Electronic Components & Devices

Electronic components can be broadly classified into passive and active devices. Passive components (resistors, capacitors) do not amplify or switch signals. Active components (diodes, transistors, thyristors, ICs) can control or amplify current flow using semiconductor materials.

Classification Tree

• Passive Components
  • Resistors – Fixed (carbon composition, carbon film, wire-wound), Variable (potentiometer, rheostat, preset), Special (LDR, thermistor)
  • Capacitors – Non-polarised (polyester, mica, ceramic), Polarised (electrolytic, tantalum), Variable (air, trimmer/preset)
• Active Semiconductor Devices
  • Diodes – Junction/Rectifier diode, Zener diode, LED, Photodiode
  • Transistors – Bipolar (NPN, PNP), Field-Effect (FET)
  • Thyristors – SCR (Silicon Controlled Rectifier)
  • Triacs – Bidirectional thyristor for full-wave a.c. control
  • Diacs – Bidirectional trigger device
  • Integrated Circuits (ICs) – Digital ICs, Linear ICs
• Transducers
  • Input – LDR, photodiode, thermistor, microphone, PIR sensor
  • Output – LED, loudspeaker, relay

British Standard BS EN 60617 specifies the graphical symbols used to represent electronic components on circuit diagrams. Below are the standard symbols you must recognise.

BS 1852 Value Abbreviation Code

To avoid confusion with decimal points, the multiplier prefix replaces the decimal point:

Tolerance letters: F = ±1%, G = ±2%, J = ±5%, K = ±10%, M = ±20%. Example: 4k7G = 4.7 kΩ ± 2%.

Testing Resistors

Use an ohmmeter across the resistor leads. The measured value should be close to the preferred value and within the stated tolerance. If the resistor is in-circuit, disconnect one lead first to avoid parallel paths giving a false reading.

Small resistors are marked with coloured bands read left to right. Bands (a), (b), (c) give the value; band (d) gives the tolerance.

Worked Examples

Preferred Values (E-Series)

Mass-produced resistors use preferred value series to cover all possible values within each tolerance band.

Multiply by factors of 10 for higher values: 47, 470, 4.7k, 47k, 470k, 4.7M, etc.

Capacitor Behaviour

Connected to d.c., a capacitor charges and then blocks further current flow. Connected to a.c., it charges and discharges continuously at the supply frequency, appearing to pass current.

Coupling capacitor: Placed in series between circuits – blocks d.c., passes a.c. Decoupling capacitor: Placed in parallel – provides a low-reactance path for a.c., leaving only d.c. to pass to the load.

Types of Capacitor

Selecting a Capacitor

Three factors to consider: value, working voltage, and leakage current.

1,000 pF = 1 nF  |  1,000 nF = 1 µF

Capacitors in Circuits

Parallel: Total capacitance = C₁ + C₂ (like resistors in series – add up).

Series: 1/C_T = 1/C₁ + 1/C₂ (like resistors in parallel – reciprocal rule).

Modern electronic devices use the semiconductor properties of silicon or germanium. Pure semiconductor atoms have four valence electrons arranged in a crystal lattice structure, forming covalent bonds. No free electrons exist, so no conduction can take place without doping.

Doping – Creating P-type and N-type Materials

Semiconductor (Junction) Diode

A P–N junction that conducts when forward biased (anode positive w.r.t. cathode) and blocks when reverse biased.

Testing: Red (+) lead to anode, black (−) to cathode = low resistance. Reversed = high resistance. A cathode band (silver) identifies polarity.

Zener Diode

A special diode with a predetermined reverse breakdown voltage (the Zener voltage). Normally used in reverse bias. When the reverse voltage reaches the Zener voltage, the diode conducts and maintains a constant voltage across itself.

Manufactured in preferred values: 2.7, 4.7, 5.1, 6.2, 6.8, 9.1, 10, 11, 12 V, up to 200 V.

Light-Emitting Diode (LED)

A P–N junction that emits light when a current of about 10 mA flows through it. Forward voltage is approximately 2 V.

Series resistor calculation:

R = (V_supply − 2 V) ÷ 10 mA

Example: For a 12 V supply → R = (12 − 2) / 0.01 = 1,000 Ω (1 kΩ)

Available in red, yellow, green. Advantages over filament lamps: lower current, smaller, no heat, last indefinitely. Seven-segment LED displays are used in calculators, digital watches and instruments.

Light-Dependent Resistor (LDR)

Photodiode

A junction diode with a transparent window. Operates in reverse bias – leakage current increases proportionally to light intensity. Can only carry microamperes but operates much faster than an LDR.

