Introduction to Circuit Components: Capacitors (Mod.4 Sect. 2 Part 1) 1 of 2

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Schematic Symbols for Capacitors

This figure shows the symbols used to represent capacitors in electrical drawings. All the symbols show the two capacitor plates separated by a space. Note that the symbols for variable capacitors contain arrows.

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How do capacitors work?

Caption: : The force of the source of potential will cause the orbits of the electrons of the dielectric material to deflect creating a stored charge in the electric field of the dielectric. The label EP shows the electric field created by the charge on the capacitor plates. The label ED shows the electric field in the dielectric.

In A, there’s no charge on the capacitor and therefore no displacement of the electrons. In B, a positive charge has been applied to the right-hand plate. You can see how the electrons in the dielectric have been attracted to and displaced toward the positive plate. In C, a positive charge has been applied to the left-hand plate. Again, you can see how the electrons in the dielectric have been displaced toward the positive plate. Figures B and C show that the electric field in the dielectric is in the opposite direction from the electric field created by the capacitor plates. As a result, the net electric field in the dielectric space decreases when a dielectric other than air or a vacuum is placed on the space between the capacitor plates. Since the value of the capacitor is equal to the charge on the plates divided by the electric field between the plates, the value of the capacitor increases when a dielectric is placed between the plates. When the value of a capacitor with a non-vacuum dielectric is divided by the value of a capacitor with a vacuum dielectric, the resulting value is called the dielectric constant of the insulating material, or K. A vacuum dielectric has a dielectric constant of 1, and all other dielectric materials have a dielectric constant greater than 1.

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Leakage Current

Caption: : All capacitors have a certain amount of leakage current due to internal resistance. This symbol is used to represent leakage current on an electrical diagram.

No dielectric is a perfect insulator. There will always be a small flow of current that escapes through a dielectric whenever a voltage is applied across a capacitor’s plates. This small amount of current is called leakage current. On an electrical diagram, leakage current is represented by a resistor drawn in parallel with the capacitor. The actual value of leakage current is very low due to the extremely high resistance value of the dielectric. The resistance value of capacitors (the insulation resistance) is measured in megohms. A typical capacitor has an insulation resistance of 100 megohms or more. How ever, some capacitors (such as electrolytic capacitors) have a lower resistance value and much larger amounts of leakage current. Leakage current is measured in milliamps (mA) or microamps (µA).