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

• 01-18-2013
sixteenornumber
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?
• 01-18-2013
bikerjay
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.
• 01-19-2013
mattthemuppet
they'd be pretty useful for 12V strip lighting, as long as they didn't provide too much current.
• 01-20-2013
mhahn@hvc.rr.com
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
• 01-20-2013
MtbMacgyver
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.