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# cut the power of a LED light from 12v to 500mA

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#### tom356

##### Newbie level 3
Hi

Is there a resistor I can use to cut the power of a LED light from 12v to 500mA or less..
I have a 12v led and want to cut the power to 500mA or less can some one tell me how to do this?
If so what parts do I need and were can get them
thank you

Ok first problem: Power is in terms of watts, or Volts * Amps
After that you need to know V = I * R
if you have a 12V supply and want to limit the current to 500mA, just say R = V/I or R=12/.5 so R = 24ohms
that means the total Resistance across the battery should be 24 ohms or more. then you need to figure out how much voltage you want across your LED and what the LED internal resistance is. If the LED has a resistance of say 5 ohms, then you need a series resistor of 20 ohms or higher. say you put that resistor between the LED and ground, this makes a voltage divider of Vresistor= 12 * (20 / 25)= 9.6V so the voltage across your LED would be 12-9.6= 2.4V
in that case your power in the LED would be p=i * v = 2.4 * .5 = 1.2watts
hope this helps you,
-Pb

Im a carpenter so i dont know the terms of watts, but thank you..
Im working on a project, I have a 12v led hooked up to my USB on my computer, USB ports puts out 500mV so i read.
'So when I plug in the UBS my computer says it drawing to much power..
So how can I cut down the drawing of power, using a 12v led?
(I need to use a 12 v LED..)

thank you

First of all the USB only provides 5 volts. Not 12 volts. The only way you can get 12 volts for your 12v LED is to add a DC-DC converter to convert 5 volts to 12 volts.

Secondly, a USB port is only guaranteed to provide 100 ma of current before enumeration. Enumeration is a complicated process that all intelligent devices have to go through to get connected to the USB. Part of the enumeration process is where the device tells the USB how much power it really needs. If the USB agrees, then and only then can the device begin drawing the higher current, up to 500 ma. Now some computers don't enforce the enumeration limit, so with those computers you might be able to get away with connecting an unintelligent LED load and draw 500 ma., but some computers do enforce the 100 ma limit.

Thirdly, I don't know why you need 500 ma, or 12 volts for that matter. Most single LEDs need only about 2 volts to light up, and they draw only about 10 ma. to 30 ma. of current. So you should have no trouble getting some small LED or array of LEDs to light up with the 5 volts and 100 ma. guaranteed from the USB. Now maybe your "12 v LED" is really an array of 8 LEDs in series, which might very well required 12 volts. And maybe it has several of these arrays in parallel so that you want 500 ma. of current. But that would be a very very bright LED array! What are you really trying to do?

I had just typed a response that Tunelabguy did better, so I will agree with his. Power of the usb is going to be your issue if you really need 6Watts of power(12V@.5A)
I know USB has different current providing capabilities that let it provide up to 500mA/900mA and in 3.0 up to 1.5A but I believe this requires your device communicating with the computer to ask for 'permission' to sink so much current (this is a fail safe implemented to prevent damaged usb devices causing further damage).
-Pb

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