Greetings, All!

After reading a few posts on the site and seeing how helpful everyone is, I have a bit of a newbie question...

I'm currently in the first stages of making a hanging LED Christmas tree made of ornaments much like the one pictured below,

though the ornaments will be a frosted clear type with green LEDs inside. The project is going to take roughly 100 LEDs (I'm planning on using a 12v power supply and 33 strings of 3 LEDs each). My big concern is that I plan to use 30AWG wire to suspend the ornaments and connect the LEDs to the power supply. (The 30AWG wire is thin enough that it looks similar to string.) With that, I have two major concerns...
1) The wire is rated for .142amps according to here. Won't my power supply need to be roughly 1A? Won't that overload my wiring?
2) If there is a way around this concern, would the fact that I may have to wire some LEDs 5-6 feet away from the power supply affect my LEDs - especially if they're wired in a string of 3? (So I'd run 5 feet to from the power supply to the 1st LED, 10 feet [5 up and 5 down] to the next LED, 10 more feet [5 up and 5 down] to the third LED and then 5 more feet back up to the power supply from the third LED.

I'm so sorry if this question has already been answered somewhere or if the answer is clearly 'no'... I had seen the project done online and can't seem to find any explanation on it anywhere.

 

Reference: 9.64 feet per ohm
To answer: An ohm every ten feet is not going to mess this up, but 30 gauge wire is so skinny that you are getting close to overheating problems. You have to break the current up into several strings to avoid that.

I had to wipe my C: drive. Who has that LED voltage /current /color chart?

 

About 2.1 volts for green?
Five of them in series is 10.5 volts.
1.5 volts left over needs a resistor of 75 ohms (minimum).
0.14 amps would support 7 strings per each 30 ga. wire.
My habit would be 5 strings per power wire.
Are we communicating?

 

Thank you so much!

I think we are... Unfortunately, my memory of wiring is a bit rusty, but I think it is jogging up. (If it isn't clear, there's nothing a bit of google wouldn't help.)

I'm worried that the LEDs I'm looking at are listed at 3-3.4v draw, limiting my number of LEDs in a series by a bit.

But looking at your suggestions, I'm finding myself a little confused. Would my 30 ga. wire limit me to 35 LEDs (based on 7 strings of 5 series - or 5 strings of 5 series to be safe?)

Sorry if I sound helpless. The planning of this project is proving to be a little more than I though it would be.

 

I'm worried that the LEDs I'm looking at are listed at 3-3.4v draw, limiting my number of LEDs in a series by a bit.

That is a very important aspect. If I used the wrong voltage, I have to do the math over.

Would my 30 ga. wire limit me to 35 LEDs (based on 7 strings of 5 series - or 5 strings of 5 series to be safe?)

5 to 7 strings of LEDs per power wire. You want more? Use another wire. You just can't run the whole amp through ONE 30 gauge wire.

 

Oh! I think I'm getting it... I could use a stronger wire as the main power wire and then use the 30 gauge wires to make up the strings of LEDs, right? I modified a schematic to have 99LEDs... the darker reddish/brown wires would be heavier gauge... the smaller wires connected to my LEDs would be the 30 gauge... Does this look like it could work or am I severely off?

 

That looks seriously good!

 

Look at that! Learning a little every day!
(I can't take credit. vthokie11 posted this on another thread and I just modified it a bit by adding lines and copying until I had enough rows.)

But that's exciting! I already feel closer to being done!

If I can throw a wrench in everything and mess it all up, I happen to have one more question... My hope is to select 9-12 of these LEDs and replace them with the slow-flashing lights shown here. The voltage doesn't seem to change much, but I've seen that they can change a bit in voltage/amps depending on the color displayed at the time. (They automatically cycle through all the colors they can, spending about 1 second per color.)

I've heard that they don't like to be wired in series with one another, as the changing voltage/amps messes with each other. The solution would be to hook each one up to its own string and use a pretty big resistor, right?

