Control Output Descriptions - Extech Instruments 48VTR User Manual

Control Output Descriptions

Relay Output Control
The control relay is a mechanical single-pole double-throw form 'C' contact that has only
two states, ON and OFF. Wire the relay as described in the Wiring Section. In order to
overcome the limitations of ON-OFF control (setpoint overshoot and oscillations, droop,
inefficiency, etc.) a technique called TIME PROPORTIONING is employed. 'ON-OFF' &
'Time Proportioning' methods are described below.
ON-OFF Control
This is the simplest form of control. For a heating application, the controller's output is
100% (ON) when the PV < SV and 0% (OFF) when PV > SV. For cooling applications,
the output is 100% (ON) when PV > SV and 0% (OFF) when PV < SV. To program the
controller for ON-OFF control, set the 'Pb' parameter in the 3
PV
PV ABOVE SV
OUTPUT = 100%
PV BELOW SV
OUTPUT = 0%
PV
ON-OFF COOLING APPLICATION
Time Proportioning Control
To program the controller for Time Proportioning control, run Auto Tune per the
instructions in the Auto Tuning section. The only parameter that must be manually set is
the CYCLE TIME parameter in the 3
Control can best be explained through example. Refer to the diagrams below and the
explanation that follows.
DIAGRAM 1
RELAY ON
0 2.5 5 7.5
TIME (in seconds)
80% Output = ON 8 seconds; OFF 2 seconds
for a 10-second CYCLE TIME (Ct)
ON
0
SV
ON-OFF HEATING APPLICATION
rd
Menu Level. Cycle Time and Time Proportioning
OFF RELAY ON
10 0
50% Output = ON 5 seconds; OFF 5 seconds
for a 10-second CYCLE TIME (Ct)
RELAY OFF
2.5 5 7.5
TIME (in seconds)
10% Output = ON 1 seconds; OFF 9 seconds
for a 10-second CYCLE TIME (Ct)
DIAGRAM 3
10 Model 48VTR - Version 5.0 – January 2006
rd
Menu Level to '0'.
PV
PV ABOVE SV
OUTPUT = 0%
SV
PV BELOW SV
OUTPUT = 100%
PV
DIAGRAM 2
RELAY OFF
2.5 5 7.5 10
TIME (in seconds)
10