Temperature Indicator : 409-4IN
REF NO: m47/om/201
Issue NO: 04
Page 15 of 34
6.
CONTROL FUNCTION
6.1 ON/OFF Control
ON/OFF Controller is the simplest form of temperature control device. The output
from the device is either on or off, with no middle state. An on-off controller will switch
the output only when the temperature crosses the set point. For heating control, the
output is on when the temperature is below the set point, and off above set point.
Since the temperature crosses the set point to change the output stage, the process
temperature will be cycling continually, going from below set point to above, and back
below. In cases where this cycling occurs rapidly, and to prevent contactors and
valves from getting damaged, an on-
off differential, or “hysteresis,” is added to the
controller operations. On-
Off hysteresis prevents the output from “chattering” or
making fast, continual switches if the cycling above and below the set point occurs
very rapidly.
Figure 8.1: Typical Relay operation
High type (H-ON):
For High type of set value, once process value reaches up to set point + Hysteresis
value, relay will be ON after few seconds (as per relay delay) and it will be ON until
process value goes down to Set point.
Low type (L-ON):
For Low type of set value, once process value reaches down to set point
– Hysteresis
value relay will be ON after nearly few seconds (as per relay delay) and it will be ON
until process value goes up toward Set point.
