8.4 Electronic Components in Electrical Systems
Key Takeaways
A diode conducts in one direction only (about 0.7 V forward drop for silicon), and four diodes in a bridge rectifier convert a.c. into full-wave d.c.
A Zener diode is used in reverse breakdown to hold a fixed voltage, and an LED needs a series resistor or a driver to limit its current.
NTC thermistors lose resistance as temperature rises and are used as temperature sensors, while light-dependent resistors lose resistance as light increases and are used in photocells.
A thyristor conducts in one direction once triggered at its gate, a triac conducts in both directions, and a diac is the trigger device used with a triac in a dimmer.
Transistors act as electronic switches or amplifiers, and inverters convert d.c. to a.c. for PV systems, UPS units and variable speed drives.
8.4 Electronic Components in Electrical Systems
Unit 202 learning outcome 6 (4 of the 40 test questions) asks you to describe the function, applications and basic operating principles of the electronic components found in electrical systems. The range lists capacitors, resistors, rectifiers, diodes, Zener diodes, LEDs, photo devices, thermistors, diacs, triacs, transistors, thyristors and inverters, used in security alarms, telephones, dimmer switches, heating and boiler controls, motor control and wireless control systems.
Passive Components
Resistors
Resistors limit current and divide voltage. They may be fixed (carbon film, metal film or wire-wound for higher power) or variable (potentiometers and presets, such as the control knob of a dimmer). A resistor is specified by its resistance and its power rating in watts. Small resistors are marked with coloured bands: for example, brown (1), black (0) and red (× 100) gives (1 kΩ).
Capacitors
A capacitor stores electric charge on two plates separated by an insulating dielectric. Capacitance is measured in farads (in practice microfarads, nanofarads or picofarads), and the charge stored is : a 100 µF capacitor charged to 12 V holds C (1.2 mC).
A capacitor blocks steady d.c. but passes a.c. Uses include smoothing the output of power supplies, start and run capacitors for single-phase motors, power factor correction, suppressing interference, and timing circuits. Electrolytic capacitors are polarised and must be connected the right way round.
Warning
Capacitors can hold a dangerous charge after the supply is switched off. Discharge them safely, or wait for the manufacturer's stated time, before touching their terminals.
Semiconductor Devices
Diodes
A diode is a p-n junction that lets current flow in one direction only. When forward biased it conducts once the voltage across it exceeds about 0.6 to 0.7 V (for silicon); when reverse biased it blocks current until its breakdown voltage is reached. On a circuit symbol the arrow points in the direction of conventional current, from the anode to the cathode (marked by a bar).
Uses include rectification, protection against reversed polarity, and flywheel (suppression) diodes across relay and contactor coils, which absorb the back e.m.f. produced when the coil is switched off (Section 7.2).
Rectifiers
A rectifier converts a.c. into d.c.:
| Type | How it works | Output |
|---|---|---|
| Half-wave | One diode passes only the positive half-cycles | Pulsating d.c. with gaps; inefficient |
| Full-wave bridge | Four diodes route both half-cycles the same way through the load | Pulsating d.c. at twice the supply frequency, then smoothed by a capacitor |
Rectifiers are found in battery chargers, LED drivers, d.c. supplies for alarms and controls, and emergency lighting.
Zener diodes
A Zener diode is designed to operate in reverse breakdown at a precise voltage (for example 5.1 V or 12 V). With a series resistor it holds a steady voltage across a load, so it is used as a simple voltage regulator or reference and for overvoltage protection.
Light-emitting diodes (LEDs)
An LED gives out light when forward biased. Its forward voltage is roughly 2 V for red LEDs and about 3 V or more for white and blue ones. Its current must be limited by a series resistor or a constant-current driver.
Example: a 2 V, 20 mA indicator LED run from a 12 V supply needs a series resistor of (a 510 Ω preferred value would be chosen).
Photo devices
- A light-dependent resistor (LDR) has a resistance that falls as the light level rises. It is used in dusk-to-dawn photocell switches for external lighting.
- Photodiodes and phototransistors respond quickly to light and are used in light sensors and beam detectors.
- Photovoltaic cells generate electricity directly from light (Section 13.5).
