Honeywell VP525C, Engineering Manual, Page: 154 / 228

Relays and Switches
146 customer.honeywell.com 70-6925
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.
Fig. 53. Typical Electronic-Pneumatic
Transducer Application.
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.
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.
Fig. 55. Typical OPEN/AUTO/CLOSED Application.
Manual Positioning Switch
A manual positioning switch is used to position a remote valve
or damper or change the setpoint of a controller. The switch
takes input air from a controller and passes a preset, constant,
minimum air pressure to the branch regardless of the controller
M B
M
HEATING
VALVE
DISCHARGE
AIR
HEATING COIL
ELECTRONIC-
PNEUMATIC
TRANSDUCER
DDC
CONTROLLER
ELECTRONIC
TEMPERATURE
SENSOR
C2378
7
9
6
8
7
9
6
8
7
9
6
8
THREE-POSITION SWITCH
TWO-POSITION SWITCH
C1887
7
9
6
8
7
9
6
8
NOTE: POSITION 1: OPEN—PORTS 2 AND 4 CONNECTED
POSITION 2: AUTO—PORTS 2 AND 3 CONNECTED
POSITION 3: CLOSED—PORTS 2 AND 1 CONNECTED
M10295
N.O. VALVE
EXH
DA
THERMOSTAT
M B
1
2
3
4
M
M
THREE-POSITION
SWITCH