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Unformatted text preview: Household Electronics
Diodes & Rectification Component Identification UCSD: Physics 8; 2007 Diodes Diodes are essentially one-way current gates Symbolized by: Current vs. voltage graphs:
acts just like a wire (will support arbitrary current) provided that voltage is positive V I I I I V V 0.6 V V plain resistor diode idealized diode WAY idealized diode
the direction the arrow points in the diode symbol is the direction that current will flow + no current flows current flows
2 UCSD: Physics 8; 2007 Diode Makeup Diodes are made of semiconductors (usually silicon) Essentially a stack of p-doped and n-doped silicon to form a p-n junction doping means deliberate impurities that contribute extra electrons (n-doped) or "holes" for electrons (p-doped) Transistors are n-p-n or p-n-p arrangements of semiconductors (like oreo cookies) p-type n-type 3 UCSD: Physics 8; 2007 LEDs: Light-Emitting Diodes Main difference is material is more exotic than silicon used in ordinary diodes/transistors typically 2-volt drop instead of 0.6 V drop When electrons flow through LED, energy taken from current by emitting a photon of light rather than vibrating lattice (heat) Anything with an LED cares about the battery orientation (it's still a diode, after all) LED efficiency is 30% (compare to incandescent bulb at 10%) Where does the other 70% go?
4 UCSD: Physics 8; 2007 Getting DC back out of AC AC provides a means for us to distribute electrical power AC is fine for bulbs, toasters, heaters, fans More sophisticated devices need DC because they have diodes and transistors that require a certain polarity rather than oscillating polarity derived from AC this is why battery orientation matters in most electronics Use diodes to "rectify" AC signal Simplest rectifier uses one diode:
AC source load input voltage diode only conducts when input voltage is positive voltage seen by load
5 UCSD: Physics 8; 2007 Doing Better: Full-wave Diode Bridge The diode in the rectifying circuit simply prevented the negative swing of voltage from conducting but this wastes half the available cycle also very irregular (bumpy): far from a "good" DC source By using four diodes, you can recover the negative swing:
B & C conduct
input voltage AC source A B A & D conduct
C D load
voltage seen by load
6 Smoothing out the Bumps UCSD: Physics 8; 2007 Still a bumpy ride, but we can smooth this out with a capacitor capacitors have capacity for storing charge acts like a reservoir to supply current during low spots voltage regulator smoothes out remaining ripple AC source A B capacitor C D load 7 UCSD: Physics 8; 2007 Voltage Regulator: Setting the Voltage Can trim down ripply voltage to precise, rock-steady value Now things get complicated! We are now in the realm of integrated circuits (ICs) ICs are whole circuits in small packages ICs contain resistors, capacitors, diodes, transistors, etc. 8 UCSD: Physics 8; 2007 Transistors: a brief glimpse Transistors have made our modern world possible Sort-of like diodes end-to-end (npn and pnp junctions but the sandwiched junction is very thin, and this is important A small current on the "base" can control a large current flowing from "collector" to "emitter" Can be used as an amplifier make a weak signal stronger Or used as a switch pedal-to-the-metal saturation regime Both are incredibly useful 9 UCSD: Physics 8; 2007 Operational Amplifiers (Op-amps) In essence, an op-amp is an idealized transistor/amplifier Has > 20 transistors inside Idyllic properties: linear amplification temperature insensitivity versatile So useful that electronics builders have trays full of them, and use them like "jelly-beans" 10 ...
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