# I need help with led resistors!!

Discussion in 'The Projects Forum' started by joshlfa, Mar 24, 2015.

1. ### joshlfa Thread Starter New Member

Mar 24, 2015
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So I'm building a led tree for my wedding. Im going to have a total of 45 led lights. So here are the specs on them.
Forward voltage: 3.2
Led current: 24mA

The power supply I'm using is 12vdc, 1amp. I am going to wire them in parallel. 15 legs with 3 leds each.
So now the million dollar question. How many ohms does the resistor(s) have to be? Where do I put them?
I've been to over 20 led calculator sites and they give me different values and some don't go up to 45 Leds. So I would really appreciate some help. Thank you

2. ### DickCappels Moderator

Aug 21, 2008
2,783
676
Each string will need it own resistor.

The LEDs will need 3 x 3.2 volts.

The voltage across your resistor will be 12 - (3 x 3.2)

The resistance of the resistor will be approximately (12 - (3 x 3.2))/current

The power rating of the resistor should be at least 20% greater than (12 - (3 x 3.2)) x current

If you are only going to use it for the wedding then you should be fine running at the maximum current rating. If you want it to stay bright for a long time it would be a good idea to run at less than maximum current.

3. ### joshlfa Thread Starter New Member

Mar 24, 2015
7
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Thank you Dick for your reply. I'm really sorry but I am a complete novice at this. When I do this equation (12 - (3 x 3.2))/current. I get 2.4
Does that mean 24 ohms?

4. ### #12 Expert

Nov 30, 2010
16,705
7,358
Vleftover = 2.4v
2.4v/0.024 amps = 100 ohms
P = 0.024^2 x 100
P= 0.0576 watts
A 1/8th watt resistor can do that but they aren't easy to find, so you will probably buy 1/4 watt resistors.

Last edited: Mar 24, 2015

Nov 30, 2010
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6. ### joshlfa Thread Starter New Member

Mar 24, 2015
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So if I had a power supply with let's say 5amps instead of 1amp how much will I have to change my resistors? Or can leave it the same?

7. ### MCU88 Member

Mar 12, 2015
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Current is proportional to voltage and inversely proportional to resistance. OHMs law states that 1 volt of electrical pressure will force 1 amphere of current into an 1 OHM load.

Please try to understand OHMs law as your very first movement with electronics.

Congratulations on getting married. I wish you all the very best for your wedding!

Nov 30, 2010
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9. ### joshlfa Thread Starter New Member

Mar 24, 2015
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Oh yes it's all coming back to me. I remember that from Auto Circuits 101. And I hope it all goes well also. I want to put the tree as a centerpiece. Thank you.

10. ### joshlfa Thread Starter New Member

Mar 24, 2015
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Very good point. Sorry for not catching onto this so easily. I appreciate the help. Thank you.

11. ### #12 Expert

Nov 30, 2010
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Read my dissertation. It was designed for people with ZERO background.
Not trying to insult you, just saying it's easy to understand when I break it down that much.
Besides, you might even think it's fun to look at it from that point of view.

Mar 24, 2015
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13. ### MCU88 Member

Mar 12, 2015
360
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Give him a break mate. Electronics is THE hardest subject that there is. The greater the genius the greater the holes in his socks. He is probably a successful hard working guy with 3 luxury cars and a million-dollar home.

14. ### #12 Expert

Nov 30, 2010
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Please don't try to interpret my emotional status at any particular moment.
Sometimes I make the effort to be all warm and fuzzy. Mostly, I'm pragmatic.
When I interpret the questioner to be capable of dealing with merely the facts, I try to get away with merely pointing him/her to the facts.

Last edited: Mar 24, 2015
15. ### MCU88 Member

Mar 12, 2015
360
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I feel the same way. Special? Gifted? Genius?... You know there is a fine line between a genius and a stark raving mad lunatic. I personally know of two in the real world. Doctors will tell you that I am special though.

But back on topic, and here is an schematic to help the OP:

File size:
78.2 KB
Views:
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16. ### #12 Expert

Nov 30, 2010
16,705
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Just lazy. It's 3:47 AM where I am. That puts me in the wrong time zone for warm fuzzies. I'm a lot nicer about 12 hours around the clock from here.

17. ### MCU88 Member

Mar 12, 2015
360
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Yes a characteristic of an genius is being lazy...

Back on topic though, and I am going to suggest an constant current source for driving LEDs. The advantage is that as the battery voltage drops the current through the LEDs remains the same, thus the brightness stays the same too.

18. ### joshlfa Thread Starter New Member

Mar 24, 2015
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Thanks guys for all of your help. Sorry for the late response. I just logged on. And no worries about what was said. No offense taken at all. I just try to be humble in any situation, and it's gotten me to where I am today. Thank you again. And I will post a picture of the finished product when I am done building this led tree.

19. ### MrChips Moderator

Oct 2, 2009
12,652
3,461
And a photo of you and the lucky bride.

atferrari likes this.