The 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 a transistor
Many microcontrollers cannot drive a
relay directly and so a driver transistor is required. A HIGH on the base
of the transistor turns the transistor ON and this activates the relay.
The relay can be connected to any electrical device via the contacts.
The 10k resistor on the base of the transistor limits the current from the
microcontroller to that required by the transistor. The 10k between base
and the negative rail prevents noise on the base from activating the
relay. Thus only a clear signal from the microcontroller will activate 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 motors are activated as the commutator
noise from the motor can get back to the microcontroller via the supply
lines. The optocoupler drives a relay and the relay activates the motor.
The figure below shows the program needed to activate the relay, and
includes some of the already discussed macros.
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