# LED Christmas Lights - Wiring question ???



## digger03 (Nov 24, 2004)

Hey,

I bought a whole bunch of 24' strands of Forever Bright LED Christmas lights - The wiring is kinda strange - two wires out of the plug, three wires from light to light, and then two wires to the plug at the far end. Can these strands be shortened ? If I cut it into two 12' strands, can I wire a new plug for the part I cut off ? Any help appreciated. Sorry, I posted this at the flashlight side first, and didn't get any responses.

Dave
[email protected]


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## Lynx_Arc (Nov 24, 2004)

Without seeing the actual wiring you got me what is going on. As for shortening them, most light chains where individual bulbs (or LEDs) are not designed for 120v alone rely on the group of them in a chain fashion to drop the voltage equally. I suspect this chain may be operating at a voltage differing from 120v across all LEDs from end to end, possibly 2-3 chains adding up to the total current or maybe even groups of a few leds wired in parallel. 

Hopefully someone with a chain of these will chime in, but for now until you know how they are wired cutting them short and rewiring may end up blowing a bunch of them.


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## digger03 (Nov 29, 2004)

Thanks for the reply - I lost my nerve to cut them, and just used them at 24' lengths - when I ran too long, I turned a corner, and started up another side of the house - it was dangerous in places (precariously perched on a steep sloped roof, trying to plug to sets together), but it all worked out. 

Dave


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## rwolff (Dec 27, 2004)

[ QUOTE ]

The wiring is kinda strange - two wires out of the plug, three wires from light to light, and then two wires to the plug at the far end.


[/ QUOTE ]

My guess is that if you look closely, only the bulb at each end of the string has 3 wires (the others have 2). What happens is this:

----+------------------------------------
|
B---b---b---b---b---b---b---b---B
|
------------------------------------+----

where "B" is a 3-wire bulb (the junction in the main branch happens in the bulb holder), and "b" is a 2-wire bulb. The 3rd wire in the string is to allow end-to-end connections.

Other than this third wire, it's a normal series-connected string, so cutting into it to reduce the number of bulbs is a BAD THING (remember what happens with ordinary minilights when a bulb burns out and the shorting link triggers to keep the others lit? They get a bit brighter because there's more voltage for each one.) Unless I miss my guess, the manufacturer is counting on the forward voltage drop for all the LEDs to avoid overdriving (I've got a 70 light string, and this results in a peak voltage of 2.4V per LED - low enough to be reasonably safe without a ballast resistor). Of course, you could add a ballast resistor to take into account the voltage that would be dropped across the missing bulbs if you shorten the string (but then you'd have to make sure the DIY connections were weatherproof). Alternately, you could shorten the string without using a ballast resistor, and be the first on your block to have SED (smoke-emitting diode /ubbthreads/images/graemlins/ooo.gif ) Christmas lights.


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