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Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 2 AC & Impedance Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 1 DC & AC DC - Direct Current • Cells/Batteries provide a source of DC power • Direct Current flows in a single direction AC - Alternating Current + • AC is easier to generate and transform - • Mains is 50Hz AC. Radio Frequencies (RF) use High Frequency AC • Simple items such as Filament Light Bulbs work with AC and DC, but many electronic components are sensitive to the direction of current Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 2 AC Frequency & Period • The Foundation course just described the shape of a sine wave whilst Intermediate requires a deeper understanding Amplitude Time One Cycle • The Period, T of one cycle, in seconds is equal to 1/f, where f is in Hertz Frequency, f = 1 / T Chelmsford Amateur Radio Society Intermediate Licence Course or Murray Niman G6JYB Period, T = 1 / f Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 3 An AC Cycle • One way of looking at the sine wave is as a cycle of 360 degrees • The voltage or current has a complete rotation (like an alternator) +Vpeak 270° N S 0° 90° Time Slipring Brush AC Volts Output -Vpeak 180° 360° • The use of degrees indicates the time or ‘phase’ within one cycle • Unlike a constant DC source, volts/current vary from zero to a +/- peak and back to zero, so we need a way of describing the average Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 4 Peak & Peak-Peak • Sine waves and other waveforms have varying amplitude with time • The variation has positive and negative values during the cycle • The peak value is the level of a positive or negative peak • The peak-peak is the difference between the negative and positive peaks Amplitude + Chelmsford Amateur Radio Society Intermediate Licence Course Vpk Time Vpk-pk One Period, T Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 5 RMS Value • RMS = Root Mean Square, which is a form of averaging • The RMS value of any varying waveform is the equivalent of the constant DC Voltage that would have the same power or heating effect • For a sine wave, the RMS value is equal to 1/2 of the peak value. Vrms = Vpk/2 or Vrms = 0.707 x Vpk Vpk + Vrms Time Chelmsford Amateur Radio Society Intermediate Licence Course Vpk-pk Example: AC Mains is 230Vrms So… Vpk = 230/0.707 = 325V Vpk-pk = 2xVpk= 650V One Period, T Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 6 Frequency & Wavelength • In air the velocity, v of radio waves is a constant ( ~3x108m/s) • So if the frequency increases, the wavelength decreases, and vice versa, determined by: v = f x • Example 7MHz=40m, 10MHz=30m, 14MHz=20m approx Frequency, v f Wavelength, Chelmsford Amateur Radio Society Intermediate Licence Course v m/s f Hertz Murray Niman G6JYB metres Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 7 AC & Transformers • Transformers consist of coils of wire sharing the same magnetic field and may have an iron or ferrite core to concentrate the field • Energy is transferred from one coil to the other by AC changing the magnetic field - which can not happen with constant DC • Voltages (such as AC Mains) can be stepped down to a lower level if fewer turns of wire are on the secondary coil than on the primary - or can be stepped up if the secondary has more turns Basic Transformer Symbol Chelmsford Amateur Radio Society Intermediate Licence Course More General Transformer Primary Murray Niman G6JYB Secondary Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 8 AC & Components • Components that store energy in electric or magnetic fields have a finite reaction time before they reach a steady state or the stored energy may oppose the change being applied – Most prominent in Capacitors and Inductors/Transformers • This behaviour is different from simple Resistance • If the input is changing (ie AC) then a time difference occurs between the current flowing and the voltage being applied – Charge rushes in or out of capacitor plates – Magnetic fields in coils create back EMFs which oppose current • You can still apply Ohms law (‘R’=V/I) to such situations to assess this form of AC Resistance, known as Reactance, X Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 9 Reactance in Inductors • As AC current flows (or tries to) Magnetic fields create ‘back EMFs’ which oppose the input current • This AC Resistance, is termed Reactance • For an inductor, the Inductive Reactance has symbol XL • Applying Ohms law:XL = Vrms / Irms Irms Vrms L AC Source Freq, Hz Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 10 Reactance in Capacitors • When an AC Voltage is applied, charge rushes in/out of a capacitor plate, attracting/moving charge on the opposite plate • This induced charge effectively enables an AC current to flow (unlike DC) • So at AC, we have Volts and Current and its AC resistance is termed Capacitive Reactance, XC • Applying Ohms law:XC = Vrms / Irms Chelmsford Amateur Radio Society Intermediate Licence Course Irms Vrms C AC Source Freq, Hz Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 11 Impedance • In combinations of capacitors, resistors, or inductors, current will result in energy transfer (into heat) in the resistors and energy storage and release in the capacitors or inductors. R C Vrms • R and X are in Ohms, but distinct in nature • When correctly combined the overall term used for the resistance is Impedance, Z (as the components impede current flow) • In such a circuit Ohms Law applies to the ratio of the overall potential difference to current so we have: Z = Vrms / Irms Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB R L Vrms Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance 12