EET 130 Lab #1
Simple Direct Current (DC) Circuit Measurements
Objectives
Learn how to:
1. Construct a simple direct current (DC) circuit.
2. Set and adjust voltage levels using a DC power supply.
3. Make voltage and current measurements using a digital multimeter (DMM).
Equipment Needed
One DC power supply
One DMM
Two 12-volt light bulbs
Miscellaneous black and red connecting wires and alligator clips
Procedure
1. Turn on the DC power supply and adjust the voltage control knobs (course and fine) full counterclockwise (this is zero volts). Turn the current control knob full clockwise. The current level should remain in this position.
2. Turn on the DMM and set it to measure DC volts. Ask the instructor for help if necessary. Connect the test leads across the power supply as shown in Figure 1.
3. Using the course control knob, adjust the DC power supply voltage to obtain a reading of approximately 18 volts on the DMM. Then use the fine control knob to obtain a reading of 18.0 volts. Next, set the reading to 10.0 volts using the same procedure. Turn off the DC power supply, but do not adjust voltage controls. Leave the DC power supply set to 10.0 volts.
4. Connect one 12-volt light bulb as shown in Figure 2. Turn on the DC power supply and set to 10.0 volts, if necessary. The light bulb should be glowing to near full brilliance. Turn off the DC power supply, but leave it set to 10.0 volts. Set the DMM to measure direct current. Ask the instructor for help if necessary. Refer to Figure 3 which shows the DMM connected in “series” with the DC power supply. Note that the DMM is connected between the positive (red) terminal of the DC power supply and the light bulb so the current has only one path to follow. The current flows from the DC power supply, through the DMM and light bulb, and then back to the negative (black) terminal of the DC power supply.
Have instructor check circuit before proceeding ______
5. Turn on the DC power supply and record the current reading which is displayed in Amperes (A). To convert to milli-Amperes (mA), multiply by 1000.
Current (I) = ______A = ______mA
6. Turn off the DC power supply. Connect another light bulb in series with the one already in the circuit as shown in Figure 4. Turn on the DC power supply, which should still be set to 10.0 volts. The DMM will now display the current flowing through both light bulbs. The current should be much less than when only one light bulb was connected in the circuit. Record the current:
Current (I) = ______mA
7. Turn off the DC power supply. Connect the DMM leads across one of the light bulbs (red lead to positive and black lead to negative). Ask the instructor for help if necessary. Turn on the DC power supply. Record the voltage across the light bulb. Repeat the procedure for the second light bulb observing the same polarity.
V1 = ______V2 = ______
8. Does the sum of the two measured voltage values approximately equal the DC power supply voltage (10 volts)? ______You have verified Kirchhoff’s Voltage Law.
9. Unscrew one light bulb from its socket until the light goes out. Observe the effect on the current and on the other light bulb. What did you observe?
Observation: ______
10. Turn off the DC power supply. Connect the two light bulbs in parallel, one across the other,
as shown in Figure 5. Use short jumper wires to make the connections.
Have instructor check circuit before proceeding ______
11. Turn on the DC power supply and record the current again. The current should have
increased significantly.
Current (I) = ______mA
12. Turn off the DC power supply. Disconnect the DMM and connect A and B with a jumper
wire. Then remove jumper connecting bottom wire and the bottom of L1. In its place,
connect the DMM in series with L1, as shown in Figure 6. This will allow you to measure and
record the current flowing through only L1. Replace the jumper wire to L1 and remove the
jumper wire from the bottom of L2 and connect the DMM in series with L2. Record the current
in L2.
L1 current = ______L2 current = ______
13. Does the sum of the two measured current values approximately equal the total supply current
recorded in step 11 above? ______
14. Unscrew one of the light bulbs from its socket so that the light goes out. Observe and record
the effect on the other light bulb and its current.
Observation: ______
15. You have verified Kirchhoff’s Current Law for parallel connected circuits. The sum of the currents in parallel connected branches will always equal the total supply current.
16. Turn off DC power supply. Completely disconnect the circuit and return all equipment.