Relays may be an old technology, but they are still essential in electronics. A relay is basically just an electrically operated switch. Some are tiny and handle only small amounts of current at low voltages, whereas other types, called contactors, are huge (the size of a small refrigerator) and can safely deal with hundreds of amperes and thousands of volts. But regardless of size and power capacity, all relays and their close cousins the contactors use the same basic principle of operation.
This chapter describes various types ranging from low-current TTL-compatible reed relays to high-power types used to control AC. Techniques for controlling a relay from a low-voltage circuit are also covered, as well as some examples of how relays can be used in control and logic circuits.
Relays have been around since about 1830. In fact, they are probably one of the oldest types of electrical components (other than perhaps switches). Relay-based switching systems replaced human telephone switchboard operators in mid–20th century, and some early computers were built using relays, such as the Zuse Z3 (1941), the Atanasoff-Berry Computer (1942), and the IBM ASCC/Harvard Mark I (1944). Although they haven’t changed much in terms of operation, they have evolved into a myriad of types over the past 180 years.
Regardless of the actual internal physical arrangement, all electromechanical relays operate on the principle of electromagnetism as the force driving ...