1418-1, Capacitors

and Capacitive

Circuits

Experiment 5: How Frequency Affects Capacitive Reactance

Objective of Experiment 5

• To verify that capacitive reactance

(XC) varies inversely with applied

frequency.

• This means that (XC) will increase as

frequency decreases

• This also means that (XC) will decrease

as frequency increases

• f = frequency of applied signal in Hertz

• C = Capacitance of the capacitor in Farads

• 2π = is a constant whose value is

approximately 6.28

Figure 18 of 1418-1

• Be aware that the frequency

calibration of the frequency generator

in your trainer is limited in accuracy.

• We are no-longer using the 30VAC

60Hz as our Input

Suggested Procedure 1. Construct the circuit

I. Connect 0.039µF capacitor between terminals A and B.

2. Apply the 200Hz Signal to terminal A

I. Your goal is to have VC equal to VR

a. Place Common (Black) lead at Terminal B.

b. Place a jumper post at point B

in your circuit.

c. Clip the Black meter probe lead to

post “B” with an alligator clip or lead.

d. Move the Red lead/meter probe

between Terminals A and C.

i. Make small adjustments on the pot.

a. 1st Measure Voltage across capacitor

b. Then the Pot, adjust pot to match

ii. Keep making fine adjustments by

measuring the AC voltage at points A

and C until A = C.

e. Turn off the trainer after the

measurements have been made and

the voltages are the same.

f. Disconnect the leads from the Pot,

measure and record the measurement

in Fig/table 17 of the lab.

3. While we are listing DC voltage (not

AC effective) measurements in our

results, these results are not really

important to the experiment.

What we are really interested in are the

resistance measurements and the trends

they show.

I. This is because the voltage

measurements can vary widely

depending on actual circuit conditions.

II. It is not necessary to look up the

effective values of the voltages.

• Remember the following:

1. The blocking capacitor is only used with

the Analog meter!

2. The voltage doubler is only needed with

the analog meter.

1. The voltage doubler can be used with a

digital meter, but is not required.

1418-1, Fig 18 Experiment 5

1418-1, Exp. 5, Fig 19 built

Suggested Procedure

1. Construct the circuit in Fig 20

I. Connect a 5600Ω Resistor for RP

II. Connect a 1000Ω Resister for RS

2. Set your meter to 10VDC at output of

voltage doubler

3. Apply the 1kHz Signal to terminal A

I. Your goal is to have VC equal to VR

4. Use the slide switch and the 100k pot so you have equal voltage readings at both positions of the slide switch.

I. It is normal for the needle of the meter to jump when you change the switch settings, but when the needle settles on the same value for both settings, the circuit is balanced.

5. Record the voltage measurements and Frequency in Figure 17.

6. Continue with the experiment

changing the components and

recording the values as indicated.

1418-1, Exp. 5, Fig 20a, adding

Parallel RP & Series RS

1418-1, Exp. 5, Fig 20b built

CIE Results

• When analyzing the CIE results, we find

that a change in frequency of 10:1

produced a change in capacitance

reactance of approximately 1:6.5.

• Under formal conditions, this data would

not be acceptable; however, with the poor

frequency calibration, we were still able to

verify capacitive reactance does decrease

with an increase in frequency.

Questions?

The End

Developed and Produced by the Instructors in the CIE Instruction

Department.

© Copyright 04/2012

All Rights Reserved / April 2012