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Help Building LED String


Aidan

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Hello, I saw a really nice looking light string at Disney where each individual bulb switched between red and green without reversing power or an extra lead. So I eventually was able to track down the string which required a bulk order over $5k! I then was able to find the through hole bulbs and wanted to build a led string out of a regular string of lights. They have a voltage of 3.0-3.5 and a max current of 30 ma. I tried dividing 120 by 3.5 and figured I would wire 35 bulbs end to end and see what happened and well... it didn't turn out well :mellow:. I did some research on using a resistor on each individual bulb as I noticed when taking apart some led strings laying around but only ended up more confused even more so when I saw some of my strings had 30 bulbs with no resistors at all. Any ideas on what I can do to get this to work? I got them to work using a 12v dc supply with groups of 4 bulbs in series with each other and multiple of those groups in parallel to the supply however I cant use that in my display in which they would be dimmed. Just looking to make these LEDS work like any other store bought LED string... Thanks!

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I think this is what you are looking for.

http://www.instructables.com/id/Choosing-The-Resistor-To-Use-With-LEDs/

With red and green you should be able to take the strands and swap every other bulb and be fine since those two colors generally are very close in voltage.  However, If I remember right the Walmart brad with replaceable bulbs does 50 runs of red, 33 for green and blue with no resistors.  I will check on that tonight.

The easiest solution is just spin two strands, 1 red and 1 green, together.  You will just have close bulb spacing.  I did this with red and white on my crepe myrtle trees this year.

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Haven't gotten to that part yet haha but I'm sure I'll figure something out. I really love how these strings look which is what drove me to (try to) make a few of them, and the one company that does make them charges way too much as they are a custom order.

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I see what you are talking about now.  The included resistor for each LED is for when you hook an individual bulb up to 12V (like for use in automotive 12V) or run a string of them in parallel.  What exactly happened why you tried 35 with 120V AC?  If you smoked them that is one thing and if they just didn't work, that is another.

Christmas lights are run in series and there shouldn't be any reason that I can see why you couldn't do the same with these, except you have proven 35 is not enough LEDs in a strand with out an additional resistor.  I would start with too many, like 50 and see what happens.  I would assume the low value on the voltage (3V) and assume 40 was the minimum for 120V.  To protect the LEDs wire in a set of alligator clips and clip them to a large resistor.  Then you can experiment by going with a smaller resistor until you get them near the same brightness as those you have hooked up to the 12V run. 

http://led.linear1.org/1led.wiz

using this wizard at 12V, 3V LED and 20 mA you get 470 ohms which is the exact resistor included in the package.  I would use 3V and 20mA in the series array calculator.  click the LED series parallel array link.  Using 120V source, 3V LED, 20mA and 40 as the number you get a 1 ohm resistor (pretty much zero).  If this is correct you should have had a 820 ohm resistor in your test....but, there could be something else going on with these flashing leds.

The fact you smoked em with 35 in line makes me believe there is something else in this scenario, those LEDs are switching color and that means they must actually have 2 separate LEDs inside them (1 red and 1 green).  The question is when they flip LEDs does it go to 0 resistance for a split second? If this is the case, then your strand is going to be seeing fluctuating voltages and could be the reason you smoked the first try.  For instance if all but one change at the exact same time that one LED sees 120V.

If they didn't work at all on 120V AC, I would try putting a rectifier in up front which converts the strand to 120V DC and try that.

Basically, welcome to the world of R&D. 

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Thanks, yeah they were smoked almost as soon as they were plugged in, also I noticed that they didn't change color to red at all, some immediately died at green while others stayed lit very brightly on green without switching color. I would assume that there are no tests I could conduct to see if the voltage drops to zero in between changing colors. I have some big resistors coming in the mail (very slowly :/) and more leds, I will try out your method once they arrive. 

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1 hour ago, Aidan said:

I would assume that there are no tests I could conduct to see if the voltage drops to zero in between changing colors

because everything is digital these days, I don't think anything would register on a voltmeter.  You would need some analog volt meter to probably see it. 

I would try this, hook up 1x red/green led with no resistor then 3 regular white leds all in series....that should be safe for 12V.   Now hook those up to 12V and see if you notice the white LEDs getting brighter when the red/green switches.  I would also look to see if the white LEDs were a different brightness while green or red was on.  That would tell me those colored LEDs inside are actually different voltages. 

When trouble shooting LED strands I use an 18V transformer and have gotten away testing only a couple LEDS for shot periods.  That is one reason I am suspicious because I don't think  you should have smoked 35 on 120V.  The other problem is I am not technically savvy enough to know exactly how they switch internally.

1 hour ago, Aidan said:

noticed that they didn't change color to red at all

I think I would grab a rectifier and try DC power for all your trouble shooting.  Maybe the fluctuating AC power interferes with the electrical switching?

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I will have to try out using the rectifier when I get the new parts in, I just swapped out 3 of the ref green bulbs for white and noticed that when the bulb is green the three leds get a little dimmer and when it is red they get a little brighter. No noticeable surge while changing colors though

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I suspect the red and green are differing voltages and you are going to have to build the strands to the lower voltage...I scoured the web looking for more info on these but everything takes me back to ebay.  But there is another add with a chart that shows green 3-3.4V 30mA and red 2-2.4 30 mA under their bi-color alternately flashing LEDs.  The chart sucks because you can't figure out what is with what, but here it is and it kind of backs up what you are seeing.  Personally I would plan to run them as 2V 30mA LEDs and put them in runs of 50 with no resistor or shorter runs with resistors.  good luck and let us know how it turns out!

Capture.JPG

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Update: today I got the new LEDs in the mail and put 50 of them into a string. To be safe I ran it through a bridge rectifier and to be extra safe wired a 68k resistor in series. They worked! Very dim, but they still lit up and flashed. They seemed like they were about half of 1/4 their full brightness so do you think I should run them with a resistor in line permanently, of lower value of course. I suppose now I will try to run them without the rectifier to see what happens. 

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50481795650__912A241C-F24C-4DE3-A540-D67FB1724BCD.MOV

image.jpg

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Another update: They do need the rectifier to switch colors, lit up green only without it

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And finally, they work perfectly with no resistor while using the rectifier! Looks like I'll have to order a few more rectifiers... Whenever they lose power and come back on they are all again in sync with each other, whereas when run off dc supply they remained out of sync unless unplugged for a length of time. Do you think adding a capacitor would allow them to "remember" where they left off after a short amount of time? If so would it go on DC or AC side of rectifier and what specs would it need? I know very little about capacitors... Thanks again for all your help! 

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2 hours ago, Aidan said:

I'll have to order a few more rectifiers

remember if you plug them end to end they are all rectified off the first one.

with the 50 and no capacitor does it seem like they are full brightness?  compared to LED + resistor on 12V?  If not, start removing them one at a time until you get the full intensity and see where you end up.  Sounds like 97% intensity and the voltage is already too low for them to operate.  50 may be too many...but it doesn't sound like you are going to be able to dim them and keep them flickering.

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2 hours ago, Aidan said:

Do you think adding a capacitor would allow them to "remember" where they left off after a short amount of time?

I don't...the dripping icicles are the same way.  They all start in sequence and then drift off to become random.

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Just forgot, do you know why when they were dimmed using the resistor they still changed colors whereas when using my LOR they stayed solid green? Do you think the lights are essentially resetting when using Ac 60 times per second and since the first color when powered on is green that's all they have time to show before being switched off again rapidly?

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