How to change voltage from 12V to 3.5V without power loss?

[CarFan70]

The title says it all..

[shorod]

Welcome to the forum!

Maybe some background in to what you're trying to do would help here. It's physically impossible to do this without power loss.

However, if you are just trying to make the DC to DC conversion as efficient as possible, your best bet would be to make a switching DC to DC converter. You could get something around 80-90% efficient this way.

What amount of current do you need to be about to source at 3.5V? If you want a quick and dirty and don't need to source more than 1.5 amps peak, you could make a quick regulator using an LM317 adjustable voltage regulator on a heat sink. It will be clean and small, but won't be the most efficient route power-wise.

If you're only looking to run a blue or white LED, then a simple series current limiting resistor would be sufficent, and at the 30 mA or less it would draw, efficiency shouldn't be a primary concern.

-Rod

[KiwiBacon]

Like Shorod said, a switch-mode power supply is the answer. Good ones can get into the high 90% conversion rate.

What are you doing?

[CarFan70]

I'm trying to connect LED diodes (3.5V) from my car socket (12V).. Thanks!

[KiwiBacon]

Originally Posted by CarFan70

I'm trying to connect LED diodes (3.5V) from my car socket (12V).. Thanks!

How many do you need to connect? I know from experience that you can drive those at up to 6v before they blow. So 3 in series would work well.
But with the tiny power draw of LED's, just using resistors to drop the voltage won't waste much power at all.

[rdjockey]

Led's run well at 1.5 to 2v up too 5v safely.. maybe 6v if your lucky, so if you put six or even eight in series they will work fine. Up to eight is ok because system voltage is up to 14v when the engine is running. If you want them on when the car is not running only connect six or seven tops, give them both a try and see how they work. Make sure you get the polarity right or you might have some probs too.

[shorod]

Since you're looking for 3.5V, I suspect you are trying to connector white or blue LEDs which have a higher forward voltage than the reds, greens, yellows and ambers. Usually it's around 3.3V for blue and white.

Are you wanting something that will remain a constant intensity regardless of battery volts (as bright when the car is not running as it is when it's on)? If so, you'll want a simply constant current regulator, which can be built using an LM317 adjustable voltage regulator.

Do you know the specs on the LEDs you're trying to connect? If so, you can easily calculate the series resistor you need to use to connect straight to the 12V system. This though will allow the intensity to vary somewhat with changing system voltage. Per Ohm's Law, V=I*R. To determine R, you have R=V/I. Your voltage will be the voltage you're trying to drop, so (14.5V - 3.3V)/0.030 = 373 ohms assuming you want a 30 mA draw through the LED at max system voltage. Standard resistor values in this range would be 330 ohms or 390 ohms. For this situation, the 390 ohm would give you better reliability since the 330 ohm would want a 13.2V supply voltage.

-Rod