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Of course you can drive a powerful LED with a power resistor. But if you have a 3V battery that will drop down to 2V over its life and the LED needs at least 3.5V to work, then you need a DC to DC LED driver circuit.
Try a 6V battery with a LED that operates at 350mA and has a 3.5V voltage drop. Ohm's Law says to use a 7.14 ohm resistor to limit the current. So you use an 8 ohm/1W current-limiting resistor for 313mA.
But what happens to the brightness when the 6V battery voltage drops to 4V over its life? With 4V, the LED in series with your 8 ohm resistor will get only 63mA and will be very dim. If you had a DC to DC LED driver circuit then the LED will be as bright as it was with a brand new battery.
To avoid too much dimming when the battery voltage runs low, you use a 12V battery that's voltage drops down to 8V over its life. You use a 25 ohm resistor for 340 ma when the battery is new. When the battery voltage is 8V at the end of its life the LED current will be only 180mA.
Using a 12V battery to supply current to a 3.5V LED wastes more than 70% of the battery's power. :lol:
It's also an issue of heat and efficiency for any linear solution (some drivers are linear). Some of those LEDs take very substantial currents and would require pretty huge resistors.
Hi Oznog,
Zach in this forum is trying to build one.
If you go to Silicon Chip's website, they want you to pay a fee to see the entire project.
But if you enter in a google search, Our Best LED Torch, you will get a link to their entire project for free.
Yup, take a look at my thread on this subject, i also have some pictures of my 1W LED being as bright as it would with fresh batteries, but battery voltage is only 2.9V! Itll be the same when the voltage drops to .95V :lol:
It works great on breadboard but i cant get the blasted thing to work on PCB, ive gone through two PCBs so far that dont work:evil:
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