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Transcript
Capacitors: Reservoirs for Electricity Quick Recap: • Capacitors are used to store electrical charge • Think of a bucket storing water, current is fed into the capacitor just like a hosepipe fills a bucket. The more current that flows into the capacitor, the higher the voltage gets. •So, if a large bucket can hold a large amount of water, a large capacitor can store a big charge. The larger the value of the capacitor the longer it will take to charge for a given flow of current. Likewise, a bigger bucket takes longer to fill with a given flow of water QUICK RECAP: • Capacitors values are expressed in submultiples of Farads(F), ranging from picoFarads(pF) to microfarads(uF) • Hence: 1,000,000,000,000pf = 1F 1,000,000,000nf = 1F 1,000,000uF = 1F QUICK RECAP • So, where are capacitors used? • Many circuits today, including your proposed project will run on extremely low currents! • If a capacitor has a large value and the current drain is small, it is possible to use capacitor as a backup power system for devices such as memory & time keeping chips inside electronic equipment. QUICK RECAP: • WORKING VOLTAGE: This is the highest voltage that a capacitor can withstand before becoming damaged. The dynamic duo of resistors and capacitors • Why do they work so well together? • The larger the value of the resistor, the less current flows through it for a given voltage, which means?…….it takes more time to fill a capacitor. By picking the combination of capacitors and resistors, you and ONLY you!! can determine your project’s charge or discharge time. Charging & Discharging Capacitors First the Theory • A capacitor stores electrical current, and a resistor controls the flow of electrical current, put these two together, and you can control how fast you want a capacitor to fill(I.e charge) and how fast you want the electrical current to empty out (discharge) from the capacitor NOW THE DEMONSTARTION!! (USING A CAPACITOR WITHIN A TV REMOTE CONTROLLER) Hence, by selecting the combination of capacitors and resistors, YOU! Can determine your project’s charge or discharge time! HOW? The diagram below shows how +V 100K + 10uF R C In this circuit, C=10uF and R = 100k (10uf 0.000001F) 0V The Maths • TIME CONSTANT = C x R = 10uF x 100K = 10 x (100x1000)/1000000 = 1 second • When a capacitor charges, it takes a certain amount of time to be fully charged and equal the power supply voltage V. The rate at which the capacitor charges will depend upon its size and the size of the limiting resistor • TIME CONSTANT This is the rate at which a capacitor charges through a resistor. Time Constant (seconds) = C(Farads) x R (ohms) Hence: The smaller the Time Constant the quicker the capacitor charges (or discharges) NOTE WELL! • After one time constant the capacitor would only be charged to about 0.6 of its full charge, and would require a further four time constants to be fully charged, hence in the example it would take 1 second x 5 = 5 seconds to be fully charged CHARGING & DISCHARGING • COPY GRAPH TABLES IN TO WORK BOOKS