# Connecting 20 3v LEDs in series?

Discussion in 'The Projects Forum' started by gfurther2, Jun 21, 2012.

1. ### gfurther2 Thread Starter New Member

Jun 21, 2012
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I need to connect 20 3 volt 20mA white LEDs. I am very new to electronics, but I want to connect them in a series circuit because they're spaced about 5 feet about and the whole circuit makes a circle, so I could save half the wire. I'm using a 3 volt power source (2AA batteries), but this is only temporary to see what the LEDs look like until I get a power adapter. Here's my question: I connected 5 LEDs in series to just make a mini model, and the LEDs didn't light! But when I connected them in parallel they lit up. So I did some research online and found out that to connect 20 of these 3v LEDs in a series circuit I'd need a 60 volt power source? (3 volts for each LED). I want to use my 2AA batteries because I know I can't get hurt using the 2AAs, but I don't know about 60 volts. So do I have to connect these LEDs in parallel and waste another 100 feet of wire, or is there a way to wire them in series. Or any other way to do this without using 2x the amount of wire!

2. ### tracecom AAC Fanatic!

Apr 16, 2010
3,878
1,396
You cannot connect LED's in series unless you provide a power supply at or above the total of the LED voltage. And if you connect them in parallel, you need a current limiting resistor for each LED, and your 2 AA batteries will last practically no time.

3. ### gfurther2 Thread Starter New Member

Jun 21, 2012
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So I've decided to use the parallel method, and I want to use a 9 volt power source (wall adapter). I know all the formulas, V=IR and all that from high school. I just have no idea how to apply it? And these current limiting resistors... Where do I put them? What size do I need. Been trying to figure this stuff out for years.

4. ### BSomer Member

Dec 28, 2011
433
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R = (Vs - Vf)/I

Vs = supply voltage
Vf = forward voltage of LED
I = forard current of LED

The best method would be to use one resistor for each LED. Where it goes does not matter.

Vs -> LED -> Resistor -> Gnd

or

Vs -> Resistor -> LED -> Gnd

5. ### gfurther2 Thread Starter New Member

Jun 21, 2012
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9 volt supply voltage, 20 3 volt LEDs at .02 amps. So (9-3)/.02 = 300 ohm resistor

Or

(9-3)/(.02 x 20) = 15 ohm resistor? I multiplied the .02 amps by 20 because I'm using 20 LEDs.

AND

Since I'm doing 20 LEDs in parallel would it look like this:

Vs(neg-) ----> Resistor ----> | ----> Resistor ----> | ----> (and so on)
Space:::::::::::::::::::::::::LED::::::::::::::::::::LED
Vs(pos+) --------------------> | --------------------> | ----> (and so on)

Or would the resistor be on the positive rail? Or does it matter?

6. ### Bernard AAC Fanatic!

Aug 7, 2008
4,239
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Use a series parallel arrangement, will " waste " just 50 ft. 9 V - 6 V = 3 v / .02 A = 150 Ω. Use any thing over 1/10 W; 1/4 or 1/2 might be easier to work with.

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7. ### gfurther2 Thread Starter New Member

Jun 21, 2012
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I want to put these LEDs around a pool, so connecting 2 LEDs in series wouldn't save me any wire in my case? I attached a drawing.

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8. ### WBahn Moderator

Mar 31, 2012
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Putting two in series will actually cost you wire, but will reduce your current requirements.

Imagine taking two wires and running them all the way around your pool. One of these is the positive rail and the other is the negative rail. Then, at each point that you want a light, you put an LED and the appropriate current limiting resistor between the two rails. Since your total current is only 400mA, the wire used to make the rails doesn't have to be too large. This will let you limit your voltage to 5V or 6V which is good from a safety standpoint around a swimming pool.

Be sure to check into the electrical codes that apply in your area for outdoor and pool area requirements.

9. ### wayneh Expert

Sep 9, 2010
12,381
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+1 Don't ignore this. Give thought to worst-case situations such as someone in the pool grabbing hold of your circuit through a flaw in the insulation, or a local pet coming along and biting a hole in the circuit. This may all seem unlikely when you install this but things happen over time. Corrosion, UV damage, moisture condensation, on and on. I predict rusting of your LED leads.

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12. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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I have experience to back up Wayne's and other's cautions. I constructed some LED strings for between plants and some backlights for my garden pond's waterfall - which all failed soon thereafter due to corrosion. I then purchased a LED lighting strip from an auto supply store (it was sealed for external use), but then my power adapter wasn't rated for outdoor use and burned up (like as in almost fire)!

I now front-light the pond garden with commercial LED garden lamps...

13. ### gfurther2 Thread Starter New Member

Jun 21, 2012
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Thank you everybody for the help! Since the last time I was on here I installed only two LEDs and after only 2 days you can see the corrosion. This has made me realize the real work of this project and I will be now holding off until I find a more reasonable outdoor solution.

Thank you all for the help though, I really learned about electronics from you guys and will be posting a lot more questions because you guys really know how to explain things unlike Wikipedia!