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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). In reality, though, there are many other circuit design issues you need to look at to build a reliable 115VAC home LED lighting fixture! We link to a few resources farther down on this page, and you can always Google up 'LED lighting circuits' for more information. Reverse polarity will not damage an LED unless the voltage is very high--it simply will not work, and will not pass current through.

However, be sure to check the manufacturer's rating for the specific LEDs you are using - there are some out there, particularly the latest models, that can be damaged by relatively low reverse voltages. The diagram on the side shows how the LED package is marked for polarity.

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, 1.2 volts x .025 amps = 0.03 watts.  A 1/4 watt resistor will work fine, but if you run a second string of 3 LEDs in parallel, each string would need its own 50 ohm resistor. It's important that each string has its own resistor....putting them in parallel with a single resistor is bad practice.

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  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).
So, 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.  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 current level needed by your strings of LEDs. See the circuit diagrams on the side.
We originally described using an LM7812 voltage regulator chip for this application, but it presents some problems--they generally won't start regulating until input voltage reaches 13.4v, and they have a 1.4 volt voltage drop, leaving you with under 12 volts at typical RE system voltages. Instead, the LM317 is a better choice, and you can adjust its output to fit your needs. Choose your current-limiting resistors as shown in the diagram below. This protects your LEDs from fluctuating system voltages.

You really need to use a multimeter for any LED circuit design and construction($10). If you have an RE system, you should already own a multimeter! 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.

DISCLAIMER: None of us here are electronics experts. We've already corrected this page numerous times thanks to real electronics experts who have emailed us. What we'd really like is for a real electronics expert to completely re-write this page for us!

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