Digital Multimeter DMM basics tutorial

- summary, tutorial or overview of the basics of the digital multimeter DMM, its makeup, operation, the use of a digital multimeter and its applications.

The digital multimeter, DMM, is one of the most common items of test equipment used in the electronics industry today. While there are many other items of test equipment that are available, the multimeter is able to provide excellent readings of the basic measurements of amps, volts and ohms. In addition to this the fact that these digital multimeters use digital and logic technology, means that the use of integrated circuits rather than analogue techniques, enables many new test features to be embedded in the design. As a result, most of today's digital multimeters incorporate many additional measurements that can be made.

DMM facilities

While the facilities that a digital multimeter can offer are much greater than their analogue multimeter predecessors, the cost of DMMs is relatively low. Digital mulimters are able to offer as standard the basic measurements that would typically include:

• Current (DC)



• Current (AC)



• Voltage (DC)



• Voltage (AC)



• Resistance

However, using integrated circuit technology, most digital multimeters are able to offer additional test capabilities. These may include some of the following:

• Capacitance



• Temperature



• Frequency



• Transistor test - hfe, etc



• Continuity (buzzer)

While some of these additional test features may not be as accurate as those supplied by dedicated test instruments, they are nevertheless very useful, especially where approximate readings only are needed.

In addition to an increase in the number of basic measurements that can be made, refinements of some of the basic measurements are also available on some models. True RMS digital multimeters are available. In many instances, AC waveforms use forms of average measurements that are then converted to RMS measurements using a form factor. This method of measurement is very dependent upon the shape of the waveform and as a result a true RMS digital multimeter may be required. In addition to the availability of a true RMS digital multimeter, similar refinements of the other basic measurements are also available in some instances.

In addition to the additional measurement capabilities, digital multimeters also offer flexibility in the way measurements are made. Again this is achieved because of the additional capabilities provided by the digital electronics circuitry contained within the digital multimeter. Many digital multimeters will offer two additional capabilities:

• Auto-range:   This facility enables the correct range of the digital multimeter to be selected so that the most significant digits are shown, i.e. a four-digit DMM would automatically select an appropriate range to display 1.234 mV instead of 0.012 V. Additionally it also prevent overloading, by ensuring that a volts range is selected instead of a millivolts range. Digital multimeters that incorporate an auto-range facility usually include a facility to 'freeze' the meter to a particular range. This prevents a measurement that might be on the border between two ranges causing the meter to frequently change its range which can be very distracting.



• Auto-polarity:   This is a very convenient facility that comes into action for direct current and voltage readings. It shows if the voltage of current being measured is positive (i.e. it is in the same sense as the meter connections) or negative (i.e. opposite polarity to meter connections). Analogue meters did not have this facility and the meter would deflect backwards and the meter leads would have to be reversed to correctly take the reading.

DMM overall accuracy

The accuracy of a digital multimeter can be very important in many electronics test applications. For most service or general electronics test applications, the digital multimeter accuracy will be well in excess of what is needed, but whatever the application it is necessary to understand the accuracy of the test equipment being used.

There are several constituents to what may be loosely termed the accuracy of the digital multimeter. Two of the major components are given below:

1. Resolution   The resolution of a digital multimeter is normally specified in terms of the number of digits displayed. Typically this will be a number consisting of an integer and a half, e.g. 3 ½ digits. By convention a half digit can display either a zero or 1. Thus a four and a half digit meter could display up to 19999. Occasionally a three quarters digit may be used instead of the half. When this is seen, it indicates that the DMM additional numeric can display a number higher than one, but less than nine. Often the range is extended to 399, 3999, etc. It is worth remembering that increased levels of resolution do not come without penalties. Longer settling times are required for the far right digits to reach their final value. Thus the time between readings is longer.



2. Accuracy   The accuracy of the meter is different to the display resolution. This represents the uncertainty of the reading due to inaccuracies in the DMM. The determination of the accuracy, itself has several constituents that may be all combined into the one "accuracy" figure. It will be found that the level of AC accuracy for the digital multimeter is normally less than that for DC measurements. The AC measurements will also be optimised for 50-60 Hz and this means that other frequencies may have poorer degree of accuracy. As with DC accuracy specifications, a number of counts (often greater than for DC) will be added to the accuracy percentage. Also, for waveforms other than a pure sine wave, additional inaccuracy will be encountered when measured with an average responding DMM. Even a true RMS responding DMM will have some accuracy limitations for waveforms with high peak amplitude components if measured near full scale.

Digital multimeters are generally able to provide very high degrees of accuracy; certainly they are far more accurate than their analogue counterparts which are normally only to guarantee accuracy levels of between 3 and 5%. A typical handheld or portable DMM should be able to provide accuracy levels of around 0.5% and many bench digital multimeters will quote figures of around 0.01%.

Summary

Digital multimeters, DMMs are widely used and very useful items of test equipment. They enable measurements of quantities such as current, voltage and resistance to be made very quickly and easily. In addition to this, many digital multimeters are able to measure other useful parameters, making these items even more useful. While they do not allow more complicated measurements to be made, if many engineers were allowed only one item of test equipment, it would probably be the digital multimeter.