Opto-devices -electronic Devices Which Produce Light Are Either Bulbs

OPTO-DEVICES -Electronic devices which produce light are either bulbs or LEDs. -Bulbs have been used for many years. -Bulbs are commonly to indicate if something is turned on. -Bulbs are also used in resistances in oscillators. -Bulbs, however, also produce heat, require high current, and are fragile. This means that they may not be used in many portable applications. -This chapter gives the symbols for bulbs and LEDs, as well as indicating the anode and cathode of the LED because the shorter lead or flat on the case is the cathode. -LEDs have solved many problems with bulbs. -LEDs use less power than bulbs—only 5 to 15 mA of current, generally. -They also produce little heat. -LEDs can be used for indicator lights, especially. They do not have the light output of a bulb, but are bright. LED SPECIFICATIONS -An LED cannot always be tested with the ohmmeter. -To determine their resistance, measure the resistance iof the rectifier diode with an ohmmeter that produces more than .7 volts at the lead tips. -The meter shows low resistance in one direction and high in another when the polarity is reversed. -With leads reversed, the diode will not conduct—it works in only one direction. If it conducts in both directions, it is shorted. -Since LEDs require 1.25 to 2 volts to produce lights, they will appear to be open if the lead voltage is not strong enough. To determine the the voltage of an ohmmeter, attach the negative and positive leads to the negative and postive leads of a voltmeter, respectively. Many ohmmeters have a low and high ohm range. -Though many rectifier diodes have high reverse breakdown voltages, many LEDs do not. Thus, the leads must be connected to the correct polarity. -When an LED begins to conduct, its resistance will drop dramatically. Current must also be limited so the LED doesn't die and resistors must be placed in series.