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Diodes and Rectification

  • Diodes only let current flow freely in one direction, becuase theres very high resistance in the other direction
  • they are made from silicone; a semiconductor
  • a semiconductor can conduct electricity, but not as well as a conductor
  • silicon diodes are made from two different types of silicon joined at a p-b junction
  • one half is made from siliscon that has extra free electrons (N for negative)
  • the other half has fewer missing electrons, containing holes from missing electrons (P for positive)
  • When theres no voltage across the diode , electrons and holes recombine, creating a region with no holes or electrons so it acts as an electrical insulator
  • when there is voltage flowing, direction is the most important
  • if it's going in the right direction, the free holes and electrons have enough energy to get across the insulaing reigon, so a current will flow
  • If it is in the wrong direction the electrons and free holes are being pulled away from the insulating reigon, they stay on the same side and no current flows
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Capacitors

  • An AC voltage that has been rectified isn't useful, as computor chips are very sensitive to input voltage, so this is where a capacitor is used
  • a capacitor stores charge by connecting it to a source of voltage, as current flows around the circuit it gets stored on the capacitor
  • the flow of current decreases as you charge for longer
  • more current on the capacitor means a large voltage across is
  • when the voltage across the capactior is equal to the battery, the current stops and the capactior is fully charged, the voltage in the capacitor wont rise above the voltage of the battery
  • if the battery is removed the capacitor discharges, the flow of current is the same for discharging as charging, but the current flows in the opposite direction around the circuit
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Diodes can be used to rectify an alternating curre

  • a single diode only lets in half of the cycle, this is called half-wave rectification
  • to get full wave rectification you need a bridge circuit, with 4 diodes. in a bridge circuit the current always flows through the component in the same direction and the output voltage always has the same sign
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Capacitors are used in 'Smoothing Circuits'

  • The output voltage from a rectified AC power can be smoothed by adding a capacitor in parallel with the output device, the component gets current alternatley from the power supply and the capacitor.
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