Relays are electronically actuated switches which allow smaller voltage, low current sources to control large voltage, high current loads.
A relay can either be mechanical or solid state. Mechanical relays can handle high current loads, while solid state relays are capable of rapidly switching. The solid state component we will use is called a transistor.
Transistors must be oriented properly to operate (see below). Great care must be taken when using handling transistors, as they are very sensitive to static electricity. It is actually possible to damage a transistor by walking around carrying it by its 3 legs! Thankfully they are very inexpensive. Transistors are often used to trigger mechanical relays.
Mechanical relays are much more resilient, though they can still be damaged by too much voltage or current. Relays are rated by the amount of voltage needed to trigger their electromagnetic coil and the maximum current they can handle. They will also list the maximum voltage rating, and whether they are AC, DC, or both. The trigger voltage can be successfully approximated by 25% and still function properly. Adiode placed between the relay’s coil terminals is used to protect the source circuit from blowback voltage generated when the relay is switched off.
Diodes are polarized components which allow electrical current to flow through them in one direction. Use a 1N4004 or similar rectifier diode.
Mechanical relays come in a variety of different types based on their connections. Some common types include SPDT (Single Pole Dual Throw) and DPDT (Dual Pole Dual Throw). Unfortunately, these configurations are not standardized, so the pin layout will vary from model to model (see below for the relays demonstrated in class). Two pins will always control the relay’s Coil. Passing current from a small source through these pins will cause the relay to control the load. A Dual Throw component will have a shared pin (Common) and a NC (Normally Closed) and NO (Normally Open) pin which allow you to control the current in opposite ways.
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