Switches and Fuses
Switches and fuses are among the most fundamental components in any electrical system. A switch controls the flow of current; a fuse protects the circuit if that current exceeds a safe level. Understanding how to choose and use both is essential for anyone building or maintaining electronic equipment.
Switch Basics
A switch is a device that can open or close an electrical circuit. In the open position, no current can flow. In the closed position, current flows through the switch contacts with minimal resistance. All switches have a contact resistance — typically a few milliohms for quality switches — and a maximum current and voltage they can safely interrupt.
Common switch types: SPST toggle (simple on/off), SPDT toggle (changeover), DPDT toggle (dual changeover), and a multi-position rotary switch for range or function selection.
View LargerSwitch Types and Configurations
Switches are classified by the number of poles (separate circuits switched simultaneously) and throws (positions each pole can connect to), using the same SP/DP/ST/DT notation as relays.
Toggle Switches
A toggle switch uses a lever that snaps between two positions. SPST toggle switches are the simplest on/off switches found on power supplies and accessories. SPDT toggle switches route a signal to one of two destinations. DPDT toggle switches control two circuits simultaneously — useful for reversing motor direction or switching both channels of a stereo signal at once.
Momentary Switches
Momentary switches only conduct while held: release them and they spring back to open (normally open, NO) or closed (normally closed, NC). PTT (push-to-talk) switches on microphones are normally open momentary switches.
Rotary Switches
A rotary switch has a shaft that rotates to select between multiple positions. A 1-pole 6-position (1P6T) switch routes one input to any of six outputs. A 2-pole 6-position (2P6T) switch routes two inputs simultaneously, maintaining the pairing across all positions. Rotary switches are used in fan-speed controls, input selectors, attenuators, and test equipment range selectors.
Slide Switches
Slide switches move a contact bar laterally between positions. They are common on toys, torches and small electronic equipment.
DIP Switches
Dual in-line package (DIP) switches are arrays of miniature SPST switches in a single package with pin spacing matching IC sockets. They are used to configure equipment options and set device addresses without requiring circuit changes.
| Type | Action | Common Uses |
|---|---|---|
| SPST Toggle | On/off | Power switch, accessory on/off |
| SPDT Toggle | Changeover | Selecting between two inputs, changeover between supplies |
| DPDT Toggle | Dual changeover | Reversing a DC motor, switching stereo signals |
| Momentary NO | Closes while held | Doorbell, reset button, microcontroller input button |
| Rotary (multi-position) | Selects 1 of N | Fan speed, range selection on meters, input selectors |
Switch Ratings
Every switch has two key ratings that must not be exceeded.
Voltage rating — the maximum voltage the open contacts can withstand without arcing across the gap. Exceeding the voltage rating causes an arc that gradually destroys the contacts.
Current rating — the maximum current the closed contacts can carry continuously without overheating. The current rating is typically specified as a DC current rating and an AC current rating, which may differ. The current rating drops for higher voltages because arcing on opening is more severe.
Fuse Basics
A fuse is a sacrificial overcurrent protection device. It contains a metal element — typically a thin wire or strip — sized to melt when current exceeds a specified threshold. When the element melts, the circuit opens and current stops, protecting the wiring and components downstream from fire and damage.
A fuse protects the wiring between the fuse and the load, not the load itself. The fuse rating must be chosen to protect the wire: if the wire can safely carry 10 A, the fuse should be rated no higher than 10 A, regardless of what load is connected.
Fuse Types
Fast-Blow (Quick-Acting) Fuses
Fast-blow fuses open almost instantly when current exceeds their rating. They are used to protect sensitive components that can be damaged by even brief overcurrent events — such as rectifier diodes, transistors, and semiconductor circuits. A fast-blow fuse rated 1 A opens in milliseconds at twice its rating.
Slow-Blow (Time-Delay) Fuses
Slow-blow fuses tolerate brief current surges — such as the inrush current when a transformer or motor starts — without blowing, while still opening on sustained overcurrents. They are used in power supply primaries and motor circuits where a fast-blow fuse would nuisance-trip on start-up. The glass body of a slow-blow fuse typically contains a spring-loaded element visible inside.
Resettable Fuses (PTC Thermistors)
Positive temperature coefficient (PTC) thermistors act as automatic resettable fuses. Under normal current their resistance is low; when current causes them to heat above a threshold, their resistance rises dramatically, limiting current. When the fault is removed and the device cools, resistance returns to normal. PTCs are used in USB charging circuits, battery packs, and equipment where nuisance fuse replacement would be inconvenient.
Fuse Sizes and Holders
Common glass fuse sizes include 5×20 mm (metric standard) and 1/4×1-1/4 inch (AGC/AGX, common in North American equipment). Automotive blade fuses (ATO, mini, micro) are used in vehicles, solar installations and much 12 V equipment. Panel-mount fuse holders allow fuse replacement without opening the equipment.
