424 Electrical Power Systems Technology
atively inexpensive and small in size. Many small, general-purpose re-
lays are mounted in octal base (8-pin) plug-in sockets. Latching relays are
another type of relay. They are almost identical to the relays discussed
previously, but they have a latching mechanism that holds the contacts
in position after the power has been removed from the coil. A latching re-
lay usually has a special type of unlatching coil connected in series with
a pushbutton stop switch. Solid-state relays are used when improved re-
liability, or a rapid rate of operation, is necessary. Electromagnetic relays
will wear out after prolonged use, and have to be replaced periodically.
Solid-state relays, like other solid state devices, have a long life expectan-
cy. They are not sensitive to shock, vibration, dust, moisture, or corrosion.
Timing relays are used to turn a load device on or off after a specific period
of time.
The operation of a pneumatic timing relay is dependent upon the
movement of air within a chamber. Air movement is controlled by an ad-
justable orifice that controls the rate of air movement through the cham-
ber. The air-flow rate determines the rate of movement of a diaphragm or
piston assembly. This assembly is connected to the contacts of the relay.
Therefore, the orifice adjustment controls the air-flow rate, which deter-
mines the time from the activation of the relay until a load connected to
it is turned on or off. There are other types of timing relays, such as solid-
state, thermal, oil-filled, dashpot, and motor-driven timers. Timing relays
are useful for sequencing operations that require a time delay between
Figure 15-6. A solenoid