1. Investigate the Characteristics of a Series DC Circuit

EET 130 Lab #3

Series DC Circuits

Objectives

The objectives are to: 1. Investigate the characteristics of a series DC circuit. 2. Verify Kirchhoff’s Voltage Law. 3. Observe the effects of short and open circuits as aids to troubleshooting.

Equipment Needed

One DC power supply One DMM One light-emitting diode (LED) (red) Various resistors (1 kΩ, 2.7 kΩ, 470 Ω) Miscellaneous black and red connecting wires and alligator clips

Procedure

Part 1: Series Circuit Resistance

1. Calculate the total series resistance (Rt) of the circuit shown in Figure 1. Show all work here.

Rt = ______

2. Build the circuit shown in Figure 1. Place the components on your Velcro board just like you see them drawn in Figure 1. Connect the resistors together with short wires labeled jumper wires in the Figure.

3. Set up the DMM to measure resistance in Ω. Connect the meter across terminals A and D and measure the total resistance. Record the total resistance value below. It should be close to the theoretical resistance value calculated in step 1. Are they close? ______

Rt = ______Ω = ______kΩ 4. Simulate a short-circuit condition by connecting a jumper wire across terminals B and C. Refer to Figure 2 below. Use the DMM to measure each of the following resistances:

RAB = ______RBC = ______(short) RCD = ______

This should illustrate that a short circuit can have close to zero resistance. Did your observation show this? ______

Obtain approval from instructor before proceeding ______

Part 2: Series Circuit Voltage:

5. Assume that 18 volts is applied to the series DC circuit in Figure 1. Calculate the total current and the voltage drop across each of the resistors. Show all work here, record your answers.

I = ______V1 = ______V2 = ______V3 = ______

6. Remove the shorting jumper wire from the circuit. Connect the DC power supply to terminals A and D as shown in Figure 3. Set the supply voltage to exactly 18.0 volts. Always use the DMM to determine the DC power supply voltage. Now measure and record the voltage drops across each resistor and record the values below. These measured values should be close to the theoretical values calculated in step 5.

V1 = ______V2 = ______V3 = ______7. Turn off the DC power supply. Set the DMM to measure current. Set the range to 20 mA on the DMM. Ask the instructor if necessary. Remove the jumper wire connecting R1 to R2 (point B) and connect the DMM in its place. Refer to Figure 4. Turn on the DC power supply. Measure and record the current at point B. I at pt. B ______8. It is recommended that the DC power supply be turned off whenever changing the circuit. Remove the DMM and replace it with a jumper wire. Then remove the jumper wire connecting R2 and R3 (point C). Connect the DMM in its place. Measure and record current at point C.

I at pt. C ______

9. The currents at points B and C should have been the same. If not, check the wiring in the circuit. The measured current should be very close to the theoretical value calculated in step 5. Are they close? ______

10.Connect a jumper wire across R2 again as done previously. Measure and record the current below. Remove the DMM and reconnect the circuit. Set the DMM to measure DC voltage. Measure and record the voltage drops across each resistor, including the short-circuited resistor.

I (with R2 short circuited) = ______VAB ______VBC ______VCD ______

Now calculate the current that should flow if R2 is short-circuited. I = ______The calculated current value should be close to the measured value. Are they close? _____

11. Remove the shorting wire.

12. Make a general statement about the current in a series DC circuit, referring to the results in steps 7 and 8.

Statement: ______

13.What happened to the current when one of the series resistors was shorted out? Did the current decrease, increase, or remain the same? ______

Why? ______

14.Remove the jumper wire connecting R1 and R2 to create an open-circuit condition. Measure the voltage across this open, which is between the right side of R1 and the top of R2. Record the voltage level below.

V (open circuit) = ______

15.Make a general statement about the voltage you would expect to observe across any open circuit.

Statement: ______

Obtain approval from instructor before disconnecting circuit ______Part 3: Series circuit with an LED

16.Turn off the DC power supply and return voltage control to zero volts. Disassemble the circuit. Now refer to the circuit shown in Figure 5. A series voltage-dropping resistor is required to operate the LED when connected to a 5-volt DC power supply. The nominal “on” voltage for a red LED is about 2 volts. Therefore, the resistor R must drop the remaining 3 volts. If the nominal current for the LED is 15 mA, calculate the value of the series voltage-dropping resistor needed for the circuit. The value of R should be between 100Ω and 500Ω. Show all calculations here.

17. Build the circuit shown in Figure 5 using the closes standard value resistor for R. Turn of the DC power supply and set it to 5 volts. The LED should be glowing. Measure and record the current and the voltage drops across the LED and resistor. Then turn off DC power supply.

I = ______VR = ______V LED = ______

Obtain final approval from the instructor ______

Questions and Problems .

1. What is the voltage drop across any short-circuited resistor? ______

2. Refer to Figure 3 and use Watt’s Law to calculate the power dissipated in each resistor and that delivered by the DC power supply. Show all calculations.

3. From your measurements in step 17, calculate the power dissipated in resistor R and the LED.

4. Use the voltage divider rule to calculate the voltage drops across each resistor in Figure 3.

5. If the DMM is set to the 2KΩ range and the display shows 0.684, what is the resistance expressed in Ω?