led issues

[henmill]

Ok, i'm working on a minor mod on my computer in which I cut out the vaio letters on the side panel, flush mount some plexiglass behind it and light it up with six 3mm blue leds. All the cutting and what-not went smooth enough, but i'm having issues with the leds. I have a fair amount of experience with wiring and led work, but I can't figure out what I'm doing wrong now. it appears that as the circuit goes around, the leds get dimmer. And one of them isn't even functioning. The first one is nice and bright as it should be, but the rest get gradually dimmer as it goes around. I have them powered on +5v connected to the hard drive via molex with a 22ohm resistor(as suggested by a calculator). should i put them on +12v and use a bigger resistor? Should I just replace them and start over? Could someone suggest a different resistor/method? I'm starting to get frustrated and any help/suggestions would be greatly appreciated. I'll try to post a pic tommorrow.
Thanks - henmill

[wingman358]

You probably have the LEDs hooked up in SERIES which would look like this:

http://img311.imageshack.us/img311/1669/series3cj.jpg


If so, then that is a circuit with the LEDs in SERIES. In order for them to have the same brightness, you need to hook them up in a PARALLEL circuit. Here's a basic diagram:

http://img311.imageshack.us/img311/8882/parallel1gy.jpg

Heres some more for you to look at with some good descriptions:
LED Circuitry (http://www.theledlight.com/ledcircuits.html)

Hope that helps!

[Malatory]

also make sure you have your forwarding voltage right for the leds.

If they are needed in parallel like wingman says. the Voltage would be at 4.6

average voltage for LED tend to be around 3.5V (Ones from Supperbrightled are rated at 3.6 with a max of 4.0V).

http://led.linear1.org/led.wiz
Soure set to 5
Forward set to 4.6V
MA = 20 (typical)
Number of LED = 6


Solution 0: 1 x 6 array uses 6 LEDs exactly
+----|>|---/\/\/----+ R = 22 ohms
+----|>|---/\/\/----+ R = 22 ohms
+----|>|---/\/\/----+ R = 22 ohms
+----|>|---/\/\/----+ R = 22 ohms
+----|>|---/\/\/----+ R = 22 ohms
+----|>|---/\/\/----+ R = 22 ohms

The wizard says: In solution 0:
each 22 ohm resistor dissipates 8.8 mW
the wizard thinks ¼W resistors are fine for your application
together, all resistors dissipate 52.8 mW
together, the diodes dissipate 552 mW
total power dissipated by the array is 604.8 mW
the array draws current of 120 mA from the source.

[henmill]

Many thanks for the quick responses guys. You're a lot of help. I don't have time to work on it now, but once i get it to work, I'll let you know. Thanks again. :D

[crazybillybob]

Parrallel setup uses less voltage then Series, but uses more current. You would need to remember this when running LEDs of the USB because you only have 500MA. In this case you want to run a Series - Parrallel setup

___ --^^^----|<---|<---|<--___
|__^^^---|<---|<---|<--|

Brightness is the same and you Draw half the current as Parrallel setup.

When running LEDs in Parrallel you can cut down on parts by running sevral LEDS off one Resistor. The only thing is that the LEDS must be the Exact Same. That means All the same color, style (UB,SB, or Reg), and have the same voltage and current ratings.
If not you'll have to have a new resistor for each one.

Just Some thing to think about.
Crazybillybob

[henmill]

Well, you can call me stupid or whatever, because I probably deserve it. The reason the one led wasn't working was because it was connected backwards(-leg to +5v and +leg to ground). However, two of the leds still appear slightly dimmer than the rest, but I'm satisfied with the way it looks now. The leds were wired in parallel, as I thought, with the correct resistor. You can check out how it looks in my photobucket account..... http://photobucket.com/albums/c167/henmill/
I would also like for you to take a look or two at my xbox and little bro's gamecube.

p.s. In case you didn't notice, I like white cases with blue leds.:D
oh yeah, thanks again for all the help guys.

[crenn]

That Xbox is a 360 wannabe.... looks cool though..... although I prefer what's next to it :D

[Frogon]

I've actually got a question about this too. I went and did that wizard thing, and it's really nice, however, I went to Radioshack, and I needed (I think) a 65 ohms resister, however they only had a 45 and a 75. I asked a guy and he said you could put together a 45 and a 20 and it'd work, however, that's a pain, and I'm not too good at sodering.

I went back, and looked at the wizard, and it says "the wizard thinks ¼W resistors are fine for your application". I then went to radioshack, and saw that they did have a large selection of different fractions of a watt resistors, and one was 1/4. Does that mean that it'll work good, or just that it will be under powering the LED? I didnt want to buy it if it would just bearly light the LED.

[Malatory]

I've actually got a question about this too. I went and did that wizard thing, and it's really nice, however, I went to Radioshack, and I needed (I think) a 65 ohms resister, however they only had a 45 and a 75. I asked a guy and he said you could put together a 45 and a 20 and it'd work, however, that's a pain, and I'm not too good at sodering.

I went back, and looked at the wizard, and it says "the wizard thinks ¼W resistors are fine for your application". I then went to radioshack, and saw that they did have a large selection of different fractions of a watt resistors, and one was 1/4. Does that mean that it'll work good, or just that it will be under powering the LED? I didnt want to buy it if it would just bearly light the LED.

1/4 watt resisters is the lowest size you should go.. if you look at the half watt vs the 1/4 watt you will see it is a size thing.

75ohms will work.. the LED wont be as bright but it should last longer than one running right at its specs. (not much as it might only drop it by .1 Volts. Look at the wizard again but drop the forwarding voltage on the LED by .1 volts) you will see a slight change in the resister needed.

[crazybillybob]

1/4 watt resisters is the lowest size you should go.. if you look at the half watt vs the 1/4 watt you will see it is a size thing.



The rating 1/4 watt 1/2 watt 1 watt etc. Is the maximum amount of power that the resistor can disipate safely. 1/4 or 1/2 Watt will be fine for you. with 1/2W you'll just have a bigger saftey margin.

Good Luck,
Crazybillybob