Thermistor

A thermal resistor whose resistance varies with temperature. Very sensitive; the semiconductor bead can be made very small for measuring temperature in inaccessible places.

Two Basic Types

Bipolar Transistor Operation

The bipolar transistor consists of three layers of semiconductor: either N-P-N or P-N-P. The NPN type is more common.

Two current paths exist: the base–emitter path (control) and the collector–emitter path (main current). A small base current forward-biases the base–emitter junction, causing a much larger collector current to flow.

Testing Transistors

A transistor can be thought of as two diodes connected together. Use an ohmmeter:

First developed in the 1960s, ICs are miniature electronic circuits containing hundreds or thousands of transistors, resistors, diodes and capacitors, all on a single silicon chip no bigger than a baby's fingernail.

Two Broad Groups

IC Packaging

ICs use DIL (Dual In Line) packaging with two rows of pins. Pin 1 is identified by a dot pressed into the encapsulation; it is the pin to the left of the cutout notch.

The thyristor consists of four layers of semiconductor (P-N-P-N) with three terminals: Anode (A), Cathode (C) and Gate (G). The word comes from the Greek thyra meaning "door".

Operation

The thyristor is like a door – it can be open or shut. It is triggered to a conducting state by applying a pulse voltage to the gate. Once conducting, the gate loses all control. To turn it off, the anode–cathode voltage must be reduced to zero or reversed.

Testing a Thyristor

The Triac

Developed to overcome the limitation of the thyristor being a half-wave device. The triac is a single device containing a back-to-back, bidirectional thyristor that is triggered on both halves of each a.c. cycle by the same gate signal.

Terminals: MT1 (Main Terminal 1), MT2 (Main Terminal 2), G (Gate). The terms anode and cathode have no meaning for a triac.

A gate current of only 50 mA can trigger a triac switching up to 100 A. Power is reduced by triggering the gate later in the cycle.

The Diac

A two-terminal device containing a bidirectional Zener diode. Terminals: A1 and A2. It turns on when a predetermined voltage (~30 V) is reached, triggering on both positive and negative half-cycles.

Thyristor vs Triac Comparison

Half-Wave Rectification

A single diode allows only the positive half-cycle to pass. The output is pulsating d.c. The lamp lights at reduced brightness.

Full-Wave (Bridge) Rectification

Four diodes in a diamond-shaped bridge configuration use the full a.c. waveform. During the first half-cycle D1 and D3 conduct; during the second half-cycle D2 and D4 conduct. The lamp lights at full brightness.

Smoothing

Rectified output is too "bumpy" for electronic equipment (though suitable for battery charging). A large-value capacitor connected across the output smooths the waveform:

The capacitor charges on rising output, then discharges into the load as output falls. Full-wave rectification charges the capacitor twice as often, giving smaller ripple and better smoothing. Increasing capacitor value reduces ripple further.

Low-Pass Filter

A resistor–capacitor filter further reduces ripple by blocking the 100 Hz ripple frequency while passing d.c. (0 Hz). Replacing the resistor with a choke (inductor) reduces output resistance.

Stabilised Power Supply

A Zener diode connected across the smoothed output provides voltage regulation. The complete chain is: Transformer → Bridge Rectifier → Smoothing Capacitor → Zener Stabiliser → Stable D.C. Output.

Inverter

An inverter converts d.c. to a.c. – the reverse of a rectifier. Static inverters (using an oscillator, amplifier and transformer) are preferred over mechanical motor-generator sets as they have no moving parts.

Voltage Divider (Potential Divider)

Two resistors in series divide the input voltage proportionally:

V_OUT = V_IN × R₂ ÷ (R₁ + R₂)

A potentiometer gives continuously variable output. When using a voltmeter on voltage dividers, its resistance must be at least 10× the resistor value to avoid "loading" the circuit.

Security Lighting

External security lighting is activated by PIR (Passive Infra-Red) detectors that sense the thermal radiation of a moving body. PIRs use a split lens system covering up to 180° field of view to reduce false alarms. They commonly switch LED or tungsten halogen floodlights for instant illumination.

Intruder Alarm Systems

Proximity Switches

Moulded plastic units containing a reed switch (in the frame) and a magnet (on the door/window). When the door opens, the magnet moves away, the reed switch changes state and triggers the alarm.

Transducers – Telephones

A transducer converts one form of energy into another. A telephone uses:

Microphone: Sound waves → vibrating diaphragm → electrical signals. Loudspeaker: Electrical signals → magnetic field → coil movement → sound.