a. Choose a range which you think contains the resistance of the element that
you are measuring or use a range in which the reading falls in the upper half of
the scale.
b. Short the leads together and zero the meter, using the zero adjustments.
If you change ranges at any time, remember to readjust to zero ohms.
c. Never attempt to measure resistance in a circuit while it is connected to
a source of voltage.
d. Connect the unknown resistance between the test leads and read its
resistance from the scale.
When applicable, disconnect at least one end of the
element being measured to avoid reading the resistance of parallel paths.
9.
In addition to measuring resistance, the ohmmeter is very useful for checking
continuity in a circuit. Often when troubleshooting electronic circuits or wiring
a circuit, you cannot visually inspect all parts of the current path. Therefore,
it is not always apparent whether a circuit is complete or whether current may be
flowing in the wrong part of the circuit because of contact with adjacent circuits.
The best method of checking a circuit under these conditions is to send a current
through it.
a. If the conductor makes a complete circuit, current flows through the
circuit.
The ohmmeter is ideal for checking circuits.
It provides the power to
cause the current and it provides the meter to indicate the amount.
To check,
first study the circuit diagram, then check the corresponding parts of the circuit
itself with the ohmmeter. The ohmmeter should indicate perfect conduction, partial
conduction (resistance), or no conduction at all.
b. Ohmmeters are not always available, and even when they are, they are often
of little value.
The most common cause of a useless ohmmeter is a dead battery,
which is usually caused by leaving an ohmmeter on the low-ohms scale.
On this
scale the meter draws current continuously.
Another common carelessness is to
leave a multimeter on the "ohmmeter" position and to permit the test leads to short
circuit.
10.
Since an ohmmeter may not be available, you must understand another way of
determining resistance.
There are two procedures you can use for measuring
resistance without an ohmmeter.
a. One is the voltmeter-ammeter method.
Connect the voltmeter and ammeter
(fig 1-11(a)). After connecting the meters, use the voltmeter and ammeter reading
to calculate resistance by Ohm's law.
b. If the resistance is high, the voltmeter method can be used.
With the
voltmeter method, a voltmeter with known resistance is connected in series with the
unknown resistance (fig 1-12).
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