LED (Light Emitting diode) lights are becoming more available on the market and are becoming more popular. There are some differing opinions around on their efficiency as compared to fluorescent lights. Some claim they are more efficient than fluorescent lights, while others refute them. The technology will no doubt improve.
One neat thing about LED lights is they are great to experiment with. It is not hard if you can handle a soldering
In order to start designing a LED light. It is handy to know the characteristics of the LED’s you are using. If possible check when buying the LEDs and get a copy of the specifications.
If you don’t know what the characteristics are, unless they are the 1W or 3V super bright Luxeon LEDs or clones, LEDs usually have a voltage requirement of 2.5 to 3 volt. The current requirements for full brightness is usually about 30 milliamps. The exact characteristics can be determined in later steps if not known.
The next step is to calculate how many LED’s in series you can put in one string. You must also work out the value of current limiting resistance. These both depend on the input voltage and the characteristics of the LED’s you are using. The is to approaches to doing this. You can use the theoretical method, or the more hands on experimental method.
Using the theoretical method, start with the forward voltage and the current requirements for full LED brightness. For example for a 12 volt nominal input you can use 3 volt LEDs. The current limiting resistor value is equal to (12 – 9) volts / 0.03 Amps = 100 ohms. The resistance of a resistor is colour coded. The colour codes are standard and lookup tables are very easily obtained.
With the experimental method, start by assuming approximately 3 volt forward voltage requirement and 30 milliamps for each LED. If the voltage can be divisible exactly be 3, you must use 1 less for example for 12 volt you can use in a single series string 3, for 24 volt you can use 7 in a single string.
Connect the number in series and a resistance substitution wheel. Start the wheel at a high value, say 1 kilo-ohm. Connect the power and then slower decrease the value. Find the value that gives you just the brightest. If the resistance is too low the LED’s will be driven too hard and have a higher likelihood of over-heating and failing. You can then read get a resistor similar to the value selected or even better measure the current selected resistor in the substitution wheel with an ohm-meter.
It is possible to put a number of strings of LEDs in parallel providing each string of LED’s has its own resistor that is set up for that string. This means you can have strings with different numbers of LED’s, still providing that you have the appropriate LED for each string.
Things to consider
In a solar power system, the voltage range can be considerable. In a 12 volt system, when the batteries are being equalized the voltage can be just over 15 volts. It is true though that when lights are operating that the voltage is never going to be that unless other sources other than the solar panels are going to be charging.
One way to compensate for that is to calculate using a nominal value like 13 volts or 25 volts (for a 24 volt) system instead of 12 or 24. If you pick a slightly higher voltage to calculate the current limiting resistor, when the voltage is lower, the worst thing that could occur is the LED’s might dim a little. However if you use to small a voltage drop resistor so that at the higher voltage the LED’s are driven too hard and get hot, you may burn the LED’s out.
Commercially, LED drivers are employed to fix this problem. These LED drivers are simply fancy voltage regulators designed to provide a constant voltage to the LED array from a large range of input voltages. Obviously when designing are unit for the masses, the designers will have no control over the input voltage and hence need to employ such drivers.
These last notes though shouldn’t deter you from playing around and seeing what you come up with. I hope this guide will give you a kick start.