Contact configurations
| Code | Name | Description |
|---|---|---|
| SPST | Single pole, single throw | Simple on/off — 2 terminals |
| SPDT | Single pole, double throw | Selects between two outputs — 3 terminals |
| DPST | Double pole, single throw | Two independent SPST ganged |
| DPDT | Double pole, double throw | Two independent SPDT ganged — 6 terminals |
| 3PDT / 4PDT | Triple / quad pole, double throw | Multi-circuit — guitar pedals, audio routing |
Switch / relay families
| Type | Typical use | Speed | Notes |
|---|---|---|---|
| Toggle switch | Panel mount | Human speed | Mechanical |
| Rocker switch | Panel mount | Human speed | |
| Push button (momentary) | Panel mount / keypad | Human speed | Normally open or normally closed |
| Tactile (SMD) | PCB / keypad | Human speed | Typically SPST-NO |
| DIP switch | PCB config | Static | 2–12 banks |
| Rotary switch | Panel mount | Human speed | Up to 12 positions common |
| Electromechanical relay | Switch mains loads | 5–20 ms | Reliable, audible click |
| Reed relay | Low-current signal | 0.5–1 ms | Hermetically sealed contacts |
| Solid-state relay (SSR) | AC / DC switching | µs | No moving parts, leakage current |
| MOSFET / BJT | PCB-level switching | ns–µs | Used as electronic switch |
| MEMS relay | Low-power signal | µs | Emerging, tiny |
Relay driver checklist
- Flyback diode across coil — mandatory. Without it, kickback destroys the driver.
- Coil current: size the transistor for steady coil current + drop in R_DS(on).
- Inrush: resistive and inductive loads have large initial current.
- Arc suppression (snubber, varistor): extends contact life when switching inductive loads.
- Derate for DC: AC ratings are higher than DC — the zero crossing helps quench the arc.
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