Time Delay Relays – Application Data
Definition:
Time Delay is defined as the controlled period between the functioning of two events. A Time Delay relay is a combination
of an electromechanical output relay and a control circuit. The control circuit is comprised of solid state components and
timing circuits that control operation of the relay and timing range. Typical time delay functions include On-Delay, Repeat
cycle (starting off), Interval, Off-Delay, Retriggerable One Shot, Repeat cycle (starting on), Pulse Generator, One Shot,
On/Off Delay, and Memory Latch. Each function is explained in the table below. Time delay relays have a broad
choice of timing ranges from less than one second to many days. There are many choices of timing adjustments from
calibrated external knobs, DIP switches, thumbwheel switches, or recessed potentiometer. The output contacts on the
electromechanical output relay are direct wired to the output terminals. The contact load ratings are specified for each
specific type of time delay relay.
Understanding the differences between all the functions available in time delay relays can sometimes be a daunting task.
When designing circuits using time delay relays questions such as:
“What initiates a time delay relay?”
“Does the timing start with the application or release of voltage?”
“When does the output relay come on?”
must be asked.
Time delay relays are simply control relays with a time delay built in. Their purpose is to control an event based on time.
The difference between relays and time delay relays is when the output contacts open & close: on a control relay, it
happens when voltage is applied and removed from the coil; on time delay relays, the contacts will open or close before
or after a pre-selected, timed interval.
Typically, time delay relays are initiated or triggered by one of two methods:
• application of input voltage (On Delay, Interval On, Flasher, Repeat Cycle, Delayed Interval & Interval/Flasher).
• opening or closing of a trigger signal (Off Delay, Single Shot & Watchdog).
These trigger signals can be one of two designs:
• a control switch (dry contact), i.e., limit switch, push button, float switch, etc.
• voltage (commonly known as a power trigger).
To help understand, some definitions are important:
Input Voltage:
Control voltage applied to the input terminals (see wiring diagrams below). Depending on the function,
input voltage will either initiate the unit or make it ready to initiate when a trigger signal is applied.
Trigger Signal:
On certain timing functions, a trigger signal is used to initiate the unit after input voltage has been applied.
As noted above, this trigger signal can either be a control switch (dry contact switch) or a power trigger (voltage).
Output (Load):
Every time delay relay has an internal relay (usually mechanical) with contacts that open & close to control
the load. They are represented by the dotted lines in the wiring diagrams. Note that the user must provide the voltage to
power the load being switched by the output contacts of the time delay relay.
The following tables contain both written and visual descriptions on how the common timing functions operate. A Timing
Chart shows the relationship between Input Voltage, Trigger Signal (if present) and Output Contacts.
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SCHNEIDER [ SCHNEIDER ELECTRIC ]
