LED question

[hookedonreefs]

I origionally posted this question to Gator. But, seeing as he in underwater right now, can anyone else help me...
This weekend I bought 5 2600 MCD blue LEDs. They're rated 20 - 30 mA. I also bought a 4.5 V power supply which produces 700 mA. Then I bought a 440 Ohm resistor. I put the LEDs in parallel, and the resistor on the positive wire of the system. The LEDs work, but they're dim. I took the resistor out and the setup still worked. (much brighter) My question is: without the resistor, am I supplying too much current? I get lost in the Ohm/volt/mA relation so I couldn't figure it out. I've read the DIY moonlighting posts and get a little lost there as well. Can anyone explain it in Layman's (sp?) terms please? Thanks.

 

[hookedonreefs]

bump

 

[seraphan mage]

Ok lets see if i can help ya. Does the system call for a resistor or you just adding one. Why the lights are so dim is when you add a resistor to the circuit your actually adding resistance to the circuit. Which is why the lights are dim. I just wouldn't use the resistor if it doesn't call for it. I could go on but thats the easiest way i think i can explain it. If you want more info please ask.

 

[hookedonreefs]

You can be a little more exact. There's no "directions" for the system as I am building it from scratch from Radio Shack. I just put the resistor in so as to not blow the LEDs. Here's my hang-up:
From what I understand - P = V/I
P = power
V = volts
I = ?amps?
and
R (also) = V/I
So my LEDs create 150ohms or resistance -> R = 4.5/.03
(My power supply is 4.5 volts and the LEDs max at 30 mA.)
add all 5 LEDs together you get 750 ohms of resistance.
My power supply -> P = 4.5/.7 **this is where I get messed up!
I can't imagine the power supply only creates 6.42ohms
(power supply is 4.5 volts at 700mA)
So I move the decimal point to 642 ohms just 'cause it sounds better. Which means I still have more resistance (750 ohms) than the power I'm producing (642 ohms) so I don't need a resistor right??
Please help. Thanks.

 

[eeldarb21]

i just put a "moonlight" on my 25 gallon tank and works great.. i was going through the same problem as you with resistors and ohm's law basically.. so i did a search engine online and found marine grade led that uses 12volts dc power, so i went to radio shack and got a 12vdc power supply and plugged it in.. good to go, mounted it and it's a light blue and white...........just enough to see some creatures! hope this helps...
i think it was called brite led .com or of this sort..

 

[seraphan mage]

Right you don't need the resistor. Ok first off if your power supply can handle 700mA and each bulb is 30mA and you have 5 thats 150mA your supply should be able to handle even more bulbs. But i would like to see the pic's and spec sheet on the lights and power supply to be sure.As far as resistance go say your put a outlet 1000 feet from where the power comes from you have to use bigger wire or you going to a voltage drop off and the outlet won't work .Smaller wire more resistance bigger wire less resistance. Thats why when you put the resistor to the circuit the lights were dim you were add more resistance to line and proably was only putting have the voltage to each light. If you have product # for lights and power supply or a website so i could look at a spec sheet that would help to .Also look at the post called attn gatorcsm down on the board may help you out more then i did .

 

[agro]

LEDs do not work like light bulbs, you need the resistor to regulate current. If you overdrive them you will shorten their lifespan. There are 2 things I need to know to tell you what you need to do.
1: Is the power supply regulated or unregulated? If it is unregultated it may be putting out upwards of 6 volts.
2: What is the forward voltage (Vf) of the LED? This info is on the data sheet. If you give me the part number and where you got it I can probably find the info (if you have that info on the power supply that would be helpful too).
You should not run LEDs in paralel through one resistor because each LED will draw a different amount of current. You will need a resistor for each LED. You can run them in series, but you won't be able to put all 5 in series off of a 4.5V supply.
Here is the formula to determine the proper resistor for each LED.
R=(Vcc-Vf)/If
R is the value of the resistor in ohms
Vcc is the output voltage of the power supply
Vf is the forward voltage of the LED
If is the forward current of the LED in amps (so 30 mA would be .030A)
So if the LED has a Vf of 1.5V and a If of 30mA, with a 4.5V supply you would have.
R=(4.5-1.5)/.03 and R works out to be 100 ohms.
Hopefully this has been helpful rather than confusing!

