what resistors do i need to step a 12V battery down to 3.25V to run a LED light?

I have a 12v battery that runs my fishfinder on my kayak.  I am making a LED 360 light and want to use the same power source so I don't have to keep using AA or rechargables.  I need to know how to step down the battery.  I no longer have my electronics books to look it up.

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Nuno7 years ago
The voltage regulator will also waste energy (plenty of it, actually). What does power your LED light normally? 3 x AA?
seandogue Nuno7 years ago
If a linear regulator is used, that would be true. But since the vast majority of converters are PWM based these days, that's just ignorant. A resistor will dump as much or more energy than a linear regulator, btw.
Nuno seandogue7 years ago
I don't know what do you mean by "that's just ignorant", will you be kind enough to explain? English is not my mother language. Personally I don't know of any commercial LED current regulator suitable for standard 5mm white LEDs, I've seen many but for "power" LEDs like 1W and more. Going with series LEDs and very low resistor values as frollard suggests might perfectly be the best solution considering time/skills/price. But the OP is still scarce on details, like how many LEDs is he wanting to light up? And judging by his question, he definitely doesn't understand LEDs, which leaves me reticent even about the origin of the 3.25V he mentions.
seandogue Nuno7 years ago
The idea that a "voltage regulator will also waste energy (plenty of it, actually)" is ignorant, ie, not stupid, but uneducated. If you had said "linear regulator", I might have agreed, but linear regulators are, while still in use, falling away with time to PWM based regulators, since they DON'T waste "plenty of it"...
Nuno seandogue7 years ago
Ah, I see.... I was actually answering Re-design's comment, so was implicit that I meant "linear regulator". Just forgot that comments here get re-ordered unless you specifically hit "Reply".
frollard7 years ago
Depends on the led voltage.  Red is typically around 3 and white/blue is typically around 4.

My recommendation:  Run multiple leds in series with the smallest possible (appropriate) resistor.  Each led in series will subtract from the total series voltage.

EX:  12 volts - 1 led = resistor has to burn off ~9 volts (75% of the power, thus 75% of the battery = waste!)
12 volts - 4 3-volt leds = leds use most of voltage, resistor can be very small, 1-10 ohms, and will burn (*waste) VERY little power as heat.

multiple leds:  http://led.linear1.org/led.wiz
resistor calculator if you REALLY want ONLY one led: http://led.linear1.org/1led.wiz
Re-design7 years ago
Use a voltage regulator like this.  They don't waste so much of the battery to heat like resistors do.