How long will my LED light last?

How can I build a led light that will run off of standard 110 house current?

• I'm wanting to find out how I can build led lights for display boxes that can be plugged into a 110 outlet, I don't want to use any batteries. I would like it to have multiple led lights so that it will be as bright as possible, and show off the art work that I want to place on top of the boxes.

• Answer:

  Relatively easy. You will need the specs on the LED's you plan on using. They have voltage and current limits which are unforgiving. Exceed a limit and the LED burns out like a fuse. A LED is simply a diode, a rectifier which happens to give off light when it is forward biased and is conducting current. Your problem is one of voltage and current. Too much of either and teh diode burns out like a fuse. A typical LED forward biases at something over 0.5 volts, and can usually tolerate about 10-30 milliamperes (MA) of current. If we These are rated in milliwatts (MW) of heat dissapation. A typical LED (the kind which is about 0.2 inch diameter and about 0.3 inch long) can dissapate about 10 MW, which means I have to adjust current and voltage according to Ohm's Law, E=IR, voltage equals curent times resistence, and P=IE, power equals current times voltage. 10 MW means I can have 10 MA and 1 volt across the diode, 5 MA and 2 V, or any combination which does not exceed any of the rated limits. So, how do you get this? Use a stepdown transformer as working with the power line directly is VERY VERY DANGEROUS. Any wall wart style will do. Radio Shack and several other electronics stores will have these at a reasonable cost. Look for one with about a 6 volt DC output. With the example of the 1 volt drop and 10 MA current, you should be able to connect 6 of these in series (anode to cathode, anode to cathode, just like batteries in series) and connect them directly across the 6 VDC supply and they all should light. If it isn't bright enough (as I am being quite conservative in my estimates), you might be able to remove one LED from the string to up the current (light output) without exceeding the current limits. Use a meter capable of measuring MA to monitor the current drawn by the string. By using a transformer to lower the voltage you use as a power source, this means you can parallel a rechargeable battery downstream of the transformer to supply power to keep the LED's lit when the power line fails. This is how a lot of emergency lights in buildings work these days. The low voatage power supply lights the lights and keeps the battery charged and if the power line fails, the battery keeps the lights lit. You will need to do more research on how to add the backup battery since there is not room here to give much in the way of detail here to charge the battery and keep it in good health (overcharging is the cause of early failure of standby batteries). Using a combination of strings in series and parallel up to the current capacity of the power supply will determine your end result. I am only giving a basic outline here since I am not legally going to allow myself to be obligated in the event you do something stupid which gets you killed working with the power line drectly. Again I say, DO NOT WORK WITH THE POWER LINE DIRECTLY! ALWAYS use a stepdown transformer to protect yourself from becoming a crispy critter and NEVER assume a circuit is dead until you verify it is really deenergized. You can DIE and there is no coming back from death as we know it.