BME 3511 Bioelectronics I - Laboratory Exercise #3
Parallel Resistive Circuits
Introduction:
Electrical measurements are essential techniques for trouble shooting electronic
equipment/circuits. The three quantities of voltage, current, and resistance are the basis for most
analyses of both constant (direct current DC) and time variant (alternating current AC) circuits.
Frequency, capacitive reactance, and inductive reactance are three additional factors related to
AC circuits.
Multimeters are the most frequently used instrument to measure current, voltage, and resistance.
Digital Multimeters (DMM) are convenient, accurate, portable, and durable. MCM DMM
Model 72-7940 is an inexpensive, yet relatively accurate DMM that will be used in BME 3511.
Note on Biomedical Electronics Lab Safety:
Electric shock can be fatal; read and heed the BME 3511 Bioelectronics Safety Guidelines.
In general, the undergraduate BME electronic laboratory experiments conducted in BME
Teaching Laboratories, do not use voltages greater than 30 V (± 15 V); therefore, the chance of
receiving an electrical shock is greatly reduced. However, all voltages do have the potential to
burn materials and start fires, destroy electronic components, and present hazards to the person
performing the operations. Common sense and an awareness of electrical circuits is important
whenever you are working on these experiments.
Objectives:
Become familiar with DMM functions and characteristics; and with the techniques for using a
DMM to measure resistance, DC voltages, and DC currents.
Be knowledgeable of voltages and current flow in parallel resistive circuits.
Laboratory Equipment and Supplies:
DMM
DC Variable Power Supply
Assorted Resistors
Breadboard
Background:
The MCM DMM Model 72-7940 features a rotary switch that can used be to select various
functions and range of measurement values. For DMM specifications, please refer to the Digital
Multimeter DMM Model 72-7940 Functions, Ranges, Resolutions, and Accuracy (page 4 of the
BME 3511 Bioelectronics I - Laboratory Exercise #1 procedures guidelines).
Refer to the DMM itself for examples of the function labels; starting at the top center and
moving clockwise: Off, AC Voltage, DC Current, Square Wave Output, Battery Tester, Diode
Test, Resistance, and DC Voltage. AC current measurement is not supported.
Note: The maximum allowable DC current is 200 mA.
Page 1
Measuring DC Voltages and DC Currents Using the MCM DMM Model 72-7940
DC Voltage Measurement Procedure
The DC voltage measurement positions are: 200mV, 2000mV, 20V, 200V, and 300V.
To measure DC voltage, connect the meter as follows:
1. Set the rotary switch to an appropriate measurement position in the DC voltage range. The
rotary switch should be placed in the desired position prior to connecting the leads. This position
should not be changed while the leads are connected.
2. Connect the test leads in parallel across with the component being measured. The measured
value shows on the display.
3. When the DC voltage measurement has been completed, disconnect the testing leads from the
circuit under test.
Notes:
DO NOT connect the DMM in series while using the DC Voltage Function.
If the value of DC voltage to be measured is unknown, use the maximum measurement position
(300V) and reduce the range step by step until a satisfactory reading is obtained.
In each DC voltage range, the DMM has an input impedance of greater then 1 MΩ. This loading
effect can cause measurement errors in high impedance circuits. If the circuit impedance is less
than 1 KΩ, the error is negligible (0.1% or less).
Source: MCM DMM Model 72-7940 Operating Manual
DC Current Measurement Procedure
The DC current measurement positions are: 2000 μA, 20 mA, and 200 mA.
To measure DC current, connect the meter as follows:
1. Turn off power to the circuits. Discharge all high-voltage capacitors.
2. Set the rotary switch to an appropriate measurement position in the DC current range. The
rotary switch should be placed in the desired position prior to connecting leads. This position
should not be changed while the leads are connected.
3. Break the circuit continuity of the current path to be tested. Connect the red lead in series to
the more positive side of the current path break and the black lead to the more negative side of
the current path break.
4. Turn on the DC power to the circuit. The measured value shows on the display.
Notes:
DO NOT connect the DMM in parallel across the load while using the DC Current Function.
If the value of DC current to be measured is unknown, use the maximum measurement position
(200 mA) and reduce the range step by step until a satisfactory reading is obtained.
After completing the DC current measurement, turn off the DC power and disconnect the
connection between the testing leads and the circuit under test.
Source: MCM DMM Model 72-7940 Operating Manual
Page 2
Procedure: Measuring Resistance, DC Voltage, and DC Current Values
Step 1.
