Elizabethtown College Department of Physics and Engineering PHY104

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1 Elizabethtown College Department of Physics and Engineering PHY104 Lab #7- Capacitors 1. Introduction The capacitor is one of the essential elements of analog circuitry. It is highly useful for its energy storage capabilities and also for its frequency-dependent impedance characteristics. In this lab, we will explore the capacitor as an electrical energy storage device and we will measure the capacitance of pairs of conductors in different geometries and with different dielectric materials. In this manner, you will hopefully pick up a better understanding of the properties of capacitors and of the trade-off in their design. A capacitor is simply a charge storage device. The capacitance (C) is the measure of how much charge (q) a capacitor can hold for a given potential difference (V), between its two conductor plates. q C. (1) V Capacitance depends solely on the geometry and materials of the capacitor. In Chapter 26 of Serway and Jewett, you can learn how to find the capacitance for some simple geometries. In this lab, we will use a parallel plate and a cylindrical geometry. Fixed-value capacitors used in analog circuits are constructed in several different manners; however, they generally have similar characteristics. The capacitor consists of two metal contacts that generally lead to pieces of metal foil separated by a dielectric material. The basic model of a capacitor is the parallel plate capacitor. The capacitance of a parallel plate capacitor is, A C 0 (2), d where capacitance (C) depends on the dielectric constant ( ) of the material separating the two metal plates and the permittivity of free space ( 0 ). The capacitance increases linearly with the area (A) of the metal plates and increases also as the distance (d) between the plates is decreased. Thus, capacitors generally consist of large area metal films separated, in a very small distance, by some dielectric material. 2. The other important characteristic of a capacitor is the breakdown voltage (V b ). This is the maximum operating voltage that can be applied to the plates of a capacitor before the dielectric will experience breakdown, and start conducting. Thus designing a capacitor is a tradeoff between minimizing size while maximizing capacitance, and maintaining a high breakdown voltage. We will examine these characteristics in this lab. Some common dielectric materials and their relevant parameters are listed in the table in Appendix 1.

2 Commercial fixed-value capacitors for analog circuits usually come in one of four forms. In all cases, they are composed of a thin metal foil separated by a thin dielectric. In some devices designed for high power applications, the dielectric material is thicker to prevent dielectric breakdown. Appendix 3 describes these common methods for construction of commercial capacitors. At the front of the classroom, we will display a sample of the different types of capacitors, cut open for viewing. 2. The Parallel Plate Capacitor A. Dependence of Capacitance on Plate Separation In this experiment, the relationship between the capacitance of a parallel plate capacitor and plate separation will be evaluated 1. Connect the multimeter to the PASCO variable capacitor. Attach small wire leads to metal posts on the back of each plate. The wires are inserted the capacitor test ports on the multimeter. 2. Set the multimeter to measure capacitance by turning the dial to the pf or----- nf range setting.(appropriate range setting to be determined by student in the experiment (Suggestion: test continuity of multimeter connections by measuring a capacitor with known value) 3. Adjust the distance between the plates of the variable capacitor to 2.0 mm. Record the capacitance on the multimeter and the distance plate separation. 4. Repeat this capacitance measurement for plate separation (d) of 3.0 mm, 5.0mm, 10.0 mm, 15.0 mm, 20.0 mm, and 25.0 mm. 5. Measure and record the diameter of the capacitor plate. Calculate and record its area. 6. Calculate the theoretical capacitance C for each of the separation distances; (see equation 2) Be sure to convert capacitance measurements to Farads, plate area in square meters, plate separation in meters. 7. Compare calculated C with measured C in table form. 8. Plot measured C vs. plate separation d. 9. On another graph, plot C versus the inverse of plate separation, 1/d. What type of graph you should see as per theory? Can you extract from this graph the value of 0? How would you do that? Compare the known value of 0 with the value extracted from your graph.

