Operation; General; Bridge Circuit Theory; Basic Bridge Circuit - Honeywell AUTOMATIC CONTROL Engineering Manual

ELECTRIC CONTROL FUNDAMENTALS

OPERATION

GENERAL

Figure 19 illustrates a basic Series 90 system including a
temperature controller, actuator, and transformer. The wiper of
the potentiometer on the controller is at the midpoint and the
actuator drive shaft is at midposition when the controlled
variable is at the setpoint. The shaft remains unchanged until
the controlled variable increases or decreases. The amount the
controlled variable must change to drive the actuator drive
shaft from full closed to full open (or vice versa) is called the
throttling range. The setpoint and the throttling range are usually
adjustable at the controller.
The controller and feedback potentiometer form a bridge
circuit which operates switching triacs through an electronic
relay. When the controlled variable changes, the potentiometer
wiper in the controller moves and unbalances the bridge circuit.
The electronic relay detects the unbalance and triggers the
appropriate switching triac. The triac drives the actuator drive
shaft and feedback potentiometer wiper in the direction
necessary to correct the unbalance. When the electronic relay
detects that the bridge is rebalanced, the triac is switched off
and the actuator drive shaft stops at the new position. If the
actuator drive shaft drives to the full open or full closed position,
the appropriate limit switch stops the motor.
For example, in a heating application a fall in temperature
causes the controller potentiometer wiper R to move from W
toward B. This unbalances the bridge and drives the actuator
toward open. The actuator drive shaft and feedback
potentiometer wiper R drives cw toward open until the bridge
is rebalanced. The actuator drive shaft and feedback
potentiometer then stop at a new position. On a rise in
temperature, the actuator drive shaft and feedback potentiometer
drive ccw toward closed stopping at a new position.
To reverse the action of the actuator, the W and B leads can
be reversed at either the actuator or the controller. The actuator
then drives toward the closed position as the potentiometer
wiper at the controller moves toward B on a fall in the controlled
variable and toward the open position as the potentiometer wiper
moves toward W on a rise in the controlled variable. These
connections are typically used in a cooling application.
NOTE: Most Honeywell Series 90 controllers move the po-
tentiometer wiper toward B on a fall in the controlled
variable and toward W on a rise.

BRIDGE CIRCUIT THEORY

The following sections discuss basic bridge circuit theory
and limit controls as applied to Series 90 control. The drawings
illustrate only the bridge circuit and electronic relay, not the
triacs, motor coils, and transformer. Potentiometers are
referred to as having 140 or 280 ohms for ease of calculation
in the examples. These potentiometers are actually 135- or
270-ohm devices.
ENGINEERING MANUAL OF AUTOMATION CONTROL

Basic Bridge Circuit

BRIDGE CIRCUIT IN BALANCED CONDITION

Figure 20 illustrates the bridge circuit in a balanced condition.
For the bridge to be balanced, R1 plus R3 must equal R2 plus
R4. R1 plus R3 is referred to as the left or W leg of the bridge,
and R2 plus R4, the right or B leg of the bridge. In this example,
each resistances in the left leg, R1 and R3, is 70 ohms. Together
they equal 140 ohms. Similarly, each resistance in the right
leg, R2 and R4, is 70 ohms. Together they also equal 140 ohms.
Since the sums of the resistances in the two legs are equal, the
bridge is in balance. This is shown in the following table:
Controller potentiometer
Feedback potentiometer
Total
CONTROLLER
POTENTIOMETER
70
W
W
ELECTRONIC
OPEN
70
FEEDBACK
POTENTIOMETER
Fig. 20. Bridge Circuit in Balanced Condition.
When the bridge is balanced, neither triac is triggered, neither
motor winding is energized, and the actuator drive shaft is
stopped at a specified point in its stroke (the midposition or
setpoint in this case).

BRIDGE CIRCUIT ON INCREASE IN CONTROLLED VARIABLE

Figure 21 illustrates the bridge circuit in an unbalanced
condition on an increase in the controlled variable. The
controller potentiometer wiper has moved to one-fourth the
distance between W and B but the feedback potentiometer
wiper is at the center. This causes an unbalance of 70 ohms
(175 – 105) in the right leg as follows:
Controller potentiometer
Feedback potentiometer
Total
108
Left Leg Right Leg
70
70
140 140
70
R1 R2
SENSING
ELEMENT
R B
R B
RELAY
DRIVE
SHAFT
R3 R4
CLOSE
70
C2521
Left Leg Right Leg
35 105
70 70
105 175
70
70