CETP Instructor PowerPoint Template 2.0

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Identifying Procedures for Measuring Resistance
Between Two Given Points in an Electrical Circuit
Continuity tests of coils and similar components
can help identify a faulty appliance part. Other
tests for resistance are useful tools for
troubleshooting individual appliance components.
In this module, you will learn to

1. Apply safety precautions while measuring
   electrical resistance
2. Identify electrical resistance
3. Identify variable resistors and their functions
4. Connect the meter to measure resistance values

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In this module, you will learn to

1. Identify the operating principle of an ohmmeter
   circuit
2. Zero Ohms adjust analog meters
3. Zero Ohms check digital meters
4. Measure resistance values
5. Perform a circuit continuity test

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Applying safety precautions while measuring
electrical resistance
Measuring resistance in circuit components calls
for great care and is an exacting operation. If
fundamental rules are violated, damage can be
done to the customers equipment, your
multimeter, and possibly to yourself.
Rule 1 Resistance measurements must always be done with the circuit and equipment power supply off.
Rule 2 Because of Rule 1, lock-out and tag-out procedures should be in place and enforced.
Rule 3 Never, NEVER attempt a resistance measurement on a printed circuit board. The voltage supplied by your meter will, in most cases, damage the circuit board,
Rule 4 Without manufacturers instructions which detail the test procedure for a particular component or circuitalong with the acceptable range of resistance values for the component(s)you will probably have no need for a resistance measurement.
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Identifying electrical resistance
Copper offers little opposition to electrical
current flow, while rubber practically blocks
electrical current flow. This opposition to
current flow is referred to as resistance (R).
Sometimes it is necessary to include a
resistance in a circuit to limit electric current
flow. Devices used for this purpose are called
resistors. There are two basic types of
resistors according to their control function
(1) Fixed resistor (2) Variable resistor The
unit of measurement for resistance is the ohm,
with the symbol O. The schematic symbol for a
fixed resistor is
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Identifying variable resistors and their
functions
The resistance value of a variable may be varied
from zero to some fixed maximum value. The
schematic symbol for a variable resistor is
Variable resistors are useful in control circuits
where a variance in output of an electrical
device is desired. Some variable resistors
specifically designed for this purpose are called
potentiometers. A familiar application is a
volume control for audio speakers on older radios
and televisions. The heat anticipator in
electro-mechanical appliance thermostats is a
combination of a fixed resistor with a variable
resistor.
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Connecting the meter to measure resistance values
Both analog and digital meters can be used to
measure resistance value. Unlike the other
functions of the multimeter, the ohmmeter uses
voltage source a built into the meter, a
battery. The ohmmeter should never be connected
to any outside electrical source.
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Identifying the operating principle of an
ohmmeter circuit
The operating principle of the ohmmeter is to
force a current to flow through an unknown
resistance, then measure the current. Applying
Ohms Law for a given voltage, the current flow
will be determined by the resistance in the
circuit.
The purpose of the battery is to force current
through the unknown resistance. The fixed
resistor R1 limits the current through the meter
to a safe level. Variable resistor R2 is called
the ZERO OHMS adjustment, and is part of the
multimeter. The purpose for the ZERO OHMS
adjustment is to compensate for voltage changes
because of battery aging.
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Zero Ohms adjusting analog meters
Each time the ohmmeter is used, check the zero
indication on the meter before measuring
resistance. Check and adjust it each time the
switch is positioned to a different range. To set
the ZERO OHMS controls on an analog meter,
proceed as follows

1. Set the range switch at one of the resistance
   range positions, switch at either - DC or DC.
2. Connect the black test lead in the COMMON - jack
   and the red test lead in the jack.

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1. Clip the contact end of the test leads together
   to short out the resistance circuit.

• Observe the meter indication. It should read
  zero on the right hand end of the Ohms arc, which
  is at the top of the dial.
• If the pointer does not read zero, rotate the
  ZERO OHMS knob until it does. If the pointer
  does not move far enough to read zero, one or
  more batteries need to be replaced.
• When the pointer shows zero, unclip the shorted
  test leads. The ohmmeter circuit is now ready to
  measure resistance.

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Zero Ohms check Digital Meters
Digital meters will typically not display a zero
reading when the test leads are shorted out.
Check the manufacturers instructions to
determine the specified readings to expect.
Figure 3. Digital Meter 0 Ohms Display
According to the manufacturers specifications
for the meter shown in Figure 3, the display
should show a reading ? 0.2? with test leads
touching (shorted).
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Measuring Resistance Values
Remember Always follow equipment and meter
manufacturer's instructions.
To measure resistance with an analog meter
proceed as follows

• Turn off the power to the circuit.
• Set the range switch in one of the resistance
  range positions.
• Use R X 1 for resistance readings from 0 to 200
  ohms.
• Use R X 100 for resistance readings from 200 to
  20,000 ohms.
• Use R X 10,000 for resistance readings above
  20,000 ohms.
• Set the function switch at either - DC or DC.
• Connect the black test lead in the COMMON - jack
  and the red test lead in the jack.
• Short the test leads together and adjust for zero
  ohms.

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To measure resistance with an analog meter
proceed as follows

1. Separate the test leads then connect them across
   the resistance to be measured. If there is a
   "forward" and "backward" resistance such as in
   rectifiers, switch back and forth between the two
   DC positions of the function switch to reverse
   the polarity.
2. Read the indication on the OHMS arc at the top of
   the dial. Note this arc reads from right to left
   for increasing values.
3. Multiply the readings by the multiplier factor at
   the switch position for resistance value in ohms.
   "K" on the dial stands for "thousandths".

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To measure resistance with an digital meter
proceed as follows

1. Turn off the power to the circuit.
2. Select resistance on the meter dial (?).
3. Plug the black test probe into the COM input
   jack. Plug the red test probe into the ? input
   jack.
4. Connect the probe tips across the component or
   portion of the circuit where you want to
   determine resistance.
5. View the reading, being sure to note the unit of
   measurementohms (?), kilohms (k?), or megohms
   (M?).

NOTE 1,000? 1 k? 1,000,000? 1
M?
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Performing a circuit or component continuity
test
A continuity test is a quick go/no-go resistance
test that distinguishes between an open and a
closed circuit. As with all resistance
measurements, a continuity test must be made with
the power disconnected. When you suspect a
particular component to be faulty and which
should have low resistance (such as a normally
closed switch, or a motor winding or solenoid
coil)a continuity test can be a very useful
diagnostic method. Likewise if you suspect a
problem in connecting wiring or a connector, a
continuity test may be a great time saver. A
continuity test should produce a zero or
near-zero ohms reading when the meter probes are
connect to each side of the component or circuit
wiring if the component or circuit allows
continuous electrical flow, (the normal condition
for a coil winding). A break in the coil
winding or circuit wiring will give the opposite
reading (infinite resistance).
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Time to See If You Got the Key Points of This
Module

• Complete the Review on pages 7 - 9.
• See if you are ready for the Certification Exam
  by checking off the performance criteria on page
  10.

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