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Published byLilian Carpenter Modified over 8 years ago
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Capacitors
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What is a capacitor? Electronic component Two conducting surfaces separated by an insulating material Stores charge Uses –Time delays –Filters –Tuned circuits
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Capacitor construction Two metal plates Separated by insulating material ‘Sandwich’ construction ‘Swiss roll’ structure Capacitance set by...
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Defining capacitance ‘Good’ capacitors store a lot of charge… …when only a small voltage is applied Capacitance is charge stored per volt Capacitance is measured in farads F –Big unit so nF, mF and F are used
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Graphical representation Equating to the equation of a straight line Q V Gradient term is the capacitance of the capacitor Charge stored is directly proportional to the applied voltage
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Energy stored by a capacitor By general definition E=QV –product of charge and voltage By graphical consideration... Area term is the energy stored in the capacitor Q V
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Other expressions for energy By substitution of Q=CV
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Charging a capacitor Current flow Initially –High Finally –Zero Exponential model Charging factors –Capacitance –Resistance I t
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Discharging a capacitor Current flow Initially –High –Opposite to charging Finally –Zero Exponential model Discharging factors –Capacitance –Resistance I t
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V or Q t t Voltage and charge characteristics ChargingDischarging
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Product of –Capacitance of the capacitor being charged –Resistance of the charging circuit –CR Symbol ‘Tau’ Unit seconds Time constant
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When t equals tau during discharge At t = tau the capacitor has fallen to 37% of its original value. By a similar analysis tau can be considered to be the time taken for the capacitor to reach 63% of full charge.
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Graphical determination of tau V at 37% Q at 37% Compared to initial maximum discharge V or Q t
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Logarithmic discharge analysis Mathematical consideration of discharge Exponential relationship Taking natural logs equates expression to ‘y=mx+c’ Gradient is -1/Tau
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Logarithmic discharge graph lnV t Gradient term is the -1/Tau
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