http://www.otherpower.com/otherpower_lighting_leds.html

 

Home Built LED Lighting

Light Emitting Diodes (LEDs) have been around for years in red, yellow and green.  New technological advances have given us incredibly bright blue and white versions--the white LEDs on our products page are state-of-the-art in brightness. The rated brightness varies by how wide the beam angle is. LEDs with a super-high brightness rating also have a very narrow beam angle. Wider-angle LEDs have a lower brightness rating, but may put out just as much light. It's important to choose the beam angle to suit your needs.

  • LEDs can last tens of thousands of hours if used at rated current
  • No annoying flicker like from fluorescents
  • LEDs are impervious to heat, cold, shock and vibration
  • No breakable glass is used, and LED lights can be waterproofed for marine use

White LEDs are perfect for replacing small, inefficient incandescent bulbs in night lights, flashlights, path lights, task lights and exit signs. Try 6-9 white LEDs for reading and task lights, and 1-3 LEDs for flashlights and path lights.

Designing LED lighting

 

LED ratings are specified by current, not voltage. For longest life, we recommend you run them at 20-25 milliamps (ma). HOWEVER, in our LED flashlight conversions (and many commercial LED flashlights), the LEDs are run at 50-60ma, twice the rated current. One of our test LEDs ran at 98ma for over 200 hours without damage or appreciable light loss. So go ahead and experiment with running them at over rated current if you are willing to take the risk of a shorter life. In my opinion, a flashlight bulb that lasts 100 hours is a huge improvement and cost saver over the incandescent alternative which gives only 15-20 hours before it dies.

You must use some method of limiting current to your strings of LEDs.  The easiest is simply using the right number of LEDs for your supply voltage.  Each white LED gives a voltage drop of 3.6 volts.  So, for a 115 volt DC light, you could use 32 white LEDs in series (115 / 3.6 = 32 +/-)  with NO current limiting (they will limit themselves by their inherent voltage drop).  Reverse polarity will not damage an LED unless the voltage is very high--it simply will not work, and will not pass current through.   The diagram below shows how the LED package is marked for polar

 

The next easiest is a simple resistor. The resistor does consume power, though, but is usually needed since an 'ideal' 3.6 volt source is rarely available. Use Ohms law (Resistance(R)=Voltage(E)/Current(I))  to calculate the value and wattage needed:  (R=E/I)

Each white LED gives a voltage drop of 3.6 volts.  As an example, for a 12 volt light, you can run a maximum of 3 white LEDs in series at full power (3.6 x 3 = 10.8 volts drop).  Subtract this from your supply voltage of 12 volts to get the additional voltage that must be dropped (in this case, 12 - 10.8 = 1.2 volts of additional drop needed).  In this case, 1.2 volts of additional drop / .025 amps (25 ma) = 48 ohms.  Use the next highest value of resistor available, 50 ohms.  You must also be sure the resistor can handle enough current.  Volts x Amps = Watts; resistors are rated in watts.  So in this case, 12 volts x .025 amps = 0.3 watts.  A 1/4 watt resistor would work fine, but if you run a second string of 3 LEDs in parallel, each string would need its own 50 ohm, 1/4 watt resistor. It's important that each string has its own resistor....putting them in parallel with a single resistor is bad practice. We didn't know this when this article was first written....thanks to all the folks that pointed this out!.

This method is cheap and works great, but there's one problem--voltages in a remote power system (or car, for that matter) tend to vary.  In our home system, voltages range from about 12 volts when the batteries are low up to 14 volts when equalizing the battery bank.  An LED lamp string designed to run at 25 milliamps at 12 volts would be pushing 64 ma at 14 volts, which would be very bright and PROBABLY last at least a few hundred hours...but then then when your batteries are low, the LEDs will pull only 10ma or so, making them very dim. If you are looking for maximum lifespan (which could be over 10 years of run time) and brightness that doesn't vary with your battery condition, try a voltage regulator circuit (below).

Therefore, we highly recommend a simple voltage regulator chip for the safety of your LEDs.  White LEDs are expensive, and it would be a shame to blow them out.  Parts for a current-limiting circuit are very cheap--less than $2.  Regulator chips are available for various voltages.  Use the Ohm's law calculations above to select the resistor for the voltage you choose.  Or, use the regulator in a current-limiting configuration to run the LEDs. You can also use an LM317 adjustable voltage regulator set to the exact voltage needed by your strings of LEDs. See the circuit diagrams below.

2 possible regulator circuits using the 7812 regulator chip

 

With this voltage regulator circuit, choose your current-limiting resistors as described above.  Output will be 12 volts DC no matter how high your input voltage goes...up to 37 VDC.  This protects your LEDs from fluctuating system voltages.

If you use the 7812 in this current-limiting configuration, make sure resistor R1 has a big enough wattage rating to handle ALL the current.  Just choose R1 for 25 ma if you are running one string of LEDs, 50 ma for 2 parallel strings, etc.

We also highly recommend using a multimeter ($10) and solderless breadboard ($5) for designing your home built LED fixtures. Both are available at Radio Shack. With the multimeter, you can check your polarity, voltages, resistors, and current draw before assembling the final version of your light by soldering.  The breadboard allows you to make changes to the circuit without soldering, and makes it easy to transfer the working circuit to a soldered version--solder-in PC boards are available that exactly match the connections of your solderless breadboard. (see photo below).

 

Large AC LED lights

Large LED clusters that run on 120VAC are extremely expensive to purchase--usually US$200 and up. You can home build them, but the electronics and design are much more complicated than the DC circuits above. Because LEDs are directional, they are not always a very good choice for room lighting...but work very well for task lighting. Here's some more information:

3-watt AC 10-LED Reading Lamp Circuit
LEDs are excellent for task lighting, and this circuit also has battery backup built in -- when AC power goes out, the LED cluster switches to battery power. Submitted by Pranab Kumar Roy, an electrical engineering student at Nagpur University, India. Designed for 230VAC European grid power, and could be used on 120VAC USA grid power by replacing the transformer. Circuit and text image is 500K bytes.

More AC LED circuit design information: http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#lineled.gif

Other LED design and handling concerns

  • If your LED mounting does not allow any air circulation, we recommend running them at 18-20ma instead of 25ma to avoid any heat buildup, which will shorten their life.
  • LED lights generally do not need reflectors, as the angles at which they emit light are set internally.  Our white LEDs emit in a 20 degree arc.  Very little light is wasted from shining in the wrong direction.
  • Although our LEDs are not powerful enough to require eye protection labelling, DO NOT look directly into the beam from a close distance, just like with a halogen lamp.
  • Solder your connections quickly and efficiently, using a small (less than 30 watt) soldering iron.  LEDs can be ruined if the internal temperature gets too high from soldering.
  • Do not place too much strain on the LED leads when bending them.  Bend the leads ONLY below the sqaure tab on each lead.
  • LEDs are sensitive to static.  The manufacturer recommends a grounding wrist strap, but we have had no problems without one during our research.  Just be careful not to drag your feet across the carpet and grab a handful of LEDs...or simply touch a grounded metal chassis before handling LEDs.