# Thread: is there any day I can use a DC-DC converter?

1. ## is there any day I can use a DC-DC converter?

im not an electrical engineer however talking with a friend at work who is, I realized I ordered the wrong parts for my homemade lighting setup. I just ordered 2 10A DC to DC converters input 8.3-14v and output 0.75 to 5.5v. for a DC-DC converter they are higher end parts and I would prefer to not throw them away. is there anyway they may still be useful for led lights?

2. Constant voltage is not ideal for LED but it will work. Knowing the voltage needed and the tolerance is a matter of picking an LED and studying the data sheet. You will want to measure the forward voltage at a given current and a worst case temperature like 90C. This will allow you to set a proper voltage.

3. they'd be pretty useful for 12V strip lighting, as long as they didn't provide too much current.

4. You could always put a resistor in series with your LEDs. You'll loose some efficiency, but if the resistor is small, not too much. Let's assume your DC/DC converter has a variable voltage output that you can fix at a specific voltage. If you are using XML LEDs you can find the current vs voltage chart in their data sheet. At 2 amps, an XML drops about 3 volts. Set your voltage converter to put out 3.1 volts and add a 0.047 ohm two watt resistor in series with your LED. The resistor will waste (as heat) about 0.2 watts, which isn't so bad. Your XML will be using up 6 watts, and wasting 0.2 watts on top of that does not seem too bad.

Where do the numbers come from:

3.1 - 3 = 0.1 volts

0.1 volts divided by 2 amps gives you a 0.05 ohm resistor (V= IR or V/I = R). I chose
0.047 because I'm pretty sure you can buy them as thru-hole parts from digikey (at least I did a couple of years ago).

2 amps times 0.1 volts equals 0.2 watts (P = IV).

Caveats:

I never tried this I usually use current limited buck converters designed to drive LEDs.

At high power, the LED will heat up. This will change the forward voltage across it for a given current. Use a good, big heatsink.

Read up on how to select a current limiting resistor for an LED.

Maybe try this experiment with a 0.1 ohm resistor to start with and see how it works. You should get less current thru the LED but things should run cooler.

Mark

5. Since you list a range for the output voltage, it must be an adjustable dc to dc converter. In that case, you can usually set it up to operate as a constant current source that'll work fine for driving LEDs. The specifics depend on how the voltage adjust pin is configured. Can you provide more specs, or better yet a link to the data sheet for the converter? Then I can tell you how to hook it up to operate as a constant current source.

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