7.5 Relay

The relay is an electromechanical device, which transforms an electrical signal into mechanical movement. It consists of a coil of insulated wire on a metal core, and a metal armature with one or more contacts. When a supply voltage was delivered to the coil, current would flow and a magnetic field would be produced that moves the armature to close one set of contacts and/or open another set. When power is removed from the relay, the magnetic flux in the coil collapses and produces a fairly high voltage in the opposite direction. This voltage can damage the driver transistor and thus a reverse-biased diode is connected across the coil to "short-out" the spike when it occurs.

Connecting a relay to the microcontroller via transistor

Since microcontroller cannot provide sufficient supply for a relay coil (approx. 100+mA is required; microcontroller pin can provide up to 25mA), a transistor is used for adjustment purposes, its collector circuit containing the relay coil. When a logical one is delivered to transistor base, transistor activates the relay, which then, using its contacts, connects other elements in the circuit. Purpose of the resistor at the transistor base is to keep a logical zero on base to prevent the relay from activating by mistake. This ensures that only a clean logical one on RA3 activates the relay.

Connecting the optocoupler and relay to a microcontroller

A relay can also be activated via an optocoupler which at the same time amplifies the current related to the output of the microcontroller and provides a high degree of isolation. High current optocouplers usually contain a 'Darlington' output transistor to provide high output current.

Connecting via an optocoupler is recommended especially for microcontroller applications, where relays are used fro starting high power load, such as motors or heaters, whose voltage instability can put the microcontroller at risk. In our example, when LED is activated on some of the output port pins, the relay is started. Below is the program needed to activate the relay, and includes some of the already discussed macros.


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