A relay can be compared to a vacuum tube
and solid state transistor amplifier because both can use a small voltage and
current in one
circuit to control a large voltage and current in another circuit. However,
unlike tubes and transistors, a relay cannot produce a variable output; It can
only switch on and off similar to a logic circuit in a computer. But the relay
has the advantage of being able to completely isolate its input circuit from its
a drawing of a Single Pole Double Throw (SPDT) relay. A single contact called
the Common contact switches between a Normally Closed contact and a Normally
Open contact. The Common contact is attached to an armature which is a strip of
metal that is hinged at one end and has electrical contacts, usually made of
silver, at the other end. A spring pulls the armature up so its contact connects
to the Normally Closed contact. An electromagnet below the armature attracts the
armature when it is energized and pulls it down so its contact disconnects from
the Normally Closed contact and connects to the Normally Open contact instead.
The electromagnet is made of a coil of
fine enameled copper wire wound on a plastic bobbin that has an iron core in its
center. A magnetic field is created in the iron core when current flows through
the coil. The number of turns in the coil and the wire size determine the
voltage that must applied to it. Relays are available for operation at
almost any voltage but the most common are 5, 6, 9, 12, 24 and 48 volts DC. The
coil current also depends on the number of turns and wire size and usually ranges from 30 to 200
There is no connection between the coil and any of the contacts. The contacts
are completely electrically isolated from the circuit that powers the coil.
a device using a relay where the relay contacts are brought out to a terminal
block or other termination, the contacts can be used to switch any type of DC or
that is within the voltage and current ratings of the contacts. This is usually
marked on the relay.
relay does not provide power for a load, it just switches power from a power source,
on and off, similar to how a wall switch turns a lamp on and off but does not
itself provide the power that lights the lamp.