Sample LED Circuits

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.

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 100 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 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, 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 OR you can use a single, half  resistance (25 ohm) but double wattage (1/2 watt) resistor to run both strings (see the diagrams below).

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 life span (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.

Other LED design and handling concerns

Courtesy of "The Other Power"