Time-Delay Relays

In some cases electrical signals need to be conveyed with a time delay. An example of this is that the light in a stairwell should stay on a certain period of time before automatically going out again. This kind of signal lag can be realized using time-delay relays. A distinction is drawn here between pick-up delayed and drop-out or release delayed relays.


Pick-up Delayed Time Relays:

In the case of pick-up delayed time relays the armature is drawn with an (normally adjustable) time lag of  tV, while release occurs without any time delay. The following graphic shows the internal design of a time relay in the left-hand segment (the area with the blue background) and the relevant circuit symbol (right-hand section of the image. If the switch S is closed, the capacitor C is charged up via the adjustable resistor R1. In this case the diode R3 is reverse biased and stops conducting. As soon as the capacitor has reached the voltage required for the relay K (an ordinary relay) to pick up in the normal way, the relay triggers. After S is released the circuit is broken causing the capacitor to discharge very rapidly via the diode, which is now forward biased, so that the relay returns without delay to its rest state. The desired delay time for the switch-on process can thus be set by adjusting the resistor R1: The greater the resistance selected for R1, the slower the capacitor charges up, i.e. the greater the lag time tV. The discharge resistor R2 also prevents any short-circuit occurring when the switch S is closed.




The subsequent graphic shows how a pick-up delayed relay is depicted in a circuit diagram (here using the example of a circuit for a time-delayed activation of a signal lamp P). This relay type differs from simple relays in that there is a box with an X in it at the far left edge of the coil as well as an actuation symbol attached to the corresponding pushbutton that hints at a "parachute" effect to symbolise delayed switch-on.




Drop-out Delayed Time Relay:

For drop-out delayed time relays the armature is drawn immediately upon application of the coil voltage, while drop-out of the armature occurs with a time lag after the coil circuit is opened This relay type demonstrates precisely the opposite behaviour to that of the pick-up delayed relay. The following graphic shows the internal design of this relay type as well as its circuit symbol. The only difference in the circuit design is in the polarity of the diode R3. After the switch S is actuated the diode is switched to forward bias so that the capacitor charges up without delay and the relay picks up immediately. After the switch is released the capacitor discharges across R1 and R2, that are then connected in series, and simultaneously via the coil of relay K. The greater the resistance of  R1, the lower the current flowing through the resistors and the the greater the current flowing through the coil, meaning that this can still retain the armature for a certain period of time. With this type of relay too, the time lag tV can be adjusted via R1.



The following diagram shows how a drop-out delayed relay is depicted in the circuit diagram (here using the example of switching a signal lamp H off with a time delay). This type of relay differs from simple relays in that it has a solid black box added to the left edge of the coil as well as the corresponding symbol for the actuated pushbutton, which is reversed with respect to that for a pick-up delayed relay.



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