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Old 01-25-2004, 03:35 PM   #1
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An Electrical Question

I'm going to be building a new canopy for my tank and I want to put some moon lights on it. I've looked and the prices for buying them commercially seem really ridiculous considering that I can buy a "super bright" LED for like a dollar at a store nearby. I know that the LEDs I would use are 5 volt LEDs. Does that mean that all I need is one of those multi-use AC adapters you can buy at like Wal-Mart? Basically, just set it to 5 volts, cut the connector off the end and solder it to the LED(s)?

Or do I need to do something more involved like using a resistor to control the current? I'm a computer science major, not an electrical engineering major. I'm an electrical retard.
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Old 01-25-2004, 05:20 PM   #2
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I think you will find your answers in the DIY forum. Search moonlights.
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Old 01-25-2004, 05:51 PM   #3
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you can also find kits, manuals, and other great diy stuff here.
http://www.thelebos.com/
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Old 01-25-2004, 08:49 PM   #4
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You got the right idea, i used three LED's on my 30 gal tank...the only thing you will need are resistors so you don't blow out the LED's. All I did was wire all the leds and resistors together and connected them to a 12 volt transformer (like the ones you mentioned) Those DIY sites should help alot.
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Old 01-25-2004, 09:23 PM   #5
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Thanks, guys. I actually busted out my father's old college electronics books to figure it out mathematically. I had the idea that any "appliance" (i.e. any electricity-using device) automatically drew the correct ammount of current, because I was once told this. I took this too literally because the "appliances" they were referring to had the necessary transformers and resistors in them.

Looking at this ancient book of my father's, I think Ohm's Law was the most applicable piece of information I found...

E=IR

Thus, if it's a 5 volt LED, I should be getting a 5 volt transformer (AC adapter) and then figuring the type of resistor I need by this variance of Ohm's Law...

R=E/R (where E is voltage and I is current)

Does this sound accurate? And how would you know what wattage the LEDs require? Is that guaranteed to be given information or not?



Thanks for all the help. I'll be looking at those DIY things, but I want to make sure my logic is correct regardless. This has become like a little puzzle or game for me.
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Old 01-25-2004, 11:04 PM   #6
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Almost,

You are on the right track, but the object with LEDs is that they are not resistive for all intents and purposes so the current in their circuit must be limited to their foward current. Typically this is 20mA (MilliAmps)

So here is the formula you should use to determine the resistor you should use ( and there is a little play here). But first you will need to know the LED's forward Bias voltage voltage - there are typlically only a few of these and they vary by the color of the LED:

Red: 1.7 Volts
Green 2.5 Volts
Blue: 3.9 volts

The Forumla is a Variation of Ohms Law (BTW I can tell your Dad's book is old as we typically do not use E - Electromotive Force, but instead use V - Volts).

Resistor's Value = Supply Volts - LED Forward Volts / LED Current Max

By using this formula you limit the current in the LED pass circuit and also appropriately adjust the voltage. So for a 5 volt supply and a BLUE LED:

R = 5 - 3.9 / .02 Amps = 55 Ohms

You could use a 12V supply as well:

R = 12 - 3.9 / .02 Amps = 405 Ohms

The thing to watch for on the supply is how many mA it is rated at. recall that each resistor/diode branch uses 20mA.


HTH

I've posted in DIY forum under '5 LED Moonlight' a diagram and a link to get a really cheap power pack - I think $3

Tom
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Old 01-25-2004, 11:09 PM   #7
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also, check out http://www.lsdiodes.com/ for some ultra cheap led's.

Jim
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Old 01-26-2004, 01:04 AM   #8
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Oops - forgot the LSdiode link - btw I bought 20 from them.

Thanks for posting this, Yaksplat.
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Old 01-26-2004, 08:24 PM   #9
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Wow, thanks for all the help, guys...

Btw, that R=E/R was a typo... I meant R=E/I, of course. I think that the only question that I've got so far is this - What are "forward volts"?

I'm sure that I can follow your awesome directions (thanks again, btw), but I'm curious to understand it a little better.


For those of you who want to see the thread TheMadNucleus was talking about here is a link
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Old 01-26-2004, 11:06 PM   #10
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Hi Guage,

np on the direction. Good to see some one so interested.

As for foward volts, it's not so complicated. First let me expand a little on the term, it is actually a transistor rating and is in short form. The full term is the valtage required to forward bias a transistor.

Transistors are, for lack of a better term, electronic (electron) switches. They work with multiple layers of silicon and other semiconductors. LED's are a type of transistor. Rather then switching electrons they pass electrons, but when they do they give off photons as a byproduct of this electron conductivity.

So that being said - what is forward voltage? Forward voltage is the voltage required to get the transistor (or LED) to move electrons from one junction to another (in the case of an LED from the negative connection to the positive connection). For example for a Red LED this is about 1.7 volts. So if you apply 1.4 volts - no electrons will pass through the LED (semiconductors) and it won't light, but if you apply 1.8 volts then a flood of electrons will begin to cross through the semiconductor and Viola - red light.

This is why LED's are not particularly dimmable. Because they are sort of sensitive to this bias - varying the voltage doesn't really affect the forward bias to much in increments (a little but not worht using as a dimmer). I.e. the thing is sort of on or off.

So how do you dim them? Well - you use a simple electronic pulse generator which turns the LED on and off so quickly that your eye can not detect that it is flashing. Since it is a semiconductor - it can do this (a regular light bulb could not). Then in this pulse generator you allow it to adjust the width if the pulse from on to just 10% of the time to on for pehaps 90% of the time. Viola - an LED dimmer.

Well - guess I'm rambling - but you asked

Lata'

Tom
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