Capacitors and Capacitance | Electronics PPT | web4study

# Capacitors and Capacitance | Electronics PPT

Category: Electronics PPT , PPT ,

Topics Covered in this ppt

How Charge Is Stored in the Dielectric

Charging and Discharging a Capacitor

Typical Capacitors

Electrolytic Capacitors

Capacitor Coding

Parallel Capacitances

Series Capacitances

Energy Stored in Electrostatic Field Capacitance

Measuring and Testing Capacitors

Troubles in Capacitors

### How Charge Is Stored in the Dielectric

• A capacitor consists of two conductors separated by a dielectric (insulator).
• Capacitors store energy in the electric field.
• The storage means the charge remains after the voltage source is disconnected.
• The measure of how much charge is stored in the capacitance C.
• Components made to provide a specified amount of capacitance are called capacitors, or by their old name condensers.
• Applying a voltage to a discharged capacitor causes a current to charge the capacitor.
• Connecting a path across the terminals of a charged capacitor causes current to flow which discharges the capacitor.
• A capacitor concentrates the electric field in the dielectric between the plates. This concentration corresponds to a magnetic field concentrated in the turns of a coil.

Charging continues until potential difference = applied voltage.

Electrons that accumulate on the negative side of the capacitor provide electric lines of force that repel electrons from the opposite side. Fig. 16-1: Capacitance stores the charge in the dielectric between two conductors. (a) Structure.

### Charging and Discharging a Capacitor

• The two main effects of a capacitor are charging and discharging.
• Accumulation of charge results in a buildup of the potential difference between the capacitor plates.
• Closing the switch allows the negative battery terminal to repel free electrons in the conductor to plate A. The positive terminal attracts free electrons from plate B.
• Charging continues until the capacitor voltage equals the applied voltage. Fig. 16-2: Storing electric charge in a capacitance. (a) Capacitor without any charge. (b) Battery charges capacitor to the applied voltage of 10 V.

• The effect of electric lines of force through the dielectric that results in storage of the charge.
• The electric field distorts the molecular structure so that the dielectric is no longer neutral.
• The dielectric can be ruptured by a very intense field with high voltage across the capacitor.
• The capacitor discharges when a conducting path is provided across the plates, without any applied voltage.
• Here, the wire between plates A and B provides a low-resistance path for discharge current.
• The stored charge in the dielectric provides the potential difference.
• When the positive and negative charges are neutralized, the capacitor is discharged and the voltage across it is zero.
• The capacitor can store an amount of charge necessary to provide a potential difference equal to the charging voltage.
• Any charge or discharge current flows through conducting wires to the plates but not through the dielectric.
• Charge and discharge currents must be in opposite directions.
• More charge and discharge current result in a higher value of C for a given amount of voltage. The value of C does not change with the voltage; it depends on the physical construction of the capacitor.

### The Farad Unit of Capacitance

• The farad (F) is the basic unit of capacitance.
• One farad of capacitance equals one coulomb of charge stored in the dielectric with one volt applied.
• Most capacitors have values less than 1 F:
• 1 μF (microfarad) = 1 × 10-6 F
• 1 nF (nanofarad) = 1 × 10-9 F
• 1 pF (picofarad) = 1 × 10-12 F
• The amount of charge Q stored in the capacitance is proportional to applied voltage. The relationship is summarized in the formulas:
• Charge on a capacitor, in coulombs: Q = CV
• Energy stored in a capacitor in joules: ε = ½CV2
• Where:
• Q = electrical charge in coulombs
• C = capacitance in farads
• V = voltage in volts
• ε = energy in joules

Characteristics of Capacitors: Three Ways to Increase Capacitance

• A larger capacitor stores more charge for the same voltage.
• A larger plate area increases the capacitance:
• More of the dielectric surface can contact each plate, allowing more lines of force between the plates and less flux leakage.
• A thinner dielectric increases capacitance.
• When the plate distance is reduced, the electric field has greater flux density so the capacitance stores more charge.

Characteristics of Capacitors:

• The dielectric constant Kε indicates an insulator’s relative permittivity, or the ability of an insulator to concentrate electric flux.
• Its value is the ratio of flux in the insulator compared with the flux in air or vacuum.
• Dielectric strength is the ability of a dielectric to withstand a potential difference without arcing across the insulator.
• This voltage rating is important because if the insulator ruptures, it provides a conducting path through the dielectric.
• Dielectric Constant Kε
• The value of a capacitor is:
• Proportional to plate area (A) in meters.
• Inversely proportional to the spacing (d) between the plates in meters.
• Proportional to the dielectric constant (Kε ) of the material between the plates.

### Typical Capacitors

• Capacitors are classified by dielectric.
• air, mica, paper, plastic film, ceramic, electrolytic.
• They can be connected to a circuit without regard to polarity (except for electrolytic capacitors).
• The polarity of the charging source determines the polarity of the capacitor voltage.
• Capacitors block dc voltages and pass ac signal voltages.

