DC Fundamentals
Unit 13 – Direct Current Meters
UNIT 13 – DIRECT CURRENT METERS
UNIT OBJECTIVE Determine how a meter movement can be used as an ammeter, an ohmmeter, or a voltmeter.
UNIT FUNDAMENTALS No Questions
CMS AVAILABLE None
FAULTS AVAILABLE None
NEW TERMS AND WORDS moving coil – The part of a meter having a pointer that deflects with current. permanent magnet – A magnet that retains its magnetism after the magnetizing force has been removed. full-scale meter current (IM) – Specified current required for full-scale deflection. meter resistance (RM) – The ohmic resistance of the wire making up the moving coil. sensitivity – The ratio of the total resistance of an instrument to the full-scale meter deflection.
EQUIPMENT REQUIRED F.A.C.E.T. base unit DC FUNDAMENTALS circuit board Multimeter DC Milliammeter Module
3-193
DC Fundamentals
Unit 13 – Direct Current Meters
Exercise 1 – The DC Ammeter
EXERCISE OBJECTIVE Determine current by using a basic meter movement. Verify ammeter operation by measuring known values of current.
EXERCISE DISCUSSION
Location: Exercise Discussion page: se1d4, Question ID: e1d4a In the circuit shown, how much current must flow through RSHUNT for a full-scale meter indication? a. 10 mA b. 9 mA c. 1 mA
EXERCISE PROCEDURE
Location: Exercise Procedure page: se1p3, Question ID: e1p3a 3. With a full-scale meter movement voltage drop (VM of 2.3) Vdc, what are the values of RM and IM that you will use in this procedure? a. RM = 2300Ω , IM = 1 mA full scale b. RM = 2300Ω , IM = 10 mA full scale c. RM = 230Ω , IM = 1 mA full scale Location: Exercise Procedure page: se1p4, Question ID: e1p4a 4. In the 1-mA circuit with VA at 10 Vdc, what is the required value of R4 for full-scale meter deflection? Use R4 = (VA – VM)/IM a. 77Ω b. 770Ω c. 7700Ω
3-194
DC Fundamentals
Unit 13 – Direct Current Meters
Location: Exercise Procedure page: se1p5, Question ID: e1p5a 5. What is the purpose of R4? a. R4 drops the voltage difference between the source and meter voltages. b. R4 shunts excess current away from the meter coil. c. R4 increases the power dissipation of the meter coil. Location: Exercise Procedure page: se1p7, Question ID: e1p7a 7. What current value is displayed on the meter and what is the circuit current? a. 10 mA and 10 mA b. 1 mA and 1 mA c. 10 mA and 1 mA Location: Exercise Procedure page: se1p9, Question ID: e1p9a 9. Slowly adjust R8 until the meter gives a full-scale indication. What current reading is now indicated by the meter (remember the circuit shunt path)? What is the actual current through the meter? a. 10 mA and 1 mA b. 5 mA and 5 mA c. 1 mA and 10 mA Location: Exercise Procedure page: se1p10, Question ID: e1p10a 10. Based on your circuit observations, what is the purpose of R8? a. R8 shunts current exceeding 1 mA away from the meter coil. b. R8 extends the meter scale to indicate a full-scale current of 10 mA. c. All of the above.
