Bridge Circuit With Low-Limit Control; Bridge Circuit With High-Limit Control - Honeywell AUTOMATIC CONTROL Engineering Manual

ELECTRIC CONTROL FUNDAMENTALS

BRIDGE CIRCUIT WITH LOW-LIMIT CONTROL

In a heating application, a low-limit controller moves a valve
actuator toward open when the low-limit setting is reached. To
do this, the limit controller is wired into the left or W leg of the
bridge. An increase in resistance in the left leg of the controller
circuit drives the actuator and feedback potentiometer toward
the open position.
In Figure 23, when the controller and the low limit are
satisfied (both potentiometer wipers at the W ends), the actuator
is at the closed position and the bridge is balanced. This is
shown in the following table:
Controller potentiometer
Low-limit potentiometer
Feedback potentiometer
Total
DISCHARGE AIR
LOW LIMIT
CONTROLLER
W R B
DRIVE SHAFT
OPEN
Fig. 23. Bridge Circuit with Low-Limit Control.
When the low-limit controller calls for more heat, the
potentiometer wiper R (shown dotted) moves halfway from W
to B. This causes an unbalance of 70 ohms (210 – 140) in left
leg of the bridge as shown in the following table:
Controller potentiometer
Low-limit potentiometer
Feedback potentiometer
Total
The unbalance causes the electronic relay to trigger the right
triac, energize the cw motor winding, and drive the actuator
drive shaft toward open. Since half of the 70-ohm unbalance
ENGINEERING MANUAL OF AUTOMATION CONTROL
Left Leg Right Leg
0 140
0 0
140 0
140 140
CONTROLLER
W R B
W R B
ELECTRONIC
RELAY
CLOSE
FEEDBACK
POTENTIOMETER
C2524
Left Leg Right Leg
0 140
70 0
140 0
210 140
has to go on each side of the bridge to rebalance the circuit, the
feedback potentiometer moves 35 ohms to the left. The table
then appears as follows:
Controller potentiometer
Low-limit potentiometer
Feedback potentiometer
Total
When the feedback potentiometer reaches the new position
(shown dotted) the bridge is rebalanced, the right triac turns
off, and the actuator drive shaft stops in the new position
(25 percent open).

BRIDGE CIRCUIT WITH HIGH-LIMIT CONTROL

In a heating application, a high-limit controller moves a valve
actuator toward closed when the high-limit setting is reached.
To do this, the limit controller is wired into the right or B leg of
the bridge. An increase in resistance in the right leg of the
controller circuit drives the actuator and feedback potentiometer
towards the closed position.
In Figure 24, when the controller and high limit are satisfied
(both potentiometer wipers at the B ends), the actuator is at the
open position and the bridge is balanced. This is shown in the
following table:
Controller potentiometer
High-limit potentiometer
Feedback potentiometer
Total
CONTROLLER
W R
W R
ELECTRONIC
RELAY
OPEN
FEEDBACK
POTENTIOMETER
Fig. 24. Bridge Circuit with High-limit Control.
110
Left Leg Right Leg
0 140
70 0
105 35
175 175
Left Leg Right Leg
140 0
0 0
0 140
140 140
DISCHARGE AIR
HIGH LIMIT
CONTROLLER
B W R B
B
DRIVE SHAFT
CLOSE
C2525