Overview Switch-on limit

This function block passes the request at its input on to the output only a limited number of times. This prevents a load from being switched on and off repeatedly (short cycling). Three conditions are applied:
  • Max. switch-ons: Once the permitted number of switch-ons within the selected period has been used up, the output stays off until the next period.
  • Restart lockout: After switching off, the output remains blocked for the time set, even if the request returns immediately.
  • Minimum on time: A run that has started is carried through, even if the request drops out briefly.
The period the number refers to can be chosen freely: a fixed number of seconds, an hour, a day, a week, a month or a year. This covers both "twice a day" and "ten times per hour".

Examples:
  • A circulation pump is to start no more than six times per hour and run for at least five minutes each time.
  • A fan is to stay off for half an hour after switching off, even if the request returns immediately.
  • A message is to be triggered no more than ten times per hour.
  • A gate drive is to be limited to no more than twenty movements per day.
If the request fluctuates, for instance because it is derived from a measured value, the minimum on time makes sure that a brief drop-out does not interrupt the current run and therefore does not count as a new switch-on.

The counter, the lockout time and the current period are stored permanently and are retained after a restart of the controller. The output is off after a restart.

Note: limiting the number alone should be set with care. If the permitted switch-ons have already been used up at the beginning of the period, the load stays blocked for the rest of the period, even if it is needed then. As a rule the restart lockout together with the minimum on time is the more effective measure, while the number per period serves as an upper limit.




Inputs

E1
 Request
 Request to switch on. As long as this input is not 0, the output is switched on unless a lock is active.
FG
 Enable
 At 0 the output is switched off immediately, even during the minimum on time. An input that is not assigned counts as enabled.
RS
 Reset
 A rising edge resets the counter and the lockout time.

Outputs
A1
 Output
 Output, 1 when switched on.
AZ
 Number in period
 Number of switch-ons during the current period.
AR
 Number remaining
 Switch-ons still permitted until the next period. With an unlimited number, 0 is output.
RZ
 Remaining lockout time
 Remaining seconds of the restart lockout.
SP
 Locked
 1 if switching on is currently not possible, that is during the restart lockout or when the number for the current period has been reached.

Parameters
Max. switch-ons
 Number of switch-ons permitted per period. With 0 the number is unlimited.
Period
 Period the number refers to. At the end of the period the counter is set to zero.
  • Seconds
  • Hour
  • Day
  • Week (changes on Monday)
  • Month
  • Year
Reset
 Shown only for the periods Seconds and Day.
  • With Seconds: length of the period in seconds, for example 600 s for ten minutes.
  • With Day: hour at which the counter is set to zero, for example 2 o'clock. With 0 the counting day changes at midnight.
Restart lockout (s)
 Time in seconds that must pass after switching off before switching on is permitted again. With 0 there is no lockout.
Minimum on time (s)
 Time in seconds for which the output stays on after switching on, even if the request drops out. The enable input FG still switches off immediately.





See also the general parameters of all function blocks.