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How do i set the output of a L7805CV regulator to 12v at 950mA

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jpoopdog

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Hi,

I recently purchased a 10W Led very cheaply in an auction, for some reason i thought it was a 5v Led, and i had a fixed value 5v 2A regulator which would be perfect, but as it turns out its 12v 950mA.

Fortunately i have quite a few L7805CV, however it apparently has variable outputs, i want to know how i set the output voltage to 12, and limit the current to 950mA, i will be running this off of 12V so i just need to limit current.

Thanks
 
LEDs set their own voltage. LEd voltages are not fixed like an incandescent light bulb, instead they are a RANGE of voltage like maybe from 10V to 14V for yours.
You just need to use a current regulator. An LM317 plus one resistor makes a good current regulator but its input voltage needs to be at lesast 3.25V higher than its output voltage.
 
If you are willing to waste some power, then use the 7805 in the same way that the LM317 would be used to create a constant current source...

The output resistor would have to be E/I = 5/0.95 = 5.1Ω. It would dissipate P=IE = 0.95*5 = 4.7W (so use a 10W resistor). The min input voltage would be the dropout voltage of the 7805 (about 2.5V) + 5V (the drop across the series resistor) + ~13V (the max expected voltage drop across your LED at 0.95A). That requires that the minimum input voltage is ~20V. The 7805 would be dissipating 2.5*.95 = 2.4W, so it would have to bolted to a heatsink,
 
As another alternative, you can use the 7805 like a LM317 in voltage regulator mode, using two resistors to set the 7805 output to 12v. You need to run about 10mA through the resistors, and the top resistor drops 5v, the bottom one drops 7v. So 470 ohms and 680 ohms gets you pretty close.

I think you should check inside the "12v LED" as it probably has an internal resistor and was designed to easily be run from 12v, like from a 12v battery, but the LED itself is much lower voltage!

If you remove it's internal resistor it will be more energy efficient and the resistor will not be needed once you are powering it from a constant current or constant voltage supply.
 
I just located a 7812ct i desoldered from a TV or something which is doing to the job wonderfully off 12v, i looked it up and it can only output 1A which is also good.
The diode is dissipating a easy to deal with amount of heat, and the 7812CT after a minute of use doesnt feel even the slightest bit warmer. I have checked and the regulator is dropping the current, direct off the battery more light is put out and more heat is generated.

Also, since im going to be doing quite allot of things with LM317's i have allot of them, whats their exact voltage drop and how do i calculate how many ohms i need to set whatever voltage or current, ive heard they can be used to limit both current and voltage
 
since im going to be doing quite allot of things with LM317's i have allot of them, whats their exact voltage drop and how do i calculate how many ohms i need to set whatever voltage or current
Check the datasheet.:D
 
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