analog and digital multimeters - los rios community...
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Analog and Digital Multimeters
Topics Covered in Chapter 8
8-1: Moving-Coil Meter
8-2: Meter Shunts
8-3: Voltmeters
8-4: Loading Effect of a Voltmeter
8-5: Ohmmeters
Chapter 8
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Topics Covered in Chapter 8
8-6: Multimeters
8-7: Digital Multimeters (DMMs)
8-8: Meter Applications
8-9: Checking Continuity with the Ohmmeter
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8-1: Moving-Coil Meter
Two Types of Multimeters
VOM
(analog)
DMM
(digital)
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8-1: Moving-Coil Meter
Types of Meters
Analog meter:
Uses a moving pointer and a printed scale to indicate
values of voltage, current, or resistance.
Volt-Ohm-Milliammeter (VOM):
Allows all three kinds of measurements on a single
scale or readout.
Digital multimeter:
Uses a numerical readout to indicate the measured
value of voltage, current or resistance.
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8-1: Moving-Coil Meter
Direct Current Meters
Direct current in a moving-coil meter deflects the pointer
in proportion to the amount of current.
A current meter must be connected in series with the
part of the circuit where the current is to be measured.
A dc current meter must be connected with the correct
polarity.
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8-1: Moving-Coil Meter
Analog instruments use a moving coil meter movement.
Current flow in the coil
moves the pointer up-
scale.
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8-2: Meter Shunts
Meter Shunts
Meter shunts are low-value precision resistors that are
connected in parallel with the meter movement.
Meter shunts bypass a portion of the current around the
meter movement. This process extends the range of
currents that can be read with the same meter
movement.
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8-2: Meter Shunts
Using Shunts to Increase Ammeter Range
Fig. 8-4: Example of meter shunt RS in bypassing current around the movement to extend
range from 1 to 2 mA. (a) Wiring diagram. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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8-2: Meter Shunts
Fig. 8-4: (b) Schematic diagram showing effect of
shunt. With RS = rM the current range is doubled.
(c) Circuit with 2-mA meter to read the current.
VM = IM x rM IS = IT - IM RS = VM
IS
VM = 50mV IS = 1 mA RS = 50 W
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8-2: Meter Shunts
Fig. 8-5: Calculating the resistance of a meter shunt. RS is equal to VM/IS. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
VM = 0.001 x 50 = 0.05V or 50 mV
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8-2: Meter Shunts
Fig. 8-5: Calculating the resistance of a meter shunt. RS is equal to VM/IS. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
IS = 0.005 − 0.001 = 0.004 A or 4 mA
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8-2: Meter Shunts
Divide VM by IS to find RS.
RS = 0.05/0.004 = 12.5 Ω
This shunt enables the 1-mA movement to be used
for an extended range from 0-5 mA.
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8-3: Voltmeters
A voltmeter is connected across two points to measure
their difference in potential.
A voltmeter uses a high-resistance multiplier in series
with the meter movement.
A dc voltmeter must be connected with the correct
polarity.
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8-3: Voltmeters
A multiplier resistor is a large resistance in
series with a moving-coil meter movement
which allows the meter to measure voltages
in a circuit.
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8-3: Voltmeters
DCV
Using Multipliers to Increase
Voltmeter Range
VM = IM x rM = 0.1 V
9.9 kW
Sensitivity = rM
VM
= 1000 W per voltRmult = VFS
IM
- rM
Rmult
10 V
For a 25 V range, change Rmult to 24.9 kW.
Note: sensitivity is not affected by the multipliers.
DCV
Using Multipliers to Increase
Voltmeter Range
VM = IM x rM = 0.1 V
9.9 kW
Sensitivity = rM
VM
= 1000 W per voltSensitivity = rM
VM
= 1000 W per voltRmult = VFS
IM
- rMRmult = VFS
IM
- rM
Rmult
10 V10 V
For a 25 V range, change Rmult to 24.9 kW.
Note: sensitivity is not affected by the multipliers.
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8-3: Voltmeters
Typical Multiple Voltmeter Circuit
Fig. 8-7: A typical voltmeter circuit with multiplier resistors for different ranges. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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8-3: Voltmeters
Voltmeter Resistance
The high resistance of a voltmeter with a
multiplier is essentially the value of the
multiplier resistance.
Since the multiplier is changed for each
range, the voltmeter resistance changes.
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8-4: Loading Effect of a Voltmeter
When voltmeter resistance is not high enough,
connecting it across a circuit can reduce the measured
voltage.
This effect is called loading down the circuit, because
the measured voltage decreases due to the additional
load current for the meter.
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8-5: Ohmmeters
Fig. 8-10: How meter movement M can be used as an ohmmeter with a 1.5-V battery. (a)
Equivalent closed circuit with R1 and the battery when ohmmeter leads are short-circuited for
zero ohms of external R. (b) Internal ohmmeter circuit with test leads open, ready to measure an
external resistance.
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8-5: Ohmmeters
Resistance RT is the total resistance of RX and the
ohmmeter’s internal resistance.
NOTE: RX is the external resistance to be measured.
The ohmmeter’s internal resistance Ri is constant at 50 +
1450, or 1500 Ω here. If RX also equals 1500 Ω, RT equals
3000 Ω.
Fig. 8-11
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8-6: Multimeters
Fig. 8-13: Analog VOM that
combines a function selector and
range switch.
Fig. 8-14: Portable digital
multimeter (DMM).
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8-6: Multimeters
Fig. 8-15: DMM with amp clamp accessory.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The problem of opening a circuit
to measure current can be
eliminated by using a probe with
a clamp that fits around the
current-carrying wire.
The clamp probe measures only
ac, generally for the 60-Hz ac
power line.
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8-7: Digital Multimeters (DMMs)
The digital multimeter has become a very popular
test instrument.
The digital value of the measurement is displayed
automatically with decimal point, polarity, and the unit
for V, A, or Ω.
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8-7: Digital Multimeters (DMMs)
Fig. 8-16: Typical digital multimeter (DMM).
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Digital multimeters
are generally
easier to use.
They eliminate the
human error that often
occurs in reading
different scales on an
analog meter with a
pointer.
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8-8: Meter Applications
Fig. 8-17: How to insert a current meter in different parts of a series-parallel circuit to read
the desired current I. At point A, B, or C the meter reads IT; at D or E the meter reads I2; at F
or G the meter reads I3. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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8-8: Meter Applications
Fig. 8-18: With 15 V measured across a known R of 15 Ω, the I can be calculated as V/R or 15 V
/ 15 Ω = 1 A.
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8-8: Meter Applications
Fig. 8-19: Voltage tests to localize an open circuit. (a) Normal circuit with voltages to chassis
ground. (b) Reading of 0 V at point D shows R3 is open.
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8-9: Checking Continuity
with the Ohmmeter
The ohmmeter is a great tool for checking the
continuity between two points.
When checking for continuity, make sure the
ohmmeter is set on the lowest ohms range.
If continuity exists between two points, the ohmmeter
will read a very low resistance such as zero ohms.
If there is no continuity between two points, the
ohmmeter will read infinite ohms.
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8-9: Checking Continuity
with the Ohmmeter
Fig. 8-20: Continuity testing from point A to wire 3 shows this wire is connected.
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8-9: Checking Continuity
with the Ohmmeter
Fig. 8-21: Temporary short circuit at one end of a long two-wire line to check continuity from the
opposite end.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.