Voltage Supply For 700Ma Led

[julesg]

Hi

I have been scratching my head over this one - first time posting on here, so "hello"!

What I am looking for is the *best* solution to provide 2.4V DC at 700mA to a high power LED from a 12V DC supply (motorcycle indicator in this case).

The spec' of the LED is:-

Max Forward Current: 700mA

Peak Pulse Forward Current: 1500mA

I would like to drive it at close to the max current for max brightness, which equates to 2.4V. Looking at the pulse curve for the LED it would appear that in my scenario (1 second on, 1 second off flashing) the pulse peak should be 1300mA falling to 900mA in 0.1 seconds.

To try out the LED I just used some 7W 15R resistors - which worked fine for a quick test, but obviously I can't use this for the long term solution.

I have considered using a voltage regulator with heat-sink to get from 12V down to close to 2.4V and then a resistor to trim to precise voltage. However, I am not sure if this is viable? The 12V supply is currently already passed through a flashing control so is switching at around once per second. I could re-arrange things so the voltage reg is supplied a constant 12V and flash the output, but would really prefer not to.

The only other option I am aware of for this kind of voltage reduction would be to use a DC/DC converter, but I believe there would be an issue with this as the LED would not provide the necessary feedback for voltage reference, without a constant current DC/DC converter which seems a little extreme.

So there it is. Any advice greatly appreciated.

Jules.

 

[OnOff]

I'm thinking you just need a resistor. A thread I started a little while ago. Post #22 might help:

http://talk.electricianforum.co.uk/topic/25575-knightriderfor-gates/

I got a bit lost as you'll see when it came to the transistor calcs but all came right in the end. The project of "blinging" up my mate's gate is on hold whilst I get back on my own front gates which I started before his!

 

[julesg]

Thanks for your reply Onoff. I have had them running with resistors (15R 7W) for testing, but even at 700mA its around 7W of power dissipation which would concern me in itself (for reliability), but with the pulse current going up to 1.3A or more that would be 25W++.... I can't see the resistors lasting too long. The 7W resistors I used for testing get insanely hot.

Maybe I am wrong??

Jules.

 

[Richard-the-Ninth]

Could you not use something like

Its desigend to run at 12v as it is.

 

[julesg]

Sadly the LED option is fixed for various reasons which have massive time/cost issues. (aka I didn't actually think providing reliable power to these LEDs would be troublesome before I fabricated the LED mounts, lenses & bezels which are part of the fabricated and now powder coated frame....)

 

[Marvo]

You can build a driver out of a couple of transistors and a couple of resistors or even a FET and a transistor. Only problem is it might take a bit of playing around to get the correct resistor value to clamp the LED's at the correct drive voltage.

I can give you a circuit but it's probably quicker for you to find one on Google.

Edit;

Here's a straight forward driver;