One of the most basic uses of a DMM is to measure voltage. There are two types of voltage, AC and DC. A typical example of a DC voltage source is a battery such as a car battery, whilst an example of an AC voltage source is a generator or mains electricity wall socket. Certain devices, such as TVs and computers convert this AC voltage to a DC voltage that is what is used to power the electronic circuits within.

 

Testing for a proper voltage supply is usually the first step when troubleshooting a circuit, basically like checking your computer is plugged in and switched on if it isn’t working! If no voltage is present (or if it is too low or high) then the circuit will not function, so correct the problem before investigating further. The waveforms associated with AC voltages are either sinusoidal (sine waves) or non-sinusoidal (sawtooth, square, ripple, etc.).

A good quality DMM will display the “rms” (root mean square) value of these voltage waveforms which is the effective or equivalent DC value of the AC voltage.

 

The majority of DMMs are ‘average responding’ which means they give an accurate rms reading if the AC voltage signal is a pure sine wave. Average responding meters are not capable of measuring non-sinusoidal signals accurately. Non-sinusoidal signals are accurately measured using specialised DMMs designated “true-rms” up to their specified crest factor. Crest factor is the ratio of a signal’s peak-to-rms value. It’s 1.414 for a pure sine wave, but is often much higher for a rectifier current pulse, for example. As a result, an average responding meter will often read much lower than the actual rms value.

 

A DMMs ability to measure AC voltage can be limited by the frequency of the signal. Most multimeters can accurately measure AC voltages with frequencies from 50 Hz to 500 Hz, but a particularly wide frequency range may ‘confuse’ the tool and make it show a much higher reading, so the specifications should state the frequency range that the DMM can see along with the range’s accuracy.

 

A DMM’s ability to measure ac voltage can be limited by the frequency of the signal. Most DMMs can accurately measureac voltages with frequencies from 50 Hz to 500 Hz, but a DMM’s ac measurement bandwidth may be hundreds of kilohertz wide. Such a meter may read a higher value because it is “seeing” more of acomplex ac signal. DMM accuracy specifications for ac voltage and ac current should state the frequency range along with the range’s accuracy.

 

1. Select V~ (ac) or V (dc), as desired.
2. Plug the black test probe into the COM input jack. Plug the red test probe into the V input jack.
3. If the DMM has manual ranging only, select the highest range so as not to overload the input.
4. Touch the probe tips to the circuit across a load or power source (in parallel to the circuit).
5. View the reading, being sure to note the unit of measurement.

Note: For dc readings of the correct polarity (±), touch the red test probe to the positive side of the circuit, and the black probe to the negative side or circuit ground. If you reverse the connections, a DMM with autopolarity will merely display a minus sign indicating negative polarity. With an analog meter, you risk damaging the meter.

Note:   1/1000 V = 1 mV,  1000 V = 1 kV

High-voltage probes are available for TV and CRT repair, where voltages can reach 40 kV (see Figure 3).

Caution: These probes are not intended for electrical utility applications in which high voltage is also accompanied by high energy. Rather, they are intended for use in low-energy applications.

 

How to Measure Resistance

 

Resistance
Resistance is measured in ohms (Ω). Resistance values can vary greatly, from a few milliohms (mΩ) for contact resistance to billions of ohms for insulators. The majority of DMMs can measure down to 0.1 Ω, whilst some measure as high as 300 MΩ (300,000,000 ohms). Infinite resistance (open circuit) is read as “OL” on the Fluke meter display, and means the resistance is greater than the meter can measure. Resistance measurements must be made with the circuit power off – otherwise, the meter or circuit could be damaged.

Certain good-quality DMMs provide protection when being used in the ohms mode, to prevent injury and damage to the machine in case of accidental contact with a voltage. For accurate, low-resistance measurements, resistance in the test leads must be subtracted from the total resistance measured. Typical test lead resistance is between 0.2 Ω and 0.5 Ω. If the resistance in the test leads is greater than 1 Ω, the test leads should be replaced. 

If the digital multi meter supplies less than 0.6 V dc test voltage for measuring resistance, it will be able to measure the values of resistors that are isolated in a circuit by diodes or semiconductor junctions. This often allows you to test resistors on a circuit board without unsoldering them (see Figure 4).

