Exercise 1: The DC Ammeter EXERCISE OBJECTIVE When you have completed this exercise, you will be able to determine current by using a basic meter movement. You will verify ammeter operation by measuring known values of current. DISCUSSION The ammeter is an instrument that measures current. The meter must be placed in series with the circuit and must be connected in the correct polarity. Any current over 1 ma causes the meter to go off scale or peg against the upper stop pin of the meter movement. Therefore, current that exceeds 1 ma must be shunted away from the meter movement. The meter used in this unit has a sensitivity of 1 ma. The range of this meter can be increased by placing a resistor (a meter shunt, R SHUNT ) in parallel with the meter movement. SHUNT for a full-scale meter indication? a. 10 ma b. 9 ma c. 1 ma FACET by Lab-Volt 273
DC Fundamentals R SERIES drops the difference between V M (2.3V) and the applied circuit voltage (V A ). R SHUNT bypasses all circuit current exceeding 1 ma, the full-scale meter current. PROCEDURE If necessary, clear the DC FUNDAMENTALS circuit board of all two-post connectors and any other connections. NOTE: Do not energize the AMPS circuit of the VOLTMETER/AMMETER/OHMMETER circuit block at this time. On the VOLTMETER/AMMETER/OHMMETER circuit block, adjust the variable 10-volt supply for 10 Vdc. Be precise (use both COARSE and FINE adjustment controls). With a full-scale meter movement voltage drop (V M of 2.3) Vdc, what are the values of R M and I M that you will use in this procedure? a. R M = 2300, I M = 1 ma full scale b. R M = 2300, I M = 10 ma full scale c. R M = 230, I M = 1 ma full scale 274 FACET by Lab-Volt
In the 1 ma circuit with V A at 10 Vdc, what is the required value of R4 for full-scale meter Use R4 = (V A V M )/I M a. 77 b. 770 c. 7700 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. Connect your dc milliammeter module in the 1 ma circuit. Energize the AMPS section of the circuit block. Use the FINE adjust control of the positive variable supply to obtain an exact full-scale meter indication. 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 FACET by Lab-Volt 275
DC Fundamentals Adjust rheostat R8 to its maximum CW position. Move the positive connection of the meter movement module to the 10- ma circuit position. NOTE: Do not move the positive supply controls for the remainder of the procedure. 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 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. Remove all circuit board connections. CONCLUSION A shunt resistor bypasses meter current and extends the range of a milliammeter. A resistor is placed in series with a basic meter movement to drop the difference between an applied circuit voltage and the meter movement voltage drop. REVIEW QUESTIONS 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. 276 FACET by Lab-Volt
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. 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. 4. To extend the range of the meter used in this exercise to 0 100 ma, make the value of the shunt a. 0.232. b. 23.2. c. 232. d. 2320. 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. FACET by Lab-Volt 277