The time delay relays are special purpose relays or logic components that have some characteristics of both relays and Timers Timing is one of the variables of any process, and often needs to be monitored or controlled. Some operatives the steps in a process may need to match or may need to be separated from some other steps by a specific time interval. The time delay relays serve to satisfy said timing requirements. The two main components of the time delay relays are the mechanism of the timing circuit that produces the required time interval and the load switching contacts that are driven at the end of that time interval. In order for a time interval to be determined, the following prerequisites must be present:
1. A power source (if needed in addition to signal power)
2. Signal power
3. A change of state of the time-determining device
4. An indication that the time-determining device has changed to the desired state
The power or signal power can be thermal, pneumatic, AC, or DC electric current. The change in the state of the timer can be mechanical (e.g., caused by the rotation of a motor, the motion of a plunger in a restricting fluid, or the bending of a bimetal strip caused by a change in temperature), electrical (e.g., caused by an accumulation of charge of a capacitor or a count of oscillations or pulses), or software-based (e.g., based on the number of program scans or interrupt-driven). The load switching can occur via snap-action switches, snap-action valves, relay contacts (e.g., SPDT, DPDT, reed hermetically sealed], or mercury [hermetically sealed]), or
solid-state devices (e.g., transistors, SCRs, or triacs).
The mode of the time delay determines the relationship between the time when the signal power is applied to the time of load switching. This relationship may also depend on the continuous presence of the power source. The four most prevalent modes are on-delay, off-delay, interval, and single-shot. A special combination of the on-delay and the off-delay is called a repeat cycle timer
Recovery time is the minimum amount of time between removal of the signal power and its reapplication, which is necessary so that the subsequent operation will have the desired repeatability. The analog or digital solidstatetiming circuits have the shortest recovery time.
An on-delay is alternatively called delay on make, delay on operate, delay on pull-in, delay on energize, slowacting,
or slow-operating. The load switching occurs a certain time after the application of the signal power. If a power source is normally required and the power is interrupted before load switching takes place, the timing cycle has
to be repeated from 0 to effect a delayed load switching (except in certain synchronous motor-driven delay relays).
An off-delay is alternatively called delay on break, delay on de-energize, delay on drop-out, delay on release, slow release, drop-out delay, or delayed drop-out. The load switching occurs a certain time after the removal of the signal power. If the power source is normally required and the power is interrupted before the minimum amount of time or after removal of the signal power prior to load switching, the amount of time delay may be inaccurate.
An interval timer is alternatively called interval on, on interval, pulse shaping, bypass timing, interval delay, and
delay on energization with instantaneous transfer. After application of the signal power and while the signal power is maintained, the load switching occurs for a certain time only, and then the load switching is de-energized. If the signal power is interrupted or the power source (if normally required) is removed before the completion of the time delay, the load switching is de-energized instantaneously.
A single-shot is alternatively called latched interval, latching off delay and latching delay on de-energization,
momentary actuation, or one-shot. Load switching occurs for a certain time only, and then the load switching de-energizes after a momentary application of the signal power The signal power may be applied longer without altering the load switching interval. If the power source is normally required and it is interrupted while the load switching is energized, it de energizes instantaneously.
A pulse detection relay monitors a contact or control input to activate an interval time period. The output contact may already be activated or on with the application of power or it may wait until the control input is active. Once
the control input is on the control input must cycle within the interval time period to either maintain the output or to
drop the output as selected.
A repeat cycle relay is alternatively called dual delay; combination delays; on-delay, off-delay; or slow acting-slow releases; or asymmetric pulser. The load switching occurs a certain time delay after the application of the signal power, and remains energized for another time delay after the removal of the signal power. If the power source is
normally required and it is interrupted while the load switching is energized, it may be de-energized prematurely. The asymmetric pulser has the added feature of two time periods, one for the on period and one for the off period.