You and your family are decorating the tree for the holidays. Of course, since you are the electrician/technician, engineer of the house, the lights fall under your purview. Of course one of the strands of miniature lights fails to, well, light. All eyes are on you. The best course of action is to dash out and get a new strand of lights for three bucks. How much is your time worth, anyway? But a strong urge to dominate anything technical clouds your better judgment and you find yourself going after the malfunctioning strand with a non-contact tester and a multimeter.
The first thing you notice is that these little lights are wired like nothing else in the electrical world. All of the lamps are wired in series. Every high school student knows that this is the worst way to wire lamps, because if one lamp goes out all of the lamps fail. But in this application it works pretty well.
A typical string of 50 miniature lights uses 2.5 V lamps. The voltage divides evenly across all 50 lamps as shown in Equation 1. By wiring the lamps in series and running them at low voltage, each lamp consumes much less power than a 120 V lamp. You can run a string of fifty 2.5 V miniature lights on less than 20 Watts. You would need 250 watts to light fifty 120V, 5 Watt bulbs. And less power means less heat.
120 V / 50 = 2.4 V
A Clever Design
As in any series system, a single open circuit causes an interruption of current to all of the loads. But mini bulbs have a mechanism that prevents an open filament from interrupting the current. A special wire is wrapped around the posts inside the lamp. The wire is coated with a thin insulation and when a filament burns out the wire shorts the posts together. This provides a shunt path for the current, so the remaining lamps can continue to function.
In order for the shunt mechanism to work, the bulb must be well seated within its socket. A pulled or twisted bulb will interrupt current flow and the whole string will go dark. This is a pretty easy problem to fix once you locate the offending bulb.
When a filament fails and a shunt shorts out a lamp, the line voltage divides over a smaller number of lamps. Each remaining lamp has a higher voltage across it. As more filaments burn out, the voltage on the lamps continues to increase. As the lamp voltage increases more filaments fail. So you can have an avalanche effect resulting in a significant number of burned out lamps. Still want to break out the testers? You can probably still make it to the drugstore.
Troubleshooting a String that Won't Light
Lay out the string on the floor. A couple of pieces of masking tape can help you mark bad bulbs.
Make sure you have some spare bulbs. You might be able to find some replacement bulbs for your set at the hardware store. Try to find bulbs that are designed for your lights, since this will make it much easier to fit the plastic bases into their snug sockets. And while you're at the hardware store, consider spending $3 on a new string and forgetting the whole idea of repairing the old string.
Check the fuses for continuity using a DMM or electrical continuity tester. Replace any open fuses.
Plug the string into a receptacle. Gently pull each socket away from the other conductors. Use a non-contact voltage tester like the Fluke 1AC to look for voltage on both sides of the lamp. If the string has a receptacle on the far end (Figure 2) one of the three conductors will be hot for the entire length of the string. Make sure you're check for voltage at the lamps and not seeing voltage from the hot conductor. Work your way from the plug, testing each side of the lamps. A lamp that has voltage on the plug side, but no voltage on the far side must be replaced. Continue until you've replaced all of the faulty lamps.
Figure 1: Fifty identical lamps in series will divide 120 volts equally into 2.4 volts each.
Figure 2: To deliver 120 V to the next string of lights, a "Hot" conductor runs to the receptacle at the end of the strand. This is the third wire you see.
Figure 3: A string of 100 lamps is usually just a combination of two 50-lamp strings.