design realization lecture 14 john canny/dan reznik 10/9/03
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Design Realization lecture 14
John Canny/Dan Reznik
10/9/03
Last Time
Composites: Fiberglass, carbon fiber and kevlar.
Hierarchical materials.
Cellular materials, honeycomb and foam.
This time
Electronics
Voltage, Current, Ohm’s 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.
Voltage, Current, Ohm’s 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 Ohm’s law:
V = I R
I in amps
V in volts
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:
Resistors
Variable resistors are called potentiometers:
Here’s a simple circuit, avoltage divider:
Note the ground and power supply symbols:
A potentiometer can actas a variable voltage divider, to control a voltage.
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
AC and Capacitors
Capacitors are charge storage devices, but don’t allow DC to flow.
AC can flow because a little charge is stored each cycle and returned.
The current flow increases with frequency.
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).
Capacitor Construction
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)
CfZ
2
1
Variable Capacitors
Capacitors can be variable. Used for tuning: Radios, antennas, crystal oscillators (to drive
computers).
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 doesn’t.
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).
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.
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).
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.
Non-Inverting Amplifier
Here is a basic non-inverting amplifier. The gain (ratio of Vo to Vi) is (Rf + Rg) / Rg
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