Design Realization lecture 14

1. Design Realization lecture 14 John Canny/Dan Reznik 10/9/03

2. Last Time • Composites: Fiberglass, carbon fiber and kevlar. • Hierarchical materials. • Cellular materials, honeycomb and foam.

3. This time • Electronics

4. 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.

5. 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

6. 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:

7. 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.

8. 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

9. 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.

10. 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).

11. Capacitor Construction

12. 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)

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

14. 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.

15. 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).

16. 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.

17. 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).

18. 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.

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