Older Holiday Light String Tips

[Ben Zonia]

These tips are mainly for older light strings. Newer LED light strings are electrically different, and use 5 V power supplies, or 3 cell battery packs, and the bulbs in parallel.

The following tips use simple electrical principles, but you may not have thought about and analyzed it before.

There are 2.5 V and 3.5 V incandescent mini lights. Don't put a 2.5 V bulb in a 3.5 V series string. It will burn out the bulb AND the parallel shunt which allows the rest of the string to still work when one or a few bulb filaments burn out under normal conditions.

How do you know whether to use a 2.5 V or 3.5 V bulb?

If the string has a multiple of 35 bulbs (35, 70, 105, etc.) use 3.5 V bulbs.

If the string has a multiple of 50 bulbs (50, 100, 150, etc.), use a 2.5 V bulb.

This makes sense if you realize that US outlet voltage, close to 120 V, divided by 35 or 50 bulbs in series, gives the voltage of each series bulb. The larger strings use separate circuits for each 35 or 50 bulb string in the whole string, typically 2 or 3 separate series strings in parallel. Some alternate the bulbs in the two or three strings.

Classic C9 and C7 incandescent bulbs in strings are in parallel. C9s are 7 watts, and C7s are 4 watts. Newer C9s and C7s use a plastic envelope, not glass, which will often separate from the base when removed if you aren't careful. This can easily cause a short in the filament, which can blow the fuse, typically 5 A automotive type fuses in the plug.

Some have a spare inside the plug, for last minute use. To avoid shorts, try unscrewing all the bulbs first to make sure they doesn't separate from the base and short out the string. Unplug the string before removing bases stuck in the sockets. Needle nose pliers usually will work, but start slow to turn the base.

[k8jd]

A few years ago I bought some of the new Fat L.E.D. strings, they have a large envelope of colored plastic with a tiny L.E.D. emitter inside them.
They seemed to fail quickly outside when the temp went blow 20 Degrees. Working OK indoors. I tried to take the parts of the string apart and found about a 50 K resistor in series with the lamps. never had much luck getting the failed strings back to life.