# Need a circuit for driving 3 watt LEDs

I have few 3 watt and 1 watt LEDs connected in my vehicle…
I dont like to power them with a resistor from 12 volt, the reason is that if I use a correct value resistor the brightness of the LED is lesser than that what it should be and if I use a little lower value resistor the LEDs kaput in every few months…

So I thought of using a driver which will drive the LEDs on 12 volt or a sort of regulator with constant current…

I thought of using the LM317, But I feel that the purpose of shifting to LED form bulbs will be in vain…
The LM317 will consume more power to pull down the voltage to 3.4 volts which will be greater that the power which is used by an LED…

So pals, is there any other way so that I can power those led with 12v without loosing their brightness as will as hope for their long life… ???

Don't high power LEDs need a constant current power supply? Check the data sheets for the LEDs as they should have some recommended driver circuits.

Which LEDs do you have?

If your LEDs burn out with the smaller resistor, and work OK with the bigger resistor, but are not as bright as you want them, then your LEDs are not as bright as you want them to be. It's that simple! If you can tell us what specific resistances you've been trying, perhaps we can give you more information. For example, if it works OK with a 1 kOhm resistor, but blows out after a month on a 100 Ohm resistor, then there's lots of space to test in between. Meanwhile, if it works OK with a 200 Ohm resistor, but blows out with a 150 Ohm resistor, then you probably have to stick with the 200 Ohm.

If you want high efficiency, you should get an adjustable switch-mode DC DC power converter. Those are typically about \$5 each, plus you need some passives around them. Tune the output voltage to the level where the LED gives as bright an output as you can get without blowing it after a while. (This still may not be as bright as you want them to be, of course.)

I always use these calculators from these websites...

I use led voltage as 3.4v and 700ma for the calculations...

If your LEDs burn out with the smaller resistor, and work OK with the bigger resistor, but are not as bright as you want them, then your LEDs are not as bright as you want them to be.

LEDs which are less bright with correct resistance are much brighter with 3.2v to 3.4v voltage supply.. So I tried little lower resistance to make it little brighter and they go bad in few months...

Moreover their is another problem, when the engine is running ar much higher RPM the voltage supply increases from 12v to 14v So I think it will be better to use a DC to DC voltage converter...

I’ve used a lm317 for a 10w led, it draws a perfect. 99amp and its perfectly bright
and it doesn’t get too hot, maybe because I have its own heatsink, but that never gets warm either,
what you can do is use a lm317 and the leds in series, the leds will all have constant current since in series current is the same, and the lm317 will be dropping less voltage
try with 3 in series, if it doesn’t work the voltage isn’t high enough, then do 2 which will definetly work
I was lucky as the 10w led needs 9-12 volts so when im running at 13.8v there’s just enough slack for the regulator

I cannot use LEDs in series... The reason is, at every place I have only 1 LED and each is far from each other, so trying to put them on series will be a mess with wires again... :(

Will it be meaningful if I an using one LM317 to drop the main voltage ( 14v ) to 9v and then use another LM317 to drop that 9v to 3.4v…

I know I may be talking like an idiot…

Joy, please provide more details on your device. Perhaps you could provide a link to these "3W LEDs"?

Like dhunt posted, given your varying voltage source, the solution is likely a current source (a current regulator).

There are few places where I am using 0.5 Watt LEDs too this is the link for it..

http://www.ebay.com/itm/10-pcs-0-5W-8mm-StrawHat-white-LED-light-SUPER-BRIGHT-/110621724679?pt=LH_DefaultDomain_0&hash=item19c1919007

There are links on those pages to complementary driver boards for those units, @ \$1.50 That's probably the best solution.

That's probably best, drop it to 5v then have a current regulator

Firstly the LED brightness depends on the current - the resistor is there to set the current, if you use the correct resistance (for the supply voltage in question) it won’t make any difference to the LEDs brightness if the supply is 5V or 100V since the currents will be the same.

Secondly if the LEDs are failing then you are either overdriving them (they simply won’t work that bright), OR you aren’t providing enough heat-sinking so they run too hot and thermally degrade. LEDs tolerate less heat than other semiconductors I believe, so heatsinking is crucial for high power LEDs. It they run too hot to touch I suggest that may be part of the problem.

I had 1w leds in my headlights(no heatsink) and one always died more often, I had a 80 ohm resistor soldered directly to the led to save space, and the dissapated heat from the resistor was actually killing the led, the other one which I had the resistor about 6 inches away never died(even after being hit 40mph by a honda civic)