 

How to make resistance measurements:

1. Turn off power to the circuit.
2. Select resistance (Ω).  
3. Plug the black test probe into the COM input jack. Plug the red test probe into the Ω input jack.
4. Connect the probe tips across the component or portion of the circuit for which you want to determine resistance.
5. View the reading, being sure to note the unit of measurement – ohms (Ω), kilohms (kΩ), or megohms (MΩ).

Note: 1,000 Ω = 1 kΩ 1,000,000 Ω = 1 MΩ

Make sure the power is off before making resistance measurements.

Continuity
Continuity is an excellent, very quick test that distinguishes between an open and a closed circuit. A DMM with a continuity beeper is a great choice because it lets you carry out many continuity tests quickly and easily. The meter simply beeps if it detects a closed circuit, so you don’t even need to be looking at the meter to find this out. Different DMMs have beepers that are triggered at different levels of resistance.

Diode test

A diode can be simply described as an ‘electronic switch’ as it will turn on when the voltage surpasses a certain level (usually around 0.6V for a silicon diode), allowing current to flow in one direction. An analogue multimeter is not the best tool for testing the condition of a diode or transistor junction as it not only gives very inaccurate readings but also drives currents of up to 50mA through the junction, as shown in the table above.

Certain digital multimeters have a diode test mode which is a useful feature to display the actual voltage drop across a junction in a circuit. A silicon junction for example should have a voltage drop of less than 0.7V when applied in the forward direction, and should allow an open circuit when applied in the reverse direction.

 

 

How to Measure Current

 

Measuring current
Current measurements are different from other DMM measurements – when taken using the DMM alone the meter must be placed in series with the circuit being measured. This requires opening the circuit and using the DMM test leads to complete and close it again, so that all of the current flows straight through the circuitry inside the DMM. You can also use a current probe however to avoid having to open the circuit – more information on current probes is below.

How to make current measurements

1. Turn off power to the circuit.
2. Cut or unsolder the circuit, creating a place where the meter probes can be inserted.
3. Select A~ (ac) or A (dc) as desired.
4. Plug the black test probe into the COM input jack. Plug the red test probe into the amp or milliamp input jack, depending on the expected value of the reading.
5. Connect the probe tips to the circuit across the break so that all current will flow through the DMM (a series connection). 
6. Turn the circuit power back on.
7. View the reading, being sure to note the unit of measurement.

Note: If the test leads are reversed for a dc measurement, a “–” will show in the display.

 

Input protection
However, a common mistake is to leave the test leads plugged into the current input jacks and then to attempt a voltage measurement. Doing so will cause a type of direct short known as a current shunt, which can cause extreme damage to both the DMM and the circuit being tested, plus possibly injury to the person using the DMM too.

The DMM used should therefore have current input fuse protection of high enough capacity for the circuit being measured. Meters without fuse protection in the current inputs should not be used on high-energy electrical circuits (> 240 V ac). The voltage rating of the fuses inside the DMM you are using should be higher than the maximum voltage you expect to measure, just in case. For example, a 20 A, 250 V fuse may not be able to clear a fault inside the meter when the meter is across a 480 V circuit. A 20 A, 600 V fuse would be needed to clear the fault on a 480 V circuit.

 

Current probe accessories

On occasion it may be necessary to take a current measurement that exceeds the rating of your DMM, or perhaps you will want to measure current on a circuit that it is not possible to open. For these high current applications (typically 2A or higher) where a high level of accuracy is less important, a current probe is very useful. This clamps around the conductor carrying the current and converts the measured value to a level that the meter is able to handle.

Two types of current probe exist that you can choose between. There are current transformers, used to measure AC current, and Hall-Effect probes that can be used to measure either AC or DC current. The output of a current transformer is typically 1 milliamp per amp. A 100 amp value is therefore reduced to 100 milliamps, which can be safely measured by most DMMs. The probe leads are connected to the “mA” and “COM” input jacks, and the meter function switch is set to mA AC for this use. The output of a Hall-Effect probe is 1 millivolt per amp, ac or dc. For example, 100 A AC is converted to 100 mV AC. The probe leads in this instance are connected to the “V” and “COM” jacks. Set the meter function switch to the “V” or “mV” scale, selecting V~ for AC current or V for DC current measurements.

 

 

Health & Safety for DMMs

 

Multimeter safety

The most important step is choosing the right DMM for the job, as well as following the correct procedures and instructions to ensure you are using it correctly so always be sure to read the user manual before the first use, paying particular attention to any warning or caution sections.

