Digital Multimeter DMM basics tutorial

- the digital multimeter, DMM is one of the most useful items of test equipment. Basic instruments can measure current voltage and resistance, but most include the capability to measure many other values.

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.

Image of a typical DMM used for measuring current, voltage, resistance and many other electronic and electrical measurements

DMM facilities

While the facilities that a digital multimeter can offer are much greater than their analogue predecessors, the cost of DMMs is relatively low. DMMs 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 DMMs 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 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 meters, similar refinements of the other basic measurements are also available in some instances.

In addition to the additional measurement capabilities, DMMs 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 instruments 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.

Digital multimeters 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 DMMs 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.

By Ian Poole

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