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Capacitance is the property of a dielectric to store electric charge.
A capacitor has the ability to store electrons and release them at a later time.
Also called accumulator and condensor.
A capacitor stores energy in an electrostatic field. The energy is stored in such a way as to oppose ang change in voltage.
Capacitors are used in filtering, coupling, and bypassing circuits.
DC circuits are discussed only.
Columb’s law explains charges. Any charged particle is surrounded by invisible lines of force called electrostatic lines of force. These lines of force have the following characteristics.
They are polarized from positive to negative.
They radiate from a charged particle in straight lines and do not form closed loops
They have the ability to pass through any known material
They have the ability to distort the orbits of tightly bound electrons.
When two unlike charges are placed on opposite sides of an electron whose outermost electrons cannot escape from their orbits the orbits of the electrons are distorted.
A Simple Capacitor
A simple capacitor is the parallel-plate capacitor, consisting of two conductors or electrodes separated by a dielectric material of uniform thickness.
Charging and Discharging a Capacitor
Each plate of an uncharged capacitor has the same number of free electrons before voltage is applied to the capacitor. There is no potential difference across the capacitor.
C = Q / V
Where C = capacitance (F)
Q = the charge per plate (C)
V = potential difference across the capacitor (V)
Capacitive Current Equation
FACTORS AFFECTING CAPACITANCE
Capacitance depends on the area of the plates, the distance between the plates, the dielectric constant of the material between the plates.
Permittivity refers to how well a dielectric material can establish electrostatic lines of force.
Dielectric strength is the ability of a dielectric to withstand a potential difference without arcing across the insulator. The breakdown voltage is a function of its material and thickness.
An alternating voltage of 120V has a peak value of 170V.
As a rule of thumb a capacitor should be selected so that its working voltage is a t least 50% greater than the highest effective voltage to be applied to it.
Stator – stationary plates
Rotor – rotating plates
Trimmer capacitors are also available.
This capacitor is polarized. It has a positive and negative lead which must be observed in connecting to a circuit for proper operation.
Oxide dielectric blocks current flow in one direction but offers low resistance in the opposite direction. This limits its use to DC applications.
A voltage of just a few volts can damage the capacitor.
Aluminum oxide capacitors have a short shelf life. The oxide layers tend to deteriorate if the capacitors are not used.
Tantalum capacitors contain either liquid or solid electrolytes. Solid tantalum is the most common because of its excellent temperature characteristics.
Solid tantalum capacitors possess a unique “healing” mechanism.
Capacitors always have their value specified in either microfarads or picofarads. As a general rule capacitor values in whole number (such as 10,47,100,470) are in pF units. PF is not usually printed. If a capacitor is label with a decimal fraction such as 0.01 the falue is interpreted as uF.
Aluminum electrolytic capacitors often have their value and voltage rating uncoded printed on them.
Class I and II Dielectrics
Class I dielectrics display the most stable characteristics.
Class II dielectrics offer much higher dielectric constants than Class I dielectrics but with less stable properties to changes in temperature, voltage, and the like. They are called general purpose capacitors.
The Three-Digit Picofarad Code
The first two digits represent significant numbers. The third digit is the multiplier or the number of zeroes to add. A letter indicates the tolerance.
Tantalum Dielectric Codes
Whole numbers a microfarad units, not the usual picofarads.
Capacitors in Series
Equivalent capacitance in series is similar to resistance in parallel calculations.
Ct = 1 / ( 1/C1 + 1/C2 + 1/C3 + … + 1/Cn)
Capacitors in Parallel
Capacitance in parallel is similar to resistance in series calculations.
Ct = C1 + C2 + C3 + … + Cn
Voltage Division Across Capacitors in Series
Two 50-uF 100V capacitors in series is equivalent to 1 25uF 200V capacitor.
ENERGY STORED IN A CAPACITOR
The capacitor is the only component besides the voltaic cell that can store electric charge.
Stray capacitance occurs in most components and in virtually all circuit configurations. Capacitance exists between any two adjacent wires or circuit traces on a printed-circuit board.
Coils have capacitance between the coil windings.
At low frequencies the stray capacitance is often insignificant
At high frequencies stray capacitance can hamper circuit performance. At microwave frequencies 1 – 100 GHz distributed effects become dominant.
For high frequency circuits lead lengths must be kept small and large spacing may be required between adjacent wires.
TROUBLESHOOTING CAPACITOR FAULTS
Common failures are short, open, and a leaky condition.
An ohmmeter may be used to test a capacitor. The capacitor must be discharged first. A good capacitor should indicate near zero when the leads are first attached. The battery of the ohmmeter will charge the capacitor. As this charging occurs the resistance moves toward infinity.
An analog ohmmeter shows this change most effectively. DMMs are often too slow.
Shorts and Opens
A short will continue to show a low resistance on an ohmmeter.
If open a capacitor indicates very high or infinite resistance when the leads of the ohmmeter are attached.
A leaky capacitor is common with aluminum oxide electrolytes. Aging usually deteriorates the dielectric causing what is effectively a partial short.
Other Types of Losses
Value changes (which includes opens) account for 25% of all capacitor failures.
Leakage accounts for 40% of all capacitor failures.
Dielectric absorption is a failure whereby a capacitor fails to discharge completely. The cap keeps a residual charge.
Equivalent series resistance (ESR)