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Discussion Starter · #1 ·
I have three packages of 50' strings (LED if it matters). I need one 70' run and one 80' run. So my thought was that I'd cut the third string into two, and then solder and heat-shrink tube those to the other sets to get the exact length strings I need. It seemed so simple at the time.

But I just opened the packages, and noticed that they have 3 wires. I've just learned that 3-wire Christmas lights are "series-parallel". So if I want to splice them together I need to figure out which wires go with which. Is that correct?

I think the diagram below depicts what's going on, except in my case, the three wires run the distance from the first light to the last (vs. this diagram, where the last series is just two wires).




Is there an easy way to do this? Can I use trial and error without blowing anything up (i.e., if it works, it's correct, if it doesn't, swap a wire and try again)? Or is that risky business?
 

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To find out which wires are which, I would do a simple continuity test on the string.

Unplugged, you touch one probe to a wire at the beginning, and the other probe to the ends, until you find which one has continuity, use different colored tape to "flag" these pairs, then you do your splicing as planned, matching the wires to each other as they were arranged at the plug ends.

This way no trial and error, but often wiring is rated for just so many "draws" (lights), and if that is the case you might "draw" more current than intended.

But with LEDS, the "draw" is less than the old style.

ED
 

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Discussion Starter · #4 ·
(probe technique)
Yes, this is sort of what I thought I needed to so, thank you so much! So of the three wires, two will have continuity to one prong on the outlet, and the third will be the lone wolf.

I just need to match the two to the two, and the one to the one, and I should be good.

And yes, I did think about the draw, and the sets say that you can connect 5 sets in series, and I'm only going ~1.6, so it should be fine.

Thanks again!
 

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You will need to remove all the lamps (if possible) to do the test otherwise you will continuity through the lamps.
 

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You need to keep the 50 lamp sections in tact. There is a current limiting element in each series loop that must remain.


Sent from my iPad using Tapatalk
 

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You asked
I have three packages of 50' strings (LED if it matters). I need one 70' run and one 80' run. So my thought was that I'd cut the third string into two, and then solder and heat-shrink tube those to the other sets to get the exact length strings I need. It seemed so simple at the time.

But I just opened the packages, and noticed that they have 3 wires. I've just learned that 3-wire Christmas lights are "series-parallel". So if I want to splice them together I need to figure out which wires go with which. Is that correct?

I think the diagram below depicts what's going on, except in my case, the three wires run the distance from the first light to the last (vs. this diagram, where the last series is just two wires).




Is there an easy way to do this? Can I use trial and error without blowing anything up (i.e., if it works, it's correct, if it doesn't, swap a wire and try again)? Or is that risky business?
curiousB replied
You need to keep the 50 lamp sections in tact. There is a current limiting element in each series loop that must remain.
and this may or may not be true.

Please consider that, with a "string of 50 lights (LEDs) "connected in series" across 120 V, the "drop" across each LED will be 2.4 Volts. This is not unreasonable, even without an additional current limiting resistor, but, one may be present.

If you wish to operate a "half-string" in parallel with a full-string, in each case you will not only need to determine how to connect these in parallel but will need to provide a current-limiting resistor for each of the half-strings. (Otherwise, each half-string will have twice its working voltage applied.)

I cannot tell you what the current in each string and half-string should be but 20 mA is usually the maximum current for these type of LEDs.
If so, the series resistor required for each "half-string' would be required to "drop" 60 V at 20 mA, so a 3000 ohm resistor would be required and the dissipation would be 1.2 W.

You will not easily find such a resistor so your best option would be to use two 1500 ohm 1 W resistors in series (if, and only if, the current in the full string is 20 mA.)

So, one of the first things that you need to do is to determine the current flowing in a full-string.


Of course, there are a number of possible other problems.

LEDs are DC devices.
Hence, if 50 LEDs are just connected in series across 120 V AC, they will actually "flash" at 60 times per second.
Because of this there, may be a bridge rectifier somewhere in the "string" (such as in the "plug") to rectify this voltage, so that the LEDs are supplied with (pulsing) DC, when they will "flash" at 120 times per second, which is less noticeable to the human eye!

This is something else that you will need to determine and, if there is a "bridge" rectifier in each full string, you will need to ensure that one is provided for each half-string, otherwise (at best) they will look about "half as bright".

Of course, these strings are operating at 120 V "Mains" potential, so all electrical connections and insulation should be "appropriate".
 

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Discussion Starter · #8 ·
Yeah, I actually did cut the wires, and used a continuity tester, and found that only two wires on each side of the cut had continuity, which blew my "2 + 1" theory out of the water, so I reversed the cuts to restore the set, and chalked up a loss.

Thanks for your thorough explanation, parts of it are a little over my head, but it does generally make sense.

I will just buy another string and conceal the excess on each side.
 

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Yeah, I actually did cut the wires, and used a continuity tester, and found that only two wires on each side of the cut had continuity, which blew my "2 + 1" theory out of the water, so I reversed the cuts to restore the set, and chalked up a loss.

Thanks for your thorough explanation, parts of it are a little over my head, but it does generally make sense.

I will just buy another string and conceal the excess on each side.
There are a few commercially made products out there that might be an option for you.

One option is a cut-to-length light string. They make them in C9 and C7 sizes. I've linked the C9 with 500 foot roll of sockets. They also come in 1000 foot rolls. You buy a roll of sockets, a male and female end, and buy the lights separately. This would be my choice, but only if C9 or C7 lights will work.

Alternatively, there is this system. I have never used it, but if 8 foot increments will work, that is an option. You'll need a power supply for this.

There are some other options out there too if you look around.

*I am in no way affiliated with 1000bulbs.com. I am merely using their site as an example*
 

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I have broken LED strings when the string was built in sections which did not overlap.
In this case there was a 2 wire portion between the sections which I cut to separate the string. Each section had its own resistor/rectifier canister.
Jack Hottel
 

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I have broken LED strings when the string was built in sections which did not overlap.
In this case there was a 2 wire portion between the sections which I cut to separate the string. Each section had its own resistor/rectifier canister.
Jack Hottel
Yes!
You "have broken LED strings when the string was built in sections which did not overlap."

It may be that these "strings" were supplied by a transformer or a "Switch Mode Power Supply" device.

This is quite "normal" in most OECD countries outside North America (using 240 V AC), where direct connection of the "Mains" voltage to such "strings" is not allowed and an "isolation" device is required.

The usual arrangement of these strings is a SMPS supplying about 36 V DC within a "controller". This voltage is a filtered DC Voltage which is then switched at a frequency much higher than the "Mains" AC supply to strings of LEDs, which are usually in reverse parallel pairs of ten each.

(By this I mean that one of these pair of LED strings is illuminated at any instant because of the "direction" of the switched DC supply, and the other of the pair of LED strings is illuminated at the next instant. Because of the switching frequency used, this all happens so fast that it can appear that all LEDs are continuously illuminated.
Since the "maximum" current with which a LED may be supplied is an average of the peak current over time, each LED may be supplied with double the "maximum" current at each half-cycle and not suffer damage as a result.
Thus, all LEDs may be run at their maximum apparent brightness.)

However the "controller" concerned has the ability to switch each half-string pair ON and OFF for varying time intervals, so that the LEDs may either be "full on" or appear to grow or dim in brightness, for various effects and to appear to "chase".
 
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