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I have 100 leds I'd like to hook up to my 12volt battery...here's the info on the leds:

Description

  • Size: 10mm Emitted
  • Colour : green
  • Lens Color : Water Clear
  • Luminous Intensity:4000mcd-5000mcd
  • Forward Voltage (V) : 3.2~3.4
  • View Angle: 20-25 Package Included: 50pcs x Superbright 10mm Round
    green LED Light Lamp Ultra Bright 10mm green led 26000MCD

Can I run 25 leds parallel then hook up four of these in a series...if so what size resistor will be needed for each parallel section or series...and how many amps will be drawn...I'm wanting to make a water tight led fishing light and have it run directly to my boat battery...

Can you please give me the best solution to hook these up to where all of them will glow the brightest with out having any that look dim... and the best way to make them last the longest...

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    \$\begingroup\$ Provide a data sheet link to the LEDs please. \$\endgroup\$ – Andy aka Oct 4 '16 at 10:12
  • \$\begingroup\$ This is all the info they'd given me.. About this item Features Size: 10mm Emitted Colour :green Lens Color : clear Luminous Intensity:22000mcd-26000mcd Forward Voltage (V) : 3.2~3.4 ;View Angle: 100-120 \$\endgroup\$ – 1life1love Oct 4 '16 at 10:37
  • \$\begingroup\$ You are contradicting what it says in your question and without better information this isn't going anywhere. \$\endgroup\$ – Andy aka Oct 4 '16 at 10:46
  • \$\begingroup\$ I cut and paste the info from the website...when the package came in there wasn't anything in it but the leds...no data sheet or anything else...that's the reason I asked... \$\endgroup\$ – 1life1love Oct 4 '16 at 11:05
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LEDs in parallel doesn't work well if you want them to all be the same brightness. Ideally you should drive each LED with the same current to make them all the same brightness. The easiest way to do that is to put them in series. If you need more than you can run in series then use a separate resistor for each series string rather than a single resistor for them all, that will minimize the brightness differences.

With 12V you and only run 3 LEDs in series not 4, that would require at least 13.6V.

3 LEDs takes 3*3.3 = 9.9V leaving 2.1V to drop over a resistor.

Assuming a drive current of 10mA (most LEDs are max 20mA but not much difference to the human eye in terms of brightness once you get to 10mA) you require 2.1/0.01 = 210 ohms series resistance.

So anything in the 200-400 ohm region should work fine, the lower end of that range will make it a little brighter but not a lot.

You can then parallel up as many of those 3 LED + resistor strings as you like assuming your power source can cope with the current. You'd be looking at ~300mA and 4W of power in total.

Keep in mind that I'm completely ignoring the change in battery voltage as it discharges. With 200 ohms if your battery is 13.8V fully charged you'll be hitting the LEDs with almost 20mA, close to their maximum rating.

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  • \$\begingroup\$ When looking up resistors on amazon it mentions wattage...1%,5%...which would be best in this 3led series... \$\endgroup\$ – 1life1love Oct 4 '16 at 9:05
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    \$\begingroup\$ The percentage is for tolerance not wattage. In your case 1/4W would be fine. \$\endgroup\$ – crowie Oct 4 '16 at 9:16
  • \$\begingroup\$ You will have 2V*10mA = 0.02W in each resistor. So as crowie said, 1/4 W is fine. 1/8W or even 1/16W would be ok if that's what you end up finding. If it's not any more expensive +/-1% would be better than 5% (they are often the same price these days). The actual value isn't that important and so 5% would work fine from that point of view however the closer the resistors are to each other the smaller the brightness differences. I doubt that within a batch of 5% parts the difference would be noticeable but all things being equal why risk it. \$\endgroup\$ – Andrew Oct 4 '16 at 10:23

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