Lesson 3/Learning Event 1
As a wheeled vehicle repairer, you will not be removing or replacing many resistors; however, you
will be working with many electrical components. Many of these components, functioning
properly, do place resistance in an electrical circuit. Earlier in this lesson we learned these
components are called the load in a circuit. If the resistance of these components is not the
specified resistance, the component or the electrical circuit will not function properly. Let's see how
the ohmmeter is used to measure the resistance of the load.
Let's assume we have a circuit that is fuse-protected. (Some circuits are circuit-breaker protected.)
Our circuit includes a set of batteries, an electromagnet, an on-off switch, and a 3-amp fuse. For
the purpose of this test, let's say the circuit blows the fuse each time a replacement fuse is installed
and the switch is closed. The circuit has been checked, and the wires are not grounded. The fuse
is of the right amperage (3 amperes) for the circuit. The battery voltage is normal (24 volts). The
fuse will not last long enough to make an amperage load test on the circuit. The only component
in this circuit that creates any measurable resistance is the load--the electromagnet. According to
the wiring schematic, the electromagnet should place 12 ohms of resistance on the circuit. We will
disconnect the electromagnet from the circuit and measure the electromagnet's resistance.
Do not forget to zero the ohmmeter first. Remember, the circuit to be tested is hooked up in
series with the ohmmeter.
For the purpose of this test, let's say the ohmmeter hand indicates 6 ohms of resistance. This is
only half of the 12 ohms the electromagnet should have. Could this difference cause the 3-ampere
fuse to blow? Remember the laws you learned so far about electricity: one law states if the
resistance increases in a circuit and the voltage remains the same, the current or amperage
decreases.
In this case, we have the opposite of that law. The resistance decreases, the voltage remains the
same, and therefore, the amperage increases. The amperage increases to more than 3 amperes, and
the fuse blows, opening the circuit. This protects the circuit from an overload that would otherwise
damage the wiring and components.
You now know how the ohmmeter can help you in testing circuits and switches and locating
electrical system troubles. The ohmmeter you will probably use will be part of a multimeter, which
is covered later in this lesson.
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