Off-Delay Timers - Programmable Logic controllers

Figure shows how a on-delay timer can be used to produce an off-delay timer. With such an arrangement, when there is a momentary input to In 1, both the output Out 1 and the timer are switched on. Because the input is latched by the Out 1 contacts, the output remains on. After the preset timer delay, the timer contacts, which are normally closed, open and switch off the output. Thus the output starts as on and remains on until the time delay has elapsed.

Some PLCs have built-in off-delay timers as well as on-delay timers and thus there is no need to use an on-delay timer to produce an off-delay timer. Figure illustrates this concept for an Allen-Bradley PLC and Figure for a Siemens PLC, illustrating the ladder diagram and the instruction list. Note that with Siemens, the timer is considered a delay item in a rung rather than a relay. In the rectangle symbol used for the timer, the 0 precedes the T and indicates that it is an off-delay timer.

Off-Delay Timers

Application of an off delay timer

As an illustration of the use of an off-delay timer, consider the Allen-Bradley program shown in Figure TOF is used to indicate that it is an off-delay rather than on-delay (TON)timer. The time base is set to 1:0, which is 1 s. The preset is 10, so the timer is preset to 10s.

In the first rung, the output of the timer is taken from the EN (for enable) contacts. This means that there is no time delay between an input to I:012/01 and the EN output. As a result the EN contacts in rung 2 close immediately when there is an I:012/01 input. Thus there is an output from O:013/01 immediately when the input I:012/01 occurs. The TT (for timer timing)contacts in rung 3 are energized just while the timer is running. Because the timer is an off-delay timer, the timer is turned on for 10 s before turning off. Thus the TT contacts will close when the set time of 10 s is running. Hence output O:012/02 is switched on for this 10 s. The normally closed DN (for done) contacts open after 10 s, so output O:013/03 comes on after 10 s. The DN contacts that are normally open close after 10 s, so output O:013/04 goes off after 10 s.

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