Switches, Connectors and Protection

Switches

A switch opens or closes a connection by hand. Two numbers name its action: the poles (how many separate circuits it switches) and the throws (how many positions each pole can connect to). SPST is single pole, single throw (on/off); SPDT is a changeover; DPDT switches two circuits at once.

Type	Use	Notes
Toggle, rocker, slide	Power on/off, mode selection	Stay in position; rocker switches often come with a lamp
Tactile (tact) button	User input to a microcontroller	Momentary; 12 V 50 mA is a typical rating; not for power
Push-button (panel)	Start, stop, emergency stop	Industrial types have N.O. and N.C. contacts and a latching or momentary action
Rotary / DIP switch	Settings, addresses	Rotary encoders turn into pulses; see Sensors
Limit / micro switch	Detecting position or a door	Lever or roller actuator; use the N.C. contact for fail-safe detection
Reed switch	Magnet detection	Sealed contacts in glass; no wear, limited current
Key switch, selector	Access control, mode	Panel mount; check the rating

Switch ratings and contact bounce

• Current and voltage rating. Every switch has a maximum current at a given voltage, often two figures: “3 A 250 V AC / 6 A 125 V AC”. The DC rating is usually much lower than the AC rating because a DC arc does not go out by itself; if only the AC rating is given, derate heavily for DC.
• Inductive and lamp loads (motors, solenoids, relays, capacitive supplies, incandescent lamps) make arcs and surges when switched. Derate to half or less of the rating, add a flyback diode or snubber, and consider a relay or MOSFET.
• Low-level (“dry circuit”) contacts. A big power switch may not make a reliable contact at a few milliamps. For signals, use a switch rated for small currents (gold-plated contacts).
• Contact bounce. When a mechanical contact closes, it bounces for 1–10 ms, producing a burst of on/off transitions. A microcontroller reads each one as a separate press.

Debouncing

• In software: ignore further changes for 20–50 ms after the first edge, or require the same reading several times in a row. This is free and usually enough.
• In hardware: an RC filter across the switch (for example 10 kΩ and 100 nF, τ = 1.0 ms) with a Schmitt-trigger input (74HC14) smooths the bounce.
• Interrupt inputs are especially sensitive to bounce and need one of the above.

Connectors

Connector	Pitch / size	Typical rating	Notes
Pin header / Dupont	2.54 mm	1 A per pin	Breadboards and hobby modules; no keying or lock, so mind the orientation
JST-PH	2.0 mm	2 A	Small LiPo batteries and sensors; polarized
JST-XH	2.5 mm	3 A	Keyed, secure; balance leads and boards
JST-SH / Qwiic / STEMMA QT	1.0 mm	1 A	I²C sensor cables
Molex KK	2.54 mm	3 A	Keyed, friction lock; widely used
Screw terminal	3.5, 5.0, 5.08 mm	8–16 A	No crimp tool; good for field wiring
XT30 / XT60 / XT90	—	15 A / 30 A / 40 A+	Hobby batteries and motors; polarized; avoid arcing on connect
Barrel jack	5.5 × 2.1 mm	2–5 A	Center-positive is the common convention, but check: there is no standard
USB-A / micro / USB-C	—	0.5–3 A (5 V); to 5 A with PD	Data and power; USB-C negotiates the voltage
Anderson Powerpole	—	15–45 A	Genderless; any pairing, so label polarity

• Polarity and keying. Choose connectors that cannot be plugged in backwards, or mark them clearly. A reversed power plug is a common way to destroy a board.
• Crimped, not soldered. A proper crimp is gas-tight and stronger than solder; use the crimp tool made for the terminal. Solder wicks up the wire and makes it brittle. See Soldering for wire and strain relief.
• Current rating is per contact and at a temperature rise; derate when many contacts carry current in a bundle or when the connector is warm.
• Locking. Use a latching connector where vibration or tugging is likely (motors, vehicles, robots).
• Wire gauge. Match the wire to the connector and to the current; see the AWG table in Soldering and System Integration for voltage drop.

Fuses and circuit breakers

A fuse is a deliberate weak link: a thin conductor that melts and opens the circuit when the current is too high for too long. It protects the wiring (so it does not overheat and start a fire) and limits the damage a fault can do.

