Electrical Systems and Power Supplies
Two Types of Electrical Power
Every electrical system needs a power supply. In engineering there are two fundamentally different types: alternating current (AC) and direct current (DC).
UK mains electricity is 230 V AC at 50 Hz. The voltage completes 50 cycles per second, producing a smooth sine wave. If you display mains AC on an oscilloscope you see a repeating wave; if you display battery DC you see a flat horizontal line. Mains AC is used for high-power fixed equipment (motors, heaters, industrial machinery). Batteries produce DC and are used for portable, low-power and safety-critical applications.
Mains AC: 230 V, sine-wave waveform, high power, fixed location, not portable. Battery DC: typically 1.5–12 V, flat-line waveform, lower power, portable, finite energy store. Electronic circuits (microcontrollers, sensors, logic gates) require DC — a power supply unit (PSU) converts mains AC to regulated DC.
Transformers
A transformer changes the voltage of an AC supply. It consists of a primary coil (input) and a secondary coil (output) wound around a shared iron core. When AC flows in the primary coil, a changing magnetic field is created in the core; this induces a voltage in the secondary coil. Transformers only work with AC — a DC current creates a static magnetic field that induces nothing.
The relationship between voltages and number of turns is: $$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$ where \(V_p\) is primary voltage, \(V_s\) is secondary voltage, \(N_p\) is number of primary turns and \(N_s\) is number of secondary turns.
Example: a transformer has 2000 primary turns and 100 secondary turns. If the primary is connected to 230 V AC: $$V_s = \frac{V_p \times N_s}{N_p} = \frac{230 \times 100}{2000} = 11.5\text{ V}$$ This is a step-down transformer (output lower than input), used to produce safe low-voltage DC from mains. Step-up transformers increase voltage (more secondary turns than primary) and are used in electricity grid transmission.
A rectifier converts AC into DC. A single diode produces half-wave rectification — it blocks every negative half-cycle, leaving only the positive pulses. A bridge rectifier uses four diodes arranged in a diamond so that both the positive and negative half-cycles contribute to the output (full-wave rectification). The output is pulsing DC, which is smoothed by a capacitor to produce a steady DC voltage.
In practice, a mains-powered PSU contains a step-down transformer, a bridge rectifier and a smoothing capacitor — this chain produces the stable DC that electronic circuits need. Many also include a voltage-regulator IC to hold the output at an exact value (e.g. 5 V for a microcontroller).
Input Control Devices: Switches
A switch is the simplest input control device. Push-to-make switches (normally open) complete the circuit when pressed; push-to-break switches (normally closed) interrupt the circuit when pressed. Toggle switches, rotary switches and limit switches (triggered by a physical stop on a machine) are all common in engineering applications. Switches are direct inputs — they require a human or a mechanical actuator to operate them.
The Relay — Electrically Controlled Switch
A relay is an electromagnetic switch. A small current in the control coil creates a magnetic field that attracts an iron armature, closing (or opening) a separate set of contacts. The relay allows a low-voltage, low-current control signal (e.g. 5 V from a microcontroller) to switch a high-voltage, high-current circuit (e.g. 230 V AC motor). The two circuits are electrically isolated — a relay is the safe interface between control electronics and mains-powered equipment.
A relay uses a small control current to switch a large load current, and keeps the two circuits electrically isolated. This protects sensitive control electronics (microcontrollers, sensors) from the high voltages and currents of mains-powered output devices like motors and heaters.
Output devices convert electrical energy into other forms of energy. You need to know four types:
- Electric motor — converts electrical energy into rotary mechanical energy. Used to drive pumps, conveyors, drills and fans. DC motors are common in battery-powered tools; AC motors power industrial machinery.
- Solenoid — an electromagnet with a moveable iron core (plunger). When energised, the plunger is pulled into the coil, producing linear mechanical movement. Used in door locks, valve actuators and automatic dispensing machines. A solenoid converts electrical energy to linear mechanical motion.
- Buzzer and bell — audible alarms that convert electrical energy to sound. A piezo buzzer uses a piezoelectric crystal that vibrates when current is applied. An electric bell uses an electromagnet and a striker on a spring.
- Lamp (light) — converts electrical energy to light and heat. Indicator lamps (signal lights on control panels) and LED assemblies are output devices. LEDs are much more energy-efficient than filament lamps.
Choosing a Power Supply
The choice between mains and battery depends on four factors: power requirement, portability, safety and cost. A mains supply can deliver unlimited power continuously but requires fixed wiring and is dangerous if mishandled. Batteries are self-contained and portable but have a finite energy store and must be replaced or recharged. For safety reasons, all control electronics (sensors, microcontrollers, logic circuits) should operate at low DC voltages (typically 3.3–12 V) — mains AC must be stepped down and rectified before it can power them.