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All About LEDs

February 28, 2016

A light-emitting diode (LED) is a semiconductor device similar to an ordinary diode, except that it’s made with a different semiconductor material.  An LED, of course, emits light when an electric current is passed through it in the forward direction.

Electronics has been a hobby of mine for many years.  I remember when the first LEDs came on the market.  They were infra-red emitters.  Next were the red LEDs.  These were followed by orange, yellow, green, and blue, in that order.  Did you notice that those colours are in spectral order?  Developing LEDs that emitted shorter wavelengths of light took more time.  Each new colour of LED used a slightly different semiconductor material, and operated at a slightly higher voltage.

LEDs are all low-voltage devices.  Their operating voltage falls in the range between 2 and 4 Volts.  They can be connected in series to run from higher voltages.  In lamps with many LEDs, they are generally connected in series-parallel.  That way, one bad LED will only shut down others that are in series with it.  LEDs are also current-operated; they need a way to limit the current passing through them.  This is usually done with either a series resistor or a constant current power supply.

I didn’t mention white LEDs before.  That’s because LEDs only emit light in a narrow spectrum, a single colour.  White sunlight, for example, is a broad spectrum extending all the way from red to violet.  LEDs can’t do that.  White LEDs are actually blue LEDs, except that they are built on a fluorescent substrate.  The blue light causes the substrate to emit light in a broad yellow spectrum.  The combination appears white, even though it’s missing red, orange, and green colours.  Small LED flashlights often have a 1-Watt LED inside.  Larger ones have a 3-Watt LED.

I recently purchased an LED security lamp that was controlled by a motion sensor.  It used a rectangular array of LEDs.  It had a small box on the back with a cord to connect to the power line.  I noticed similar lamps without this box that operated from 12 Volts.  My lamp was rated to operate from power lines ranging from 100 to 250 Volts.  What does that range suggest?  Right!  A switching power supply that produced 12 Volts.  That must be what’s in the box.  That’s how they do it.  That’s how the lamp can operate from either North American or European power lines.

LED light bulbs must be the same.  The LEDs must be connected in series-parallel.  There must also be a tiny switching power inside the bulb that operates from the 120 Volt line voltage.  I don’t know how else they could do it.  A dropping resistor would consume more power than the LEDs did.  Connecting all the LEDs in series would risk all of them going out at once, just like the old-fashioned Christmas lights used to do.  Only a switching power supply would work correctly.


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