ABCs of DMMs

How do you make measurements safely? What features do you need? What is the easiest way to get the most out of your meter? Which meter is best suited to the environment you're working in?

Let's begin by explaining what a DMM is. A DMM is simply an electronic tape measure for making electrical measurements.

It may have any number of special features, but mainly a DMM measures volts, ohms and amps.

Choosing a DMM for the job requires not only looking at basic specifications, but also looking at features, functions, and the overall value represented by a meter's design and the care taken in its production.

Reliability, especially under tough conditions, is more important than ever today.

User safety is a primary consideration.

Providing adequate component spacing, double insulation and input protection helps prevent injury and meter damage when they are used improperly.

Fluke offers many DMMs with different combinations of features like Touch Hold, analog bargraphs and enhanced resolution.

Accessories for high current and temperature measurements are available to extend the capability of DMMs.

Resolution refers to how fine a measurement a meter can make.

By knowing the resolution of a meter, you can determine if it is possible to see a small change in the measured signal.

For example, if the DMM has a resolution of 1 mV on the 4 V range, it is possible to see a change of 1 mV (1/1000 of a volt) while reading 1 V.

The terms 'digits' and 'counts' are used to describe a meter's resolution. DMMs are grouped by the number of counts or digits they display.

A 3½-digit meter can display three full digits ranging from 0 to 9, and one 'half' digit which displays only a 1 or is left blank.

A 3-½ digit meter will display up to 1999 counts of resolution. A 4½-digit meter can display up to 19,999 counts of resolution.

It is more precise to describe a meter by counts of resolution than by digits. Today's 3½-digit meters may have enhanced resolution of up to 3200, 4000 or 6000 counts.

For certain measurements, 3200-count meters offer better resolution.

For example, a 1999-count meter won't be able to measure down to a tenth of a volt if you are measuring 200 V or more.

However, a 3200-count meter will display a tenth of a volt up to 320 V.

This is the same resolution as a more expensive 20,000-count meter until you exceed 320 V.

Accuracy is the largest allowable error that will occur under specific operating conditions.

It is an indication of how close the DMM's displayed measurement is to the actual value of the signal being measured.

Accuracy for a DMM is usually expressed as a per cent of reading. An accuracy of 1% of reading means that for a displayed reading of 100 V, the actual value of the voltage could be anywhere between 99 and 101 V.

Specifications may also include a range of digits added to the basic accuracy specification. This indicates how many counts the digit to the extreme right of the display may vary.

So the preceding accuracy example might be stated as ± (1% + 2). Therefore, for a display reading of 100 V, the actual voltage would be between 98.8 and 101.2 V.

Analog meter specifications are determined by the error at full scale, not at the displayed reading.

Typical accuracy for an analog meter is ±2% or ±3% of full scale. At one-tenth of full scale, these become 20 or 30% of reading.

Typical basic accuracy for a DMM is between ± (0.7% + 1) and ± (0.1% + 1) of reading, or better.

Voltage, current and resistance in any electrical circuit can be calculated by using Ohm's law, which states that voltage equals current times resistance (see Figure 1).

Figure 1:

Thus, if any two values in the formula are known, the third can be determined.

A DMM makes use of Ohm's law to directly measure and display either ohms, amps or volts.

For high accuracy and resolution, the digital display excels, displaying three or more digits for each measurement.

The analog needle display is less accurate and has lower effective resolution because you have to estimate values between the lines.

A bargraph shows changes and trends in a signal just like an analog needle, but is more durable and less prone to damage.

One of the most basic tasks of a DMM is measuring voltage.

Testing for proper supply voltage is usually the first step when troubleshooting a circuit. If there is no voltage present, or if it is too high or too low, the voltage problem should be corrected before investigating further.

The waveforms associated with AC voltages are either sinusoidal (sine waves), or non-sinusoidal (sawtooth, square, ripple, etc).

Figure 2: Three voltage signals: DC, AC sine wave and non-sinusoidal AC signal.

Quality DMMs display the RMS (root mean square) value of these voltage waveforms. The RMS value is the effective or equivalent DC value of the AC voltage.

Most DMMs are 'average responding', giving accurate RMS readings 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 DMMs designated 'true-RMS' up to the DMM's 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 DMM's ability to measure AC voltage can be limited by the frequency of the signal.