 

Right. Otherwise they will fight.
That's going to need about 470 ohms each.

 

I can't say thank you enough. You've cleared up a ton of confusion and made me look forward to the next steps that originally seemed incredibly difficult. Next, I'm off to doing some calculations (which I can't promise that I won't stop back by for help with if I get completely lost), and then on to ordering the resistors so that I can wire it all up and test it out!

Edit: I didn't see that you had included the resistance needed, too. You're seriously a hero! Thank you tremendously!

 

The, "right" way is to actually measure a few LEDs so you aren't betting the advertiser got it right...THEN order resistors!

 

To measure Vf of changing color LEDs, might need to use oscilloscope as digital meter might be too slow. As suggested 470 ohm looks to be about right, 21 mA for red, 19 mA for green.

 

One last thought...There is no law that says you have to run LEDs at their highest rated current. You could use maybe up to 1.5 times the calculated resistance and not see much difference in brightness. Double the resistance and you can probably see the difference.

I ran the numbers to the max because this is a seasonal project. You won't be running it 24/7 for ten years.

 

Does this mean I can't keep my Christmas Tree up year-round? (Kidding!)

Thank you both for all of your support. It's phenomenal to find people who are interested in helping others and educating them on this work!

I'm stumbling into another question now... When it comes to resistors, there are so many more options than I remembered. I see carbon vs. metal, options in percentages, and (likely most importantly) different wattages. I can see 1/8W, 1/4W, 1W, fireproof, etc. choices... Do these matter all that much? If I buy 1W resistors, is it too much? Is a 1/8W too small?

 

I do not believe the resistor composition makes much difference, ' not even sure of my stock. From post #3, R of 75 ohms would be about 30 mW, so 1/8 th W would be fine but usually more expensive, so I would go with 1/4 W, 5% if size is not an issue.
Cost est. 5% carbon film 1/4W, US $ .65/ 10 or $ 3.75/100. $2.60/40, or 3.75/100.

 

Thank you so much! I didn't remember much about composition and didn't have anything written down about it.

But now I know enough to do the re-calculations for the resistors. 470 OHM is best for the multi-colors LED's... The majority of the project uses solid green LEDs with a Vf of 3.2-3.4V and a draw of 20mA, meaning in a series of 3LEDs, I would have 2.4 Volts left over.

Just to double check myself...

V=I times R, so 2.4V = .06A (gathered by adding the 20mA from each of the three LEDs) x Resistance.

So I would need a 40 OHM resistor?

That sounds off to me... Any thoughts on where I went wrong?

(Also, sorry for the simplicity of a math problem... I just learn better by interacting.)

 

All three LEDs in series use the same 0.02 amps.
2.4V/0.02A = 120 ohms

P = I^2R
P = .048 watts
Double it for a safety factor.
0.096 watts

Any kind of resistor will do. Go cheap. 1/4 watt 5% is a good suggestion.
An eighth watt would do if they are cheaper. Just, nobody buys those for hobby work so they might not be cheaper.

 

Thank you again! I see my error now... didn't need to add together the amperage. .02 was what the equation needed. Whoops!

 

Also, now that I'm getting into final details... Wouldn't I need at least a 2A power supply? 100 LEDs at 20mA? Wouldn't I multiply it all out to be 2A total? Or is the math taking 33 strings (of 3 LEDs each, rated at 20mA each) times the 20mA draw, equaling .66A, so I would need a 1A supply? Or would a 1A be too much?

Sheesh... Amperage, resistance, voltage, Watts? How do you all keep up?!

 

33 strings (of 3 LEDs each, rated at 20mA each) times the 20mA draw, equaling .66A,

That's it. The power supply doesn't shove a whole amp through the 33 strings. It ALLOWS up to a whole amp.

Everybody has their specialty. Try memorizing all the keys, modes, and scales of music. Something like 708 of them, and that's just the language you need to START understanding!