A relay is an electrically operated switch used to isolate one electrical circuit from another. In its simplest form, a relay consists of a coil used as an electromagnet to open and close switch contacts.
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Switch Contact Designations
Relay switch contacts can be “normally open” (NO) or “normally closed” (NC), that is, when the coil is at rest and not energised (no current flowing through it), the switch contacts are given the designation of being NO or NC. In an open circuit, no current flows, such as a wall light switch in your home in a position that the light is off. In a closed circuit, metal switch contacts touch each other to complete a circuit, and current flows, similar to turning a light switch to the “on” position.
Basic Single Pole Single Throw Relay
In the accompanying schematic diagram, points A and B connect to the coil. Points C and D connect to the switch. When a voltage is applied across the coil at points A and B, an electromagnetic field is created which attracts a lever in the switch, causing it to make or break contact in the circuit at points C and D (depending on the design). The switch contacts remain in this state until the voltage to the coil is removed. Whether the contact is made or broken depends on the design of the relay (NO or NC). Relays come in different switch configurations. The switches may have more than one “pole,” or switch contact. The diagram shows a “single pole single throw” configuration, referred to as SPST. This is similar to a wall light switch in your home. With a single “throw” of the switch, the circuit is closed. Other configurations include double pole single throw, double pole double throw, and even three or four sets of contacts.
Relays are especially valuable in circuits where a small voltage can control the on/off state of a separate circuit that uses high voltage or high amperage. For example, closing a switch that turns on a furnace or some other heavy-duty machine can be more safely done if only a low voltage is needed for the switch circuit to energise a relay, whose contacts then complete the circuit with much higher power requirements.
A reed switch is an integral part of a reed relay. Invented in 1936 at Bell Telephone Laboratories, a reed switch is a simple on/off switch whose metal contacts are closed by the presence of a magnetic field. Usually, reed switch contacts are NO until a magnetic field is applied to the coil. The metal switch contacts, or “reeds,” are specially shaped blades positioned close to each other but not touching. They're sealed inside a small glass tube to protect them from elements such as dust and dirt, ensuring that the contacts make a good connection every time. Reed switches are small in size, very lightweight, have a long life, are inexpensive and have a quick response time. They're used for many different types of electrical circuit control, from the communications field to children’s toys to automobiles. The famous Hammond organs use reed switches to activate the notes from the keyboard and foot pedals.
A reed relay is an electrical switch that incorporates a reed switch and a coil that creates a magnetic field for tripping the switch. The whole unit can be housed in a tiny case, which is usually moulded metal or plastic. Like any traditional relay, when voltage is applied to the coil, the reed switch contacts change position, either from open to closed or closed to open, depending on the design. Because the entire device is sealed in an enclosure, it is protected from the environment, and can be used in the most rugged environments. Reed relays also benefit from all the positive attributes of reed switches, namely compact in size, low cost, light weight, long life, high reliability and a quick response time. They come in different configurations, with various voltages needed to energise the coils, a different number and configurations of contacts, and the amount of voltage and current those contacts can handle. Reed relays are most noted for their use in telephone exchanges before the advancement of today’s solid state technology.