LED question - The Planted Tank Forum
Old 01-02-2012, 08:38 PM Thread Starter
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LED question

I purchased a cheap battery powered LED "strip". it's basically just a wired strand of 30 blue LEDs powered by 3 C batteries (4.5v total) and has a small resistor that I believe is gold black black brown. which, according to http://www.dannyg.com/examples/res2/resistor.htm, is a 10 ohms +/-5% resistor...

now... this thing has 30 leds and is about 10' long, i want to wire up three or four of these into my t5ho unit... ultimately powered by a single, smaller battery... AA or AAA to hide within the light unit and toggled on with a push button.

I've got no problem wiring things up... my problem is figuring out the power aspect... how much power do i need for 3 or 4 LEDs, and do i need a resistor?

this is pretty much what i've got, only more LEDs

http://www.amazon.com/Battery-Operat...ref=pd_sim_k_3
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Old 01-03-2012, 12:01 AM
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You need to know how the LEDs are connected - series, parallel, or both - and what the forward voltage is for one LED. Then you can determine how to connect them up to run on a battery. Usually no resistor is needed, since the battery voltage drops as the current goes up, so that limits the current well enough. But, if those are very low current limited LEDs, you would need a resistor.

Hoppy
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Old 01-03-2012, 01:09 PM Thread Starter
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Originally Posted by Hoppy View Post
You need to know how the LEDs are connected - series, parallel, or both - and what the forward voltage is for one LED. Then you can determine how to connect them up to run on a battery. Usually no resistor is needed, since the battery voltage drops as the current goes up, so that limits the current well enough. But, if those are very low current limited LEDs, you would need a resistor.
i'm thinking parallel, there is one wire in and one wire out from battery box. so positive to positive to positive, etc on the leds. I don't know how to get the forward voltage though
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Old 01-03-2012, 06:53 PM
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If you run 3 or 4 of these in parallel you will need a D cell, at least. That adds up to a lot of current for a battery to supply. I doubt that an AA battery could drive a single string for more than a few minutes, if that. You could experiment until you find a good combination with an acceptable lifetime.

Hoppy
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Old 01-03-2012, 06:57 PM Thread Starter
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Originally Posted by Hoppy View Post
If you run 3 or 4 of these in parallel you will need a D cell, at least. That adds up to a lot of current for a battery to supply. I doubt that an AA battery could drive a single string for more than a few minutes, if that. You could experiment until you find a good combination with an acceptable lifetime.
maybe we're confusing each other now lol

I only want to run 3 or 4 LEDs, 30 are being ran off of three C's right now.

i'm confused about batteries too, since i'm reading aaa's are 1.5v each... and C's are rated at that as well. I assume its the capacity that changes in size? so i could run three AAA batteries on this 30 strand, but they'll lose charge much quicker.
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Old 01-03-2012, 07:00 PM
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i hope your not intending to grow anything off these led's....

also i hope its just for added lighting effects.. because ur not going to get any growth on those leds.
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Old 01-03-2012, 07:12 PM Thread Starter
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i hope your not intending to grow anything off these led's....

also i hope its just for added lighting effects.. because ur not going to get any growth on those leds.
my initially reaction is to respond with a post dripping of sarcasm. However, no, these will be used for moonlights. I have a t5ho fixture for the plant growth, along with dosing ferts and pressurized co2.

I figured this would be a nice easy project that could be hidden within the t5ho housing.
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Old 01-03-2012, 08:23 PM
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Originally Posted by scapegoat View Post
i'm thinking parallel, there is one wire in and one wire out from battery box. so positive to positive to positive, etc on the leds. I don't know how to get the forward voltage though
Just because there are only 2 wires you can't tell if it's parallel or series. Both will have just 2 wires from the battery box.

Here's a decent calculator for the resistors and it will design arrays for you for multiple leds.

http://led.linear1.org/1led.wiz
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Old 01-03-2012, 09:27 PM
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Originally Posted by scapegoat View Post
my initially reaction is to respond with a post dripping of sarcasm.
lolol...

yeah moon light is lighting effect, and it should look nice....

Also if a 1.5V battery can power the array, it has to be in parallel..
I cant think of any LED's which u could string up like that in serial and have it run 1.5V.
LED's of low wattage class typically have a power req of 1.5V-3V. Automotive LED's like the ground effect strips done in serial and require 12V.

So im going with paralell wiring on the LED's if a 1.5V battery can provide it with enough current.
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Old 01-04-2012, 02:19 AM
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I wouldn't bother trying to run these off of batteries.

I would run them off of a small DC power supply. You probably could use a 5 volt cell phone charger to drive them if you need 4.5v. The 5.0v shouldn't be a problem, and I doubt they'd take more current that the phone charger can output.

You can use automotive LEDs on any common 12v wall charger. I've done it. They make great moonlights but you may have to use a lower voltage--- I used a 9v with the moonlights I used to have and the brightness was dead-nuts perfect.

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Old 01-04-2012, 08:10 PM
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wait also its 1 battery? or 2 batteries?

2 batteries in series = 3V
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Old 01-04-2012, 10:14 PM Thread Starter
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I may have made my original post more confusing than it needs to be. I have a 30 led strand powered by 3 C batteries in series(4.5v) w/ an unknown resistor.

I want to remove 3 or 4 of the leds from the strand and wire them into my t5ho hood and power them from a single, smaller, battery.
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Old 01-04-2012, 10:56 PM
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is there a resister after each series?

id say its probably safe to cut b4 after each resistor... however ur still going to need a 4.5V feeding current to it..
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Old 01-05-2012, 01:56 AM Thread Starter
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nope, there is a single resister

so it's three C batteries, switch, resistor. negative and positive out to first led, then a neg and positive from that led to the next, and so on until the last led where the wiring stops.

the batteries are reverse order, as one usually see's... so

- + ----------------------\ /--------\
+ -.................................={led}.....={led} (+28 more LEDs)
- + ---[switch]--[resistor]-/ \--------/

judging by the fact that there are 30 LEDs being ran by 4.5v (- whatever from the resistor) I think a single 1.5v battery would be appropriate for ~10 LEDs... so i would need a resistor, or a few, to run 3 or 4 LEDs off of a single 1.5v, correct?
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Old 01-05-2012, 02:11 AM
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Wow, this really got complicated. Your original 30 LED chain needs 4.5 volts to operate. Assuming they are wired in parallel, which they seem to have to be, you would need 1/10 the voltage to power 3 LEDs, or 1/2 volt, roughly. So, you would need a resistor to drop a 1.5 volt battery voltage down to .5 volts. Assume the current needed is ??. The resistor needs to be 1/?? ohms. This isn't critical because an AA battery voltage would droop considerably with the amount of current you need. Unfortunately I don't know how much current those LEDs need. I think I would look for a trim pot to put in the circuit and use it as a rheostat to adjust the current up from zero to where the LEDs light up.

EDIT: I'm confused! You need 1/10 the current to power 3 LEDs, not voltage. But, you still need the same voltage, or 4.5 volts. So, you need to determine the forward voltage drop across one LED, and the desired current in order to figure out what size resistor is needed. But, it used a 10 ohm resistor for 30X one LED's current, to drop the voltage down to equal the forward voltage drop. So, perhaps (?) you need 100 ohm to get the same voltage drop when using only 3 LEDs (1/10 the current)

Hoppy

Last edited by Hoppy; 01-05-2012 at 04:38 PM.
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