Build Your Own Combat Robot

FIGURE 7-1
Typical automotive
surplus SPDT relay.
Chapter 7: Controlling Your Motors 129
Relays also comprise two kinds of contacts: normally open (NO) and normally
closed (NC) contacts. Normally open contacts (also known as Type A contacts)
do not allow power to flow until the relay coil is energized. Normally closed contacts
(also known as Type B contacts) allow power to flow when the relay is de-energized,
but they break the connection when the relay is energized. Both of these
types of relays are called single-throw (ST) relays. Many relays contain an NO and
an NC contact with one common wire (known as the COM contact) between
them so that the relay will make one contact and break another when it is energized.
This is known as a double-throw (DT) relay (also known as a Type C contact).
Most relays are either single- or double-pole relays, and each of these can be either
single- or double-throw relays. So relays are usually given a four-letter designation—the
first two letters are the number of poles, and the second two are the
number of throws. The SPDT relay shown in Figure 7-1 is a single-pole (circuit)
double-throw relay.
Figure 7-2 shows the schematic drawings of SPST, SPDT, and DPDT relays.
The dashed line between the two contacts in the DPDT relay shows that both contacts
move together, but they are not electrically connected to each other.
Current Ratings
When choosing relays to use in your robot, you should first look at and compare each
relay’s current and coil voltage rating. Relays will have a rating for the amount of
current their contacts are designed to switch. The current holding capacity of a relay
is much greater than its current switching capacity, and manufacturers usually
don’t bother giving a rating for the relay’s holding capacity.
When a relay breaks the circuit with a significant current flowing, a momentary
electrical arc will result between the relay contacts as they separate. The relay contacts