How does the True Off Delay Timer work?
What is the difference between "off-delay" and "true-off-delay" delay relays, and when to use the true-off-delay function?
Pneumatic time delay relays work well, but they are usually very large and very expensive. Additionally, they only have two functions available: normally open (N.O.T.C.) and normally closed (N.C.T.O.) contact delays; and normally open (N.O.T.O.) and normally closed (N.C.T.C.) contact delays.
sometime in the 1970s, manufacturers developed solid state time delay relays, initially using some type of RC (resistor capacitor) circuit to set the time delay. Currently, most integrated circuits or microprocessors are used to implement functions and delays.
One disadvantage of solid state time delay relays is that they are intended to replace closing time delay pneumatic devices. No need to maintain input voltage during the shutdown delay of the old pneumatic delay: when the input voltage is applied to the pneumatic delay, the voltage change of the contacts is removed and the contacts remain in the changed state until air is passed from one room to another room, so the completion time delay returns the contacts to their normal state.
Solid state turn-off delay products require a continuous input voltage during the turn-off delay, so both the logic circuit and the output relay remain energized. Otherwise, the relay will remain grounded when the input voltage is removed. The relay will exit immediately instead of completing the off delay.
Turn off delay function operation
After the input voltage is applied, the delay relay is ready to accept triggering. When the trigger is applied, the output is activated. After the trigger is removed, the time delay (t) begins. The time delay (t) is followed by a drop in output power. During the delay, any application of the flip-flop resets the delay (t) and the output remains powered.
To solve this problem, manufacturers have come up with a unit commonly referred to as a "true-off delay". In these products, the logic circuits and relays are powered by the on-board power supply. This can be a capacitor that discharges during the off delay to keep the logic and output relay energized. Or it could be a latching relay that keeps the latch closed during the off delay, and then a small capacitor discharges during the off delay to release the latch. Using a small battery is another option.
In all cases, the idea is to have the device have its own power supply as the shutdown delay, rather than an external power supply like a standard shutdown delay solid state device.
True Off Delay Function Operation
After the input voltage is applied, the output is energized. The time delay (t) begins when the input voltage is removed. The output is de-energized after the delay time (t). Input voltage must be applied for at least 0.1 seconds to ensure proper operation. Delay (t) will be reset by applying the input voltage during delay (t). No external trigger is required.
There is also a version called delayed on/off delay. Here's how it works:
Delay on/off delay function operation
After the input voltage is applied, the time delay (t1) begins. The output powers up after a delay time (t1). When the input voltage is removed, the output remains powered during the delay (t2). At the end of the delay (t2), the output is de-energized. Input voltage must be applied for at least 0.1 seconds to ensure proper operation. During the delay (t2), any effect of the input voltage keeps the output energized and resets the delay (t2). No external trigger is required.
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