For You Electrical Gurus

[Pappy]

Three years ago I posted a thread about modifying some LED Christmas lights. No one could answer my question and the project got shelved. This time I have a more specific question.

Background: I had bought two 100' strings. I want to splice parts of the 2 together so I have one custom string that will circle the front of my roof. I'm sure it is possible since they sell 200+ foot strings using the same 22ga wire. That is too long and I would have to cut a section out to make them the right length.

Sitting on the porch yesterday I pulled the lights out and started playing with them. I got the wiring combination figured out so they are wired in the right color sequence. Problem is that it appears the transformer doesn't have enough umph to power the extended length. It is 29V, 248mA, 7.2W. I can find 29V transformers that are both higher amperage and voltage. Which one do I need to increase? How much? Or should I skuttle the whole idea and go with plan B, shorten a 200' string to the length I need? I don't mind the $$$, just hate to admit an inanimate object beat me!

[leehljp]

I'm curious to see what the answer is.
Not knowing the correct answer for a transformer, I would get a 100ft 14 gauge extension cord to hook up the second set of 100 ft. At least I think the 100 ft 14 gauge extension cord would work. Of course plug the first 100 ft into the nearest outlet.

[Nick Keenan]

LED's have a voltage threshold below which they don't light at all. It's typically around 2v, although it varies with color. Much above that voltage and they burn out. If you have LED's in series you have to add up the voltages, so if you have two in series you need 4V. A 29V power supply is going to be around 15 LED's, depending on color. So you can't just string lights together, you have to match the number of LED's to the power supply.

You can put multiple strands in parallel, so long as the power supply can provide enough current (amperage) to light all of the strands.

[LCHIEN]

Pappy, I an an electrical engineer with nearly 50 years experience.

However the advent of LEDs has brought on a flood of different and widespread ways of hooking up 50 to 100 lights in a string for Christmas strings. There are those that directly plug into AC and some that have control boxes that have all kinds of flasher patterns, some that are DC power pack powered, and each one probably has several ways of doing it.

So its really hard to help with seeing what you have.
And some statements doesn't even make sense... like " I can find 29V transformers that are both higher amperage and voltage." Really you can find 29 V transformers that are not 29 volts? That's like getting 5 gallon buckets that hold 10 gallons.

Did you get these at lowes or HD? Where I can at least look them up by number on the web or in person?

Anyway, there's not enough info. Sounds like you have a power pack for the string? 29 VAC or DC?
Maybe a phone call to discuss it? The information you gave simply somehow is not enough.

[LCHIEN]

Some quick research shows that there are at least 9 common LED string schematics. ANd I've seen some more described on You tube for 100 lites (12VDC powering 4 series groups of 25 paralleled LEDs with no resistors.

Can you narrow yours down?


https://midimagic.sgc-hosting.com/chrsleds.htm

[Pappy]

Loring, my screw up on the available transformers. I meant to say higher amperage and watts. Pictured is the power box/transformer that came with one of the strings. The other one says 29V, .02A and doesn't show the watts. The link is to the lights I bought. The current Amazon listing shows 30V but mine are both 29V.



Amazon.com: Twinkle Star 300 LED Christmas Mini String Lights, 99 FT Fairy Lights with Safe Adapter for Indoor Outdoor Home Garden Party Christmas Decoration, Multicolor : Everything Else

[LCHIEN]

does the string only have one power connector, on one end?
If that's the case its probably meant for one power pack to power one string only.
Does the string comprise of just a pair of wires or more than that?
From the Amazon description it has several modes of twinkling and chasing lights, usually that has multiple wiring since groups of lights have to be controlled separately.

Honestly I'd have to look at it carefully to tell much about it.

Running a 100 foot extension cord to it would not be a problem at the current/power level being used. just 24 watts on the AC side.

Shortening a string may not be a good idea. some of these have lights in series to save wiring and costs and the series voltage has be be just right or it won't work.

[Pappy]

Power only on one end. Two wires from the power unit go to a box that controls the various functions. Two wires from there to the lights. The light color sequence is red-green-yellow-blue' Red/yellow and green/blue light up together. I'm not sure where the third wire on the string starts/ends.

Tuesday is trash day. I think the best bet might be to send these to the landfill and find some as close to circling my roof as possible. Amazon sells one now that is 181 feet that is fairly close. Better yet, solder them back to the original strings and donate them to Goodwill.