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why are the resistors heating up? Answered

So i have 48 UV LEDS and each has a 470 ohm resistor on the anode so in a sense all leds are connected in parallel but it has a resistor on it. when i connect my 12 volt battery with 6500 MA the resistors begin to heat up. is there anyway to prevent them from heating up? or what is it that im doing wrong. ill provide a picture on the way the leds and resistors are connected if you want me to but hopefully what i said gives you an idea on how they are connected.


That is what they do. Resistors will heat up as they slow down the current running through the circuit. If they are getting very hot to the touch and start to boil or smell then you need to get higher power resistors.

Assuming you are using a typical 5mm UV LED with a forward voltage of about 3V and forward current of about 30mA you should be using 1 ohm resistors in your array. Your array should look something like the image below.

If your LEDs are using different specs then go to this LED calc and it will help you find the right resistors to use and how to wire it all together.

led array.jpg

Hmmm. 12V * 6.5A = 78 watts of input power. You probably used plain old 1/4 watt resistors into your circuit, didn't you? 48 * 0.25 = 12 watts rated power consumption. 78 = 6.5 * 12, so you're giving those resistors 6.5 times as much power as they can handle.

After your resistors burn through, you can either replace them with high-power (2W) versions, or you can build a current-limiting circuit for your system.

would this current-limiting circuit prevent me from having the hassle of taking off the 470ohm 1/4 watt resistors?

In principle, yes. You might also get away with using a power supply which provides a voltage that directly matches what the LEDs need (in which case you wouldn't need the resistors at all).

The resistors are providing what is called a "voltage drop", so the LEDs don't get blown out. That technique is not efficient, since the excess power (voltage * current) is converted to waste heat.

are there any tutorials on building a power supply for my LEDS? im actually interested in making one now that you say that it would be beneficial to go with that approach.

I'm sure there are, and a search for "LED power supply" should find them for you. You might not even need to build anything special!

1) Check the data sheets for your LEDs. Find out what voltage they need to operate, and how much current they draw at that voltage.

2) Multiple the current per LED by the size of your array (x 48), to get the total current your power supply would need to handle.

3) Get a wall-wart (or batteries) with as close to that voltage (1) as you can find, and with a maximum current rating higher than (2).

In principle, that should be all you need.

"Well, there's your problem." :-) You definitely shouldn't be driving them with 12V. Try 4.5V (e.g., three C cells or something), or a dedicated 3.5V wall wart. If they only need 20 mA each, then you need something that can deliver 20 mA * 48 = 0.96 A, so a 1A supply would be fine.