Design Realization lecture 14

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Design Realization lecture 14

• John Canny/Dan Reznik
• 10/9/03

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Last Time

• Composites Fiberglass, carbon fiber and kevlar.
• Hierarchical materials.
• Cellular materials, honeycomb and foam.

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This time

• Electronics

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Voltage, Current, Ohms law

• Voltage is analogous to pressure, and is measured
  naturally enough, in volts.
• Current is analogous to flow, and is measure in
  amperes or amps for short.
• Direct current (DC) is a constant voltage, e.g. a
  single C or D battery produces 1.5 volts.
• Alternating Current (AC) is a voltage that
  reverse rapidly, at 60 cycles/second in the US.
  An electrical outlet gives 110 volts AC.

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Voltage, Current, Ohms law

• Resistors are used to produce desired voltage or
  current, independent of frequency.
• Resistance is measured in ohms, and the current
  through a resistor satisfies Ohms law
• V I R
• I in amps
• V in volts

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Resistors

• Resistors have a power rating as well, ½, ¼, 1/8
  watt etc. (P V I)
• Resistors used to all look like this(axial lead
  type)
• But increasingly are surface-mount
• Or grouped in chip packages

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Resistors

• Variable resistors are called potentiometers
• Heres a simple circuit, avoltage divider
• Note the ground and power supply symbols
• A potentiometer can actas a variable voltage
  divider, to control a voltage.

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AC and frequency

• Alternating current most often has a sinusoidal
  shape over time
• The frequency is thenumber of completecycles
  per second.
• Its measured in Hertz (Hz).
• Waveform is
• V sin 2 ? f t

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AC and Capacitors

• Capacitors are charge storage devices, but dont
  allow DC to flow.
• AC can flow because a little charge is stored
  each cycle and returned.
• The current flow increases with frequency.

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Capacitor Construction

• Capacitors are sandwiches of dielectric between
  two conductors.
• The dielectric is an insulator, usually a
  polymer.
• Performance determined by dielectric constant
  and electrical breakdown strength (kV/mm).

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Capacitor Construction
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Capacitor Reactance

• A capacitor limits AC current rather like a
  resistor does.
• The reactance Z of the capacitor determines how
  much current flows, V Z I where
• C is the capacitance in Farads.
• A Farad is a huge unit. Most capacitors are
  measured in micro-farads or pico-farads (10-12)

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Variable Capacitors

• Capacitors can be variable. Used for tuning
• Radios, antennas, crystal oscillators (to drive
  computers).

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Inductors

• Inductors are coils of wire, sometimes around a
  ferrite or iron core.
• The ferrite core is a composite with small
  magnetic particles. Works at high frequencies
  where iron doesnt.

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Transformer

• Two coils of wire around the same magnetic core
  create a transformer.
• An AC voltage in one coil induces a voltage in
  the other.
• Ratio of voltages ratio of turns.
• (more turns highervoltage).

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A simple R/C circuit

• This circuit is a voltage divider, with one leg
  which is a capacitor, one a resistor.
• Discuss what high-pass and low-pass would
  mean in this circuit.

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Amplifiers

• Amplifiers are an important class of active
  component (resistors, capacitors and inductors
  are passive they cant strengthen a signal).
• Amplifiers boost small signals from radio
  antennas, microphones, sensors etc. to larger
  values.
• Ex stereo amplifier.
• There is a popular component for building
  amplifiers called an Operational Amplifier
  (Op-Amp).

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Inverting Amplifier

• Here is a basic inverting amplifier.
• The gain (ratio of Vo to Vi) is - Rf / Ri
• The OpAmp has very high gain, which makes it
  change output until its two inputs are nearly
  equal you can assume they are.

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Non-Inverting Amplifier

• Here is a basic non-inverting amplifier.
• The gain (ratio of Vo to Vi) is (Rf Rg) / Rg