4
ing the sensor temperature. (If you don’t want to wait 10 seconds, press the center button once to adjust the differential,
or press the center button twice to immediately display the sensor temperature).
Adjusting the Differential
• The differential controls how many degrees the system can fluctuate around the set point. It is used to prevent short cycling
of the unit being controlled.
• Very stable systems can run with small differential values without short cycling the equipment. Systems where the tempera-
ture changes quickly should have larger differential values. A differential value of at least 2 is recommended.
• In the heating mode, the relay will energize when the sensor temperature falls to the set point minus the differential setting.
For example, with a set point of 70 and a differential of 2, the temperature must fall through 69 and then through 68 before the
relay is energized. The relay will de-energize when the temperature rises to the set point.
• In the cooling mode, the relay will energize when the sensor temperature rises to the set point. The relay will de-energize
when the temperature falls to the set point minus the differential. For example, with a set point of 40 and a differential of 3,
the temperature must fall through 39, 38, and then through 37 before the relay will de-energize.
• To set the differential, use the following steps:
1. If you have just powered up the unit, set up the operating modes (see previous page). If the SPC has been running and is
displaying the sensor temperature, move on to the next step.
2. Press and release the center button twice. The display will change to show dIF. Wait 2 seconds or press the UP or
DOWN button and the differential will be displayed.
3. Press and hold either the UP or DOWN button until the desired differential is displayed.
4. Wait approximately 10 seconds. If the differential was changed, the display will flash. Then the SPC will return to
displaying the sensor temperature. (If you don’t want to wait 10 seconds, press the center button once and the sensor
temperature will be displayed immediately.)
TROUBLESHOOTING
No Display: Check the power to the SPC. The SPC can run off any of the power
sources described on page 2. Turn the power off and back on.
OPN Display: The SPC does not see a sensor connected. Check the wires from
the sensor are continuous from the sensor to the SPC controller. Then follow
the procedure for Incorrect Temperature Display.
SHT Display: The SPC sees a short across the input terminals. If you remove the
sensor wires from the SPC terminals, the display should change to read OPN.
If the display does not change to OPN, the SPC may be damaged.
Incorrect Temperature Display: Remove the wires from the SENSOR screws.
The display should change to read OPN. If it doesn’t, the SPC may be
damaged. Take an ohm reading across the detached sensor wires. The ohm
reading should correspond to the chart on the right. If the ohm reading is
significantly different, the sensor may be damaged.
Output Red Light does not come on at the desired temperature: Check
the set point and differential values. In a heating application, the red light will
not come on until the temperature drops below the set point minus the
differential. In a cooling application, the red light will not turn off until the
temperature drops below the set point minus the differential.
SPC does not activate the output: First remove all connections to the Red,
White, and Black output wires. If the Output Red Light is on, the relay should
be energized; the Red to White wires should be continuous, and the Red to
Black wires should be open. If the Output Red Light is off, the relay should
be de-energized; the Red to White wires should be open, and the Red to Black
wires should be continuous. If the above two conditions are met, the SPC is
working normally. Check the unit the SPC is controlling.
TEMPERATURE
(in degrees F)
VALUE
(in Ohms)
-10 59075
0 42683
10 31215
20 23089
30 17264
40 13040
50 9944
60 7653
70 5941
80 4649
90 3667
100 2914
110 2332
120 1879
130 1524
140 1243
150 1021
160 842
170 699
180 583
190 489
200 412
210 349
220 297
230 253
Heat-Timer Corporation
20 New Dutch Lane Fairfield, NJ 07004
Phone (973) 575-4004 Fax (973) 575 -4052
http://www.heat-timer.com
HT 059198-00 REV B
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