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, a voltage divider:  Note the ground and power supply symbols:  A potentiometer can act as a variable voltage divider, to control a voltage.

AC and frequency  Alternating current most often has a sinusoidal shape over time:  The frequency is the number of complete cycles 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 ( )

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 = higher voltage).

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