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. It was invented by Joseph Henry in 1835. 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.
A relay has two components. One is the controlling the part and the other the contolled part. The controlling part is a coil which creates a magnetic field. This coil requires a low voltage (around 5-12V) and a low current (around 50mA) (The actual ratings are given on the relay). The controlling coil has two terminals. Generally there is no polarity for the controlling coil, but on some relays there might be polarity for which you have to be careful. Due to the field generated by this coil, the controlled part which is made up of magnetic material is activated (by attraction to this magnetic field). The controlled circuit generally has a high voltage and current rating (which is again given on the relay).
Since relays are switches, the terminology applied to switches is also applied to relays. According to this classification, relays can be of the following types:
In Double Throw relays (Single Pole or Double Pole), there are two poles (A,B in figure). The common terminal (C) of the relay by default (when relay is not powered) is connected to one of these terminals (A). This is called Normally-Connected pole. The other pole (B) is called Normally-Open pole. You can always use a Double Throw relay as a Single Throw relay. Just don't connect one of the A or B.
Relays are used:
How to Use
A method of using a relay to drive a motor is shown. It uses a SPST relay. When pt. A is given logic high, the relay switches on and current flows through the motor and it runs. This is an application of using relay to drive a high current circuit from a low-current signal. Although the control signal is low-current, you still need a transistor or a buffer to drive the relay coil, or else get a relay with low driving current.
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