8. Intro To AC

• Alternating Current


  Current that reverses direction periodically
• Alternating Voltage


  Voltage that reverses polarity periodically
• Sine Wave


  Waveform that varies sinusoidally with time

  
• Change of polarity


  Periodic reversal of the direction of current or voltage
• Period (T)


  Time taken for one complete cycle of a waveform
• Instantaneous Values


  Voltage or current at a specific point in time, denoted by lowercase letters

  
• Peak value


  Maximum positive or negative value of a waveform, denoted Vp or Ip

  
• Peak to Peak Value


  Difference between maximum positive and maximum negative values, denoted Vpp or Ipp
• RMS value


  Root mean square effective value, the DC voltage that produces the same amount of heat
• 240V plug socket voltage is RMS
• Average value of a sine wave

  Total area under the waveform over half a cycle, divided by the distance in radians

  
• Ohm's Law in AC analysis of resistive circuits

  1. Use Ohm's Law with sine wave values (Vrms, Irms)

  2. Use Ohm's Law with peak values (Vp, Ip)

  3. Use Ohm's Law with peak-to-peak values (Vpp, Ipp)
• Ohm's Law Example


  • Total resistance RT = R1 + R2
  • Irms = Vs(rms) / RT
  • V1(rms) = Irms * R1
  • V2(rms) = Irms * R2
• Kirchhoff's Laws apply to AC circuits as well as DC circuits
• AC Generator


  Vin = -N * dΦ/dt, where N is the number of turns and Φ is the magnetic flux
• Angular Measurement


  • Degree = 1/360 of a circle
  • Radian = angular measurement along the circumference of a circle equal to the radius

  
• Phase of a sine wave


  Position with respect to a reference, described by y = A sin θ

  
• Nonsinusoidal Waveforms


  Pulse waveforms, triangular waveforms, sawtooth waveforms

  
• Pulse Waveforms


  • Ideal pulses have instantaneous rise and fall times
  • Practical pulses have finite rise and fall times, typically 10-90%

  
• Repetitive Pulses


  • Have a fundamental frequency and a pulse repetition frequency
  • Ratio of pulse width to period is the duty cycle

  
• Average value of a pulse waveform

  Vavg = baseline + (duty cycle) * (amplitude)

  
• Harmonics


  Odd harmonics and even harmonics present in nonsinusoidal waveforms
• Fourier Analysis


  Technique to analyze nonsinusoidal waveforms in terms of their harmonic content

  
• Cathode Ray Oscilloscope


  Instrument used to display voltage versus time waveforms
• Cathode Ray Tube (CRT)


  Emits electrons which strike a phosphorescent coating to produce a visible trace