads are switched.ie red test lead connected to cathode and black test lead connected to the
anode (reverse bias), the multimeter reads infinity (infinite values) or open (‘’OL’’/’’1’’).

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If the multimeter displayed values as in 1 and 2, the diode is said to be OK (good).

NB. If the diode reads infinity both-ways or zero ohm (continuity) both ways, the diode is considered defective
(bad) and must be replaced.

If you see any crack on a diode, just replace the diode because it is an evidence of a bad diode.

NB. Multimeter will always display the forward voltage drops in millivolt (mV) but not in volt (V).

Conversion.

1V = 1000mV.

1mV = 1/1000V = 0.001V.

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 TESTING SCHOTTKY DIODES USING DMM.

Schottky diodes has lower forward voltage drops than the normal (ordinary) dioes.eg power diode and signal
diodes. For Schottky diodes, voltage drops are between 0.15V and 0.45V.

Apply the same procedures for testing ordinary diodes but in the case of Schottky diodes, in forward bias, a lower
forward voltage drops is displayed on the multimeter screen.eg 115mV, 150mV, 315mV etc. while in reverse bias
it will also reads infinity/open.

TESTING DUAL PACKAGE SCHOTTKY DIODES USING DMM.

(i). Switch the multimeter dial to diode test mode.

(ii). label the pins as 1, 2 and 3. Where pin2 is the common cathode pin for D1 and D2. Pin 1 and pin 2 is anode
of D1 and D2 respectively.

(iii). Test D1 and D2 separately as when testing a single diode.

Testing across D1.

Use pin 1 and pin 2.

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 Multimeter will read infinity (open/OL or 1) with red test lead of the multimeter connected to pin 1 and black test
lead to pin 2 (reverse bias) and reads forward voltage drops with red test lead connected to pin 2 and black test
lead to pin 1 (forward bias).

Testing across D2.

Use pin 2 and pin 3.

Multimeter will read infinity (open) with red test lead connected to pin 2 and black test lead to pin 3 (reverse bias)
and forward voltage drops with red test lead connected to pin 3 and black test lead to pin2 (forward bias).

NB. Testing across pin 1 and pin 3 the multimeter should read infinity both ways.

TESTING BRIDGE RECTIFIERS USING DMM.

Diagram A. Diagrams B.

Bridge rectifier is made of four diodes (as shown in diagram A) and it is integrated in one package as shown in
diagrams B.

To test the bridge rectifier, consider the diode arrangement in diagram A and relate to rectifiers in diagram B. the
diodes are tested separately as when testing a single power diode.

Procedures.

(i). Switch the multimeter dial to diode test mode position.

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(ii). Test the diodes separately.

a) To test diode D1, test across BA.

b) To test diode D2, test across BC.
c) To test diode D3, test across CD.
d) To test diode D4, test across AD.

In the above test (ii) a-d, the multimeter shows infinity (open/OL or 1) when the diode is in reverse bias and shows
forward voltage drops when it is in forward bias.

(iii). Test across AC and DB, in both cases, there should be NO reading in either way.ie the multimeter should
show infinity (open/OL or 1).

NB. If you find any reading across AC and DB, the bridge rectifier is defective and must be replaced.

NB. If in procedure (ii) a-d, any test reads short or open, the bridge rectifier is BAD and must be replaced.

TESTING ZENER DIODES USING DMM.

There is special meter for testing Zener diodes. DMM can only be used to test Zener diodes for open circuit and
short circuit.

Zener diodes also sometimes reads as normal diode when testing with DMM set in diode test mode position but
will show higher forward voltage drops than the normal diode.eg 680mV etc.

NB. A shorted Zener diodes usually reads low forward voltage drops near zero ohms (continuity) either way when
testing.

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 DIODES INSTALLATION IN CIRCUIT BOARD.

To install a diode in circuit boards, a technician must be able to interpret and identify the circuit symbols, reference
and signs that represents diodes on circuit boards.

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NB: Marked part is the cathode part.

Diodes must be installed in circuit board with the right polarity.ie forward bias except Zener diodes that operates
in reverse bias.

Technician must also be able to identify the cathode and anode parts of the actual diode and its circuit symbol so
as to installed diodes in the right way in circuit boards.

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NB. When installing diode in circuit, make sure its ratings is higher than the supply line voltage it is to be connected.

NB. Diode allow current to flow through it if it is connected in forward bias. In reverse bias, diode do not conduct
current through it. Therefore, when installing diode in circuit make sure it is connected in forward bias so that it
can allow current to flow through it.

To understand how to connect diodes in the right way (forward bias) in circuit boards, study the following diagrams
carefully.

DC CONNETIONS.

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 AC CONNECTIONS.

NB. When AC is connected to a diode such as power diodes, the input AC will be converted into pulsating DC.
This process is called rectification.

This will only happen if the connection is in forward bias. Otherwise, the diode will not conduct.

NB. When connecting diode in voltage lines, the polarity of both the diode and the voltage line must be considered.
When diode is wrongly connected (wrong polarity), there will be no flow of current. Wrong polarity connection may
also cause damage to the diode (failure).

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Reference;

Basic electronic 2nd edition. Sp sharma.

Grob’s basic electronic. 12th edition. Mitchel e. Schultz.

Basic electronic. Humphrey kimathi.

Electronic circuit fundamental. Mike tooley.

Electronic fundamental. Thomas L. Floyd. David L. buchla.

Component testing. Jestine yong.

Internet research (images downloaded from internet).

NB; These notes is made for training students on electronic device repair program under NERS-GULU. If found
return to NERS-GULU or contact +256 775 442 110.

Please where you find typing error, contact us.

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