In the world of microcontrollers and semiconductors, one thing will never go away: The need to open or close a circuit. There are multiple types and formats of these electromechanical interfaces.
Voltage and Current Ratings
Almost all switches, relays, and solenoids have current ratings. Sometimes they are given in both AC and DC, sometimes they are only given in AC.
Almost all contactors have a lower DC rating than AC rating. This is because an AC circuit has a current null in the waveform, which means that when the contactor changes its connection, there is a lower chance of an arc as the circuit opens or closes. A DC circuit always has current flowing since there is no real waveform, so the connection will almost always arc to some amount. Every time an arc occurs, the contactor is damaged as it melts. The more power flowing through the connection, the stronger the arc, the more the contactor erodes.
It is not uncommon to see a toggle or rocker switch or relay rated as “125VAC / 10A … 12VDC / 5A”.
Switches
There are four primary types of switches: Toggle or Rocker, Pushbutton, Slide or DIP, and Rotary.
A Toggle or Rocker switch is like a light switch: It is either up, down, or in the middle. You can easily verify the position visually.
A Pushbutton switch is exactly as the name says: you push it, it makes contact.
Toggle/Rocker and Pushbutton switches can either be Latching or Momentary. A latching switch will stay where you put it. A Momentary switch will spring back. A Momentary function is usually denoted using (PARENTHESIS).
A Slide or DIP switch slides along a small linear track to open or close the circuit.
A Rotary switch rotates a contactor around the knob’s axis.
Poles and Throws
Switches and Relay contacts are classified using 2 attributes: How many Poles and Throw they have.
Poles: The number of contactors in the switch. This may also be referred to as “How many circuits are switched”, however that reference can be confusing when it comes to using multi-pole switches backwards. A better phrase might be “How many banks of circuits are switched”.
In rotary switches, this is commonly referred to as a “Deck”.
Throws: The number of terminals available to each contactor.
Common Switch Formats
 | SPST – Single Pole Single Throw 1 contactor with 1 terminal option OFF – ON OFF – (ON) |
 | SPDT – Single Pole Double Throw 1 contactor with 2 terminal options ON – OFF – ON ON – ON (ON) – OFF – ON (ON) – OFF – (ON) |
 | DPST – Double Pole Single Throw 2 contactors, each with 1 terminal option |
 | DPDT – Double Pole Double Throw 2 contactors, each with 2 terminal options |
Other Switch Formats
 | 3PDT – Three Pole Double Throw 3 contactors, each with 2 terminal options |
 | 4PDT – Four Pole Double Throw 4 contactors, each with 2 terminal options |
 | 1P8T – One Pole Eight Throw 1 contactor (“Deck”), each with 8 terminal options |
 | 2P8T – One Pole Eight Throw 2 contactor (“Deck”), each with 8 terminal options |
 | 3P8T – One Pole Eight Throw 3 contactor (“Deck”), each with 8 terminal options |
Relays
Relays are a type of switch which can be activated by an external circuit. Like switches, they are categorized by Poles and Throws. The most common formats are SPST, SPDT, and DPDT.
Relays operate by having an external circuit supply current into a coil of wire. This creates a magnetic field which pulls the contactor together. Nearly all 4- and 5-pin relays use the same “icecube” format. Pins 85 and 86 are connected to the coil, pin 30 is the common, and pin 87 is the Normally Open (closed when energized) contact. 5-pin relays add pin 87a, which is Normally Connected. The circuit breaks when energized as the contactor swings over to pin 87.
 | Single Pole Single Throw |
 | Single Pole Double Throw |
 | Double Pole Double Throw |