3-195
DC Fundamentals
Unit 13 – Direct Current Meters
REVIEW QUESTIONS
Location: Review Questions page: se1r1, Question ID: e1r1 1. The dc resistance of a meter's moving coil a. has no effect on a circuit. b. protects the coil from overload. c. develops a voltage drop. d. is about zero ohms. Location: Review Questions page: se1r2, Question ID: e1r2 2. The addition of a shunt resistor a. extends the full-scale indication of the meter. b. allows the meter coil to pass more current. c. has no effect on the full-scale indication of the meter. d. reduces the full-scale indication of the meter. Location: Review Questions page: se1r3, Question ID: e1r3 3. If a dc milliammeter is rated at 10 mA full scale, then a. the meter can indicate only a 10 mA current. b. the meter pointer will be at full scale at 10 mA. c. a shunt is needed to divert 9 mA of current. d. any current below 10 mA will indicate full scale. Location: Review Questions page: se1r4, Question ID: e1r4 4. To extend the range of the meter used in this exercise to 0–100 mA, make the value of the shunt resistor that is required for full-scale deflection (100 mA) a. 0.232Ω . b. 23.2Ω . c. 232Ω . d. 2320Ω . Location: Review Questions page: se1r5, Question ID: e1r5 5. The meter used in this exercise has a permanent magnet and a moving coil, and it is used as a dc milliammeter. Based on this description, a. the meter can be directly used for dc and ac current measurements. b. polarity is not a consideration for connection. c. the meter can be used only to measure current. d. the meter must be connected for proper meter movement deflection.
3-196
DC Fundamentals
Unit 13 – Direct Current Meters
CMS AVAILABLE None
FAULTS AVAILABLE None
3-197
DC Fundamentals
Unit 13 – Direct Current Meters
Exercise 2 – The DC Ohmmeter
EXERCISE OBJECTIVE Measure resistance by using a basic meter movement. Verify ohmmeter operation by measuring known resistor values.
EXERCISE DISCUSSION
Location: Exercise Discussion page: se2d4, Question ID: e2d4a In the circuit, the meter current and pointer deflection are a. 1 mA and full- scale deflection. b. 0.5 mA and half- scale deflection. c. zero current and no deflection.
EXERCISE PROCEDURE
Location: Exercise Procedure page: se2p5, Question ID: e2p5a 4. Why is the ohmmeter (meter movement) not reading zero ohms? a. R8 is a maximum resistance and less than 1 mA flows through the circuit. b. R8 must be adjusted for a full-scale meter deflection (1 mA), which indicates zero ohms (zero adjust connection). c. All of the above. Location: Exercise Procedure page: se2p8, Question ID: e2p8a 7. Based on your meter indication, the resistance of R6 is about a. 49.9 kΩ . b. 10 kΩ . c. 1 kΩ . Location: Exercise Procedure page: se2p10, Question ID: e2p10a 9. Based on your meter indication, the resistance of R7 is about a. 49.9 kΩ . b. 10 kΩ . c. 1 kΩ .
3-198
DC Fundamentals
Unit 13 – Direct Current Meters
REVIEW QUESTIONS
Location: Review Questions page: se2r1, Question ID: e2r1 1. An ohms adjust rheostat is usually a part of the internal resistance of an ohmmeter to compensate for a. large variations in meter movement resistance. b. ohms scale errors during manufacture. c. variations in ohmmeter supply voltage. d. current variations when small values of resistors are measured. Location: Review Questions page: se2r2, Question ID: e2r2 2. What is the current flow through a series ohmmeter when it indicates zero resistance? a. maximum b. minimum c. halfway between minimum and maximum d. None of the above. Location: Review Questions page: se2r3, Question ID: e2r3 3. The total internal resistance of an ohmmeter having a 1.5V internal battery and using a 0-1 mA meter movement would be a. 3000Ω . b. 2500Ω . c. 2300Ω . d. 1500Ω . Location: Review Questions page: se2r4, Question ID: e2r4 4. If you used the ohmmeter from question 3 and an unknown resistor caused the pointer to go to half-scale, the value of the unknown resistor would be a. 2500Ω . b. 2300Ω . c. 1500 Ω . d. 500Ω . Location: Review Questions page: se2r5, Question ID: e2r5 5. When using an ohmmeter, you should disconnect the applied voltage from the circuit being checked because a. the voltage source will increase the resistance. b. the current will decrease the resistance. c. the ohmmeter has its own internal battery. d. no current is needed for the meter movement.
