Mini-light strings, the marvel of modern decorating, can be difficult to repair when their bulbs begin to burn out, especially if a whole set of them goes dark at once. To help find and replace these small incandescent candle-shaped bulbs gone bad, here is a description of how they work.
Starting from the basic 3-wire whole string and working outward to the bulb sockets from there, these bulbs and their sockets are installed and work as follows.
Main sockets. These sockets are the permanent ones attached to the main wire that holds them and their bulbs in series. The bulbs themselves, which are seated in their own small plastic bases, fit into these main sockets where contact is made with the wire. The inner cores of these sockets are somewhat rectangular in shape. The final string of lights, which includes this main wire of sockets and their bulbs, are twisted together with the two extension-cord-like 120-volt circuit wires in a rope-like manner.
Bulb set. The bulb set is the bulb wire above, which usually contains 50 bulbs connected in series on one wire. Each end of this set is connected in parallel to the two circuit wires that plug into an electrical outlet or into each other to make the strings longer. Often, a single string of 100 mini-bulbs consists of two 50-bulb sets, each one connected separately in parallel to the two circuit wires. Thus, each set operates separately from the other one in the same string.
Bulb base. To confuse things a bit, the bulbs are held by two sockets so to speak: 1) the small plastic rectangular base holding the bulb itself, and 2) the permanent one on the bulb wire into which the bulb and its base are seated. Thus, before a bulb can be seated into its main socket, its two bare wires must first be inserted through the two holes in the bottom of the plastic base, and then wrapped vertically around the two grooves on the outer side of this base. This bulb and its base are then seated into the main socket where the two bare wires make contact with the bulb wire.
Bulb. When a new candle-shaped glass bulb stands alone, it has two bare lead wires protruding straight down from its lower end. These are the ones that wrap around its plastic base. From there, upward within the sealed bulb, these leads become small posts held steady buy a glass bead attached between them. Just above this glass bead a small wire is wrapped horizontally around both posts several times. It is called a bypass shunt. Above the shunt, the two posts protrude further up into the bulb where they are connected together by a fine-wire filament.
The filament produces the light when the string is plugged in or turned on. In time, however, the filament will burn itself out. At that moment, the current or electricity to the bulb starts crossing through the shunt instead to keep the rest of the bulbs in the series set lit. However, if that shunt goes bad after the filament does, the entire 50-bulb set will go dark because this bulb is no longer allowing electricity to pass through it properly. In this case, to repair the bulb set, one needs to find that bulb having the bad filament and shunt, and then replace it with a new one. Normally, a new one will be rated near 2.4-volts for a 50-bulb set.
Finding the bad bulb. Unless one buys a special bulb-set testing device, the fastest way to find a bad bulb with a broken or missing filament is to look at each bulb with a magnifying glass in front of a background light. The background light enhances this process for the colored bulbs whose filaments can be hard to see with naked eye. Also, a darkened or color-changed bulb might indicate a burnt-out filament, which will be easier to spot outright than by using the magnifier.
Otherwise, to locate the bad bulb, the repair person might have to remove and replace each bulb in the set one at a time temporarily with a new one known to be working okay while the string is plugged in. Normally, one or two extra bulbs already mounted in their bases come with a new string of mini-lights. When the bad bulb has been replaced with a good one, presto, the light set comes back on.
Conclusion. Much repair time can be saved by replacing a bad bulb soon after it goes out, i.e., before the whole set goes out. That is, this dark bulb is much easier to spot and replace while its shunt is still passing electricity to the rest of the set, which is still lit.
For information and diagrams on incandescent mini-lights and the testing of their bulbs, see the following sites.
