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Chapter 11 Electrical Engineering 233
Resistors
The resistor is one of the most common
and reliable electrical components. Resistors are
used to limit current fl ow and divide voltage
in a circuit. Resistors come in a variety of sizes,
shapes, and confi gurations. Most are made from
carbon. Resistors are available in values ranging
from less than one ohm to many millions of ohms.
A system of color-coding resistors with their
values was developed because it is not always
possible to print values on very small resistors.
Resistors typically have three or four (sometimes
fi ve) color bands. The fi rst two colors represent
digits. The third band is the multiplier and tells
what power of ten the digits should be multiplied
by. The fourth band is the tolerance, which indi-
cates the accuracy of the resistor value. If there
is no fourth band, a twenty percent tolerance is
assumed. On a fi ve-band resistor, the fi rst three
are digits, the fourth is the multiplier, and the
fi fth is the tolerance.
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Figure A.
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Figure B.
Volt-Ohm-Milliammeter (VOM)
Volt-ohm-milliammeters (VOMs) are common
multimeters used in electronics. Most are handheld
and digital, making them easier to use than stationary
or analog meters. VOMs can test voltage, current, and
resistance. Many VOMs can also test capacitors, tran-
sistors, and continuity.
Continuity Tester
Continuity is a term used to describe a com-
plete path for current flow. If a circuit is complete, it
has continuity. A continuity test can be made with an
ohmmeter. Low or no resistance indicates continu-
ity. Infinite resistance indicates an open circuit or a
break in the circuit and no continuity. Continuity tes-
ters typically beep or use an indicator light if there is
continuity. A continuity tester could be used to see
if an incandescent lightbulb has burned out. If there
is continuity, the light is still good. Figure A shows
a modern digital multimeter capable of measuring
voltage, current, resistance, continuity, and more.
See Workbook Activity 11-9 for an opportunity
to troubleshoot a circuit using an ohmmeter.
Oscilloscope
The oscilloscope is one of the most widely
used electrical test devices because it is so ver-
satile. Oscilloscopes can show the exact shape
of a wave on their screens, which will show any
possible distortion. They can measure voltage, fre-
quency, pulses, and the timing of multiple signals.
See Figure B.
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