Select three resistors (560Ω, 2200Ω, 6800Ω) that were used in Laboratory Exercise #1.
Record the nominal values on the Laboratory Exercise # 3 Report Form.
Use the appropriate range scale on the DMM to measure the resistor values;
and record your readings in Table 1.
Using the measured resistor values, calculate and record each of the following values:
Equivalent resistance for all three resistors if they were placed in parallel with one another.
Circuit current if the circuit voltage source were 12 VDC.
Current through each of the three individual resistors in parallel.
Step 2.
Construct a parallel circuit using a breadboard and the three resistors.
Do Not connect any voltage source.
Using the DMM Resistance Function:
Measure across all three parallel resistors and confirm the equivalent resistance value.
Step 3.
Confirm that the DC power supply is turned off.
Connect the DC power supply in series with the resistors.
Turn on the power supply and set the output voltage to 12 VDC.
Using the DMM DC Voltage Function, select the appropriate VDC Range:
Measure the voltage across all three resistors in parallel and confirm the 12 DC voltage.
Turn off the power supply and disconnect it from your circuit.
Step 4a. (See Figure A, Page 5)
Confirm that the DC Power Supply is turned off.
Connect the power supply in series with the DMM and the first of the three parallel resistors.
Connect the DMM in series with the resistors.
Select the DC Current Function and Range = 200 mA.
Do Not use any function other than DC Current.
Turn on the power supply and set the output voltage to 12 VDC.
Record the current reading.
Turn off the power supply and disconnect it from your circuit.
Step 4b & 4c. Repeat Step 4a for each of the other resistors.
Page 3
Procedure: Measuring Resistance, DC Voltage, and DC Current Values - continued
Step 5. (See Figure B, Page 5)
Confirm that the DC Power Supply is turned off.
Connect the power supply in series with the DMM and the three parallel resistors.
Connect the DMM in series with all three of the parallel resistors together.
Select the DC Current Function and Range = 200 mA.
Do Not use any function other than DC Current.
Turn on the power supply and set the output voltage to 12 VDC.
Record the current reading.
Turn off the power supply and disconnect it from your circuit.
Laboratory Report Comments and Observations
Safe guard your resistors from Laboratory Exercise #3.
You will need these same resistors for later laboratory exercises.
You must annotate any references you consulted in answering the questions below.
You may wish to use a word processor to complete your answers and attach the printout to your
Laboratory Exercise #3 Report Form.
1. In addition to completing Table 1, provide comments regarding any differences between the
nominal, calculated, and measured values for resistance, current, and voltage.
2. If you connected the three resistors in series (as in Lab #2) and added another resistor in series,
2a. Would the total current have increased or decreased?
2b. Would the current through each individual resistor have increased or decreased?
2c. Would the voltage drop across each resistor have increased or decreased?
3. If you connected the three resistors in parallel (as in Lab #3) and added another resistor in parallel,
3a. Would the total current have increased or decreased?
3b. Would the current through each individual resistor have increased or decreased?
3c. Would the voltage drop across each resistor have increased or decreased?
Page 4
Connecting Current Measuring DMM in Series with Resistive Load and DC Power Supply
+ Red Probe to + of the Power Supply
+
A
- Black Probe
Ammeter in Series
with Power Supply
-
Steps 4a, b, c. Connecting the DMM in series with each of parallel resistors.
Figure A
+ Red Probe to + of the Power Supply
+
A
- Black Probe
Ammeter in Series
with Power Supply
-
Step 5. Connecting the DMM in series with all three of the parallel resistors together.
Figure B
Page 5
BME 3511 Bioelectronics I Laboratory Exercise #3 Report Form
Parallel Resistive Circuits
I affirm that I personally participated in the collection and analysis of the data for this laboratory
exercise and that I personally contributed to the completion of this laboratory report.
Student Name: ___________________________________________
Signature: ___________________________________________ Date: __________________
I affirm that I personally participated in the collection and analysis of the data for this laboratory
exercise and that I personally contributed to the completion of this laboratory report.
Student Name: ___________________________________________
Signature: ___________________________________________ Date: __________________
Grade: ___________________
Grader Comments:
BME 3511 Bioelectronics I Laboratory Exercise #3 Report Form
Nominal
Value
DMM
Measurement
Calculated
Value
DMM
Measurement
R1
IR1
R2
IR2
R3
IR3
REQ
IREQ
Calculated Parallel Current (12 VDC) _________
Measured Total Current (12 VDC) ___________
Table 1 Recorded DMM Nominal, Calculated, and Measured Values