CAPACITANCE

DEFINED Capacitance is the measure of how much electrical charge a device can store. Capacitance is measured in Farads (F) The device used in electronics for storing this charge is called the capacitor.

THEORY - + Power Source Metal Plates Dielectric A B Charging The capacitor is made from two metal plates with an insulator type material in the center called a dielectric. When power is applied to the circuit the capacitor will attract electrons to plate A. The electrons can not pass through the dielectric so the plate starts storing electrons. The electrons on plate B are repelled from plate A and attracted to the power source leaving the plate positively charged. This process lasts until the capacitor has been fully charged. Charging

THEORY A B - + Consuming Device Metal Plates Dielectric Discharging Once the capacitor is charged it may now be used to power a consuming device. There is one limitation. The device can only be powered as long as the charge is in the capacitor. The electrons on plate A will attract to plate B once there is a path for them to flow. Once all the electrons have been neutralized, the capacitor has lost all of its charge.

Example Circuit When S1 is closed the capacitor is charging. When S1 is open and S2 is closed the capacitor will discharge.

RATING CAPACITORS Capacitors are rated in Farads and Volts - One farad is equal to 1amp being charged by an EMF of 1volt per second. The formula is shown below. C = Capacitance in Farads I = Current in amps E=Voltage in volts t=time in seconds -Typically capacitors are in Microfarads (µF) or Picofarads (pF). -The voltage rating is a working rating the capacitor can handle before being damaged.

CAPACITORS IN SERIES The total capacitance in a series circuit adds up inversely just as resisters do in parallel.

CAPACITORS IN PARALLEL The capacitance of capacitors in parallel work like resistors in series. The capacitance of each of the capacitors add up to the total capacitance of the circuit.

CAPACITORS & SCHEMATICS Ceramic Disc Variable Capacitor Electrolytic Radial Lead Axial SCHEMATICS Polyester Film Electrolytic Variable Non-polarized

CERAMIC DISC CAPACITORS Capacitors are named for the material that make up the dielectric. Ceramic Disc capacitors are rounded in shape and are usually a light brown color. The advantage to the Ceramic Disc capacitor is that it can work at small capacitance and high voltages.

ELECTROLYTIC CAPACITORS Electrolytic Capacitors are capacitors that have their plates polarized. One plate is positive and one negative. It is very dangerous to reverse the polarity of an electrolytic capacitor and should not be done. Electrolytic capacitors can handle very large capacitances for their size. These capacitors look like tin cans and their leads can be radial or axial. The rating of these devices is easily read on the device itself - no code. The rectangular shape in the lightly shaded area on the capacitor indicates which lead is negative. On the axial capacitor an arrow points to the negative lead. Electrolytic Radial Lead Axial

VARIABLE CAPACITORS Variable Capacitors are used in tuning circuits. The capacitor can be used as a filter in order to tune in a specific frequency.

OTHER CAPACITORS Polyester Film There are many other types of capacitors. Their names come from the material that makes up the dielectric. The ratings for these capacitors are usually labeled on the device. Mylar Tantalum Polypropylene Film Chip Capacitor

RC TIME CONSTANT The RC Time Constant in the time it takes, in a series resistor capacitor circuit, for voltage to rise to 63.2% or fall to 36.8% of the peak voltage value of the circuit. When five of these time constants occur, the capacitor will be fully charged. The formula below can be used to predict this value. t = Time in seconds R = Resistance in Ohms C = Capacitance in Farads

RC TIME CONSTANT SAMPLE CALCULATION With a 500kΩ resistor and a 10μF capacitor placed in series, what is the time constant of the circuit and how long will it take to fully charge the capacitor? Time Constant Calculation Full Charge Time = 5s x 5 Full Charge Time = 25s Capacitor Fully Charged

RC TIME CONSTANT GRAPHIC REPRESENTATION The first cursor proves that at five seconds the voltage is 6.32V The second cursor is showing that after 5 time constants the capacitor is fully charged. .IC allows the simulation package to have an initial condition so that the graph can be calculated in transient analysis The charge of the capacitor will be measured at A. RC TIME CONSTANT GRAPHIC REPRESENTATION

The number you obtain is in picofarads. CAPACITOR CODE Ceramic Disc Capacitors have a code that shows their value 100,000 ρF or .1uF The number you obtain is in picofarads.

USING A MULTIMETER The MASTECH MY-64 Multimeter can read the capacitance of capacitors. Place the capacitor in the holes as shown in the figure to the right. Set dial for the F(Farads) setting then read the screen. Change range if necessary. The capacitor below is .02uF.