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R<br />

G<br />

cally this law relates voltage, current, and resistance by I � V/R. For<br />

the 12-V bulb at a rated current of 300 mA, the resistance is R � V/I �<br />

12 V/300 mA � 40 �. For the 6-V bulbs it is 6 V/300 mA � 80 �. Further<br />

comment regarding the power levels and resistance levels will be<br />

reserved for the chapters to follow.<br />

The preceding description assumed an ideal level of 12 V for the battery.<br />

In actuality, 6.3-V and 14-V bulbs are used to match the charging<br />

level of most automobiles.<br />

Strain Gauges<br />

Any change in the shape of a structure can be detected using strain<br />

gauges whose resistance will change with applied stress or flex. An<br />

example of a strain gauge is shown in Fig. 3.41. Strain gauges are semiconductor<br />

devices whose terminal resistance will change in a nonlinear<br />

(not a straight-line) fashion through a fairly wide range in values when<br />

they are stressed by compression or extension. Since the stress gauge<br />

does emit a signal, a signal processor must also be part of the system to<br />

translate the change in resistance to some meaningful output. One simple<br />

example of the use of resistive strain gauges is to monitor earthquake<br />

activity. When the gauge is placed across an area of suspected<br />

earthquake activity, the slightest separation in the earth will change the<br />

terminal resistance, and the processor will display a result sensitive to<br />

the amount of separation. Another example is in alarm systems where<br />

the slightest change in the shape of a supporting beam when someone<br />

walks overhead will result in a change in terminal resistance, and an<br />

alarm will sound. Other examples include placing strain gauges on<br />

bridges to maintain an awareness of their rigidity and on very large generators<br />

to check whether various moving components are beginning to<br />

separate because of a wearing of the bearings or spacers. The small<br />

mouse control within the keyboard of a portable computer can be a<br />

series of stress gauges that reveal the direction of stress applied to the<br />

Typical Installation<br />

30 AWG or ribbon leads<br />

Model SGN<br />

_<br />

4/12<br />

12-Ω terminal resistance<br />

Overall length: 5.5mm ≈= ≈ 0.22"<br />

20-26 AWG instrumentation wire<br />

Terminal pads for stress relief and junction for different gauge wire<br />

FIG. 3.41<br />

Resistive strain gauge. (© Copyright Omega Engineering, Inc. All rights<br />

reserved. Reproduced with the permission of Omega Engineering, Inc.,<br />

Stamford, CT 06907.)<br />

APPLICATIONS ⏐⏐⏐ 89

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