Most DMMs can accurately measure AC voltages with frequencies from 50 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 a complex AC signal. DMM accuracy specifications for AC voltage and AC current should state the frequency range along with the range's accuracy.

Figure 3: Accessories, such as Fluke 80K-40 and 80K-6 high-voltage probes, extend the voltage measurement range of a DMM.

Most DMMs measure down to 0.1 ohm and some measure as high as 300 Mohm (300,000,000 ohm). Infinite resistance (open circuit) is read as 'OL' on a 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.

Some DMMs provide protection in the ohms mode in case of accidental contact with voltages. The level of protection may vary greatly among different DMM models.

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 and 0.5 ohm. If the resistance in the test leads is greater than 1 ohm, the leads should be replaced.

If the DMM supplies less than 0.6 VDC 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 (Figure 4).

Figure 4: For measuring resistance in the presence of diodes, DMM test voltages are kept below 0.6 V so the semiconductor junctions do not conduct current.

Continuity is a quick go/no-go resistance test that distinguishes between an open and a closed circuit.

A DMM with a continuity beeper allows you to complete many continuity tests easily and quickly. The meter beeps when it detects a closed circuit, so you don't have to look at the meter as you test.

The level of resistance required to trigger the beeper varies from model to model of DMM.

A diode is like an electronic switch. It can be turned on if the voltage is over a certain level, generally about 0.6 V for a silicon diode, and it allows current to flow in one direction.

When checking the condition of a diode or transistor junction, an analog VOM not only gives widely varying readings but can drive currents up to 50 mA through the junction. (Table 1.)

Some DMMs have a diode test mode. This measures and displays the actual voltage drop across a junction.

A silicon junction should have a voltage drop less than 0.7 V when applied in the forward direction and an open circuit when applied in the reverse direction.

Current measurements taken with the DMM alone require placing the meter in series with the circuit being measured.

An indirect method of measuring current can be performed using a current probe that clamps around the outside of the conductor, avoiding opening the circuit and connecting the DMM in series.

A common mistake is to leave the test leads plugged into the current input jacks and then attempt a voltage measurement.

This causes a direct short across the source voltage through the low-value resistor current shunt.

A high current flows through the DMM and, if it is not adequately protected, can cause extreme damage to both the DMM and the circuit and possible injury to the operator.

Extremely high fault currents can occur if industrial high-voltage circuits are involved (240 V or higher).

A DMM should 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 VAC).

Those DMMs that do use fuses should have a fuse with sufficient capacity to clear a high-energy fault. The voltage rating of the meter's fuses should be greater than the maximum voltage you expect to measure.

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.

Sometimes you may have to make a current measurement that exceeds the rating of your DMM or the situation does not allow you to open the circuit to measure the current.

In these higher current applications (typically over 2 A), where high accuracy is not needed, a current probe is very useful.

This clamps round the conductor carrying the current, and it converts the measured value to a level the meter can handle.

There are two basic types of probes: current transformers, which are used to measure AC current only, and Hall-Effect probes, which are used to measure AC or DC current.

Figure 5:

The output of a current transformer is typically 1 mA per amp. A 100 A value is reduced to 100 mA, 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.

The output of a Hall-Effect probe is 1 mV per amp, AC or DC.

For example, 100 A AC is converted to 100 mV AC. The probe leads 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. The meter displays 1 millivolt for every amp measured.

Making measurements safely starts with choosing the proper meter for the application as well as the environment in which the meter will be used.

Once the proper meter has been chosen, you should use it by following good measurement procedures. Carefully read the instrument user manual before use, paying particular attention to the 'warning' and 'caution' sections.

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.

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.

Some meters will detect an overload 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 organisation approval/listing.

One very important requirement of a DMM is that it can be used with a wide variety of accessories.

Many accessories are available that can increase your DMM's measurement range and usefulness, while making your measurement tasks easier.

High-voltage probes and current probes scale down high voltages and currents to a level the DMM can safely measure.

Temperature probes convert your DMM into a digital thermometer. RF probes can be used to measure voltages at high frequencies.

A selection of test leads, test probes and test clips can help you connect your DMM to the circuit.