Experiment 7: Relay-Driven LEDs
58 Chapter 2
FundAmentAls
Inside a relay
A relay contains a coil of wire wrapped around an iron core.
When electricity runs through the coil, the iron core exerts
a magnetic force, which pulls a lever, which pushes or pulls
a springy strip of metal, closing two contacts. So as long as
electricity runs through the coil, the relay is “energized” and
its contacts remain closed.
When the power stops passing through the coil, the relay
lets go and the springy strip of metal snaps back into its
original position, opening the contacts. (The exception to
this rule is a latching relay, which requires a second pulse
through a separate coil to flip it back to its original posi-
tion; but we won’t be using latching relays until later in the
book.)
Relays are categorized like switches. Thus, you have SPST
relays, DPST, SPDT, and so on.
Compare the schematics in Figure 2-58 with the schematics
of switches in Figure 2-38. The main difference is that the
relay has a coil that activates the switch. The switch is shown
in its “relaxed” mode, when no power flows through the coil.Figure 2-58. Various ways to show a relay in a schematic. Top
left: SPST. Top right and bottom left: SPDT. Bottom right:
DPDT. The styles at bottom-left and bottom-right will be used
in this book.The contacts are shown as little triangles. When there are
two poles instead of one, the coil activates both switches
simultaneously.
Most relays are nonpolarized, meaning that you can run
electricity through the coil in either direction, and the relay
doesn’t care. You should check the data sheet to make sure,
though. Some relay coils work on AC voltage, but almost
all low-voltage relays use direct current—a steady flow of
electricity, such as you would get from a battery. We’ll be
using DC relays in this book.
Relays suffer from the same limitations as switches: their
contacts will be eroded by sparking if you try to switch too
much voltage. It’s not worth saving a few dollars by using
a relay that is rated for less current or voltage than your
application requires. The relay will fail you when you need it
most, and may be inconvenient to replace.
Because there are so many different types of relays, read the
specifications carefully before you buy one. Look for these
basics:
Coil voltage
The voltage that the relay is supposed to receive when
you energize it.
Set voltage
The minimum voltage that the relay needs to close its
switch. This will be a bit less than the ideal coil voltage.
Operating current
The power consumption of the coil, usually in milliamps,
when the relay is energized. Sometimes the power is
expressed in milliwatts.
Switching capacity
The maximum amount of current that you can switch
with contacts inside the relay. Usually this is for a “resis-
tive load,” meaning a passive device such as light bulb.
When you use a relay to switch on a motor, the motor
takes a big initial surge of current before it gets up to
speed. In this case, you should choose a relay rated
for double the current that the motor draws when it is
running.