Multimeters Charleston SC
Electric Contractors Residential, Electric Wire & Cable
The Multi-Talented Multimeter
To troubleshoot household, appliance or automotive wiring, you can buy all sorts of devices: receptacle analyzers, continuity testers, circuit finders and voltage testers. Or you can buy a multimeter. These multi-talented little devices are widely available for as little as $20 in both analog (swinging needle) and digital form. Even if you prefer your clocks to have hands, get the digital version of the multimeter. It is more accurate and more rugged. There are about as many uses for a multimeter as there are devices in a home. Among other things, you can use one to troubleshoot a hot water heater element (high resistance means it's burned out), a television cable (low resistance means it's shorted) and an extension cord (ditto). If you can use a simple four-function calculator, you can master the multimeter. Using one requires knowing only three concepts:
V (volts) The voltage difference between two points.
A (amps) The electrical current flowing through a conductor.
W (ohms) The resistance to current flow between two points. For an electrical device or appliance to be useful, it has to be part of an electrical circuit. The term circuit comes from the fact that electrons have to have some place to go. They can't just pile up at the end of a wire. Illustration 1 shows a simple circuit. A power source (in this case it's the power company, but it could just as well be a DC battery) pumps electrons through the black wire. The electrical "pressure" in the black wire is measured in volts, while the rate of flow of electrons through the wire is measured in amps. What keeps the electron flow from being infinite is the electrical resistance of the power-consuming device, measured in ohms. The circuit is completed by the white wire, which provides the electrons a path back to the source. If either black or white wire is broken, all current flow ceases.
Electricity is the most useful of all energy forms because it is so easily transformed into other forms of energy. In the case of a lamp, the resistance of the lamp's filament to the flow of electrons converts the electrical energy into heat energy and light energy. A motor changes electrical energy into motion energy, and loudspeakers change electrical energy into sound energy. Regardless of the purpose of a circuit, somewhere in it you will always find a device characterized by its electrical resistance, measured in ohms. The three measurable quantities in a circuit - volts, amps and ohms - are related by the amazingly simple Ohm's law: Amps equals volts divided by ohms. Picturing the electrical current through a wire as the flow of water through a garden hose makes Ohm's law almost intuitive:
The greater the pressure (volts), the greater the current flow (amps).
The greater the resistance (ohms), the weaker the flow (amps). The common table lamp serves as a perfect example of an electrical circuit. The narrower of the two slots in the wall receptacle is hot. Its voltage alternates between plus and minus 110 volts at a rate of 60 times per second, providing the alternating electrical pressure to move electrons back and forth through the wire. The wider slot is neutral. It is connected to the earth and remains always at zero volts relative to the earth. To keep things straight, the National Electric Code specifies that insulation on the hot wire must be black, and the neutral wire must be white. With the multimeter, finding the problem in a broken lamp is simple.
First, check to see if there is electricity at the source. Plug the red and black leads into the multimeter's AC volts and common sockets, turn the meter's function switch to AC volts, and insert the test probes into the two slots of the receptacle. If there is power at the receptacle, the meter should display a number between 110 and 125 (110 to 125 volts AC). Next, check the bulb. Remove it, plug the red and black leads into the W and common sockets, turn the switch to W, and touch one test probe to the bulb's threaded base and the other test probe to the center contact of the bulb. If the meter reads anything over 1,000, you have uncovered the problem: a burned-out filament. If not, by elimination, it lies somewhere between the receptacle and the bulb, i.e., in the plug, cord or switch/socket. Now check the plug and cord. Unplug the lamp from the wall. Slide the outer socket shell off the socket body (and the cardboard insulating sleeve, if there is one) to expose the two terminal screws. Touch one test probe to the darker screw and the other probe to the wider tongue of the plug. The meter should read less than 10 ohms. Repeat with the lighter screw and the narrower tongue, looking for the same result. If either test fails, either the plug is defective or one of the cords is broken. Finally, check the switch and socket. With the lamp still unplugged, touch one test probe to the lighter screw and the other to the threaded portion of the socket body.
You should get less than 10 ohms, indicating electrical continuity. Next, touch one test probe to the darker screw and the other to the center tab inside the socket body. If the meter reads less than 10 ohms (continuity), click the switch, and the meter should change to more than 1,000 ohms (disconnected). If it first reads over 1,000 ohms (disconnected), click the switch, and it should change to less than 10 ohms. If any of these tests fail, the switch/socket is defective. I recommend that every homeowner purchase an inexpensive multimeter. To familiarize yourself with it and with household electricity, take a lamp apart - even if it isn't broken - and repeat the troubleshooting tests above. I guarantee you will never again feel quite so helpless.