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INDUCTANCE Benchmark Companies Inc PO Box Aurora CO 80047

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DEFINED When a length of wire is formed onto a coil, it becomes a basic inductor

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DEFINED Magnetic lines of force around each loop in the winding of the coil effectively add to the lines of force around the adjoining loops, forming a strong electromagnetic field within and around the coil

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DEFINED A inductor is a device which stores energy in a magnetic field

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DEFINED Inductor consists of a coil of wire, usually around a metallic or ferromagnetic core which creates an electromagnet

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DEFINED A current through an inductor creates a magnetic field around the coil which resists any changes in current

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DEFINED The unit of inductance is the henry (H), defined as the inductance when one ampere per Second through the coil, induces one volt across the coil

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DEFINED The unit of inductance is the henry (H), defined as the inductance when one ampere per Second through the coil, induces one volt across the coil N= Number of Turns µ = dielectric constant A = cross-sectional area of the coil l = length of the coil

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DEFINED Schematic Symbols Fixed InductorVariable A couple of symbols for the inductor are illustrated below

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INDUCTOR TYPES Toroidal core inductorAxial LeadVariable Inductor

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Rules of Inductor Behavior The current and voltage relationship in an inductor is If the current isn’t changing, then the voltage change across the inductor is zero

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Rules of Inductor Behavior An inductor is a short circuit to DC The current through an inductor cannot change instantaneously If current changed quickly, then we might have infinite voltage Contradicts conservation of energy

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Example Problem 1 What are the values of I and V, the current through and voltage across the inductor? The easier value to find is the voltage V. In this case, the current through the inductor isn’t changing, so the voltage must be 0 V. So V = 0 V In DC conditions, an inductor acts like a short circuit – so we need to find the current through the resistor and it will be the same as the current through the inductor.

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INDUCTOR CODE Use the color code guide in your handout as a means to Identify the value of the inductor.

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RL TIME CONSTANT The RL Time Constant is the time it takes, in a series resistor inductor circuit, for current 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 inductor will be fully discharged. The formula below can be used to predict this value. t = Time in seconds R = Resistance in Ohms L = Inductance in Henry’s

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RL TIME CONSTANT The current across an inductor cannot change instantaneously because a finite time is required to move charge from one point to another (limited by circuit resistance) t = Time in seconds R = Resistance in Ohms L = Inductance in Henry’s

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RL TIME CONSTANT example With a 1kΩ resistor and a 1mH inductor are placed in series, what is the time constant of the circuit and how long will it take to fully discharge the inductor? Discharge Time = 1us x 5 Full Charge Time = 5us Time for DischargeTime Constant Calculation t =L/R t=1mH/1kΩ t =.001/1000 t=1us

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The first cursor proves that at 1us the current is 6.32V The second cursor is showing that after 5 time constants the inductor represents a short circuit. RL TIME CONSTANT GRAPHIC REPRESENTATION

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When a DC voltage source is connected to the inductor, voltage is maximum across the inductor because of the magnetic field caused by the maximum rate of change of current in the circuit. Note: at t = 0 seconds Current is 0 Amps Voltage is Maximum Volts THEORY

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As the magnetic field “relaxes” due to the continuous DC, current begins to flow through the inductor. When this occurs, the voltage across the inductor begins to decrease and current through the inductor begins to rise.

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THEORY Eventually the inductor represents a short circuit. In an ideal inductor the voltage drop becomes 0 Volts and the current through the inductor becomes maximum. Note: at t = infinity seconds Current is Maximum Amps Voltage is 0 Volts

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Series and Parallel Inductors Inductors in Series With several Inductors in series, they all act together to affect the current

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Series and Parallel Inductors Inductors in Parallel With several Inductors in parallel, we have to split the current, just like in resistors

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