Thermistor Control - Taylor 358 Service Manual

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Thermistor Control

Function
The thermistor control maintains product
temperature in the freezing cylinder by monitoring
the resistance of the thermistor probe.
Thermistor Probe (038061- -20)
The resistance value of the thermistor probe
corresponds with the product temperature in the
freezing cylinder. As the product becomes colder,
the probe resistance increases. As the product
becomes warmer, the probe resistance decreases.
Approximate probe resistance readings:
S 10,000 OHMS at room temperature
(77°F/25°C)
S 46,012 OHMS at product temperature
(20°F/-6.6°C)
Operation
The thermistor probe is positioned in the bulb-well
located at the front of the freezing cylinder. The
thermistor control becomes operational when
powered by the 24 VAC transformer.
When the desired product is achieved (control
set-point), the thermistor control relay relaxes and
discontinues sending L1 power to the compressor
relay coil.
When the product in the freezing cylinder warms up
.5°F (.3°C) above the control set-point, the
thermistor control relay closes, sending L1 power to
the compressor relay coil. The refrigeration system
will run until the control set-point is achieved.
Controls & Systems
Anticipator
The anticipator signals the thermistor control to
activate the refrigeration system whenever product
is drawn. As the draw valve is raised and the freezer
draw switch closes, continuity is created between
the thermistor control anticipator terminals. The
thermistor control relay will close within .5 seconds
to start the refrigeration system. The control set
point will shift 8°F (4.4°C) approximately 5 seconds
after the draw switch is released. The set point shift
will change 3.5°F (2°C) and remain in that state for
20 seconds. The set point will then return to its
original setting.
Setting Temperature
1. Position the thermistor fine adjustment at
mid-range. This will limit the fine adjustment
temperature range to ±2_F (±1_C).
2. Turn the coarse adjustment clockwise to the
coldest setting.
3. With the freezer correctly primed, place the
power switch in the "AUTO" position.
4. After the appropriate freezing time, test the
product temperature. When a sample portion
temperature is approximately 1_ above the
desired temperature setting, slowly turn the
coarse adjustment counterclockwise (warmer)
until the refrigeration system cycles off.
5. Allow the refrigeration system to cycle through
at least two "off" cycles. After the unit cycles
off, draw a sample of product and check the
temperature. Readjust the coarse adjustment
as required, but make only small adjustments.
Note: The anticipator automatically activates the
refrigeration system within 5 seconds after the draw
valve is opened. If several small samples are drawn,
the temperature may drift lower. To accurately set
the control, let the product temperature stabilize by
allowing the thermistor control to cycle the freezer
on and off by the control set-point, not the
anticipator.
12
Models 358 & 359