Inductance The property of inductance might be described as "when any piece of wire is wound into a coil form it forms an inductance which is the property of opposing any change in current".

Inductance Alternatively it could be said "inductance is the property of a circuit by which energy is stored in the form of an electromagnetic field".

Inductance We said a piece of wire wound into a coil form has the ability to produce a counter emf (opposing current flow) and therefore has a value of inductance.

Inductance The standard value of inductance is the Henry, a large value which like the Farad for capacitance is rarely encountered in electronics today Typical values of units encountered are milli-henries mH, one thousandth of a henry or the micro-henry uH, one millionth of a henry.

Inductance A small straight piece of wire exhibits inductance (probably a fraction of a uH) although not of any major significance until we reach UHF frequencies. The value of an inductance varies in proportion to the number of turns squared.

Inductance If a coil was of one turn its value might be one unit. Having two turns the value would be four units while three turns would produce nine units although the length of the coil also enters into the equation.

Inductance formula The standard inductance formula for close approximation - imperial and metric is:

Imperial measurements L = r 2 X N 2 / ( 9r + 10len ) where: L = inductance in uH r = coil radius in inches N = number of turns len = length of the coil in inches

Metric measurements L = 0.394r 2 X N 2 / ( 9r + 10len ) where: L = inductance in uH r = coil radius in centimetres N = number of turns len = length of the coil in centimetres

Reactance Reactance is the property of resisting or impeding the flow of ac current or ac voltage in inductors and capacitors. Note particularly we speak of alternating current only ac, which expression includes audio af and radio frequencies rf.

Reactance NOT direct current dc. This leads to inductive reactance and capacitive reactance.

Inductive Reactance When ac current flows through an inductance a back emf or voltage develops opposing any change in the initial current. This opposition or impedance to a change in current flow is measured in terms of inductive reactance.

Inductive Reactance Inductive reactance is determined by the formula: 2 * pi * f * L where: 2 * pi = ; f = frequency in hertz and L = inductance in Henries

Capacitive Reactance When ac voltage flows through a capacitance an opposing change in the initial voltage occurs, this opposition or impedance to a change in voltage is measured in terms of capacitive reactance.

Capacitive Reactance Capacitive reactance is determined by the formula: 1 / (2 * pi * f * C) where: 2 * pi = ; f = frequency in hertz and C = capacitance in Farads

Some examples of Reactance What reactance does a 6.8 uH inductor present at 7 Mhz? Using the formula above we get: 2 * pi * f * L where: 2 * pi = ; f = 7,000,000 Hz and L = Henries Answer: = 299 ohms

Some examples of Reactance What reactance does a 33 pF capacitor present at 7 Mhz? Using the formula above we get: 1 / (2 * pi * f * C) where: 2 * pi = ; f = 7,000,000 Hz and C = Farads Answer: = 689 ohms

Resonance Resonance occurs when the reactance of an inductor balances the reactance of a capacitor at some given frequency. In such a resonant circuit where it is in series resonance, the current will be maximum and offering minimum impedance.

Resonance In parallel resonant circuits the opposite is true. Resonance formula 2 * pi * f * L = 1 / (2 * pi * f * C) where: 2 * pi = ; f = frequency in hertz L = inductance in Henries and C = capacitance in Farads

Resonance Which leads us on to: f = 1 / [2 * pi (sqrt LC)] where: 2 * pi = ; f = frequency in hertz L = inductance in Henries and C = capacitance in Farads

Resonance A particularly simpler formula for radio frequencies (make sure you learn it) is: LC = / f 2 where: f = frequency in Megahertz (Mhz) L = inductance in microhenries (uH) and C = capacitance in picofarads (pF)

Resonance Following on from that by using simple algebra we can determine: LC = / f 2 and L = / f 2 C and C = / f 2 L

Impedance at Resonance In a series resonant circuit the impedance is at its lowest for the resonant frequency whereas in a parallel resonant circuit the impedance is at its greatest for the resonant frequency. See figure.

Resonance in series and parallel circuits

Impedance Electrical impedance describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits,

Impedance describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.

Impedance The symbol for impedance is usually Z and it may be represented by writing its magnitude and phase in the form |Z|< θ

Combining impedances The total impedance of many simple networks of components can be calculated using the rules for combining impedances in series and parallel.

Combining impedances The rules are identical to those used for combining resistances, except that the numbers in general will be complex numbers. In the general case however, equivalent impedance transforms in addition to series and parallel will be required

Series combination For components connected in series, the current through each circuit element is the same; the total impedance is the sum of the component impedances

Impedance

Parallel combination For components connected in parallel, the voltage across each circuit element is the same; the ratio of currents through any two elements is the inverse ratio of their impedances

Parallel combination

Hence the inverse total impedance is the sum of the inverses of the component impedances

Diodes Diodes are semiconductor devices which might be described as passing current in one direction only. The latter part of that statement applies equally to vacuum tube diodes.

Diodes Diodes can be used as voltage regulators, tuning devices in rf tuned circuits, frequency multiplying devices in rf circuits, mixing devices in rf circuits, switching applications or can be used to make logic decisions in digital circuits.

Diodes There are also diodes which emit "light", of course these are known as light-emitting-diodes or LED's.

Schematic symbols for Diodes

Types of Diodes The first diode in figure is a semiconductor diode Commonly used in switching applications You will notice the straight bar end has the letter "k", this denotes the "cathode" while the "a" denotes anode.

Types of Diodes Current can only flow from anode to cathode and not in the reverse direction, hence the "arrow" appearance. This is one very important property of diodes

Types of Diodes The second of the diodes is a zener diode which are fairly popular for the voltage regulation of low current power supplies.

Types of Diodes The next is a varactor or tuning diode. Depicted here is actually two varactor diodes mounted back to back with the DC control voltage applied at the common junction of the cathodes. These cathodes have the double bar appearance of capacitors to indicate a varactor diode.

Types of Diodes When a DC control voltage is applied to the common junction of the cathodes, the capacitance exhibited by the diodes (all diodes and transistors exhibit some degree of capacitance) will vary in accordance with the applied voltage.

Types of Diodes The next diode is the simplest form of vacuum tube or valve. It simply has the old cathode and anode. These terms were passed on to modern solid state devices. Vacuum tube diodes are mainly only of interest to restorers and tube enthusiasts

Types of Diodes The last diode depicted is a light emitting diode or LED. A led actually doesn't emit as much light as it first appears, a single LED has a plastic lens installed over it and this concentrates the amount of light.

Types of Diodes Seven LED's can be arranged in a bar fashion called a seven segment LED display and when decoded properly can display the numbers as well as the letters A to F.