This lab exercise is ©2002 by Ren Colantoni. You are welcome to use it for any non-profit educational purpose to which it may
apply. Just don't try to make money with it.
El 04- Electronics 4
Laboratory Exercise 11 – Parallel Circuits, Part 2
OBJECTIVE: To examine the various aspects of current found in parallel circuits.
MATERIALS: Protoboard; power supply; multimeter; resistors: 4.7K, 10K, 22K, 33K,
68K.
DISCUSSION: In parallel circuits, we have multiple current paths available as we add
resistors or "branches" to the circuit. As a result, the total current that the power supply
must provide is going to be greater than the current through any individual branch. This is
explained in the formula IT = I1 + I2 + I3 + IN …, where the total current in the circuit is
equal to the sum of the individual currents of each branch.
In a manner similar to the voltage divider approach that we used to determine individual
voltages in a series circuit, we can make use of a current divider formula that allows us
to determine the proportion of the total current of a parallel circuit that flows through an
individual branch. In all cases, we find that the parallel branch with the smallest
resistance carries the largest current, that is, the largest amount of the current that goes to
make up the total current of the circuit. This formula appears as I1 = R2 / (R1 + R2) × IT,
where R1 and R2 are in parallel, and IT is the total current provided by the power supply.
Note the inverse relationship of the selected resistor to the sum of the resistors. If we
wish to calculate the current through R1, we put R2 on the top of the formula. If we
wanted to calculate the current through R2, we would put R1 on the top of the formula.
In this exercise, we will build a variety of circuits and apply the formulas to determine
their current flow and to apply the formulas to see if they are valid.
PROCEDURE:
1. Connect the circuit of Figure 1 below. Note that the lead on the right side of the
ampmeter is to be free to connect to any one of the three resistors – it is not
attached only to R1.
2.
3.
4.
5.
Measure the current through R1: __________
Measure the current through R2: __________
Measure the current through R3: __________
Compute IT = I1 + I2 + I3: __________
6. Next, connect the same circuit again, with all three branches hooked up – see
Figure 2:
7. Measure IT in the circuit of Figure 2: __________
8. What is your conclusion when you compare the measured current to the
calculated?
__________________________________________________________________
___
9. Connect the circuit of Figure 3 below:
10. Measure IT in the circuit of Figure 3: __________
11. Now, reconnect the circuit so that it looks like Figure 4. Note that they are the
same circuit, we have just moved the ampmeter from the main leg of the circuit to
a branch.
12. Measure the current through R1: __________
13. Calculate the current through R2 using the formula IR2 = IT – IR1: __________
14. Now, move the meter to the R2 branch of the circuit of Figure 4. Don’t forget to
close the gap where the meter was inserted in the R1 branch.
Measure the current in R2: __________
15. How do the calculated and measured values of IR2 compare?
______________________
16. Connect the circuit of Figure 5.
17. Adjust the power supply until the ampmeter reads 1.5mA.
18. Using the current divider formula IR1 = R2/(R1 + R2) × IT, determine the current
through R1:
IR1 Calculated : __________
19. Without changing the power supply, move the meter so that you measure the
current through R1.
IR1 Measured: __________
20. Similarly, use the current divider formula to determine the current through R2:
IR2 Calculated: __________
21. Move the meter again to measure the current through R2. Don’t touch the power
supply.
IR2 Measured: __________
22. Add the measured and calculated currents of steps 18, 19, 20, and 21 and then
compare these to the current of step 17. How do these compare or relate?
__________________________________________________________________
____________
__________________________________________________________________
____________
23. Remove power and return the equipment and supplies to their proper storage.
Then, answer the following questions in writing for credit.
REVIEW QUESTIONS:
1. In a parallel circuit, which resistor branch draws the most current? The least
current?
2. Two resistors are connected in parallel. A third resistor then is added in parallel to
the first two. What happens to IT? Why?
3. Two resistors of 10K and 30K are connected in parallel. The total current for the
circuit is 100mA. Write the formula for finding the current through the 10K
resistor.
4. How much current flows in 30K resistor of question 3?
5. You have a circuit of 15 resistors of equal value in parallel. The current through
one of these is 2.74mA. What is the total current in the circuit?