Page 435 - Robot Builder's Bonanza

404 COMMON ELECTRONIC COMPONENTS FOR ROBOTICS

NORMALLY OPEN, NORMALLY CLOSED
The contacts of a momentary switch can be either normally open or normally
closed. The “normally” has to do with the position of the contacts when the switch
is not being depressed.
Normally open
In a normally open (NO for short) switch, the circuit is normally broken.
Depressing the push button closes the contacts. It’s just the opposite with a nor-
mally closed (NC) switch: depressing the button opens the contacts.
Normally closed
Relays

Relays work like switches, but they are changeable under electronic control. Instead of a
human (or dog, or whatever) pressing a switch to change the contact settings, in a relay it’s all
done via electrical signals. Figure 31- 22 shows the basic construction of a relay and how it
works. As shown, the two basic parts of the relay are the coil and the contacts. In operation,
when the coil is energized, the magnetic field that is created activates the switch contacts.
Because relays are essentially switches, they are defined in the same way: the poles and
throws indicate the number of contacts. But unlike switches, most (but not all) relays are by
their nature momentary. The switch contacts in a relay are activated when the relay is ener-
gized. Remove the juice from the relay, and the spring- loaded contacts go back to their origi-
nal position.

COMMON RELAY TYPES
There are lots of different kinds and styles of relays out there, but most fall into one of the
following three types (see Figure 31- 23):

• The SPST (single- pole, single- throw) relay has four connections: two for the coil and two
for the switch contacts. The contacts may be normally open (the most common) or nor-
mally closed.
• The SPDT (single- pole, double- throw) relay has five connections: two for the coil, as usual,
and three for the switch contacts. The contacts are often labeled as Common, NC, and
NO. You wire your circuit to Common and either the NC or the NO connection.
• The DPDT (double- pole, double- throw) relays are an extension of the SPDT. They have a
separate set of pole contacts, for a total of eight connections.

Common

Coil not Figure 31- 22 Relays are electrically
energized
controlled switches. Energizing a coil with
Coil Contacts current activates the switch contacts. Many
relays have two contacts, in addition to a
common connection. These contacts are
Coil
energized marked normally open (NO) and normally
closed (NC). The notation refers to when
Relay the relay is not energized.

31-chapter-31.indd 404 4/21/11 11:56 AM