Types of Capacitors:

• Mica: Typically used for small capacitance values of 10 to 5000 pF.
• Paper: Typically used for medium capacitance values of 0.001 to 1.0 μF.
• Film: Very temperature-stable. Frequently used in circuits where this characteristic is a necessity, such as radio frequency oscillators and timer circuits.
• Ceramic: Available in a wide range of values because Kε can be tailored to provide almost any desired value of capacitance. Often used for temperature compensation (to increase or decrease capacitance with a rise in temperature).
• Surface-mount: Also called chip capacitors. Like chip resistors, chip capacitors have their end electrodes soldered directly to the copper traces of the printed circuit board.
• Variable capacitors:
• Fixed metal plates form the stator.
• Movable plates on the shaft form the rotor.
• Air is the dielectric.
• Capacitance is varied by rotating the shaft to make the rotor plates mesh with the stator plates.

Voltage Rating of Capacitors

• The voltage rating of capacitors specifies the maximum potential difference of dc voltage that can be applied without puncturing the dielectric.
• The potential difference across the capacitor depends upon the applied voltage. It is not necessarily equal to the voltage rating.
• A voltage rating higher than the potential difference applied provides a safety factor for long life in service.
• The breakdown rating is lower for ac voltage because of the internal heat produced by continuous charge and discharge.

### Electrolytic Capacitors

• Electrolytics provide the most capacitance in the smallest space with the least cost.
• Electrolytics have a very thin dielectric film, which allows it to obtain very large C values. Fig. 16-9: Construction of aluminum electrolytic capacitor. (a) Internal electrodes. (b) Foil rolled into a cartridge.

Polarity

• Electrolytes are used in circuits that have a combination of dc and ac voltage. The dc voltage maintains the required polarity across the electrolytic capacitor to form the oxide film.
• If the electrolytic is connected to opposite polarity, the reversed electrolysis forms gas in the capacitor. It becomes hot and may explode.
• This phenomenon only occurs with electrolytic capacitors.

Leakage Current

• A disadvantage of electrolytes is their relatively high leakage current, caused by the fact that the oxide film is not a perfect insulator.

Tantalum Capacitors

• This type of electrolytic capacitor features:
• Larger C in a smaller size.
• Longer shelf life
• Less leakage current than other electrolytes.
• Higher cost than aluminum-type electrolytes.

### Capacitor Coding

• The value of a capacitor is always given in either microfarads or picofarads.
• The coding depends on the type of capacitor and its manufacturer.

Chip Capacitors

• Make sure it is a capacitor and not a resistor.
• Capacitors feature:
• A solid-color body.
• End electrodes completely enclose the end of the part.
• There are three popular coding systems for chip capacitors. All systems represent values in picofarads. Examples of the systems follow on the next slides.

Parallel Capacitances

• Connecting capacitances in parallel is equivalent to increasing plate area.
• Total C is the sum of individual Cs:
• CT = C1 + C2 + … etc.
• Voltage is the same across parallel capacitors.

### Series Capacitances

• Connecting capacitances in series is equivalent to increasing the thickness of the dielectric.
• Total C is less than the smallest individual value.
• CEQ = 1/ 1/c1+1/c2
• Capacitors are used in series to provide higher working voltage rating for the combination (e.g., each of 3 equal Cs series has 1/3 the applied voltage).
• The voltage across each C is inversely proportional to its C. A smaller C has a larger proportion of applied voltage.
• All have the same charge because they’re in one current path. With the equal charge, the smaller C has the greater potential difference.
• Charging current is the same in all parts of the series path.

### Energy Stored in Electrostatic Field Capacitance

• The electrostatic field of the charge stored in the dielectric has electric energy supplied by the voltage source that charges C.
• Energy = ε = ½ CV2 (joules)
• V = voltage across the capacitor
• ε = electric energy (joules)
• Stored energy is the reason why a charged capacitor can produce electric shock even when it is not connected to a circuit.

### Measuring and Testing Capacitors

• A capacitance meter is a piece of test equipment specifically designed to measure the capacitance value of capacitors.
• For nonelectrolytic capacitors, lead polarity does not matter.
• Discharge the capacitor before applying the meter.
• It is important to know conversions from nanofarads to micro- or picofarads because meters do not measure nanofarads.

Leakage Resistance of a Capacitor

• Leakage resistance is a resistance in parallel with a capacitor that represents all leakage paths through which a capacitor can discharge.
• There are three leakage paths of possible discharge:
• Through the dielectric.
• Across the insulated case or body between the capacitor leads.
• Through the air surrounding the capacitor.

### Troubles in Capacitors

• Open- or short-circuited capacitors are useless because they cannot store charge.
• The leaky capacitor is equivalent to a partial short circuit: it loses its insulating properties gradually, lowering its resistance.
• Except for electrolytes, capacitors do not deteriorate with age while stored, since there is no applied voltage.
• All capacitors can change value over time, but some are more prone to change than others. Ceramic capacitors often change the value by 10 to 15% during the first year.

Checking Capacitors with an Ohmmeter

• The highest ohm range, such as R x 1 MΩ is preferable.
• Disconnect one side of the capacitor from the circuit to eliminate any parallel resistance paths that can lower the resistance.
• Keep fingers off the connection, since body resistance lowers the reading.
• For a good capacitor, the meter pointer moves quickly toward the low-resistance side of the scale and then slowly recedes toward infinity.
• When the pointer stops moving, the reading is the dielectric resistance of the capacitor which is normally very high.
• Electrolytic capacitors will usually measure a much lower resistance of about 500 kΩ to 10 MΩ.
• NOTE: In all cases, discharge the capacitor before checking with the ohmmeter.
• When the ohmmeter is initially connected, its battery charges the capacitor.
• This charging current is the reason the meter pointer moves away from infinity.