VAV-SVX03A-EN
15
Zone Sensor Operation
This section explains how the zone
sensor operates.
Temperature is sensed using a
thermistor. The thermistor changes
resistance with a change in
temperature. The resistance of a
thermistor decreases as temperature
increases. By using a voltage divider
circuit, the deviation from the room
setpoint is determined.
1. 12-volts DC is supplied from TB1-5 on
the circuit card to terminal 1 on the
zone sensor.
2. The zone sensor output signal is
connected from terminal 2 of the zone
sensor to TB1-4 on the circuit card.
3. A common (ground) is supplied from
TB1-3 on the circuit card to terminal 3
on the zone sensor. At setpoint, the
input to the circuit card is 6 volts
measured across terminals 2 and 3.
Therefore, with 12 volts input between
terminals 1 and 3, the voltage drop
across the thermistor will equal the
voltage drop between terminal 2 and
Zone
Sensor
terminal 3. With the voltages being equal, the room temperature is considered
satisfied and all output relays are de-energized. If the temperature in the zone goes
up, the resistance of the thermistor will go down, causing a voltage increase, which
will drive the air damper open. Conversely, if the room temperature goes down, the
thermistor resistance goes up, causing a decrease in the output signal, which will
drive the air damper to minimum and energize the relay output. To determine the
setpoint, the setpoint potentiometer is moved, changing the resistance between
terminals 2 and 3. By changing the setpoint potentiometer, the temperature at which
the output signal equals 6 volts will be changed. Therefore, the setpoint potentiometer
moves the setpoint up and down.
Degrees Fahrenheit
Thermistor Nominal Resistance (Ohms)
65
4070
66
3960
67
3850
68
3750
69
3660
70
3580
71
3500
72
3420
73
3330
74
3250
75
3170
76
3080
77
3000
78
2940
79
2870
80
2810
