How to Use a DMM, Digital Multimeter

- Digital Multimeters, DMMs are an essential tool for the electronics engineer. Find out how to use a digital multimeter to measure voltage, current, resistance and many more quantities.

Digital multimeters are easy to use. They can be used to measure the basic three electrical parameters of current (amps), voltage (volts) and resistance (ohms).

Knowing how to use a DMM can enable the best to be gained from them - the most accurate readings and also knowing the limitations of the readings.

Knowing how to use a digital multimeter also enables some tricks of the trade to be used to measure parameters that may not normally be known about, and can be very useful.

Digital multimeter measurement basics

When using a digital multimeter, there are a number of initial steps and precautions that should be observed. These guidelines are always best to follow, and if incorporated into a workflow they will help to make measurements more accurate and prevent damage to the instrument or reduce safety risks for the user.

  • Check battery regularly:   DMMs require a power for their operation. For portable instruments, this is provided by a battery. Regular checks of the battery state are very advisable to ensure that there is sufficient voltage to adequately power the DMM. Also if batteries are left in situ for long periods they can leak and dame the contacts in the instrument as the contents of the battery are corrosive. To achieve this a sticker such as one indicating the data the battery is due for replacement or a calibration due sticker could be used.
  • Return meter to high voltage setting after use:   To prevent the possibility of accidental damage by the meter being set to a current or low voltage range, it is always wise to leave the meter set to read a high voltage, even if there is an Off button. It is too easy to automatically connect a digital multimeter to the circuit without thinking about the range. This can lead to damage of both the equipment under test and the meter if it is set to a current range when voltage is to be measured, for example. Although some meters are auto-ranging for voltage, current etc, others are not, and therefore it is always wise to leave the meter set to the highest voltage range possible.
  • Ensure probes are in good condition:   Poor probes may not only result in poor readings, because it is not possible to connect to any test points properly, but also there can be the risk of injury if they are cracked and broken leaving exposed conductive areas when making a high voltage measurement.

How to use a digital multimeter for current, voltage, resistance

The three main measurements made by digital multimeters are voltage, current and resistance.

These measurements are normally very easy to make using digital multimeter.

  • How to use a digital multmeter for voltage:   One of the most widely used measurements for a DMM is that of voltage. Knowing how to use a digital multimeter to make the best voltage measurements can ensure the most accurate measurements are made.

    Using a digital multimeter to measure voltage by placing the probes from the instrument across the points where the voltage reading is required
    A typical digital multimeter voltage measurement

    When making a voltage measurement the probes from the digital multimeter are placed across the points where the voltage is to be measured.
    Read more about DMM voltage measurements

  • How to use a digital multmeter for current:   When making a current measurement, the current flowing within the circuit needs to be detected. The traditional method of achieving this is to break the circuit and place the digital multimeter acting as a current meter in circuit. In this way the current flows through the meter. the level of current can then be detected and displayed.

    Using a digital multimeter to measure current showing how the circuit needs to be broken to insert the instrument
    A typical digital multimeter current measurement

    Read more about DMM current measurements

  • How to use a digital multmeter for resistance:   When making a resistance measurement with a digital multimeter, it is necessary to place the item under test across the probes of the multimeter. It must be removed from the circuit, otherwise stray conduction paths will be present. Even if measuring continuity the unit must be off and power removed. Should any voltages be present, these will at the very least distort the readings, and at worst they could damage or even destroy the instrument. Care must be taken.

    Read more about DMM resistance measurements

These are the basic three measurements that can be made using a digital multimeter. They are by far the most widely used, and they are the three that were combined into the previous generation analogue multimeters as well.

Using a DMM for other measurements

In addition to the basic voltage, current and resistance measurements, many digital multimeters offer a range of other measurements that they can make. The actual measurements depend upon the actual DMM being used. However knowing how to use the digital multimeter to make these measurements means that the results can be assessed within the capabilities of the instrument.

Other measurements that may be incorporated into digital multimeters may include:

  • Continuity sounder:   This is one of the most common additions found in a digital multimeter. This capability is sometimes included on a low value Ohms range, or may have its own switch position. The idea is that it is possible to test for continuity and listen for a buzz, rather than having to continually turn away from looking at the unit under test. When using the continuity tester, the same precautions that should be implemented when using the ordinary resistance ranges should be observed, especially the unit should not be powered up..
  • Frequency :   Some digital multimeters can be used to measure frequency. This is one of the less commonly included ranges, but can be used to give a rough indication of frequency up to a few hundred kilohertz. Normally the ranges do not extend very high, and they are not normally very accurate. However they are useful for some low frequency measurements, where accuracy is not paramount.
  • Capacitance :   With some digital multimeters, it is possible to use them to measure capacitance. Again this facility is not available on all meters, but some have the ability to measure it. Like resistance measurements, capacitance measurements should ideally be undertaken on the component when it is not in circuit. Normally the capacitance ranges are limited, and they are not able to measure small levels of capacitance.
  • Transistor tester :   Occasionally multimeters have the facility to measure transistor parameters. In particular they measure the He or Β of the transistor. Typically there are three connections for each of the PNP and NPN varieties, so you need to know what sort it is before testing.

By Ian Poole

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