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Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 2 AC & Impedance

Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 2 AC & Impedance. . . + -. 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

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Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 2 AC & Impedance

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  1. Chelmsford Amateur Radio Society Intermediate Course(3) Technical Basics - 2AC & Impedance

  2.  + - 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

  3. Amplitude Time One Cycle AC Frequency & Period • The Foundation course just described the shape of a sine wavewhilst Intermediate requires a deeper understanding • The Period, T of one cycle, in seconds is equal to1/f, where f is in Hertz • Frequency, f = 1 / T or Period, T = 1 / f

  4. +Vpeak 270° 90° 0° Time -Vpeak N S 360° 180° Slipring ACVoltsOutput Brush 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) • 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

  5. + Vpk Amplitude Vpk-pk Time - One Period, T 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

  6. + Vpk Vrms Vpk-pk Time - One Period, T 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 Example: AC Mains is 230Vrms So… Vpk = 230/0.707 = 325V Vpk-pk = 2xVpk= 650V

  7. v vm/s Frequency, f fHertz metres Wavelength,  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

  8. More General Transformer Basic TransformerSymbol Secondary Primary 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

  9. 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

  10. Irms L Vrms AC Source Freq, Hz 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

  11. Irms Vrms C AC Source Freq, Hz 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 Currentand its AC resistance is termedCapacitive Reactance, XC • Applying Ohms law:-XC = Vrms / Irms

  12. R C Vrms L R Vrms 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 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

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