Type	Behavior	Notes
Fast-blow (F)	Opens quickly on overload	Protects delicate electronics
Slow-blow / time-delay (T)	Tolerates a short surge (motor start, supply inrush)	For loads with inrush current
Blade (automotive)	32 V DC, 1–40 A	Standard in vehicles; color-coded by rating
Resettable polyfuse (PTC)	Resistance rises sharply when hot, resets when cool	Convenient for USB and small boards; slow and not for mains
Thermal fuse	Opens above a temperature, once	In heaters and transformers
Circuit breaker	Resettable switch that trips	Panels and larger supplies

• Choose the current: above the normal running current (typically 125–150 % of it, or higher for loads with inrush), and always below the safe current of the wire and connectors. The fuse protects the weakest part of the path.
• Choose the voltage: the fuse’s voltage rating must be at least the circuit voltage (DC ratings are lower than AC ratings).
• Place it near the source (the battery or supply), so the whole run of wire is protected.
• Interrupt rating: a fuse must be able to break the maximum fault current the supply can deliver (a car or lithium battery can supply hundreds of amps). Use a fuse with a suitable breaking capacity.
• Replace a fuse with the same rating, never with a bigger one or a wire. Find out why it blew first.

Reverse-polarity protection

Connecting a battery or supply backwards destroys polarized parts in an instant. Where it can happen, add protection.

Method	Loss	Comment
Series diode	0.3–0.7 V	Simplest. At 1 A the loss is 0.5 W for a 0.5 V drop: poor for a low-voltage battery
Bridge rectifier	1.4 V	Accepts either polarity (and AC) but wastes the most
P-channel MOSFET in the positive line (source toward the load; gate to ground through a resistor)	I² × RDS(on): 0.03 W at 1 A and 30 mΩ	The body diode conducts initially, then the gate turns the MOSFET on: almost no loss. Check the gate-source voltage limit (typically ±20 V)
Diode across the supply plus a fuse	None in normal use	A reversed supply blows the fuse; slower and needs replacing
Polarized connector	None	Prevents the mistake; always use as well

Surge, static and spike protection

• TVS diode across the supply or a data line clamps spikes (long cables, motors, hot-plugging, mains-borne surges). Pick a standoff voltage above the normal voltage and a clamp voltage below the maximum the circuit can survive.
• ESD protection on exposed connectors (USB, buttons, headers) with low-capacitance TVS arrays. Static discharges of several kilovolts are common on a dry day.
• Inrush limiting. A large capacitor charges with a big surge when power is first applied. An NTC thermistor in series, or a soft-start circuit, limits it.
• Series resistors (100 Ω to 1 kΩ) on I/O lines limit the current that a spike or a mistaken connection can push into a pin.
• Clamp diodes. Two diodes (one to the supply, one to ground) catch any input that goes beyond the rails.
• Flyback diode across every relay and motor coil (see Inductors, Transformers and Relays).

Good practice

• Label polarity and voltage at every power connector, and use red for positive and black for ground.
• Power switch and fuse first. The path from the source goes: connector, fuse, switch, protection, then the circuit.
• Star grounding. Bring noisy grounds (motors) back to the supply separately from quiet analog grounds, rather than daisy-chaining.
• Strain relief at every connector and joint, so the wire cannot be pulled out or flexed where it is soldered.
• Test before connecting a load. Measure the supply voltage and polarity with a meter before plugging in a board (see Test Tools and Fault Finding).
• Enclose mains wiring and keep it separate from low-voltage wiring. Follow the electrical code for any mains work.

Related

• Electronics Fundamentals: the overview of this guide.
• Batteries and Power Supplies: power supplies, batteries and regulators.
• Soldering: wire, crimping, connectors and strain relief.
• Workshop Safety: electrical safety and fire.

FAQ

Why does my button press register several times?

The contacts bounce when they close, making a burst of transitions for a few milliseconds. Debounce in software by ignoring changes for 20–50 ms, or add an RC filter and a Schmitt-trigger input.

Can I use an AC-rated switch on a DC circuit?

Only with heavy derating. DC arcs do not extinguish at a zero crossing, so a switch rated 10 A at 250 V AC may handle only a fraction of that at 24 V DC. Use the DC rating, or a switch made for DC.

Which fuse should I use?

Choose one above the normal current (125–150 %) but safely below the rating of the wire and connectors; check the voltage rating and the interrupt rating; and use slow-blow if the load has inrush.

Is a series diode good enough for reverse protection?

It works, but it drops 0.3–0.7 V and dissipates heat. For a battery-powered circuit where every volt counts, use a P-channel MOSFET instead; for a simple mains adapter input, a diode is fine.

What is the difference between a TVS and a varistor?

Both clamp surges. A TVS clamps faster and more precisely and recovers; a varistor (MOV) handles bigger energy and is used across mains inputs, but degrades with repeated surges.

References

• Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press.
• Manufacturer datasheets and selection guides: JST, Molex, Amass (XT connectors), Littelfuse and Bussmann (fuses), Omron and C&K (switches).
• UL 248 and IEC 60127, fuse standards; NFPA 70 (NEC) for wiring and overcurrent protection.
• Ganssle, J., A Guide to Debouncing, The Ganssle Group (contact bounce measurements and fixes).

Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. Follow the electrical code for any mains work.