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About LEDs

A Quick and Dirty History of Light Emitting Diodes (LEDs)

Electric lighting has not changed much in one hundred years or so, but it has brightened our lives. Since the first practical and reliable incandescent bulbs appeared in the early 1900s, the designs have not changed much at all. Incandescent bulbs are simple, cheap to produce, cheap to buy, but certainly not cheap to operate. With efficiencies averaging 5% and the remaining 95% going to heat production, they make better space heaters than lighting devices.

Becoming widely available as early as the 1940s, florescent lighting proved to be a better option. But because of its unpleasant light output, noise, and the low cost of electricity, they found their way into very few homes. They were, however, used in many commercial buildings early on, just as they are commonly found today.

Florescent lights have received a number of improvements through the years. Modern florescent bulbs are offered in an array of light color temperature outputs, some appearing equal to that of an incandescent bulb. Electronic ballasts have also become the standard, and are smaller, more efficient, and quieter than the magnetic ones they replaced. These changes have increased their residential use and enable the now poplar compact florescent to be a suitable direct replacement for incandescents.

Compact fluorescents are four to five times more efficient and last five to nine years. Unfortunately, they do have a downside. Florescent lights contain mercury, and they were even originally known as “mercury vapor lights.” So they have to be disposed of properly, and a broken lamp would not exactly be the greatest boon to your health either.

I am by no means saying you should get rid of your fluorescents; they're still a good product. I am suggesting you take a look at an alternative:

An LED, or light emitting diode, is a form of solid state light that was realized in practical form in the 60s by General Electric. Then the popular red LED was created, following from yellow, orange and green. These low-light output LEDs were seen in many electronics, mostly used as indicators, with the exception of some displays. The earliest LED numeric displays were incorporated into HP calculators in the late 60s. LEDs available to average consumers did not change much for many years and they continued to be used increasingly in numeric displays and indicators. The biggest change to happen to consumer LEDs came after several years with the invention of the blue LED. In the early 90s Nichia® came out with the blue LED, which was followed by the release of higher brightness red and blue LEDs later in the decade.

Soon LEDs were finding their way into increasingly higher light output applications such as traffic lights, flashlights, and automotive brake lights. What was needed most was a light that could compete with our current incandescent and florescent task lighting, and we got our solution in the 21st century with the white LED.

In recent years LEDs affordable to the average consumer have become available in exceptionally high outputs, and many have switched their ratings from millicandela (mcd) to Lumens. An increasing number of LEDs are available with light outputs close or equivalent to their halogen and florescent rivals. Many manufactures, such as Cree®, are offering LED modules with outputs in the several lumen range, available in pure white (~6000K) to warm white (~3000K) color temps. These modules contain one or more LEDs on a single circuit board, and some even have multiple LEDs on a single chip.

 

Working With LEDs

LEDs have different requirements when compared to incandescent bulbs.  They need DC (direct current) and are polarity sensitive. The power in your home is typically AC (alternating current) and runs most appliances in your house, but the computer in front of you, your cell phone, stereo, and many other electronic devices typically use DC. To go from 120 volt AC to your DC device, the voltage is usually first reduced with a magnetic device (called a transformer) to a lower AC voltage, which varies depending on lighting need. You then take your AC and rectify it. A rectifier is a device that functions as a one-way check valve for electrons; they can only travel one way, emitting only positive voltage. The check valve is a diode, a semiconductor device that is related to our friend the LED, and the positive voltage emitted is DC. This operation of a rectifier is in the simplest of terms; there are a number of different designs. In addition to rectifying AC, they usually filter the voltage and possibly regulate it.

LEDs are polarity sensitive, which means that they need a positive voltage at the anode and negative or ground at the cathode. When an LED is connected correctly with the appropriate supply voltage (one to four volts, depending on type) it will illuminate. With polarity reversed, no light will illuminate. LEDs have a voltage limit in the correct and reversed polarity, this limit is specified by manufacturer. When connecting an LED you must consider its operating voltage and current.

To have a better understanding of LED voltage and current supply, you must understand Ohm's Law (some will be able to skip this step, and others should read closely). Amps are a measure of current, a measure of the amount of electricity flowing in a circuit. Volts are a measure of the amount of electrical potential. Ohms are a measure of resistance to current flow. Watts are a measure of power, you may know this as the measurement your utility company uses to charge you. To get volts when you know watts and amps or any two out of the four, here are some formulas:

V=I*R
R=V/I
I=V/R
P=I*V
V=Volts I=Amps R=Ohms P=Watts

Let's say I have a bright white LED with an operating voltage of 3-3.4 volts. For overall longevity, I am going to drive this LED at 3.2 volts at its rated current of 0.018 amps. I have an 8 volt regulated supply voltage, and I am going to have to add a resistor in series with the LED to get the correct voltage. Now I subtract the LED voltage from the supply voltage 8-3.2V= 4.8 volts. Now I divide that voltage by the LED current 4.8/.018 = 266.6 Ohms. I then choose the next highest standard resistor value of 270 Ohms. This resistor in effect drops the excess voltage across it, but there is a small power loss with the resistor and this has to be considered when aiming for efficiency.

Now if you're building a light, odds are that you're going to use more than one LED. To do this, you can wire them in series, parallel, or a combination of the two. Series is when you connect electrical components such as LEDs in a string, end to end, negative to positive to negative to positive, et cetera. When you connect LEDs in series each one drops X volts and adds up the total voltage, but the current needed remains the same. To connect LEDs in parallel, you connect each positive terminal together and each negative terminal together. The total voltage required remains the same, but the current required adds up.

When doing large LED strings with lower voltages, it may be necessary to wire the LEDs in a series-parallel configuration. An example of this would be a 12 volt lead acid battery with an actual voltage of 12.7 and a charging voltage of 13+. In this situation, we need to use a voltage regulator to ensure a safe voltage for the LEDs. Let's say the LED has an average voltage requirement of 3.3 volts, then we'll use a 10 volt voltage regulator and we'll wire series strings of 3 LEDs, dropping approximately 10 volts total. In addition, we can wire equal series strings in parallel with this string, keeping in mind that the polarity must junction at the correct point. With this arrangement, you can wire as many LED strings in parallel as you wish (or as the available current will allow). The design should also include the appropriately rated fuse, wired in series with the voltage regulator and the LED string set. This is a design I often use and favor because of its safety and flexibility for use in different off the grid homes.

If you are using AC line voltage, your safest bet is using a DC wall transformer (lovingly nicknamed “wall wart”). These come in both regulated and unregulated forms. Keep in mind that the unregulated version will have huge voltage swings depending on load, so adding a voltage regulator may be necessary for certain applications. You could, however, use the LEDs directly as an AC voltage rectifier, keeping in mind that the forward and reverse voltage limit will add up to more than the AC voltage. The number you should be working with is the peak value, which is the AC voltage multiplied by 1.414. You can wire two strings of LEDs in opposite polarity connections to your AC terminals for the appropriate calculated voltage. During the rising edge of the AC one string will light, during the falling edge, the other. This will happen at 60 cycles per second in most countries and should be hard to detect with the naked eye. This method however is very crude and basic, does not give the best LED life, and it is not very safe because you are dealing with AC line voltage and many terminal connections.

 

For my templates of the homemade LED light (construction viewable on YouTube), please click here to view as a PDF.

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