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

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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JerryE4

5 months ago

Here is what I use when such problems confront me 10pcs Mini LM2596s 3A DC to DC Buck Converter Power Supply Step Down Module http://r.ebay.com/FdwDOV These are damn cheap and very useful for plenty of projects so long as you don't mind waiting for the sellers to send from China, and there are sellers that ask a bit more for them in the USA if time is a factor. These little gems will do exactly what you are asking for at a decent price, this add is selling 10 of them for just about 10 bucks US with free shipping, so you can afford to experiment with them to see exactly how you wish to use them in your setup. I always keep a half dozen or so around for such problems in working on our Motor Home, which we use to avoid the cold and snow of the Dakota's winters. You surely don't need to buy from this seller, he is just the one that came to the top when I did my search, there are plenty of sellers on Ebay with this product some in the US for a few bucks more but much quicker shipping, so look around and buy as your needs dictate, should you decide to go this route. I currently have a rather large box containing all sorts of step up and step down power converters that I have played with in my experementing with LEDs from the little 3mm up to the 100 Watt monster lights that can light up a yard.

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Nuno

8 years ago

The voltage regulator will also waste energy (plenty of it, actually). What does power your LED light normally? 3 x AA?

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seandogueNuno

Answer 8 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.

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Nunoseandogue

Answer 8 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.

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seandogueNuno

Answer 8 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"...

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Nunoseandogue

Answer 8 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".

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frollard

8 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

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Re-design

8 years ago

Use a voltage regulator like this.  They don't waste so much of the battery to heat like resistors do.