What is a multi-meter

A multimeter is a device used to measure electrical properties such as voltage, current and resistance. A multimeter is a multi function device that works as a Voltmeter, Ammeter and Ohmmeter. You can switch between these applications by turning a switch. 

Caution: Use of multimeter in testing electrical equipment must only be done by trained individuals. The guideline below is only for battery operated devices and educational experiments without any connection with electrical wires of a building.

The common multimeter is a small handheld device with an indicator needle over a measurement scale or a numeric LCD display. The device has a switch to select the type of test to be performed. A multimeter also has two wires, one red (+) and one black (-), with metal tips. They are called probes.

If the probes are not already attached, you must plug the red test lead into the + socket and the black test lead into the - COM socket of the multimeter.

Described below are some common tests using a multimeter.

Testing the voltage

Voltage is the difference in potential of electrons between two points. For example the difference in potential between the positive pole and negative pole of a AA battery is usually about 1.5 volts. Difference in potential happens when the concentration or accumulation of electrons in one point is more than another point. If such two points are connected to each other using a wire, the electrons will flow from one side to the other until the potential of electrons equals. Such a movement of electrons from one point to the other is called a Direct Current of electricity. 

To measure the voltage of a battery you set the range selector switch to any of the DCV positions, connect the black test lead to the negative pole of the battery, connect the red wire to the positive pole of battery and watch the movement or disposition of the indicator needle. Relative disposition of the needle shows the voltage. For example if you set the range selector to 10 volts DC (10 DCV) and needle goes half way and stops on the center of display, that shows the voltage of your battery is 5 volts. 

Display of a multimeter has different scales. It is better if you look at the scale line that is the same or closest to your range. For example if you set the range selector switch to 2.5 VDC (2.5 volts Direct Current) you can look at the scale numbered from 0 to 250. This scale is 100 times more than your range, so you must divide your results by 100. So if the display shows 175, it really means 1.75 Volts. 

Testing for Continuity

This test should be done when current is NOT present. Always unplug the device or turn off the main circuit breaker before attempting a continuity test.

A continuity test is done to determine whether a circuit is open or closed. 

Set the multimeter to the ohm setting. The symbol for ohm is , the Greek letter omega. If there is more than one ohm setting, choose X1. 

Note that while the probes are not touching anything, the multimeter will indicate a reading of infinity. A reading of infinity indicates that the circuit is open and cannot conduct current. When you touch the two probes together, the reading changes to zero. A reading of zero indicates that the circuit is closed and can conduct current. Touch each probe to one of the terminals (or poles) of the device. If the reading changes to zero the device has continuity.

Analog Multimeter
To test a switch, place a probe on each pole of the switch. When you move the switch from the off to the on position, the meter reading should change from infinity to zero, which implies that the switch is working. To test a component such as a motor, touch a probe to each pole. A reading of zero indicates that motor has continuity and current can pass through it.

Continuity test can also be used to test light bulbs. If the light bulb is burnt, it fails continuity test.

Testing for Ground

This test should be done when current is NOT present. Always unplug the device or turn off the main circuit breaker before attempting a ground fault test.

A ground fault test is done to determine if current is passing from a circuit inappropriately. A ground fault is a potentially dangerous electrical shock hazard. A ground fault can also cause a device to malfunction.

Set the multimeter to the ohm setting. If there is more than one ohm setting, choose X1. Touch one probe to a terminal and touch the other probe to the device's housing or mounting bracket. Now move the first probe to another terminal. If the multimeter displays anything other than a reading of infinity for any of the test combinations, a ground fault exists and the device should be repaired or replaced. Do not use a component that has a ground fault.

Measuring Resistance

This test should be done when current is NOT present. Always unplug the device or turn off the main circuit breaker before attempting to measure resistance.

Resistance is how much the flow of current in a circuit is impeded. Resistance is necessary for heat to be generated in heating elements like those used in an electric stove or oven, dryer or hair dryer.

It is necessary to know what the proper resistance rating should be for a particular device in order to determine if it is functioning properly.

Set the multimeter to the ohm setting. Touch a probe to each of the terminals. The reading on the multimeter should change from infinity to the level of resistance detected in the element. Compare the measurement to the manufacturer's specification for the element.