Page 294 - Electromechanical Devices and Components Illustrated Sourcebook

256 Electromechanical Devices & Components Illustrated Sourcebook

Primary Fixed Mount
Secondary B
Secondary A
Moving Core
Coil Form Inner Plate
Potentiometer
Shaft
Coiled Bimetal Strip
Lock Screw
+
Calibration Scale
0
Output
Base Plate −
Input
Figure 15-28 Linear variable Differential Transformer Terminals
Figure 15-30 Bimetal Strip Temperature Transducer

Temperature Sensors
Pivot
Temperature sensors generally fall into three different cate- Preload Spring
Diaphragm Housing Lock Nut
gories, bimetal strips, bulb-type, and thermocouples. Bimetal
strip sensors are the most common, being used in most Set Point Adjustment
Capillary
adjustable thermostats and temperature controllers. The most Lever Arm
Tube
common example of a bimetal strip unit is found in an ordi-
Limit Switch
nary home thermostat.
Figure 15-29 shows a bimetal strip thermostat with a dou-
ble throw mercury switch. The trip temperature is set by Base Plate
adjusting the pointer to the desired setting on the reference
Bulb
scale. If the temperature rises, the mercury switch closes in
one position and if the temperature drops, the switch closes in
the opposite position. Figure 15-31 Bulb-Type Thermostat

Reference Scale Degrees F
10 bulb, capillary tube, and diaphragm housing are filled with a
0 20 30 40 50
−10 fluid with a high thermal expansion rate. As the temperature of
−20 60
Pointer the bulb increases, the fluid expands and forces the diaphragm
to extend. The set point is adjusted by tensioning the preload
Adjustable Knob
spring, which counters the movement of the diaphragm,
Terminals
Figure 15-32 shows a bulb-type temperature transducer.
Coiled Bi-
Metal Strip The bulb and capillary tube feed a bellows set. As the fluid
Flexible Cable
Actuation
Angle
Mercury Switch Bellows
Switch Mount
Figure 15-29 Adjustable Bimetal Strip Thermostat Bellows Housing
Pivot
Lever Arm
A bimetal strip temperature transducer, as shown in Capillary
Figure 15-30, is configured by fixing the outer end of a coiled Tube Return Spring
element to a base plate and allowing the inner end to rotate the
shaft of a potentiometer. The potentiometer is mounted to the
center of a secondary base plate whose position can be
adjusted in reference to the outer base plate. To calibrate the Linear
Potentiometer
transducer, the inner plates are rotated against one another
until the output voltage matches the current temperature con- Base Plate
Bulb
dition. The output of the potentiometer can drive either a volt-
age divider or a Wheatstone bridge.
Bulb-type temperature thermostats, as shown in Figure 15-31,
operate in reference to the expansion of a captured fluid. The Figure 15-32 Bulb-Type Temperature Transducer