PDA

View Full Version : Power supply question


Daniel G
01/21/2011, 12:36 AM
Okay, so if i was to connect two strings of leds to a 700mA power supply, would the current be halved or would both strings be getting the 700mA.
Thanks.

kcress
01/21/2011, 12:42 AM
That question is not easy to answer.

If both strings were perfectly balanced, yes.
They won't be perfectly balanced so actually, no.

It's your duty to balance them so they are close.

That's done by adding fixed resistors that you use to monitor the current.

Then, using that information and more, you need to manually balance them by swapping around LEDs - if needed.

Daniel G
01/21/2011, 01:00 AM
I will be having a power supply of 24v and 700mA so i will be connecting two strings of 7 leds. Now i didn't get your answer. Si is it yes to 700mA or is it yes to 350mA?

der_wille_zur_macht
01/21/2011, 08:43 AM
I will be having a power supply of 24v and 700mA so i will be connecting two strings of 7 leds. Now i didn't get your answer. Si is it yes to 700mA or is it yes to 350mA?

It's rougly yes to 350mA. In a theoretical sense, yes, when you put two strings of LEDs in parallel, each string gets half the current.

In a practical world, that never happens, because (even when from the same model/bin/batch) LEDs vary considerably. So, if you just randomly grab 14 LEDs and make two 7-LED strings, you'll end up with at least slightly imbalanced strings, and hence imperfect current sharing - you might get 300mA in one string and 400mA in the other. Or, 100mA in one string and 600mA in the other.

This is why parallel strings must be approached with a different mindset than the plug-and-play approach to a single string of series LEDs.

Daniel G
01/21/2011, 12:18 PM
Okay, i got it now. Thank you very much. You guys are always great help.