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FORM 160.87-OM1
ISSUE DATE: 8/24/2018
party devices. Refer to SC-EQ Installation Instructions
(Form 450.50-N1) for details on the SC-EQ (Smart
Chiller - Equipment) communications card.
Digital Inputs
The I/O Board converts the 115VAC inputs to logic
level inputs for the microboard at J19. A 115VAC input
to the I/O Board is converted to a logic low (less than
1VDC). A 0VAC input to the I/O Board is converted
to a logic high (greater than 4VDC). Refer to Form
160.54-M1 for more details on the I/O board.
Digital Outputs
The microboard controls 115VAC relays and solenoids
via the I/O Board (via J19). The I/O Board contains
+12VDC relays that isolate the microboard low volt-
age circuits from the 115VAC device coils. Solid state
switching devices are used to control the relays. The
microboard energizes the +12VDC relays by applying
a ground to the coil input. They are de-energized by
opening the ground path. The contacts of these relays
switch 115VAC to system relays and solenoids. The
outputs that control the chilled liquid pump and com-
pressor motor starter have anti-chatter (anti-recycle)
timers associated with them. The output that controls
relay K0 is not allowed to change at a rate greater than
once every 10 seconds. The output that controls relay
K13 is not allowed to change at a rate greater than once
every 20 seconds.
The microboard controls actuator motors via Triacs on
the I/O Board. Each actuator has an open winding and
a close winding. Current flowing through a winding
causes the actuator to rotate in the respective direction.
Each winding is controlled by a Triac. The Triac is
turned ON to allow current to flow through a winding.
The microboard turns on the Triac by applying a logic
low (less than 1VDC) to the Triac driver on the I/O
Board. It turns it OFF by applying a logic high (greater
than 4VDC).
Analog Inputs
System pressures, in the form of analog DC voltages,
are input from Pressure Transducers. Refer to Form
160.54-M1 for more details. Formulas and graphs are
included to calculate the expected transducer output
voltage for a given pressure input.
System temperatures, in the form of analog DC voltag-
es, are input from thermistors. Refer to Form 160.54-
M1 for more details. Included are tables to convert the
expected output voltage for any temperature applied to
the thermistor.
JOHNSON CONTROLS
Flow Switch
The chillers are supplied with factory-mounted Flow
Sensors on the evaporator and condenser. These are
electronic thermal-type sensors. The operating prin-
ciple of the sensor is thermal conductivity. It uses the
cooling effect of a flowing liquid to sense flow. The
temperature of the heated sensor tip is sensed by a
thermistor located in the tip. A second thermistor, lo-
cated higher in the tip in a non-heated area, is only af-
fected by changes in liquid temperature. The tempera-
tures sensed by the thermistors are compared. Flowing
liquid carries heat away from the heated sensor tip,
lowering its temperature. The higher the flow rate, the
lower the tip temperature and therefore a lower differ-
ential between thermistors. Lower flow rates remove
less heat from the tip allowing a higher tip temperature.
The lower the flow, the greater the differential between
thermistors. The sensor is vendor-calibrated to turn ON
its output at a flow rate of 20cm (0.6 ft.)/second.
The sensor operates from a 24VAC power source and
has a solid state relay output. On each sensor, one side
of the solid state relay (pin 4) is connected to +5VDC
on the microboard and the other side (pin 2) is con-
nected to an Analog Input of the microboard (Refer to
Form 160.54-M1 for more details). After power is ap-
plied, there is a thermal warm-up period of up to 20
seconds. During this time, the output could be unsta-
ble. When the setpoint (or greater) flow rate is sensed,
the solid state relay output is turned ON causing it to
conduct current through the 7.5K ohm Microboard
load resistor to the +5VDC. This applies greater than
+4VDC to the microboard input. When a flow rate less
than the setpoint is sensed, the solid state relay output
is turned OFF, resulting in no conduction through the
load resistor. This applies less than 1VDC to the mi-
croboard input. To determine the state of the solid state
relay, first confirm that +5VDC is present at pin 2 of
the flow sensor. Then connect a voltmeter between the
microboard TP1 (GND) and the respective flow sensor
input to the microboard.
The software accommodates either the Paddle type
sensors connected to TB4 of the I/O Board or the Ther-
mal type sensors connected to J14 on the microboard.
To assure the program reads the correct input, the Flow
Switch setpoint on the OPERATIONS Screen must be
set appropriately .
SECTION 5 - SERVICE
5
209
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