How to Troubleshoot a Lamp With a Multimeter
A lamp that works is usually taken for granted, while those that don't are frequently cast aside. Unless you've been looking for a reason to buy a new one anyway, it would be a shame to add yet another nonfunctioning but repairable floor or table lamp to the thrift store clearance isle. Assuming the lamp is plugged into a working receptacle and is equipped with a bulb that is known to be good, a bit of troubleshooting with a multimeter is in order. Does this Spark an idea?
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Instructions
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Remove the lampshade. Remove the bulb. Plug in the lamp and turn it on.
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Set the multimeter to measure 120 volts AC. Touch one of the multimeter probes to the metal contact at the bottom of the socket and the other probe to the metal side of the socket. If there is no reading on the multimeter or if the reading is intermittent, unplug the lamp and clean the bottom contact with a pencil eraser or the tip of a screwdriver. Plug in the lamp and test the socket again. Continue troubleshooting if the multimeter doesn't read a steady voltage.
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Unplug the lamp. Remove the socket cover to expose the electrical wiring and terminals. Ensure that the wires are in the proper position and firmly soldered to the terminals or that the terminal screws are tight. Set the multimeter to measure resistance (conductivity) in ohms. Touch one probe to the gold-colored terminal and the other to the metal contact at the bottom of the lamp socket. Observe the multimeter as you turn the lamp on and off. A steady reading of high or infinite resistance indicates a faulty socket or switch; they are replaced as a unit. A reading that alternates from zero resistance to high indicates a functioning switch, in which case further troubleshooting is required.
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Leave the multimeter set to measure resistance. Hold one of the multimeter probes against one prong of the power cord plug. Touch the other multimeter probe to the gold-colored terminal while observing the multimeter. If the multimeter reads high or infinite resistance, touch the probe to the silver-colored terminal. If the reading is still high resistance, that wire is broken somewhere along its length, and the power cord must be replaced. A reading of low resistance in both positions indicates a short, in which case the power cord must be replaced. One high and one low reading means the wire is good. Test the other wire in the same manner.
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Tips & Warnings
A lamp with a switch that is separate from the socket requires additional troubleshooting. Test the switch by checking the resistance across the switch terminals; the multimeter should swing from low to high as the switch is turned on and off. You must also test the wires between the switch and socket in the same manner as the lamp power cord, by checking the resistance of each wire.
If a lamp passes these multimeter tests but still doesn't function, it has an intermittent failure that might be hard to pinpoint. Try wiggling the wires, socket and switch while repeating the tests to see if you can reproduce the problem.
Lamps are simple devices, but they are still energized to potentially fatal line voltage. When the lamp is plugged in, never touch the socket contacts or terminals with anything other than the insulated multimeter probes.
Intermittent problems with a lamp can often be traced to a power cord plug or wire that is almost but not quite broken completely. Both are hazardous conditions that could cause a fire. Unplug the lamp until the problem can be located and repaired.
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