Switches, Relays & Fuses

Why Switches, Relays & Fuses Matter

Every circuit needs a way to be turned on and off, and every circuit needs protection against excess current. Switches give you manual control, relays let a small signal control a large load, and fuses sacrifice themselves to save the rest of the circuit. These are among the first components you'll use in any real project.

Mechanical Switch Types

Switches are described by the number of poles (independent circuits they control) and throws (positions they can connect to). The naming convention uses abbreviations like SPST, SPDT, DPDT.

• •SPST (Single Pole, Single Throw) — the simplest on/off switch. One input, one output. Like a light switch.
• •SPDT (Single Pole, Double Throw) — one input, two outputs. Can select between two circuits. Used for A/B switching.
• •DPDT (Double Pole, Double Throw) — two independent SPDT switches in one package. Can reverse motor direction or switch stereo audio.
• •Momentary — returns to its default position when released (e.g., push-button). Can be Normally Open (NO) or Normally Closed (NC).
• •Latching (toggle) — stays in the position you set it to until you move it again.

Physical Switch Styles

• •Toggle switch — metal lever that flips between positions. Common in panel-mount applications.
• •Push-button (tactile) — small PCB-mount button, great for microcontroller inputs. Very cheap and compact.
• •Rocker switch — wide paddle that rocks back and forth. Common in power strips and appliances.
• •Slide switch — slides between positions. Often used for mode selection.
• •Rotary switch — knob that selects between multiple positions. Used for multi-way selection (e.g., range on a multimeter).
• •DIP switch — tiny switches in a row, mounted on PCBs for configuration. Each switch is an independent SPST.

Contact Bounce (Debouncing)

When a mechanical switch closes, the metal contacts don't make a clean single connection. They physically bounce for a few milliseconds, rapidly opening and closing. A human doesn't notice, but a microcontroller counting pulses will see dozens of false triggers.

• •Hardware debouncing — an RC filter (resistor + capacitor) on the switch output smooths the bounces. Typical values: 10 kΩ + 100 nF gives about 1 ms time constant.
• •Software debouncing — after detecting a state change, wait 10–50 ms before reading again. If the state is stable, accept it.
• •IC debouncing — dedicated chips like the MAX6816 handle debouncing cleanly.

Electromagnetic Relays

A relay is an electrically-operated switch. It uses a small current through a coil to create a magnetic field that pulls metal contacts together (or apart). This lets a low-voltage, low-current signal (e.g., 5 V from a microcontroller) control a high-voltage or high-current load (e.g., 230 V mains or a motor).

• •Coil side — the control circuit. Typical coil voltages: 5 V, 12 V, 24 V DC. Draws 30–80 mA.
• •Contact side — the switched circuit. Contacts are rated for a maximum voltage and current (e.g., 250 VAC / 10 A).
• •NO contact — open when relay is de-energised, closes when coil is powered.
• •NC contact — closed when relay is de-energised, opens when coil is powered.
• •Changeover (SPDT) relay — has both NO and NC contacts plus a common terminal.

Flyback Diode Protection

A relay coil is an inductor. When you suddenly cut power to it, the collapsing magnetic field generates a large voltage spike (Lenz's Law) that can destroy your transistor or microcontroller pin. A flyback diode (also called snubber diode or freewheeling diode) placed across the coil in reverse absorbs this spike.

Solid-State Relays (SSR)

A solid-state relay uses semiconductor switching (typically a TRIAC for AC or a MOSFET for DC) instead of mechanical contacts. SSRs have no moving parts, so they don't suffer from contact bounce, are silent, switch much faster, and last longer. However, they have a small voltage drop when on, generate heat under load, and can't handle inrush currents as well as mechanical relays.

Fuses

A fuse is a sacrificial component designed to break the circuit when current exceeds a safe level. Inside the fuse is a thin wire or metal strip that melts when too much current flows, permanently opening the circuit and protecting the rest of the system from damage or fire.

• •Fast-blow — melts quickly when rated current is exceeded. Used for sensitive electronics.
• •Slow-blow (time-delay) — tolerates brief surges (like motor starting current) but blows on sustained overcurrent. Used for inductive loads.
• •Key ratings: Current rating (e.g., 2 A), voltage rating (e.g., 250 V), and breaking capacity (maximum fault current it can safely interrupt).
• •Glass cartridge fuses — transparent body, easy to visually check. Common sizes: 5×20 mm (metric) and ¼×1¼ inch (American).
• •Blade fuses — colour-coded, used in automotive. Each colour = a current rating.
• •PCB fuses — small through-hole or SMD fuses soldered directly onto circuit boards.

Resettable Fuses (PTC Thermistors)

A PTC (Positive Temperature Coefficient) resettable fuse — often called a polyfuse or PolySwitch — is a polymer device whose resistance skyrockets when it heats up past a threshold. When overcurrent flows, it heats itself, resistance jumps to thousands of ohms, and current drops to a trickle. Once the fault is removed and it cools down, it resets automatically. Great for USB ports, battery protection, and situations where you don't want to physically replace a fuse.

Circuit Breakers

A circuit breaker performs the same function as a fuse but can be reset (flipped back on) instead of replaced. The two main types are thermal (bimetallic strip bends from heat) and magnetic (electromagnet trips a latch on high current). Most home electrical panels use thermal-magnetic breakers that combine both mechanisms. Circuit breakers are more expensive than fuses but more practical for high-power systems.

Choosing the Right Protection

• •For hobby PCBs — PTC resettable fuse (convenient, reusable) or small glass fuse.
• •For power supplies — Fuse rated slightly above the expected maximum current (e.g., if circuit draws 1.5 A max, use a 2 A fuse).
• •For motors — slow-blow fuse to handle startup surge.
• •For mains wiring — circuit breaker in the electrical panel.
• •For sensitive equipment — fuse + TVS diode for overvoltage protection.