Selecting the Right Fuse
Choosing the correct fuse requires three decisions: current rating, voltage rating, and blow characteristic.
Current rating: Select a fuse rated at 125–150% of the normal operating current. This provides a safety margin without nuisance blowing. If a device draws 20 A at full load, a 25 A fuse is appropriate. Do not install a larger fuse to stop nuisance blowing — investigate the root cause instead.
Voltage rating: The fuse voltage rating must equal or exceed the circuit voltage. Fuses have a minimum voltage rating for reliable arc interruption, and a maximum rating above which the arc cannot be extinguished safely.
Blow characteristic: Use fast-blow fuses for semiconductor protection and slow-blow fuses for transformer and motor primaries.
Circuit Breakers
A circuit breaker is a resettable overcurrent protection device that uses a bimetallic strip or electromagnetic mechanism to trip and open the circuit. After a fault is cleared, the breaker is reset manually by pressing a button or flipping the lever. Circuit breakers are common in household distribution panels, solar and marine installations, and in the positive lead from a battery to the equipment it supplies.
Thermal circuit breakers respond to sustained overcurrent via heat. Magnetic (hydraulic) circuit breakers respond to fast, high-magnitude faults. Thermal-magnetic breakers combine both mechanisms. For 12 V DC power distribution, thermal circuit breakers are most common; they double as a convenient power switch.
Practical Applications
12 V Power Wiring
The positive lead from a battery or power supply to the equipment should be fused as close to the battery (or supply) as possible. The fuse protects the wire against short circuits anywhere along its length. A load drawing 20 A, such as an inverter or a car audio amplifier, requires at least 10 AWG wire and a 25–30 A fuse at the battery.
Fused Distribution
Camper vans, boats, solar installations and workbenches often feed several 12 V loads from one battery or supply. A fused distribution block, with one blade fuse per branch circuit, protects each piece of equipment and its wiring separately. Anderson Powerpole and XT60 connectors are popular for making these connections.
Mains Switching and Protection
A mains-powered project needs a fuse in the live conductor where it enters the enclosure, and a double-pole switch rated for the voltage. Mains rocker switches with a built-in fuse holder and IEC inlet make this tidy and safe.
Hands-On: Verify Fuse Ratings with a Multimeter
Practice reading fuse markings and verifying fuse condition using a multimeter's continuity or resistance function.
- Assorted glass fuses (various ratings)
- One known-blown fuse
- Multimeter
- Fuse holder (optional, for in-circuit testing)
- Read the marking on each fuse. Identify the current rating (e.g., 1A, 5A) and, if present, the voltage rating and speed code (F=fast, T=slow).
- Set the multimeter to continuity or resistance mode. Touch the probes to each end of a known-good fuse. The multimeter should beep (continuity) or show near-zero resistance.
- Test the known-blown fuse. The multimeter should show open circuit (OL or no continuity beep).
- If you have a glass fuse, examine the element visually: a good fuse has an intact wire; a blown fast-blow fuse shows a clean break; a blown slow-blow fuse often shows a melted spring or carbon deposit.
- Record the resistance of three different-rated fuses. Notice that even good fuses have slightly different resistances based on wire gauge — lower-rated fuses use finer wire with slightly higher resistance.
Frequently Asked Questions
What does SPDT mean on a switch?
SPDT stands for Single Pole Double Throw. A single pole means one set of contacts; double throw means the common contact can connect to either of two output terminals. This gives you a changeover switch: in one position it connects COM to terminal A; in the other position it connects COM to terminal B. It is useful for selecting between two signal sources or two power supplies, for example.
What is the difference between a fast-blow and slow-blow fuse?
A fast-blow fuse opens almost immediately when current exceeds its rating, protecting components that are quickly damaged by overcurrent. A slow-blow fuse tolerates brief current surges, such as the inrush when a transformer energizes, before opening on sustained overloads. Using a fast-blow fuse where a slow-blow is required causes nuisance trips; using a slow-blow where fast protection is needed may allow component damage before the fuse acts.
Where should a fuse be placed in a DC circuit?
Place the fuse in the positive (ungrounded) conductor, as close to the power source as possible — ideally within 18 inches (45 cm) of the battery or power supply terminals. This minimizes the length of unfused wire that could carry fault current. Do not fuse the negative (ground) conductor.
Can I use a circuit breaker instead of a fuse?
Yes, for most 12 V and workbench applications a resettable circuit breaker is a practical alternative to a fuse. Breakers are convenient because they do not need replacement — you simply reset them after clearing the fault. They are common in power distribution panels. For very high fault currents or semiconductor protection, fuses may respond faster and provide better protection than thermal breakers.
Why does my fuse keep blowing even after I replace it?
A fuse blows because current exceeded its rating. If a replacement fuse blows too, there is a fault — a short circuit, a failed component drawing excess current, or an undersized fuse for the actual load. Do not install a larger fuse. Disconnect the load, test the wiring for shorts, and check each component in the circuit. Find and fix the fault before restoring power.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.