LED Blue/UV Moonlight Mod...Tutorial needs Help!!!!

[saxman]

why 4.5v? is it cuz the 3v setting is not enough to power the LED's and 4.5v is the next one up? and u want to pump at least 3.6/3.8v to that specific LED?
oops...my bad...i thought your LED's were rated at 5V...if they're 3.6V nominal, use the 3V setting.

 

[scsinet]

Originally Posted by asharp13
http:///forum/post/3187349
sax, im am beginning to understand this stuff...honestly its awesome but i firmly believe that the only stupid question is the one that you dont ask...so i have to ask:
why 4.5v? is it cuz the 3v setting is not enough to power the LED's and 4.5v is the next one up? and u want to pump at least 3.6/3.8v to that specific LED?
does it matter how many LED's you put in that diagram? will each one get that 4.5v being pumped out by the power supply? and is this asumption correct: you could use any power supply setting (4.5v-12v) as long as it is above the voltage required by the LED...the only difference in the circuit being the resistor that is needed to bring the supply voltage down to that which is required by the LED to operate efficiently.
Put as simply as possible, when you use a resistor in a voltage drop configuratation like you do with LEDs, the resistor has to burn all of the current across the line that is not needed by the LED. If your LEDs only require 3.6v of forward voltage (like in Sax's parallel diagram), there is no need to run the supply at 12 volts. The LEDs can't use it, and you're just wasting energy and creating heat in the resistor, not to mention having to use gigantic sandstone power resistors to handle it. Sax suggested 4.5v because it's the most efficient way to do it... you only need to exceed the forward voltage of the LED by a bit, not blow way past it.
In my series diagram, since you need to add up all of the forward voltages, you need a higher supply voltage. In this case, I diagrammed a 12v supply, but since 2 LEDs a 3.6v is 7.2v, then a 9v supply would work fine. Just plug the new values into the above forumlas to adjust the resistor sizes.

 

[asharp13]

what if the forward voltage(3.6)* the number of LEDs(4) is greater than the supply voltage? you would get a (-) resistor value. what does that mean? that its not possible??

 

[scsinet]

Originally Posted by asharp13
http:///forum/post/3187440
what if the forward voltage(3.6)* the number of LEDs(4) is greater than the supply voltage? you would get a (-) resistor value. what does that mean? that its not possible??
Here is where I am not understanding what Saxman is telling you. He's gonna have to jump back in and go into more detail where he says that if the LEDs were rated at 3.6v, then use the 3v setting.
IME, the supply voltage has to be greater than the voltage required by the LEDs. If you have parallel LEDs, the supply voltage has to be greater than the forward voltage of one LED. If you have series LEDs, the supply voltage has to be greater than the sum of the forward voltages of all the LEDs in the string.
You're exactly right, the resistor cannot be a negative value. I think what Saxman is trying to do is underdrive the LED a bit for longer life, but this is where he and I disagree.
I am trying to drive the LED at 3.6v, which is it's "typical" voltage rating, not it's maximum. I am trying to drive the LED at 25ma, which is less than the "maximum" current rating of 30ma.
So driving the LED at 3.6v and 25ma should be fine. Derating the voltage from 3.6v to 3v (and even less once you add the resistor) will cause a serious degredation in light output I would expect.
Anyhoo, he'll probably see this today and chime in with details of what he's talking about.

 

[asharp13]

ok that makes a lot more sense. i think i actually understand this now!!!! i cant wait to order these LED's and start putting this thing together!!!!

 

[asharp13]

oh i forgot to ask. i am trying to figure out the wiring in the setup and have the basic idea in my head, but do you guys have any advice for covering the exposed wires i.e. where i soder one wire to the LED itself...i dont want any metal wires just hanging around ya kno. do you have any insight or advice as to how to make the wiring as clean and professional as possible?

 

[scsinet]

Look at Radio Shaft for breadboard. You could cut it into strips, and mount the LEDs on that.

 

[scsinet]

Originally Posted by saxman
http:///forum/post/3187318
SCSI: nice dwg! are you using a schematic pgm?
Visio Professional with the electrical diagramming shape packages.
I've used EWB and Eagle, and get so frustrated with them that I don't both anymore. Visio works for what I need.

 

[asharp13]

this may be a silly question but how could i find a good electrical supply store that would have all these kind of things laying around for cheap? what type of store am i looking for? what should i google search?

 

[scsinet]

Jameco, MCM Electronics, Parts Express, Newark, DigiKey, or Mouser are all suppliers that will at least have the switches, power supply, wire, and resistors. There are gazillions of suppliers out there for this stuff, but the ones I just listed are the ones I keep catalogs on hand for.
The LEDs... Digikey and Mouser almost certainly have them as will one or more of the others, but you are better off buying from a supplier that specializes in LEDs as you'll get the best selection and lowest prices. Digikey/Mouser tend to get really expensive for less commonly sold items.
For moonlights and other aquarium related LED purposes, I personally use Luxeon LEDs that I buy from that famous site where you bid on stuff. Seller LED_HK (or ledhk or led-hk, can't remember exactly) sells that stuff for really good prices, and even though it's shipping from HK, it's cheap and reasonably fast.