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Ongoing thoughts and experiments on powering LEDs
They're slightly tricky to power, especially as I want full brightness all the time from batteries and wall power...
I've been using the ballast resistor method -
Batteries by themselves are a little hard to work with, especially alkalines. They produce 1.6 volts to start, but the voltage decreases throughout their working lifetimes. Simply putting a resistor in series with the LED - working with 1.6 volts to not overdrive the LED - means that the light will only be brightest with absolutely fresh batteries. Totally intolerable.
I've been using 6V/500mA (2.1mm/5.5mm dc connector) adapters from Mouser. They're pretty small and inoffensive as wall warts go. Part #106054. They're not regulated, so they still require some kind of current/voltage regulation. I did my first design barrel-positive, but the rest of the world (and the warts I got) are barrel-negative, so that's how it will be.
Mouser has nice power jacks: ME163-4303.
A page on wall warts: http://www.powerstream.com/Wall-mount-FAQ.htm
My current approach. I've been slapping low-drop-out linear voltage regulators in front of the LEDs, and adding ballast resistors from there. Unfortunately, low-drop-out regulators aren't. The cheap ones drop more than a volt - no good when using batteries. They let me run on 6v and 12v both, though.
I'm trying 'ultra LDO' regulators - LP3964ET from digi-key. Not quite as LDO as I'd like (how much?) but the shutdown pin works good... One serious problem - it only goes up to 7.5 volts Vin. Another regulator before it?
*Question* - if an LDO is below-voltage, does it still drop out? *Another* - How about reverse current protection? Rectifier?
Voltage regulators can be configured as current regulators... small-package LDOs instead of ballast-resistors, for LED chains?
Also MAX1916 - 3 current-regulators in one package. Maybe also facilitates pwm-less dimming?
Step-up DC-DC converters
If we're running off of battery or wall voltage above that of the LED voltage, will need some kind of step-down - or an additional linear voltage regulator.