LED light set up

alexp90

alexp90

alexp90

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Not sure we're to post this and after trawling site and google haven't found an answer :(

I want to make LED circuit to light up my case. It will be made up of 4 (possibly 5 but work off 4 for now) sets of 4 LEDs all rated at 2.2v and 20ma
If they were set up as follows

LEDsetup.jpg


IE each set of 4 would be in parallel within a parallel circuit, if that makes any sense :S
I'm trying to work out the volts and amps that would be running through it as i'm trying to run it off the PSU, either 5v or 12v
Would i be correct in thinking the volts would be 2.2v and there would be 0.32A in the loop?
I know i'd need a resistor so how do i work out what ohm? :S

All help appriciated!
Alex
 
You can assume the rails will be at a constant voltage, and that the leds are identical (same colour yeah?).

So each block of four has 12V across it, so 3V per resistor. Whatre the tolerances on the leds like? As in maximum/minimum voltage permissable? With that knowledge you can probably avoid using any resistors at all by using a cunning wiring pattern of leds.
 
I think the way you have it arranged at the minute, all 12V will be going to each LED. If you connect the four LED's in each parallel group in series (ie 4 parallel groups of 4 series LED's), you will need a single 47ohm resistor (as each LED will have 3V across it). I used an online calculator for that, as I can't be bothered to do it myself, but it does seem strange, I remember using a 1Kohm resistor for two series LED's off a 9v battery for my guitars LED's. I suggest you try a 1Kohm first if you do it this way, just to check the brightness. (this same calculator says you need a 560ohm resistor for EACH led if you connect them up as shown). the link is http://led.linear1.org/1led.wiz

I'll have a better think tomorrow when I'm awake and not watching blades of glory!

Just checked the site, it even designs the circuit for you! Go to http://led.linear1.org/led.wiz
and enter 12,2.2,20,12 in the 4 boxes and voila! the circuit I suggested is the second one, although it now recommends a 180ohm resistor in each group, that seems more believable to me!
 
Why even do it the difficult way?

Use 5v.

Wire 2 LED's in series = 4.4V. Its close enough. If you find the specs for the LED's you plan on using, you will find the max forward voltage. Dont exceed that.
As long as you wire them in pairs then you will not need a resistor.
 
im looking on ultraleds. Planning to get all the same colour (at the moment red but once i work out how to do the calculations on red it may change)

specs given

6,000 Mcd
Red 630 - 640 nm
20 ma
1.8 - 2.2 Volts

The drawing is a simple version of what i was planning. The 'main' wire you see would plug into PSU and then the 4 sets would separately plug into that, wouldn't all be soldered together (so i can manage the wires within the case)
I was thinking of doing the sets in series as suggested, do you think thats the best choice?
Thanks for the help so far (thanks for the link death). I'll keep working on it but i'll probably be back with a question!

Edit: Already Back! :P Can someone explain how they got 180ohms? I'm trying to work off my limited knowledge of my AS physics, not going well, i'm getting 160 but know im going wrong somewhere.
Edit 2: Ah! is it because 160ohm resistors aren't realy sold?

Alex
 
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alexp90 said:
1.8 - 2.2 Volts
Click to expand...
While LED (or any diode) will basically act quite much like short circuit after forward voltage treshold has been exceeded there's still some extra voltage loss when current rises.
Datasheet should have graph showing that

The 'main' wire you see would plug into PSU and then the 4 sets would separately plug into that, wouldn't all be soldered together (so i can manage the wires within the case)
Click to expand...
So you're intending to have one main plug for connecting everything into PSU and then chains of four LEDs are connected to that using connectors so that they can be removed/added when necessary?
In that case best (only sensible) way is putting separate serial resistor into every chain.
Again if you're just having all LEDs with fixed wiring then one serial resistor is enough.



death_looms said:
Just checked the site, it even designs the circuit for you! Go to http://led.linear1.org/led.wiz
and enter 12,2.2,20,12 in the 4 boxes and voila! the circuit I suggested is the second one, although it now recommends a 180ohm resistor in each group, that seems more believable to me!
Click to expand...
You forgot one diode per chain: (12V-4x2.2V)/20mA=160ohm



PhillyDee said:
As long as you wire them in pairs then you will not need a resistor.
Click to expand...
And we'll have SEDs aka Smoke Emitting Diodes until they turn to friodes.
We're not talking about some crappy alkalines batteries with lousy max output current but computer PSU.
 
LEDsetup-1.jpg


Ok thanks so far, after taking on board comments i have come up with another plan, Similar to the last but the sets are in series and its now got the resistors in. The resistors will be 180ohms. Does that look like it will work ok?
Also if i was to add another set of 4 in as the rest it would be the same wouldn't it? 180ohm resistor in the loop and just add to the parallel 'main'

Hypothetically could i add as many sets of 4 in as i wanted like this? (within reason of course, staying within the PSUs current limits)

Thanks
Alex
 
alexp90 said:
Also if i was to add another set of 4 in as the rest it would be the same wouldn't it? 180ohm resistor in the loop and just add to the parallel 'main'

Hypothetically could i add as many sets of 4 in as i wanted like this? (within reason of course, staying within the PSUs current limits)
Click to expand...
Yep, in case of just one serial resistor for whole thing it would have to be changed when adding extra LEDs.

50 chains would make 1A current draw so calculate from that.

ledchains.png
 
Yep, that circuit looks much better. Whenever you get them, make a mockup circuit where you can change the resistor value, get a feel for the LED's brightness, if they're superbright, you may need to run them cooler than rated (higher value resistor)! My superbrights are under my guitar pickups (wired to the selector ;) ), so only ambient light escapes, and they're still fairly bright, and they're not running at max!
And yes, you can add more 'sets' to that circuit, only the current draw will increase, the current and potential per set will b constant. Have fun, post pics!

EDIT: btw if you can't get the resistor value you want, get the next HIGHER one, too little current won't hurt the LED's, too much may!
 
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V=IR

V=3v (12v/4 because there are four sets)
I=20mA (20x10^-3)

R=V/I

R=3/20x10^-3

R=150 ohms

I'm pretty sure that's what you need I've just finished A level electronics and I am hopefully looking at an A. Otherwise I'm fudged. :D Someone else to confirm this would be nice though :D

I'd use a 160 to be safe or the closest one up form that you can find.
 
Brilliant! Thanks for all the help, Will be a while before its all done as i havn't even started choping the case they will be in up yet. But will put some pics in a build log. Hopefully will be getting started on saturday if i have the time
 
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