T2 - Return-Air Thermistor; T4 - Saturated Condensing Temperature, Circuit; T10 - Reset Temperature; Compressor Operation - Carrier 48DK024 Controls Operation And Troubleshooting

P1 LEAVING AIR TWO-DIGIT
SET POINT DISPLAY
POTENTIOMETER
Fig. 6 — Display/Set Point Board
T2 — RETURN-AIR THERMISTOR — This thermistor is
located in the mixed-air portion of the unit cabinet. The ther-
mistor's primary function is to provide morning warm-up
information. This sensor will also provide differential infor-
mation for the processor during cooling operation (such as
the rate of change for a capacity step).
T3 — SATURATED CONDENSING TEMPERATURE, CIR-
CUIT 1 — This thermistor is used only on 034-074 units,
and is located on the condenser coil return bend. See Fig. 17
and 18. It controls the staging of the unit condenser fans based
on the condensing temperature of the refrigerant at the des-
ignated position on the condenser coil. On 024-030 units,
this thermistor is replaced with a fixed resistor.
T4 — SATURATED CONDENSING TEMPERATURE, CIR-
CUIT 2 — This thermistor is used only on 034-074 units,
and is located on the condenser coil return bend. See Fig. 17
and 18. It controls the staging of the unit condenser fans based
on the condensing temperature of the refrigerant at the des-
ignated position on the condenser coil. On 024-030 units,
this thermistor is replaced with a fixed resistor.
T10 — RESET TEMPERATURE — This thermistor is used
only if the accessory temperature reset package is used. It
provides occupied space temperature information to the pro-
cessor, which determines whether or not reset is required.
The thermistor is remotely mounted outside the unit in the
conditioned space.
Fig. 7 — Thermistor T1 Location, 48DK024-030 Units

Compressor Operation

DISPLAY
BUTTON
CONTROL RELAY (CR) — This relay provides informa-
tion to the processor about compressor operation (one con-
trol relay per compressor). The relay controls and protects
the compressor and also controls the crankcase heater.
the compressor is sent from the relay board. This signal trav-
els through all of the safeties: the high-pressure switch, the
low-pressure switch (024-030 units), and the internal pro-
tector (06D compressors) or the discharge gas thermostat
(06E compressors) and on to the control relay coil. Once the
control relay coil has been energized, the control relay com-
pletes a feedback circuit for the processor, informs the pro-
cessor of the status of the compressor safeties, energizes the
compressor contactor coil, and deenergizes the crankcase heat-
ers. A fault will be detected by the processor if the control
relay opens during operation or start-up. The processor will
lock the compressor or the circuit off by deenergizing the
appropriate relay(s) on the relay board and energizing an alarm
signal.
Accessory Board —
(factory supplied) in the VAV rooftop units. See Fig. 19. This
board is located in the control box of each unit. Each board
has a prewired connector supplied with it to connect directly
to the processor board. It has 3 potentiometers: P3, P5, and
P6. See Table 3.
P3 — RESET LIMIT — The processor board is programmed
for occupied space temperature reset. In order for reset to
work, the accessory temperature reset board must be used.
Potentiometer P3 is the maximum number of degrees (F) to
which the supply air can be reset.
P5 — ECONOMIZER MINIMUM POSITION — This po-
tentiometer controls the set point for the minimum position
of the economizer.
P6 — MORNING WARM-UP TEMPERATURE — This po-
tentiometer controls the morning warm-up temperature set
point.
Fig. 8 — Thermistor T1 Location, 50DK024-030 Units
7
A control signal to check the safety statuses and to start
The accessory board is standard