The International Electrotechnical Commission (IEC) established safety standards for working on electrical systems. Make sure you are using a meter that meets the IEC category and voltage rating approved for the environment where the measurement is to be made. For instance, if a voltage measurement needs to be made in an electrical panel with 480 V, then a meter rated Category III 600 V or 1000 V should be used. This means the input circuitry of the meter has been designed to withstand voltage transients commonly found in this environment without harming the user. Choosing a meter with this rating which also has a UL, CSA, VDE or TÜV certification means the meter not only has been designed to IEC standards, but has been independently tested and meets those standards. 

 

Common situations that lead to DMM failure:

1. Contact with ac power source while test leads are plugged into current jacks
2. Contact with ac power source while in resistance mode 
3. Exposure to high voltage transients
4. Exceeding maximum input limitations (voltage and current)

Types of DMM protection circuits:

1. Protection with automatic recovery. Some meters have circuitry that detects an overload condition and protects the meter until the condition no longer exists. After the overload is removed, the DMM automatically returns to normal operation. Usually used to protect the ohms function from voltage overloads.
2. Protection without automatic recovery. Some meters will detect an overload condition and protect the meter, but will not recover until the operator performs an operation on the meter, such as replacing a fuse. 

Look for these safety features in a DMM:

1. Fused current inputs
2. Use of high-energy fuses (600 V or more)
3. High-voltage protection in resistance mode (500 V or more)
4. Protection against voltage transients (6 kV or more)
5. Safety-designed test leads with finger guards and shrouded terminals
6. Independent safety organization approval/listing (e.g., UL or CSA)

Safety checklist

✓ Use a meter that meets accepted safety standards for the environment in which it will be used.
✓ Use a meter with fused current inputs and be sure to check the fuses before making current measurements.
✓ Inspect test leads for physical damage before making a measurement.
✓ Use the meter to check continuity of the test leads.
✓ Use only test leads that have shrouded connectors and finger guards.
✓ Use only meters with recessed input jacks.
✓ Select the proper function and range for your measurement.
✓ Be certain the meter is in good operating condition.
✓ Follow all equipment safety procedures.
✓ Always disconnect the “hot” (red) test lead first.
✓ Don’t work alone.

✓ Use a meter that has overload protection on the ohms function.
✓ When measuring current without a current clamp, turn the power off before connecting into the circuit.
✓ Be aware of high-current and high-voltage situations and use the appropriate equipment, such as high-voltage probes and high-current clamps.

 

DMM Accessories

Ensure that you choose a DMM that can be used with a range of accessories, so that it can be adapted to suit different uses and occasions rather than requiring that you buy another DMM. Various accessories are available that increase the usefulness of your DMM whilst making the tool easier to use, such as high-voltage probes and current probes that scale down high voltages or currents to a level that is safe to use with the DMM. This prevents any injury to both yourself and the tool.

Temperature probes are also available that convert your DMM into a handy thermometer, whilst RF probes allow it to be used to measure voltages at very high frequencies. Other accessories such as test leads, test probes and test clips allow you to connect your DMM to a circuit easily. Finally, don’t forget to buy a soft or hard carrying case to protect your DMM and store the accessories, prolonging the life of the tool and ensuring that you really get your money’s worth!

 

 

Glossary

 

 

• Accuracy. How close the DMM’s displayed measurement is to the actual value of the signal being measured. Expressed as a percentage of reading or as a percentage of full scale.
• Analogue meter. An instrument that uses a needle movement to display the value of a measured signal. The user judges the reading based on the position of the needle on a scale.
• Annunciator. A symbol that identifies a selected range or function. 
• Average responding DMM. A DMM that accurately measures sinusoidal waveforms, while measuring non-sinusoidal waveforms with less accuracy.
• Count. A number used to specify a DMM’s resolution.
• Current-shunt. A low-value resistor in a DMM for measuring current. The DMM measures the  voltage drop across the current shunt and, using Ohm’s Law, calculates the value of the current.
• DMM, digital multimeter. An instrument that uses a digital display to show the value of a measured signal. DMMs feature greater durability, resolution, and far more accuracy than analog meters. 
• Non-sinusoidal waveform. A distorted waveform such as a pulse train, square waves, triangular waves, sawtooth waves and spikes.

• Resolution. The degree to which small changes in a measurement can be displayed. 

• RMS The equivalent dc value of an ac waveform.

• Sinusoidal waveform. A pure sine wave without distortion.

• True-rms DMM. A DMM that can accurately measure both sinusoidal and non-sinusoidal waveforms.