Course: EET110 (Electric Circuits I)

Instructor: Mr. Cosby

Title: Lab Overview: DC Circuit Measurements

Name: Erich Cosby

Lab Partners: none

Date: 5/18/2012

I pledge to support the Honor System of ECPI. I will refrain from any form of academic dishonesty or deception, such as cheating or plagiarism. I am aware that as a member of the academic community, it is my responsibility to turn in all suspected violators of the honor code. I understand that any failure on my part to support the Honor System will be turned over to a Judicial Review Board for determination. I will report to the Judicial Review Board hearing if summoned.

Signed_____________ehc____________ Date Signed ____5/18/12__

I. Abstract

The first EET110 lab was held as an introduction to building circuits on the breadboard and using DC test equipment. First, circuit calculations are performed to predict voltages and currents. Then, a circuit was built and measurements were taken. A MultiSim simulation was also built and the same measurements were made. A comparison of results between calculated, MultiSim, and the breadboard circuit did show differences in measurements. These differences are discussed in the report.

II. Introduction

In order to analyze electrical circuits, there are some mathematical equations that must be used. The primary equation for basic operation is called Ohm’s Law. Ohm’s Law is stated as follows:

I=VR

I is current (amps, A )V is voltage (volts, V )R is resistance (ohms, Ω)

This equation states that the current in a circuit is directly proportional to the voltage applied and inversely proportional to the circuit resistance. If voltage increases, then current increases. If resistance increases, then current decreases.

Using Ohm’s Law, predictions can be made as to how an electric circuit will behave.

III. Procedure (ECPI Faculty, 2010)

Analyze the circuit shown in Figure 1 using calculations, Multisim, and a prototype circuit.

Figure 1: DC Electric Circuit

a. Calculations

i. Current through R1

I = V/R = 12 V / 1 kΩ = 12 mA

ii. Voltage across R1

V = IR = 12 mA x 1 kΩ = 12 V

b. MultiSim

i. Current through R1

Insert a DMM in the circuit to measure current. The DMM must be in series with the current being measured and the DC Amps scale must be selected.

ii. Voltage across R1

Insert a DMM in the circuit to measure voltage. The DMM must be in parallel with the device being measured and the DC Volts scale must be selected.

Figure 2: MultiSim Circuit with DMMs in Place for Measurements

c. Prototype Circuit

i. Parts needed:

1 kΩ resistor (1) Prototype board (1) Test wires (as needed) DC Power Supply DMM

ii. Build the circuit as shown in Figure 1

iii. Adjust the DC Power Supply for 12 V output. Set the current limiter knob to mid-point.

iv. Verify the DC Power Supply output using the DMM. Adjust the DMM for DC volts scale and place the leads across the power supply output (black to black and red to red).

v. Measure the voltage across R1

Insert a DMM in the circuit to measure voltage. The DMM must be in parallel with the device being measured and the DC Volts scale must be selected.

Figure 3: Prototype Circuit Voltage Measurement

vi. Measure current through R1

Insert a DMM in the circuit to measure current. The DMM must be in series with the current being measured and the DC Amps scale must be selected.

Figure 4: Prototype Circuit Current Measurement

IV. Results

A comparison of results is shown in Table 1:

Parameter Measured

Calculated Value MultiSim Prototype

V1 12 V 12 V 12.076 V

I1 12 mA 12.001 mA 12.221 mA

Table 1: Comparison of Voltage and Current Results

V. Conclusion

The results showed that both the Ohm’s Law calculation and the MultiSim simulation provide good methods to predict how a real circuit will work. The actual measured values of the prototype circuit were not exactly the same as calculation or MultiSim. Some reasons for this are as follows:

a. DC supply voltage not adjusted to exactly 12 V DC

b. Resistor resistance not exactly 1 kΩ

c. Resistance in the test probe wires and the prototype sockets

d. Calibration of the test equipment

If the prototype circuit parameters were used in the calculations and in MultiSim instead of the exact, ideal values, then the results would be expected to match more closely.

Note that MultiSim uses circuit models and calculations in a similar, but more complete, way to how hand calculations are done. Multisim does not take into account things like wire resistance and imperfect resistor values.

Some problems did occur during the prototype testing. These problems included:

a. During the current measurement, the test probe was, at first, left in the volts/ohms input. It had to be moved to the 100 mA input.

b. When adjusting the DC power supply, the current limiter knob was set at minimum so the voltage was inhibited. The current limiter had to be set higher to allow the circuit to operate. The current limiter is important in that it will turn the supply off if too much current (a short circuit) occurs.

VI. References

a. EET110 DC Measurement Lab, ECPI EET Faculty, 2010, pages 1-5