3 . B. Dependence of Capacitance on Filling In this portion of experiment, the effect of filling the space between the plates of a parallel plate capacitor with an insulator (dielectric) will be examined. 10. Place various sheets of paper between the capacitor plates, filling about 5 mm of space. 11. Record the capacitance at the same distances as with the empty capacitor that was done before, starting with d = 5 mm. 12. Compare the results with the paper inserted with the readings for the capacitor with just air filling. This means you have to prepare the appropriate tables and graphs. 13. Can you extract from your results the value of the relative dielectric constant of paper? Explain how you can do it and calculate the relative dielectric constant of the inserted paper from your measurements. 3. Building a Film Canister Cylindrical Capacitor In this part of the lab you will construct a film canister capacitor and perform the following steps: 14. Construct the capacitor following the directions listed below. 15. Calculate its theoretical capacitance using the dimensions measured and taking in consideration the materials of construction as for the dielectric constant. 16. Measure the capacitance with the multimeter and compare with calculated value. How to Build a Film Canister Cylindrical Capacitor ( ( ) We will build this capacitor in the lab. The capacitor consists of aluminum foil taped to the inside and outside wall of the film can. A wire (paper clip) is used for leads from the two conducting surfaces. Be sure to leave a few millimeters free of aluminum foil at the top to prevent a conducting path between the inner and outer foil. You can find more details (if needed) in the online sites cited above.

4 Figure 1: Schematic drawing and picture of a constructed film can capacitor.

5 Appendix 1: Table of dielectric constants and strengths - Dielectric Constants & Strengths - Values presented here are relative dielectric constants (relative permittivities). As indicated by e r = for a vacuum, all values are relative to a vacuum. Multiply by e 0 = x F/m (permittivity of free space) to obtain absolute permittivity. Can't find what you are looking for - click here or here for very extensive lists. Substance Dielectric Constant (relative) Dielectric Strength (MV/m) Max Temp ( F) ABS (plastic) Air Enamel Epoxy glass PCB Glass 4-10 Mica Mica, Ruby Mylar Oil (mineral, squibb) Paper (bond) Phenolica (glass-filled) 5-7 Phenolics (cellulose-filled) 4-15 Phenolics (mica-filled) Plexiglass Polyethylene LDPE/HDPE Polyamide Polypropylene Polystyrene Polyvinylchloride (PVC) Porcelain Pyrex glass (Corning 7740) Quartz (fused) Silicon Strontium titanate 233 Teflon (PTFE) Vacuum (free space) Water (32 F) (68 F) (212 F) Water (distilled) Wood

Appendix 2: Fixed value capacitors (QUOTED DIRECTLY from http://www.tpub.com/neets/book2/3f.

6 Appendix 2: Fixed value capacitors (QUOTED DIRECTLY from A PAPER CAPACITOR is made of flat thin strips of metal foil conductors that are separated by waxed paper (the dielectric material). Paper capacitors usually range in value from about 300 picofarads to about 4 microfarads. The working voltage of a paper capacitor rarely exceeds 600 volts. Paper capacitors are sealed with wax to prevent the harmful effects of moisture and to prevent corrosion and leakage. Many different kinds of outer covering are used on paper capacitors, the simplest being a tubular cardboard covering. Some types of paper capacitors are encased in very hard plastic. These types are very rugged and can be used over a much wider temperature range than can the tubular cardboard type. Figure 3-15(A) shows the construction of a tubular paper capacitor; part 3-15(B) shows a completed cardboard-encased capacitor. Figure Paper capacitor. A MICA CAPACITOR is made of metal foil plates that are separated by sheets of mica (the dielectric). The whole assembly is encased in molded plastic. Figure 3-16(A) shows a cut-away view of a mica capacitor. Because the capacitor parts are molded into a plastic case, corrosion and damage to the plates and dielectric are prevented. In addition, the molded plastic case makes the capacitor mechanically stronger. Various types of terminals are used on mica capacitors to connect them into circuits. These terminals are also molded into the plastic case.

Mica is an excellent dielectric and can withstand a higher voltage than can a paper dielectric of the same thickness. Common values of mica capacitors range from approximately 50 picofarads to 0.

7 Mica is an excellent dielectric and can withstand a higher voltage than can a paper dielectric of the same thickness. Common values of mica capacitors range from approximately 50 picofarads to 0.02 microfarad. Some different shapes of mica capacitors are shown in figure 3-16(B). Figure Typical mica capacitors. A CERAMIC CAPACITOR is so named because it contains a ceramic dielectric. One type of ceramic capacitor uses a hollow ceramic cylinder as both the form on which to construct the capacitor and as the dielectric material. The plates consist of thin films of metal deposited on the ceramic cylinder. A second type of ceramic capacitor is manufactured in the shape of a disk. After leads are attached to each side of the capacitor, the capacitor is completely covered with an insulating moisture-proof coating. Ceramic capacitors usually range in value from 1 picofarad to 0.01 microfarad and may be used with voltages as high as 30,000 volts. Some different shapes of ceramic capacitors are shown in figure Figure Ceramic capacitors.

