Application — Thermistors NTC thermistors decrease resistance with an increase in temperature and are the most common type of thermistor used in applications. PTC thermistors increase resistance with an increase in temperature. Thermistors come in a variety of resistance values. Resistance is typically specified at a room temperature of 25°C (77° F) to make it easier to test the resistance of a thermistor using a DMM before it is placed into a heated/cooled application. Although thermistors are available in numerous resistance ranges, standard NTC thermistors rated at 25° C (77° F) include the following resistance values: 2 kΩ, 3 kΩ, 5 kΩ, 10 kΩ, 30 kΩ, 50 kΩ, and 1 MW Changes in thermistor resistance are not linear with temperature changes. Manufacturers provide a chart showing the specific resistance at each temperature above and below the thermistor’s rating (typically 25° C). Photoconductive cells are used in many applications in which it is important to know whether light is present. Such applications include headlights that automatically turn on, automatic glare reduction on rear view mirrors, and security lighting.
TEMPERATURE vs. RESISTANCE—2000 Ω NTC THERMISTOR Temperature °F –40.00 –22.00 –4.00 14.00 32.00 41.00 59.00 77.00 95.00 113.00 131.00 149.00 167.00 185.00 203.00 212.00
°C –40.00 –30.00 –20.00 –10.00 0.00 5.00 15.00 25.00 35.00 45.00 55.00 65.00 75.00 85.00 95.00 100.00
Resistance Nominal Ohm 67,021 35,305 19,338 11,058 6,530 5,079 3,143 2,000 1,306 873 597 416 296 214 157 136
Minimum Ohm
Maximum Ohm
Resistance Tolerance +/– %
64,412 34,230 18,944 10,880 6,464 5,029 3,114 1,983 1,295 867 593 414 293 211 154 133
69,747 36,418 19,843 11,240 6,597 5,130 3,172 2,018 1,317 880 601 419 299 218 161 139
4.07 3.15 2.35 1.65 1.03 1.00 0.94 0.88 0.83 0.79 0.74 0.70 1.00 1.59 2.12 2.37
Resistance
Temperature Tolerance +/– °C 0.60 0.50 0.40 0.30 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.30 0.50 0.70 0.80
Application — Photoconductive Cells Photoconductive cells are used in many applications in which it is important to know whether light is present. Such applications include headlights that automatically turn on, automatic glare reduction on rear view mirrors, and security lighting.
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ELECTRICAL MOTOR CONTROLS for Integrated Systems APPLICATIONS MANUAL
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Name_______________________________________________ Date________________________
Activity — Thermistors From the given 2 kΩ thermistor, draw the approximate graph showing the characteristics of the thermistor. R×6
R×4
RESISTANCE (Ω)
R×2
R
0.75R
0.5R
0.25R
–40
–30
–20
–10
0
10
20
30
40
50
60
70
80
90
100
TEMPERATURE (°C)
Activity — Photoconductive Cells In an automobile electrical system, a MANUAL/OFF/AUTO switch is used to place the low beam headlights in a manual position or an automatic position. In the automatic position, the photoconductive cell automatically turns on the lights at night by returning the SSR contact to its NC position. An OFF/HIGH BEAMS switch is used to manually turn on the high beam headlights. An adjustable resistor, R, is used to set the sensitivity that determines when the lights automatically turn on. © 2014 American Technical Publishers, Inc. All rights reserved
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ELECTRICAL MOTOR CONTROLS for Integrated Systems APPLICATIONS MANUAL
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–
12 VDC
SSR R
PHOTOCELL
SSR
AUTO
MANUAL OFF HIGH BEAMS ON 1
2
3
LIGHTING CIRCUIT TERMINAL STRIP
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HEADLIGHT ELEMENTS HIGH BEAM
LOW BEAM
HIGH BEAM
LOW BEAM
HEADLIGHTS
AUTOMOBILE ELECTRICAL SYSTEM
Complete the following steps and answer the questions using the automobile electrical circuit. 1. Connect DMM 1 to measure the incoming power from the battery/charging circuit. ������������������ 2. What should DMM 1 read if the circuit is working properly? 3. Connect DMM 2 to measure the voltage out of the AUTO switch and into the SSR contact to determine whether voltage is present at that point. ������������������ 4. What should DMM 2 read if the circuit is working properly, the switch is in the AUTO position, and it is dark outside? ������������������ 5. What should DMM 2 read if the circuit is working properly and the switch is in the MANUAL position? 6. Connect DMM 3 to measure the voltage at the low beams. ������������������ 7. Can there be a voltage reading at both terminal 2 and 4 at the same time?
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Chapter 21 — Semiconductor Input Devices
DMM 1
DMM 2
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3
DMM 3
4
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ELECTRICAL MOTOR CONTROLS for Integrated Systems APPLICATIONS MANUAL
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