Relays are switches that are turned on and off by a small electrical current.
Inside a relay is an electromagnet. When a small current energizes this electromagnet, it attracts an armature blade and closes contact points. Current that the relay is designed to switch on or off, can then flow across the points.
As long as the small switching current flows to the relay, the much larger current will flow through its contact points.
A relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of electrical amplifier.
These contacts can be either normally-open, normally-closed, or change-over contacts.
- Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive.
- Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive.
- Change-over contacts control two circuits: one normally-open contact and one normally-closed contact.
Operation
When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current is switched off, the armature is usually returned by a spring to its resting position. Latching relays exist that require operation of a second coil to reset the contact position.
By analogy with the functions of the original electromagnetic device, a solid-state relay operates a thyristor or other solid-state switching device with a transformer or light-emitting diode to trigger it.
Types of relay
- A latching relay is mechanically arranged so that the armature can rest in either of two positions. There are two coils that pull the armature in opposite directions, so the relay can be switched to one position or the other and then left in that state indefinitely. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. (Some common relays may be wired to electrically latch, which offers no power saving but does ensure that the relay returns to a known state during and after a power outage.)
- A reed relay has two, usually normally open, contacts inside a vacuum or inert gas filled glass tube. This protects the contacts against atmospheric corrosion. The two contacts are closed by magnetism from a coil around the glass tube, or a permanent magnet moved towards it.
- A mercury wetted relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less), or for high-speed counting and timing applications where the mercury eliminated contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment.
- A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. Such devices are often used for motor starters, and may be built up with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip to open contacts, or where a solder pot melts, releasing a spring to operate contacts.
- A Solid State Relay (SSR) is a solid state electronic component that behaves like a mechanical relay but does not have any moving components, increasing long-term reliability.
Applications
Relays are used:
- to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,
- to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),
- as oscillators, also called vibrators. The coil is wired in series with the normally-closed contacts. When a current is passed through the relay coil, the relay operates and opens the contacts that carry the supply current. This stops the current and causes the contacts to close again. The cycle repeats continuously, causing the relay to open and close rapidly. Vibrators are used to generate pulsed current.
- to generate sound. A vibrator, described above, creates a buzzing sound because of the rapid oscillation of the armature.
Source: CDX Global & Wikipedia - en.wikipedia.org
