A switch opens and closes a circuit by hand. A relay does the same electrically: a small current in a coil moves contacts that switch a different, possibly much bigger, circuit. A fuse (or circuit breaker) is a deliberate weak point that opens by itself when current is too high. Switches and relays control; fuses protect.
Switches
Click a switch to flip it. Pole = how many circuits. Throw = how many positions each can reach.
Describe a switch by its poles and throws: SPST is a plain on/off, SPDT picks between two paths (such as two antennas), DPDT switches two circuits at once. Switches are also momentary (spring-return, like a push-to-talk) or latching. Check the contact ratings: a switch rated for mains AC can fail quickly on a DC load of similar current, because a DC arc is harder to snuff out.
Relays
The coil is an electromagnet. Its small current closes a switch in a different circuit.
The coil is an electromagnet, so a relay isolates the control circuit from the switched one. That lets a microcontroller key a transmitter, or a front-panel button switch an antenna. Relays come in several forms:
General-purpose and power relays switch DC or AC supplies.
RF (coaxial) relays are built to switch RF with a controlled impedance, for transmit-receive changeover or antenna selection. An ordinary relay is not a substitute at VHF and above.
Latching relays hold their position with no coil current.
Solid-state relays use semiconductors instead of moving contacts: silent and fast, but with a voltage drop and different limits.
A coil stores energy in its magnetic field. When the current is cut, the field collapses and drives the voltage sharply the other way. A flyback diode across the coil, with its cathode toward the positive side so it does nothing while the coil is energised, gives that current a path.
Without the diode, the collapsing field drives the transistor's collector many times above the supply. Sketch only: not to scale.
Fuses and breakers
The wiring can take 5 A. A fuse must open before the wire overheats, so never use a larger fuse.
Fuse5A
A fuse protects the wiring, and by extension the equipment and the building. Its rating must be no higher than the current the thinnest wire in the circuit can safely carry; see Wire gauge and fusing for the numbers. It is wired in series with the hot (positive) lead, as close as practical to the source, so the whole length of wire is protected.
A fast-acting fuse opens quickly on an overload; a slow-blow (time-delay) type tolerates the brief surge when a motor or large capacitor starts.
A circuit breaker does the same job and can be reset. Use ones rated for the voltage and for DC if it is a DC circuit.
Replace a blown fuse with the same type and rating only after finding out why it blew. Never use a larger one, foil or a wire strap.