Thermistors
A thermistor is a resistor whose resistance changes sharply with temperature:
- NTC (negative temperature coefficient): resistance falls as temperature rises. Used as temperature sensors in boilers, hot water cylinders and electronic thermostats, and to limit inrush current.
- PTC (positive temperature coefficient): resistance rises sharply above a set temperature. Used in motor winding protection and self-regulating heaters.
Transistors
A transistor has three terminals (base, collector and emitter for a bipolar transistor; gate, drain and source for a MOSFET). A small current or voltage at the base or gate controls a much larger current through the device, so a transistor can act as an electronic switch or as an amplifier. Transistors switch relays from low-power control circuits, amplify signals in telephones and door-entry systems, run alarm panels, and are the switching devices in trailing-edge dimmers and inverters.
Thyristors
A thyristor (silicon-controlled rectifier, SCR) has an anode, a cathode and a gate. It blocks current until a small pulse is applied to the gate; it then conducts in one direction until the current falls below its holding current, for example when an a.c. supply passes through zero. By delaying the gate pulse in each half-cycle, a thyristor controls the power delivered to d.c. motors, heaters and battery chargers, and it is used in soft starters.
Diacs
A diac is a two-terminal trigger device. It does not conduct until the voltage across it reaches its breakover voltage (typically around 30 V), in either direction, and then it conducts suddenly. Its main job is to deliver a sharp trigger pulse to the gate of a triac.
Triacs
A triac behaves like two thyristors connected back to back with a common gate, so it can be triggered to conduct in both directions. It is the power-control device in leading-edge dimmers, speed controllers for fans and small universal motors, heater controls and solid-state relays.
Inverters
An inverter converts d.c. into a.c. by switching transistors (such as IGBTs or MOSFETs) rapidly. Inverters connect PV panels and batteries to the a.c. supply, provide mains power in UPS units and central battery systems, and form the output stage of variable speed drives, which rectify the supply to d.c. and then invert it to a variable frequency to control motor speed.
Where You Will Meet These Components
| System (from the Unit 202 range) | Typical components | What they do |
|---|---|---|
| Security alarms | PIR detectors, transistors, microcontrollers, LEDs, rechargeable battery with a rectifier-based charger | Detect movement, process signals, drive sounders and keep the system running in a power cut |
| Telephones and door entry | Capacitors, diodes, transistors, integrated circuits | Amplify and route signals and protect against reversed polarity and surges |
| Dimmer switches | Diac and triac (leading edge), or MOSFET/IGBT (trailing edge), potentiometer and capacitor | Phase control: switch on part-way through each half-cycle to vary the power to the lamp |
| Heating and boiler controls | Thermistors, transistors, relays, microcontrollers | Sense temperatures and switch pumps, valves and burners |
| Motor control | Thyristors, triacs, rectifiers, inverters (variable speed drives), transistors | Soft starting, speed control, reversing and braking |
| Wireless control systems | Radio transmitter and receiver modules, transistors, batteries | Switch lighting, heating, blinds and alarms remotely |
Important
Electronic components can be destroyed by a 500 V insulation resistance test. Disconnect them, or use the alternative test method in Section 14.3, and always match dimmers and controls to the load they are switching (Section 11.1).
Which component allows current to flow in one direction only?
Capacitor
Resistor
Diode
Diac
In a traditional leading-edge dimmer switch, which pair of components controls the lamp current?
A diac triggering a triac
A Zener diode and an LED
A thermistor and a capacitor
A photocell and a relay
What happens to the resistance of an NTC thermistor as its temperature rises?
It rises
It stays constant
It becomes infinite
It falls
What is the main purpose of an inverter in a grid-connected solar PV system?
To store surplus energy from the panels so that it can be used at night
To convert the d.c. from the panels into a.c. at the supply voltage and frequency
To increase the supply frequency to 400 Hz so that smaller cables can be used throughout
To protect the panels and wiring from lightning strikes and voltage surges
An indicator LED with a forward voltage of 2 V is to run at 20 mA from a 12 V d.c. supply. What value of series resistor is needed?
100 Ω
600 Ω
500 Ω
240 Ω
Sections you finish are checked off in the contents.