TorchBoy
Flashlight Enthusiast
The LM317 is great if you have enough voltage headroom and you're not worried about top efficiency. It sounds like you don't have enough volts for it to still be running in regulation.
I'll mention a couple others that are my present favourites.
Just over a week ago I replaced my car's interior light with a couple of Crees driven by a KD "Kennan" buck board. Efficiency is around 85%, fixed 750mA current. That board handles 12V nicely and steps down the voltage as required. The current can be tweaked by changing a surface mount set resistor - I've thought of using a multi-position switch with several resistors of different values. The board needs the odd volt or two above the forward voltage of the LED series it's driving.
You mentioned dimming, so an interesting option is the AMC7135, which is a fixed 330mA linear regulator with a 0.12V dropout voltage - much lower than the LM317 used as a current regulator. It comes in multi-mode varieties (example) quite inexpensively. The main drawback is that it can only cope with 6V maximum (preferably lower for efficiency and heat reasons). Don't let that stop you - just use something in series with the board to step the voltage it needs to handle down to less than 6V. Normal power diodes might drop 0.6V each, so use something around 3.6V and you'll need fewer of them. (Have you figured where this is going?) You might just as well get some return for those dropped volts, so use LEDs instead of ordinary diodes - just keep in mind that they'll have a lower forward voltage when running at lower current.
If you want to go up-market there are lots of expensive drivers, but I haven't found much of a need for them for my uses.
Merry Christmas.
I'll mention a couple others that are my present favourites.
Just over a week ago I replaced my car's interior light with a couple of Crees driven by a KD "Kennan" buck board. Efficiency is around 85%, fixed 750mA current. That board handles 12V nicely and steps down the voltage as required. The current can be tweaked by changing a surface mount set resistor - I've thought of using a multi-position switch with several resistors of different values. The board needs the odd volt or two above the forward voltage of the LED series it's driving.
You mentioned dimming, so an interesting option is the AMC7135, which is a fixed 330mA linear regulator with a 0.12V dropout voltage - much lower than the LM317 used as a current regulator. It comes in multi-mode varieties (example) quite inexpensively. The main drawback is that it can only cope with 6V maximum (preferably lower for efficiency and heat reasons). Don't let that stop you - just use something in series with the board to step the voltage it needs to handle down to less than 6V. Normal power diodes might drop 0.6V each, so use something around 3.6V and you'll need fewer of them. (Have you figured where this is going?) You might just as well get some return for those dropped volts, so use LEDs instead of ordinary diodes - just keep in mind that they'll have a lower forward voltage when running at lower current.
If you want to go up-market there are lots of expensive drivers, but I haven't found much of a need for them for my uses.
Merry Christmas.
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