3-199
DC Fundamentals
Unit 13 – Direct Current Meters
CMS AVAILABLE None
FAULTS AVAILABLE None
3-200
DC Fundamentals
Unit 13 – Direct Current Meters
Exercise 3 – The DC Voltmeter
EXERCISE OBJECTIVE Measure volts by using a basic meter movement. Verify voltmeter operation by measuring known values of voltage.
EXERCISE DISCUSSION No Questions
EXERCISE PROCEDURE
Location: Exercise Procedure page: se3p6, Question ID: e3p6a 5. What is the full-scale voltage range of the meter? a. 100 volts b. 10 volts c. 1 volt Location: Exercise Procedure page: se3p7, Question ID: e3p7a 6. Monitor the positive variable supply with an external multimeter. Slowly adjust the positive supply between zero and 10 Vdc. Does your F.A.C.E.T. meter agree with the reading displayed on your multimeter? a. Yes. The readings are about the same. b. No. The readings do not agree. c. No. The F.A.C.E.T. circuit meter has a milliampere scale. Location: Exercise Procedure page: se3p9, Question ID: e3p9a 8. Why does your F.A.C.E.T. meter indicate half-scale (5 Vdc)? a. The series circuit composed of R8 and R1 drops 5 volts. The remaining 5 volts dropped across the meter resistance generates a half-scale meter deflection. b. R8 and R1 shunt 5 volts around the in-circuit meter. c. The circuit is calibrated for 10 volts full scale. Therefore, a five-volt external voltage should generate 0.5 mA of circuit current. This 0.5 mA generates a half-scale meter deflection.
3-201
DC Fundamentals
Unit 13 – Direct Current Meters
Location: Exercise Procedure page: se3p10, Question ID: e3p10a 9. Move the positive lead of the F.A.C.E.T. meter from the R1/R8 circuit to the R2/R8 circuit. Adjust R8 for a full-scale meter indication. What is the full-scale voltage reading, and what is the meter current at this reading? a. 10V indicated at a current of 1 mA b. 5V indicated at a current of 1 mA c. 5V indicated at a current of 0.5 mA Location: Exercise Procedure page: se3p11, Question ID: e3p11a 10. Moving the F.A.C.E.T. meter from the R1/R8 circuit to the R2/R8 circuit a. changed the meter calibration to about 5 Vdc full scale at 1 mA of full-scale current. b. changed the meter full-scale current from 1 mA to 0.5 mA. c. had no effect on the meter's full-scale voltage indication.
REVIEW QUESTIONS
Location: Review Questions page: se3r1, Question ID: e3r1 1. The dc resistance of a meter's moving coil a. has no effect in a voltmeter current. b. must be considered as part of the meter multiplier resistance. c. is connected in parallel with the voltmeter. d. is used as a meter protector. Location: Review Questions page: se3r2, Question ID: e3r2 2. Multiplier resistors are used in a voltmeter to a. shunt voltage around the meter movement. b. limit the voltage at the input of the voltmeter. c. extend the range of the meter. d. shunt the current from the meter movement. Location: Review Questions page: se3r3, Question ID: e3r3 3. In the figure, R8 is set for 1050Ω . The meter range must be extended from 10V to 100V fullscale. What is the required (new) R1 value? a. 99.5 kΩ b. 99 Ω c. 96.65 kΩ d. 96 kΩ
3-202
DC Fundamentals
Unit 13 – Direct Current Meters
Location: Review Questions page: se3r4, Question ID: e3r4 4. If the meter used in this exercise were set to the 0-5V range, and if a measured voltage deflected the pointer to 0.7 on the scale, then the measured voltage would be a. 1.5 volts. b. 2 volts. c. 3 volts. d. 3.5 volts. Location: Review Questions page: se3r5, Question ID: e3r5 5. In the figure, R8 is set for 1050Ω . R2 is 1650Ω . What is the sensitivity (ohms/volt) of the meter used in the 0-5V range? a. 1000 b. 1500 c. 2000 d. 2500
CMS AVAILABLE None
FAULTS AVAILABLE None
3-203
