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Tech-Chek Name ______________________________________________________ Date ________________

Section 21-1: Thermistors ������������������ 1. A thermistor is a(n) ___-sensitive resistor. A. airflow C. temperature B. sound D. light ������������������ 2. The operation of a thermistor is based on the ___-hole theory. A. electron C. neutron B. proton D. positron ������������������ 3. ___ heated thermistors are used for precision temperature measurement and temperature compensation. A. Directly C. Chemically B. Indirectly D. Mechanically ������������������ 4. With a(n) ___ thermistor, an increase in temperature causes the resistance of the thermistor to decrease. A. NTC C. NPN B. PTC D. PNP ������������������ 5. Cold resistance is measured at ___°C. A. 5 C. 25 B. 15 D. 35

Section 21-2: Photoconductive Cells ������������������ 1. A photoconductive cell (photocell) is a device that conducts current when energized by ___. A. temperature C. sound B. humidity D. light ������������������ 2. A photocell is, in effect, a variable ___. A. amplifier C. transistor B. resistor D. transformer ������������������ 3. ___ and contamination are the primary causes of photocell failure. A. Humidity C. Voltage swells B. Overheating D. Transient voltages

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2 ELECTRICAL MOTOR CONTROLS for Integrated Systems WORKBOOK

Section 21-3: Photoconductive Diodes ������������������ 1. The primary difference between a photodiode and a regular diode is that a photodiode has an addition of a ___ in its housing for focusing light on the PN junction. A. prism C. lens B. condenser D. mirror ������������������ 2. Without light, the ___ of the photodiode is high. A. power C. voltage B. current D. resistance ������������������ 3. The ___ flowing through a photodiode increases as the resistance decreases. A. capacitance C. voltage B. current D. frequency

Section 21-4: Pressure Sensors ������������������ 1. A voltage-output pressure sensor typically outputs 0 VDC to ___ VDC proportional to the rating of the pressure sensor. A. 5 C. 15 B. 10 D. 20 ������������������ 2. A current-output pressure sensor typically outputs 4 mA to ___ mA proportional to the rating of the pressure sensor. A. 10 C. 30 B. 20 D. 40 ������������������ 3. Pressure sensors are suited for a wide variety of pressure measurements on ___, pumps, and other similar equipment. A. transformers C. AC motors B. DC motors D. compressors

Section 21-5: Flow Detection Sensors ������������������ 1. A flow detection sensor is a sensor that detects the movement (flow) of liquid or gas using a(n) ___. A. amplifier C. solid-state device B. flow switch D. flapper ������������������ 2. A flow detection sensor operates on the principle of ___ conductivity. A. thermal C. fluid B. magnetic D. variable

© 2014 American Technical Publishers, Inc. All rights reserved


1 ELECTRICAL MOTOR CONTROLS for Integrated Systems WORKBOOK

Worksheets Name ______________________________________________________ Date ________________

Section 21-1: Thermistors In a thermistor application, a fan motor is controlled by an NTC thermistor through an SSR. The fan motor turns on at a set lowering temperature and off at a set rising temperature. 1. Design a circuit using a timer so that the fan does not turn on until the thermistor has reached the lower set point and 30 sec has passed, eliminating false turn-on. Also, add a second timer that keeps the fan motor turned on 2 min after the thermistor has reopened the SSR contacts, allowing the fan to clear any heat away. L1

L2 115 VAC

T

FAN MOTOR

SOLID-STATE RELAY FAN MOTOR TURNS ON AUTOMATICALLY AT HIGH TEMPERATURE

THERMISTOR APPLICATION DC

SSR

T

L1

L2

115 VAC

1

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2 ELECTRICAL MOTOR CONTROLS for Integrated Systems WORKBOOK

Worksheets Worksheets Name ______________________________________________________ Date ________________

Section 21-2: Photoconductive Cells A photoconductive cell can be used to detect the presence of a pilot flame before a gas valve is opened. 1. Design a circuit so that the heating valve cannot be energized unless a photoconductive cell detects a pilot flame. An SSR is used as the interface between the photoconductive cell and the gas valve. This is required because the photoconductive cell cannot carry the higher current of the gas valve. The SSR coil is rated for a few milliamperes (mA), but the SSR contacts can switch several amperes (A).

ELECTRONIC ROOM THERMOSTAT 24 VAC IN

74°

18 VDC OUT

Thermostat

DC POWER SUPPLY

ELECTRONIC HEATING VALVE

ELECTRONIC COOLING VALVE

TO THERMOSTAT

TO THERMOSTAT

ELECTRONIC HEATING VALVE

+

– 18 VDC CUT OFF POWER SUPPLY

© 2014 American Technical Publishers, Inc. All rights reserved

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