Messed up with LED driver calc, can't think, brain to mush mode....

[Rockingit]

Hi Troops,

not so much an electrical issue as an electronics one, just realised that I've dropped a hairy one with making some crimbo led display lights (for me, for home). LED driver I got isn't up to the job, and my brain has ceased to function (mixture of cider and soldering fumes).... 10 x varied 105-130mW 5mm leds @ 3.7vf - 4.2vf in series. What size driver/s both current and voltage?

R.

 

[Rockingit]

OK, a couple of days later and I finally get back on it....now have a new problem! Having gotten my head around the principle and the formula, I'm now stuck again - because - the led drivers I have are constant current at 350mW AND will auto-sense the required voltage between 3 and 36V. So what do I use as my start values for the maths to derive my resistor value?!

If, for example, I've got 4 in series each at 4v / 100mA, can I just assume a supply value of 17v (needing an extra volt or two to get things moving) and therefore a 12ohm resistor? There's not much info with the drivers.

 

[topquark]

You'd use the maximum output voltage 36V. The problem you may get though (which is why I don't like them in series), is when one blows the circuit will fail (and you'll need a difference resistance to get it started again, having replaced). It's much easier to drive devices like this in parallel....with the same voltage across them (and possibly a small resistor if you're dealing with unequal forward resistance, just to make them all "appear" the same). They are often done in strings or arrays, to even out the voltage drop across them.

Say if you had four 4V ones and two 3.6V, then you'd arrange them as two strings of 2x4V and 1x2.6V with say 12V across them and a limiting resistor for each string. You would then base your calculations on using the 12V source.

What you can get with series is the problem when one blows they all blow as well (as the voltage drop across the first to fail will decrease during failure).

 

[Rockingit]

What you can get with series is the problem when one blows they all blow as well (as the voltage drop across the first to fail will decrease during failure).

Yep, just did. Beginning to wish I'd never started this ****ing thing now. Bah!! Humbug!!!

 

[topquark]

Unlucky, if I hadn't mentioned it, it prob wouldn't have happened (so blame me ). Stick with it! (a lot easier in parallel though, did I mention that? ).

[FONT=Verdana]6V _____________________________________[/FONT]
[FONT=Verdana]       |[/FONT]
[FONT=Verdana]       /[/FONT]
[FONT=Verdana]       \  <--- resistor value to match with the LED forward voltage to 6V, repeat in parallel for the next LED.[/FONT]
[FONT=Verdana]       /[/FONT]
[FONT=Verdana]       |[/FONT]
[FONT=Verdana]      LED[/FONT]
[FONT=Verdana]       |[/FONT]
[FONT=Verdana]0V__|___________________________________
[/FONT]

 

[Rockingit]

IT IS FINISHED!!
Can you tell I'm happy?!

And more's the point, Mrs Rockingit is pleased with the results, so I'm in with a chance.

Right you lot, Merry Crimbo to you and your's, I'm off to drown in various pleasant intoxicating liquids for a few days, see you when I surface!!

R.

 

[topquark]

Happy unwrapping