Multimeter. The multimeter needed to do these tests is an inexpensive () handheld analog one having a needle-deflection readout, and run by an AA-battery. Although multimeters can make several electronic measurements, the only two needed here will be the 150-VAC (volts alternating current) and the resistance (ohms) ones. Otherwise, a multimeter is kept in the off setting to save the battery.
For mini-lights, a multimeter can be used to test the following issues.
The circuitry voltage (120-VAC) while plugged into an outlet. These two wires work independently from the bulb wire attached to them in a three-wire string.
The continuity of the two 3-amp buss fuses in the male plug if the circuitry was not working when tested while plugged in.
The continuity of each circuitry wire (hot & cold) if the circuitry was not working when tested.
The continuity of individual bulbs (i.e., for non-working filaments and bypass shunts, and for slightly corroded lead wires).
Safety. Since 120-volts of electricity pass through most mini-light strings, basic safety and correct procedures are paramount in testing them. Persons who know nothing about electricity, nor how-to-use a multimeter, should at least read-up on these subjects. Better yet, get professional hands-on training on them. Otherwise, the string and multimeter could be damaged, not to mention the tester him or herself.
Step 1. Test the circuitry. Plug the non-working string into a working outlet. Set the multimeter to150-VAC. Insert one multimeter probe into each of two female-plug sockets. (Most mini-light plugs are two prong.) The readout should be about 120-VAC, which means the circuitry is okay. If no reading occurs, double check the probe contacts in the female plug. If still no reading, unplug the string, and do the next step.
Step 2. Test the fuses for continuity (unplugged). Slide open the panel door on the male plug. Set the multimeter to a moderate resistance setting (inexpensive multimeters will have only one setting). Touch its probes together. The reading should go to zero, i.e., the needle will defect to zero.
Touch one multimeter probe to each end of one fuse simultaneously. If the deflection reading is zero for each fuse, they are okay. If not, replace the ones not having continuity; they can be found in hardware stores. It might be easier to do this test by removing the fuses first, and then testing each one outside the male plug.
Step 3. Test the circuitry wires for continuity (unplugged). If the fuses are okay at this point, insert one multimeter probe (still set to resistance) into one of the female plug sockets while the string is still unplugged. Touch the other probe to one of the male plug-in leads at the other end of the string. If no reading occurs, touch the other male lead instead. One of these two male leads should yield a zero reading for a good (continuous, unbroken) wire at this time. Perform the same test from the second female-plug socket.
If both wires do not give a zero continuity reading, something is wrong with the circuit wiring. Visually check these wires for damage. Repair any damage, if found, and retest them. If the circuit wires now show continuity, and the bulbs are still out, move to the next step.
Step 4. Test the individual bulbs for continuity (unplugged). Because the bulbs in question must be removed for this test, this process is much slower than by examining them with a magnifier and background light. Still, surprisingly, a few bulbs that look okay with the magnifier might not test okay with the multimeter.
Thus, the multimeter's probes must make good contact with each of the bulb's lead wires for a successful test. That kind of contact can be assured by scratching the bulb's wire leads with a sharp-edged tool until shiny before testing them. Replace any bulbs testing bad (no continuity).
Conclusion. Experts indicate the lifetime of the incandescent mini-light bulb is 3-to-5 years, depending on its amount of usage. After maintaining these light strings for that length of time, it is time to get new strings. If the same kind are purchased, the old ones can be used for spare parts and bulbs. Yet, the LED (light-emitting diode) strings might be a good option to consider at this point. They last longer than the incandescent ones, and they use up to 10-times less electricity.
To learn more about multimeters and how to use them, see the following sites.
