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Defining a Capacitor

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Defining a Capacitor

Introduction

A capacitor is an electronic component that stores energy in the form of an electric field. Without delving too deeply into the physics of electricity, capacitors achieve an electric field by accumulating charge on two conductive plates at different voltages (as in Fig. 1). These conductive plates are separated by a special type of insulator called a dielectric. Different factors, like the properties of the dielectric, area of the plate, and the distance between the plates, affect how much energy can be stored. The amount of energy that can be stored is measured in Farads. Capacitors typically hold somewhere between a few picofarads (pF; one picofarad is 10-12 of a Farad) to hundreds of microfarads (µF one microfarad is 10-6 of a Farad). Large capacitors that hold tens of farads or more are referred to as “super-capacitors” or “ultra-capacitors”. They are not very common due to their impractical size. Using one would be comparable to building a car with 1000 gallon gas tank.

Figure 1. Parallel plate capacitor.

Hydraulic Capacitor Analogy

  • Water Analogy: Elastic wall in a pipe
  • Unit: Farads (F), a Farad equals one Coulumb / Volt
  • Equation Variable: C
  • Brief Description:

It is difficult to understand how a capacitor reacts in a circuit based by trying to theorize it. It is simpler to describe its properties using a water analogy. Capacitors work very similar to an elastic wall sealed in the middle of a pipe. When this hydraulic capacitor is placed between a pressure drop, the elastic divider will stretch and store the energy. The hydraulic capacitor analogy illustrates a few important properties of capacitors.

  1. As charge accumulates on one plate in an electric capacitor, the electric field increases, expelling an equal amount of charge from the other plate. Similarly, as the elastic in the hydraulic capacitor expands to make room for the water being pumped in. The elastic also must push an equal amount of water out the other side (see Fig. 2).
  2. As an electric capacitor charges, the voltage drop across its terminals approaches the voltage applied to it by the circuit. Similarly, as the elastic in a hydraulic capacitor stretches further, it will begin to press back with greater force. Eventually, it will press back with enough force to be equal to the pressure drop applied by the system, as in Fig. 3.
  3. Once an electric capacitor is fully charged, it will look like a gap in a circuit, or a resistor with infinite resistance. This is because once fully charged, no DC current can flow through a capacitor. The same thing happens with a hydraulic capacitor. As the elastic stretches it pushes water out one side, creating a flow. Once the elastic has stretched to equal the pressure of the system, it will stop moving. At this point the elastic cannot displace any more water (as shown in Fig. 2). Since no water can breach the stretched barrier, no liquid can flow through the fully charged capacitor.
Figure 2. How current flows through a capacitor.
Figure 3. Charging a capacitor over time.

Determining the Values of Common Capacitor Types

There are many different kinds of capacitors. This is because a wide variety of different materials can be used for the dielectric. Both electrolytic capacitors and ceramic capacitors are fairly common, so we will focus on them.

Electrolytic Capacitors

Electrolytic capacitors are the most common type of capacitor. Generally, they are cylindrical in shape and are used when the circuit calls for a large capacitance. Electrolytic capacitors have a polarity, so they must be connected correctly to avoid damage. The negative side of the capacitor is denoted by a white stripe. The capacity and maximum voltage values are also printed directly on the capacitor, as shown in Fig. 4.

Figure 4. Identifying electrolytic capacitor values.

Ceramic Capacitors

Ceramic capacitors are much smaller than electrolytic capacitors. Generally, they are shaped like small disks and have no polarity. Their capacitance is printed as a code on the component. The code will either be two or three digits and may have a tolerance letter on the end (see Fig. 5).

Figure 5. Identifying ceramic capacitor values.

All ceramic capacitor codes indicate the capacitor value in picofarads (pF). Two digit codes do not need to be decoded, as they simply specify what the capacitance is in pF. Three digit codes work the same way (the first two digits represent a number not a code), except the third digit represents the power by which you should multiply (see Fig. 6). Finally, a letter on the end of a three digit code designates the tolerance (also shown in Fig. 6). Capacitor tolerance is how much accepted error there is in the capacitance value due to the manufacturing process. Below are some examples of converting capacitor codes to their values.

Example:

  • Two Digit Code: “39” = 39 pF

  • Three Digit Code: “102” = 10 × 100 = 1000 pF
  • (Note:The third digit, 2, corresponds to 100 on the table, which has two zeros in it.)

  • Three Digit Code with Tolerance Letter: “103M” = (10 × 1000) ± 20% = 10000 ± 20%
  • (Notice:The third digit 3 corresponds to 1000 on the table which has three zeros in it.)

Figure 6. Capacitor codes.
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