LM317 question, regarding minimum voltage

[Daravon]

I am following this mod with an LED I already have
http://www.candlepowerforums.com/vb/showthread.php?t=155553
and that LED is good for 1A, but I just measured my light and it's actually pulling 1.2A (3.5V V_f rather than 3.7 spec).

1. Is this really a big deal that is, is it going to burn my LED out real fast

2. I would like to use an LM317 as a constant current source, with a pot in the tailcap, for dimmage. But many websites like this recommend at least 2-3V across the LM317 for proper functioning. Since I have 4.5V from the batteries and 3.5v across the LED, will it work? Otherwise I could get dealextreme 3AA adaptors I guess.

 

[TigerhawkT3]

Although you didn't specify which LED you're using, it's probably "good for 1A" assuming proper heatsinking. That mod has none. The LED will fry. I'd recommend a resistor of about 20-50Ω for extreme low output that will preserve the life of the LED (as well as give you a great long-running emergency light).

If you have your heart set on a dimmable, high-output LM317 build, you could build one of these, or I could build one for you.

 

[Al Combs]

I would like to use an LM317 as a constant current source, with a pot in the tailcap, for dimmage. But many websites like this recommend at least 2-3V across the LM317 for proper functioning. Since I have 4.5V from the batteries and 3.5v across the LED, will it work? Otherwise I could get dealextreme 3AA adaptors I guess.

I logged onto National's site to have a look... They have a graph on page 6 of their LM317's 28 page pdf called, "Dropout Voltage". At 25°C, the 1 amp I/O differential curve is just a hair over 2 volts. So your 3.5 vF LED requires at least a 5.5 volt source to drive it to 1 amp. Really you need a tiny bit more voltage for headroom. It means since current is everywhere the same in a series circuit, at 5.5 volts the LM317 will dissipate 2½ watts of power as heat. It's only a linear voltage regulator. You can wire the LED in series with a sense resistor and have the LM317 monitor the voltage drop to make it behave like a constant current regulator. But that will cost even more power loss at the sense resistor. I'm guessing you meant a 3D Mag like the post you linked to. If you use 3 of the 3*AA D cell adapters for a total of ≈10.8 volts, the regulator will use more power than the LED!:thumbsdow

TigerhawkT3 is right, unless you really need something for dimming, use a power resistor instead of an LM317. The only good thing is you can run down to any Radio Shack and buy a 276-1778 for $2.29. Other than that, something like the 5 mode 1,000ma AMC-7135 regulator from DX will only cost $3.19. Of course you will have to wait a month or so for shipping. These chips are also linear, but the pdf on them says their dropout is only 0.15 volts @ 350ma. There are three of them wired in parallel on the board. So you only need 3.65 volts for your particular LED to reach 1 amp. That would be perfect for 3 NiMH D cells.:thumbsup:

 

[VidPro]

yup the LM317 is going to blow a lot of voltage out, when they have new mosfet low resistance stuff, and you arent connected to the wall socket no use blowing so much power out, unless you have the voltage to begin with, like a series setup.