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EE221L — Circuits II Lab

Exercise #11

Capacitor Characterization via Frequency Response

I.  Objectives

A. The objectives of this exercise are:

The objective of this laboratory exercise is to gain insight into non-ideal (true)

capacitors, including quality and the significance of parasitics (inductance and 

resistance) at various frequencies. This will be accomplished as follows:

a)  You will create your own test procedure to fully characterize a capacitor,including parasitic resistances and inductances

 b)  You will perform this test on two separate capacitors of different design (one

an oil/paper electrolytic, the other a polystyrene based capacitor)

c)  You will write your own background/theory and test procedure, as thoughthey were to be given to students in future lab sections

II. Introduction

Capacitors

Capacitors are notoriously non-ideal. Based on materials used and the method of 

construction and design, capacitors inherently have parasitic impedances due to

lossy material and lead inductance. Figure 1 gives an equivalent model of a

nonideal capacitor. In series with the capacitor is an equivalent series resistance

(ESR) and inductance (ESL), and in parallel is a resistor (Rp) that slowly

discharges stored energy (no capacitor will store charge forever).

C0

ESL

ESR

RP

 Fig. 1. Capacitor equivalent circuit model

How does each of these elements influence the performance of a capacitor over a

wide frequency range?

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Your task will be to devise a method for determining each of these parameters.

You can use a multimeter for determining at least one parameter – hint: the

multimeter applies a DC source to your circuit under test when measuring ohms.

The other parameters will be determined through the use of a signal generator and 

your trusty oscilloscope.

Before getting started though, you should read pages 650-651 of your textbook.

Once you are done taking measurements, estimate as many performance

 parameters from this data as you can, e.g., Q, etc. A plots of data over source

frequency can be very important for this experiment, as well as for your report.

Finally, use the LCR Bridge meter to have it provide its estimate of various

 parameters. You can compare these to your own estimates.

III.  Procedure

1.  Test Setup. Evaluate the model given in Fig. 1. Determine a test procedure to

allow for the calculation of each element (ESR, ESL, C, and Rp). At your 

disposal, you have an oscilloscope, DMM, and waveform generator to perform the

exercise.

2.  Perform the test on the two different capacitors provided and record enough

information to make the necessary calculations.

3.  Test on the LCR Bridge meter.

IV.  Post Analysis and Report

Report Content – Format – Experiment (see EE webpage for directions)

1.  Write an objective, give a background/theory discussion section, and write

the procedure that you came up with to determine the values of the parasitic

impedances.

2.  Produce a results section, which will only give the results of your experiment,

computation of the values of the parasitics found and all relevant graphs/tables.

3.  Write your own set of in-depth conclusions answering the question about the

level of quality of your circuit model as compared to the values provided by the bridge meter. Compare the quality of one capacitor to the other. Does one lend 

itself to higher frequency applications than the other?