Running American C7/C9 Christmas lights (110V) directly on a 220V supply

Whew, long time without another post – it’s been a busy time, glad to get back to blogging.
This post is going to explain the basics of using American 110V Christmas lights on a 220V electrical supply, by splitting the light string in half and putting them in series instead of using a dedicated voltage converter.

Last Christmas I was in the U.S. and, as a fan of large bulb C7 Christmas lights, I picked up a nice old set for a grand total of $9.49.
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They looked and worked great in the hotel room, I just needed to modify them to work on a 220V supply that we use in South Africa (and most of the rest of the world). I could have used a voltage converter but this string uses over 100W, a large transformer would be needed ($$$).
A cheap and easy solution is just to split the string in half, and have both sections connected in series; each section would receive 110V.

A regular string of C7 or C9 lights is constructed with all the bulbs in parallel, this is great as if one bulb fails the rest of the set will work uninterrupted:c7circuit

We can modify this circuit to work on a 220V supply fairly easily by splitting it in half as described:
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As long as all the incandescent bulbs are of the same wattage (i.e. each “series-parallel string” is of the same resistance), power will be distributed evenly.
If a bulb blows then the resistance and subsequent current distribution will be unequal, and one side will receive more power than normal – not good for the bulbs.
Best replace the burnt bulb when this happens (or remove one from the other section to balance it out), but, otherwise, it should work fine.
NB: This will only work with a string of incandescent bulbs of identical wattage only, it will NOT work with LED bulbs as they are not resistive loads. Expect bad things to happen with LEDs.

To modify the string to work in this configuration, it’s easiest to split one of the wires in the center and solder an extra cable running from this center to the plug end. See the diagram below for how the final circuit should look:
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DISCLAIMER: Please only do this if you are familiar with live wiring and soldering. This may not meet code requirements in your area, and I am using this indoors only and wouldn’t recommend outdoor use. I (the Author) won’t be held responsible for any damage to the light set or other items/persons/property. You are doing this at your own risk.

As my string has 25 bulbs, I used 2 sections of 12 bulbs and cut off the 25th bulb to keep everything even. After splitting the one center wire and soldering in our new cable, I insulated it well with rubber sealant and electrical tape. Be careful here, you don’t want to take any risks.
The new wire was then neatly run down the line to the plug.
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And here they are all powered up, looking snazzy!

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These C7/C9 strings have the ability to be chained up (they have a standard US output socket on the end), but don’t use this as it will be drawing current from only one of the sections, putting the string out of balance. Best to just remove it.
Good luck and happy holidays!

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