Basic Instrumentation and Measurement - Multimeter

A multimeter is a device used to find the electrical problems. It can be used to test resistance and to measure voltage. Resistance can be safely tested with the power off. However, voltage can only be measured with the power on. Because of the risk of electric shock, an individual trained in the use of a voltage tester should only conduct voltage tests.

Safety Information.

-          Don’t try to adjust , test or repair if you don’t have basic repair skills.

-          Always follow the manufacturer’s instructions.

-           Electric Shock can cause injury or death. Never assume the electricity is off. Before servicing an electrical device , disconnect it from its electrical source either by unplugging or turning off the power at the breaker or fuse box.

-          Mark the service panel with a note so that no one will restore power while you are working.

-          After turning off the power to the circuit, test the circuit to be certain that there is no power.

-          Some devices can store a hazardous electrical charge even when disconnected from an electrical source, always discharge these devices before attempting service. If you are unfamiliar with a device or its components, consult with a professional before attempting service.

-          When working with electrical equipment wear rubber-soled shoes, avoid damp locations and hold all tools by their insulated handles. Never touch conductive surfaces such as pipes or metal bracing.

-          Use a non-conductive fiberglass ladder when working with electricity. Avoid shorting electrical circuits.

-          The tests should be done when current is NOT present. Always unplug the device or turn off the main circuit breaker before attempting these tests. Always test your test equipment for proper operation before use.

-          Some multimeters include a fuse, which will sometimes prevent damage to the multimeter if it is overloaded. However the fuse often only protects the highest current range on the multimeter.

-          A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source; meters without protection are quickly damaged by such errors, and can sometimes explode causing injury to the operator.

-          Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard. There are four categories:

o   Category I: used where current levels are low.

o   Category II: used on residential branch circuits.

o   Category III: used on permanently installed loads such as distribution panels, motors, and appliance outlets.

o   Category IV: used on locations where current levels are high, such as service entrances, main panels, and house meters.

Multimeter Layouts

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.

1.      Digital Multimeters

·         More accurate , durable and have extra features.

·         Display the measurement in numbers to avoid parallax error.

·         Auto ranging – select the correct range for the device under test to the most significant value are displayed.

·         Auto-polarity – for direct current reading , shows negative sign if it is opposite polarity.

·         Contemporary multimeters can measure many quantities. The common ones are:

                               i.      Voltage in volts.

                              ii.      Current in amperes.

                             iii.      Resistance in ohms.

                             iv.      Additionally, multimeters may also measure:

                             v.      Capacitance in farads.

                            vi.      Frequency in hertz

                           vii.      Duty cycle as a percentage.

                          viii.      Temperature in degrees Celsius or Fahrenheit.

                            ix.      Conductance in siemens.

                             x.      Inductance in henrys

                            xi.      Audio signal levels in decibels.

                            xii.      Digital multimeters may also include circuits for:

                            xiii.      Continuity that beeps when a circuit conducts.

                             xiv.      Diodes and Transistors

2.      Analog multimeters

·    Resolution is limited by scale pointer , vibration of the pointer , accuracy of printing scales , zero calibration , number of ranges and errors due to non-horizontal use of the mechanical display.

·    Error is due miscounting division markings , errors in mental arithmetic , parallax observation errors and less than perfect eyesight.

·    To improve resolution – mirrored scales and larger meter movements.

Probes

There are variety of test probes to connect to the circuit and device under test.

·         Crocodile clips

·         Retractable hook clips

·         Pointed probes

The connectors are attached to flexible, thickly-insulated leads that are terminated with connectors appropriate for the meter. Handheld meters typically use shrouded or recessed banana jacks, while benchtop meters may use banana jacks or BNC connectors. 2mm plugs and binding posts have also been used at times, but are not so common today.

Meters which measure high voltages or current may use non-contact attachment mechanism to trade accuracy for safety. Clamp meters provide a coil that clamps around a conductor in order to measure the current flowing through it.

Application Mode

 

Continuity Test

Continuity is to make sure the thing is electrically connected. We can use a resistance-tester (ohmmeter) to confirm the continuity because the resistance of wire is very small less than 100 Ohms. Sometimes there is buzzer to make the job easier during troubleshooting. This check is important for  :

·         To make sure the soldering is good.

·         To confirm if the wire is broken in the middle e.g inside the insulator.

·         To make sure something isn’t connected. E.g short circuit.

·         To do reverse engineering e.g to verify the design back to a schematic.

 

Analogue Multimeter

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. To test the continuity of a circuit requires the use of a multimeter or multitester.

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.

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.

Digital Multimeters

Put the multimeter into the correct mode. Choose the icon of sound wave. Please refer to the picture.

 

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.

Analogue Multimeters

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.

 

Digital Multimeters

Set the multimeter to the Ohm setting. Touch a probe to each of the terminals. If it has different ranging meters it will have sub mode for different range but if it is auto ranging there will be only one.

 

Measuring for Voltage

·         To test whether the power supply is working.

·         To verify that the circuit is getting enough power.

·         To verify signals to and from chips.

·         Testing batteries , solar cells and power outlets.

·         Test the circuit only when the power is on.

·         Measure the voltage between 2 points. If we need to measure one point , put the negative probe (black) at ground.

·         If we are getting odd readings , we can use reference voltage. (e.g measure a battery)

·         Voltage is directional so make sure the black probe is put correctly at ground.

·         Please select the mode for AC and DC correctly unless told assume it is DC voltage.

Digital Multimeter

There are 2 separate modes for AC and DC. The V with 2 lines (dashed and one solid) is DC and the one with wave line is AC.

Select the correct range. When the probes are not connected it should display 0V.