Electronic-Pneumatic Transducer; Pneumatic Switch - Honeywell AUTOMATIC CONTROL Engineering Manual

For commercial buildings
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TEMPERATURE
CONTROLLER
M B
M
P/E RELAY
BELLOWS
SPRING
SETPOINT
ADJUSTMENT
Fig. 52. P/E Relay Application.

ELECTRONIC-PNEUMATIC TRANSDUCER

The electronic-pneumatic transducer is a proportional relay
that varies the branch air pressure linearly 3 to 15 psi in
response to changes in an electrical input of 2 to 10 volts or 4
to 20 ma. Electronic-pneumatic transducers are used as the
interface between electronic, digital, or computer-based control
systems and pneumatic output devices (e.g., actuators).
Figure 53 shows discharge air temperature control of a
heating coil using digital control for sensing and control. The
output of the transducer positions the valve on a heating coil.
DDC
CONTROLLER
ELECTRONIC-
PNEUMATIC
M B
TRANSDUCER
M
HEATING
VALVE
HEATING COIL
Fig. 53. Typical Electronic-Pneumatic
Transducer Application.
N.O.
N.C.
PUMP
VOLTAGE
C
PUMP
C2384
ELECTRONIC
TEMPERATURE
SENSOR
DISCHARGE
AIR
C2378
83
PNEUMATIC CONTROL FUNDAMENTALS
A resistance-type temperature sensor in the discharge air
duct is the input to the controller, which provides all of the
system adjustments and logic requirements for control. The
controller output of 2 to 10 volts dc is input to the electronic-
pneumatic transducer, which converts the signal to a 3 to 15
psi output to position the heating valve.

PNEUMATIC SWITCH

The pneumatic switch is available in two- or three-position
models (Fig. 54). Rotating the switch knob causes the ports to
align in one of two ways in a two-position switch, and in one
of three ways in a three-position switch. The two-position
switch is used for circuit interchange. The three-position switch
sequentially switches the common port (Port 2) to the other
ports and blocks the disconnected ports.
1
3
2
4
TWO-POSITION SWITCH
1
3
2
4
1
2
THREE-POSITION SWITCH
Fig. 54. Pneumatic Switches.
Figure 55 shows a typical application for sequential
switching. In the OPEN position, the valve actuator exhausts
through Port 4 and the valve opens. In the AUTO position, the
actuator connects to the thermostat and the valve is in the
automatic mode. In the CLOSED position, the actuator
connects to main air and the valve closes.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
1
3
2
4
1
3
2
4
3
4
C1887

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