Capacitors have the ability to absorb and store an electrical charge and then release it into the circuit. Capacitors are frequently used in timers which will keep a circuit or device in operation for a period of time after the circuit has been shut off. An example of this is a dome light circuit that stays on for a specified length of time after the door has been closed.

A capacitor is constructed from two conducting plates separated by an insulating material called a dielectric. This insulating material can be paper, plastic, film mica, glass, ceramic, air or a vacuum. The plates can be aluminum discs, aluminum foil or a thin film of metal applied to opposite sides of a solid dielectric. These layered materials are either rolled into a cylinder or left flat.

The operation of a capacitor is relatively simple. When the capacitor is placed in a circuit, a charge builds on the plates until the plates are at the same potential as the power source. When the source potential is removed, the capacitor will discharge and cause a current to flow in the circuit. If the potential of the source changes, the capacitor will either charge or discharge to match the source, thereby smoothing voltage fluctuations in the circuit.

Since current can flow into a capacitor only until the charge reaches the potential of the source, a capacitor will block current in a DC circuit. AC currents are not blocked by a capacitor because the polarity of the AC circuit is continually changing.

The unit of measure of capacitance is the "farad." Most capacitors are much less than one farad, and are rated in microfarads or picofarads. When capacitors are connected in series their total capacitance is reduced, like resistors connected in parallel. When capacitors are connected in parallel their total capacitance increases, like total resistance when resistors are connected in series.


® Terminal

© Terminal

Metal oxide

surface — Aluminum film

Paper & ^electrolyte

CERAMIC CAPACITOR Silver elect [_ Ceramic insulator

Insulating coating




There are three types of capacitors: ceramic for electronic circuits, paper and foil for noise suppression in charging and ignition systems, and electrolytic as used in turn signal flashers. Ordinary and electrolytic capacitors are designated by different symbols in wiring diagrams.




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As stated, capacitors have three uses:

Noise suppression—Noise in an audio system is often caused by AC electrical voltage riding on top of the DC voltage supplying power to a radio or tape player. A capacitor connected to the circuit will filter out the AC voltage by allowing it to pass to ground. Most alternators on Toyota vehicles have a capacitor built in for this purpose.

Spike suppression—A capacitor can absorb voltage spikes in a circuit. This application has been used in conventional ignition systems to prevent an arc from jumping the breaker points when they are opened.

Timers—A resistor put in series with a capacitor can keep current flowing in a circuit for a specified amount of time after power from the source has been removed. This can be used to keep dome lights on after the vehicle doors are closed. The resistor-capacitor or RC circuit in the example above is used to keep a transistor turned on, so the transistor allows current to remain flowing to the system.

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