 

[hookedonreefs]

This is a HEADACHE!!! (It's a good thing my company makes advil)
I have no clue weather my power supply is regulated or unregulated. I threw out the packaging, so is there a way to tell off the supply itself?
The LEDs have a 3.7 forward voltage rating. If you can't do a parallel system than I have a problem - the grounds. eg:
O

[hr]
|

[hr]
|

[hr]
|

[hr]
|

[hr]
|
| | | | | |
| V V V V V
| | | | | |
G X X X X X
| | | | |
G G G G G
I know, ugly but you get the idea. O is power supply, V is LEDs, X is resistors, G is grounds, - & | is wiring. So according to this picture there are 6 grounds, each having to be separate. (If they're connected then it's a full parallel circuit right?) So where in the He** do you find six separate grounding locations on the average SW tank without "leaking" electriciy into the tank?
Regardless, thanks for all the help, it's been an interesting discussion.

 

[hookedonreefs]

OK the spacing got totally screwed up... I'll try it again, ignore the periods (.)
O----|----|----|----|----|
.|.....|.....|.....|.....|.....|
.|.....V....V....V....V.....V
.G....|.....|.....|.....|.....|
.......X....X....X.....X....X
.......|.....|.....|......|....|
.......G....G....G.....G...G

 

[agro]

Just tie all the G's together All the grounds should tie to the negative side of the power supply. I should clarify that I meant you can't run the LEDs in paralel off of one resistor. But if each LED has it's own resistor you are fine.
O----|----|----|----|----| Positive Lead
.|.....|.....|.....|.....|.....|
.|.....V....V....V....V.....V LED
.|.....|.....|.....|.....|.....|
.|.....X....X....X....X.....X Resistor
.|.....|.....|.....|.....|.....|
.|----|----|----|----|----| Negative Lead
The power supply should have a sticker on it somewhere that gives voltage and current on it someplace, and if it does not say 'regulated' on there somewhere, it is probably unregulated. It should have a company name and model # on there too.
Where did you get it from? If it's from Radio Shack (I saw they have 2600mcd blue LEDs and a 4.5V 700ma supply ) then it is regulated. If it is regulated then you need 40 ohm resistors. If you can't find any just round up to the next value (43 ohms I believe).

 

[hookedonreefs]

Thanks again Agro. Yes it is regulated, and I have 47 ohm resistors that I'll be using. I'll solder and tape this week. My tank went cloudy again, once it settles and the light are hooked up I'll post a pic. for anyone that's interested.

 

[dockery07]

hey hookedonreefs,
I think your calculations are a little off, but not enough to probably matter. Just make sure your power supply is capable of supplying 675 mWatts, and the resistors you use are 2 Watt. Otherwise they will overheat and burnout, or worse yet catch on fire.

 

[agro]

Actually with 47ohm resistors there will be less than 30mA going through each branch of the circuit. With this current each resistor is only dropping 90mW of power, so the typical 1/4 watt (250mW) resistors will be fine. The LEDs are dropping 45mW of power, for a total of 675mW like you said . Anyway, since the power supply is capable of 4.5V at a current of 700mA, it's power rating is 3150mW which is plenty.
If anyone is interested the formulas are:
W = V x I or,
W = I^2 x R or,
W = V^2 / R
where V=voltage, W=watts and I=current

 

[gatorcsm]

I have not been following this thread. Just returned from sea for a short visit, but I'd recommend using only 1 resistor. You may have completed this project. But one resistor in series with the LED bank would accomplish this same task, without lowering the entire circuit's resitance. Also, it'd be a little cheaper.
I'm not sure what your are using, but I'd just guestimate, by what I've glanced over, that a 50ohm resistor in series with the entire LED bank would be sufficient.
I didn't calculate it out, but based on the 47 ohm you would use for each leg, that should be suitable for regulating the entire circuits current prior to splitting off.
I will concur that the resistor for each LED will provide more back-up (that if one LED branch fails, the rest won't be adversely affected), but as moonlights are generally a fairly inexpensive and quick project, once you get the hang of the resistors, the ease and inexpensive nature of one LED would be beneficial.
Last note: I've been meaning to experiment with a resin to coat my moon lights in to make them totally encased and water proof. This may be something to look into...
Hope it does/did work out for you.
Gator