Some relays are constructed with a kind of "shock absorber"
mechanism attached to the armature which prevents immediate, full motion
when the coil is either energized or de-energized. This addition gives the
relay the property of time-delay actuation. Time-delay relays can be
constructed to delay armature motion on coil energization, de-energization,
or both.
Time-delay relay contacts must be specified not only as either
normally-open or normally-closed, but whether the delay operates in the
direction of closing or in the direction of opening. The following is a
description of the four basic types of time-delay relay contacts.
First we have the normally-open, timed-closed (NOTC) contact. This type
of contact is normally open when the coil is unpowered (de-energized). The
contact is closed by the application of power to the relay coil, but only
after the coil has been continuously powered for the specified amount of
time. In other words, the direction of the contact's motion (either
to close or to open) is identical to a regular NO contact, but there is a
delay in closing direction. Because the delay occurs in the direction
of coil energization, this type of contact is alternatively known as a
normally-open, on-delay:
 The following is a timing diagram of this relay contact's
operation:
 Next we have the normally-open, timed-open (NOTO) contact.
Like the NOTC contact, this type of contact is normally open when the coil
is unpowered (de-energized), and closed by the application of power to the
relay coil. However, unlike the NOTC contact, the timing action occurs upon
de-energization of the coil rather than upon energization. Because
the delay occurs in the direction of coil de-energization, this type of
contact is alternatively known as a normally-open, off-delay:
 The following is a timing diagram of this relay contact's
operation:
 Next we have the normally-closed, timed-open (NCTO)
contact. This type of contact is normally closed when the coil is unpowered
(de-energized). The contact is opened with the application of power to the
relay coil, but only after the coil has been continuously powered for the
specified amount of time. In other words, the direction of the
contact's motion (either to close or to open) is identical to a regular NC
contact, but there is a delay in the opening direction. Because the
delay occurs in the direction of coil energization, this type of contact is
alternatively known as a normally-closed, on-delay:
 The following is a timing diagram of this relay contact's
operation:
 Finally we have the normally-closed, timed-closed (NCTC)
contact. Like the NCTO contact, this type of contact is normally closed when
the coil is unpowered (de-energized), and opened by the application of power
to the relay coil. However, unlike the NCTO contact, the timing action
occurs upon de-energization of the coil rather than upon energization.
Because the delay occurs in the direction of coil de-energization, this type
of contact is alternatively known as a normally-closed, off-delay:
 The following is a timing diagram of this relay contact's
operation:
 Time-delay relays are very important for use in industrial
control logic circuits. Some examples of their use include:
Flashing light control (time on, time off): two time-delay relays are
used in conjunction with one another to provide a constant-frequency
on/off pulsing of contacts for sending intermittent power to a lamp. Engine autostart control: Engines that are used to power emergency
generators are often equipped with "autostart" controls that allow for
automatic start-up if the main electric power fails. To properly start a
large engine, certain auxiliary devices must be started first and allowed
some brief time to stabilize (fuel pumps, pre-lubrication oil pumps)
before the engine's starter motor is energized. Time-delay relays help
sequence these events for proper start-up of the engine. Furnace safety purge control: Before a combustion-type furnace can be
safely lit, the air fan must be run for a specified amount of time to
"purge" the furnace chamber of any potentially flammable or explosive
vapors. A time-delay relay provides the furnace control logic with this
necessary time element. Motor soft-start delay control: Instead of starting large electric
motors by switching full power from a dead stop condition, reduced voltage
can be switched for a "softer" start and less inrush current. After a
prescribed time delay (provided by a time-delay relay), full power is
applied. Conveyor belt sequence delay: when multiple conveyor belts are
arranged to transport material, the conveyor belts must be started in
reverse sequence (the last one first and the first one last) so that
material doesn't get piled on to a stopped or slow-moving conveyor. In
order to get large belts up to full speed, some time may be needed
(especially if soft-start motor controls are used). For this reason, there
is usually a time-delay circuit arranged on each conveyor to give it
adequate time to attain full belt speed before the next conveyor belt
feeding it is started.
The older, mechanical time-delay relays used pneumatic dashpots or
fluid-filled piston/cylinder arrangements to provide the "shock absorbing"
needed to delay the motion of the armature. Newer designs of time-delay
relays use electronic circuits with resistor-capacitor (RC) networks to
generate a time delay, then energize a normal (instantaneous)
electromechanical relay coil with the electronic circuit's output. The
electronic-timer relays are more versatile than the older, mechanical
models, and less prone to failure. Many models provide advanced timer
features such as "one-shot" (one measured output pulse for every transition
of the input from de-energized to energized), "recycle" (repeated on/off
output cycles for as long as the input connection is energized) and
"watchdog" (changes state if the input signal does not repeatedly cycle on
and off).
 The "watchdog" timer is especially useful for monitoring of
computer systems. If a computer is being used to control a critical process,
it is usually recommended to have an automatic alarm to detect computer
"lockup" (an abnormal halting of program execution due to any number of
causes). An easy way to set up such a monitoring system is to have the
computer regularly energize and de-energize the coil of a watchdog timer
relay (similar to the output of the "recycle" timer). If the computer
execution halts for any reason, the signal it outputs to the watchdog relay
coil will stop cycling and freeze in one or the other state. A short time
thereafter, the watchdog relay will "time out" and signal a problem.
REVIEW: Time delay relays are built in these four basic modes of contact
operation: 1: Normally-open, timed-closed. Abbreviated "NOTC", these relays open
immediately upon coil de-energization and close only if the coil is
continuously energized for the time duration period. Also called
normally-open, on-delay relays. 2: Normally-open, timed-open. Abbreviated "NOTO", these relays close
immediately upon coil energization and open after the coil has been
de-energized for the time duration period. Also called normally-open,
off delay relays. 3: Normally-closed, timed-open. Abbreviated "NCTO", these relays close
immediately upon coil de-energization and open only if the coil is
continuously energized for the time duration period. Also called
normally-closed, on-delay relays. 4: Normally-closed, timed-closed. Abbreviated "NCTC", these relays
open immediately upon coil energization and close after the coil has been
de-energized for the time duration period. Also called normally-closed,
off delay relays. One-shot timers provide a single contact pulse of specified
duration for each coil energization (transition from coil off to
coil on). Recycle timers provide a repeating sequence of on-off contact
pulses as long as the coil is maintained in an energized state. Watchdog timers actuate their contacts only if the coil fails
to be continuously sequenced on and off (energized and de-energized) at a
minimum frequency.
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