Examples of ceramic capacitors. An ELECTROLYTIC CAPACITOR is used where a large amount of capacitance is required. As the name implies, an electrolytic capacitor contains an electrolyte.

A dry electrolytic capacitor consists essentially of two metal plates separated by the electrolyte.

8 Examples of ceramic capacitors. An ELECTROLYTIC CAPACITOR is used where a large amount of capacitance is required. As the name implies, an electrolytic capacitor contains an electrolyte. This electrolyte can be in the form of a liquid (wet electrolytic capacitor). The wet electrolytic capacitor is no longer in popular use due to the care needed to prevent spilling of the electrolyte. A dry electrolytic capacitor consists essentially of two metal plates separated by the electrolyte. In most cases the capacitor is housed in a cylindrical aluminum container which acts as the negative terminal of the capacitor (see fig. 3-18). The positive terminal (or terminals if the capacitor is of the multisection type) is a lug (or lugs) on the bottom end of the container. The capacitance value(s) and the voltage rating of the capacitor are generally printed on the side of the aluminum case. Figure Construction of an electrolytic capacitor.

9 An example of a multisection electrolytic capacitor is illustrated in figure 3-18(B). The four lugs at the end of the cylindrical aluminum container indicates that four electrolytic capacitors are enclosed in the can. Each section of the capacitor is electrically independent of the other sections. It is possible for one section to be defective while the other sections are still good. The can is the common negative connection to the four capacitors. Separate terminals are provided for the positive plates of the capacitors. Each capacitor is identified by an embossed mark adjacent to the lugs, as shown in figure 3-18(B). Note the identifying marks used on the electrolytic capacitor are the half moon, the triangle, the square, and no embossed mark. By looking at the bottom of the container and the identifying sheet pasted to the side of the container, you can easily identify the value of each section. Internally, the electrolytic capacitor is constructed similarly to the paper capacitor. The positive plate consists of aluminum foil covered with an extremely thin film of oxide. This thin oxide film (which is formed by an electrochemical process) acts as the dielectric of the capacitor. Next to and in contact with the oxide is a strip of paper or gauze which has been impregnated with a paste-like electrolyte. The electrolyte acts as the negative plate of the capacitor. A second strip of aluminum foil is then placed against the electrolyte to provide electrical contact to the negative electrode (the electrolyte). When the three layers are in place they are rolled up into a cylinder as shown in figure 3-18(A). An electrolytic capacitor has two primary disadvantages compared to a paper capacitor in that the electrolytic type is POLARIZED and has a LOW LEAKAGE RESISTANCE. This means that should the positive plate be accidentally connected to the negative terminal of the source, the thin oxide film dielectric will dissolve and the capacitor will become a conductor (i.e., it will

10 short). The polarity of the terminals is normally marked on the case of the capacitor. Since an electrolytic capacitor is polarity sensitive, its use is ordinarily restricted to a dc circuit or to a circuit where a small ac voltage is superimposed on a dc voltage. Special electrolytic capacitors are available for certain ac applications, such as a motor starting capacitor. Dry electrolytic capacitors vary in size from about 4 microfarads to several thousand microfarads and have a working voltage of approximately 500 volts. The type of dielectric used and its thickness govern the amount of voltage that can safely be applied to the electrolytic capacitor. If the voltage applied to the capacitor is high enough to cause the atoms of the dielectric material to become ionized, arcing between the plates will occur. In most other types of capacitors, arcing will destroy the capacitor. However, an electrolytic capacitor has the ability to be self-healing. If the arcing is small, the electrolytic will regenerate itself. If the arcing is too large, the capacitor will not self-heal and will become defective. OIL CAPACITORS are often used in high-power electronic equipment. An oil-filled capacitor is nothing more than a paper capacitor that is immersed in oil. Since oil impregnated paper has a high dielectric constant, it can be used in the production of capacitors having a high capacitance value. Many capacitors will use oil with another dielectric material to prevent arcing between the plates. If arcing should occur between the plates of an oil-filled capacitor, the oil will tend to reseal the hole caused by the arcing. Such a capacitor is referred to as a SELF-HEALING capacitor. COPIED DIRECTLY FROM THE WEBSITE: (

Trade of Electrician Standards Based Apprenticeship Capacitance Phase 2 Module No. 2.1 Unit No COURSE NOTES

Trade of Electrician Standards Based Apprenticeship Capacitance Phase 2 Module No. 2.1 Unit No. 2.1.8 COURSE NOTES Certification & Standards Department Created by Gerry Ryan - Galway TC Revision 1 April