BASICS OF ELECTRICITY - 4
CALCULATING THE TIME CONSTANT OF AN INDUCTIVE CIRCUIT:
The time constant is designated by the symbol ìtî. To determinethe time constant of an inductive circuit use one of thefollowing formulas:
T( in seconds) = L(henrys) / R (ohms)
T(in milliseconds) = L(millihenrys)/ R(ohms)
In the following illustration, L1 is equal to 15 millihenrys andR1 is equal to 5 W. When the switch is closed, it will take 3milliseconds for current to rise from zero to 63.2% of itsmaximum value.
FORMULA FOR SERIES INDUCTORS:
Lt = L1+L2+L3+L4
Lt = 2mh+2mh+1mh+1mh = 6mh
FORMULA FOR PARALLEL INDUCTORS:
In the following circuit, an AC generator is used to supplyelectrical power to three inductors. Total inductance is calculatedusing the following formula:
1/Lt = 1/5 + 1.10 + 1/20 = 7/20
Lt = 2.86 mh
CAPACITANCE AND CAPACITORS:
Capacitance is a measure of a circuitís ability to store an electrical charge. A device manufactured to have a specific amount of capacitance is called a capacitor. A capacitor is made up of a pair of conductive plates separated by a thin layer of insulating material. Another name for the insulating material is dielectric material. When a voltage is applied to the plates, electrons are forced onto one plate. That plate has an excess of electronswhile the other plate has a deficiency of electrons. The plate with an excess of electrons is negatively charged. The plate with a deficiency of electrons is positively charged.
Direct current cannot flow through the dielectric material because it is an insulator. Capacitors have a capacity to hold aspecific quantity of electrons. The capacitance of a capacitor depends on the area of the plates, the distance between theplates, and the material of the dielectric. The unit of measurement for capacitance is farads, abbreviated "F". Capacitors usually are rated in mF (microfarads), or pF (picofarads).
CAPACITOR CIRCUIT SYMBOLS
Capacitance is usually indicated symbolically on an electricaldrawing by a combination of a straight line with a curved line,or two straight lines.
SIMPLE CAPACITIVE CIRCUIT:
In a resistive circuit, voltage change is considered instantaneous.If a capacitor is used, the voltage across the capacitor does not change as quickly. In the following circuit, initially the switch is open and no voltage is applied to the capacitor.When the switch is closed, voltage across the capacitor will rise rapidly at first, then more slowly as the maximum value is approached. For the purpose of explanation, a DC circuit isused.
CAPACITIVE TIME CONSTANT
The time required for voltage to rise to its maximum value in a circuit containing capacitance is determined by the product of capacitance, in farads, times resistance, in ohms. This is the time it takes a capacitor to become fully charged. This product is the time constant of a capacitive circuit. The time constant gives the time in seconds required for voltage across the capacitor to reach 63.2% of its maximum value. When the switch is closed in the previous circuit, voltage will be applied.During the first time constant, voltage will rise to 63.2% of its maximum value. During the second time constant,voltage will rise to63.2% of the remaining 36.8%, or a total of 86.4%. It takes about five time constants for voltage across the capacitor to reach its maximum value.
Similarly, during this same time, it will take five time constants for current through the resistor to reach zero.
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