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Measuring Voltage with a Digital Multimeter

One of the primary measurements made by digital multimeters is that of voltage or volts.

Using a digital multimeter to measure voltage is a key function, and one that is particularly easy. However even for the experienced engineer there are a number of precautions, tricks of the trade that can be used.

DC voltage measurement basics

When measuring voltage with a digital multimeter, the process is normally very straightforward, and second nature to many users of this site.

When measuring voltage using a digital multimeter, the probes are simply placed across the two points where the voltage is to be measured.

A typical digital multimeter voltage measurement

Virtually all digital multimeters have an auto-polarity function, and therefore there is normally no need to worry about which way round the probes are connected. However it is normally best practice to connect the COM or Common connection that is normally the negative connection to the lower voltage or the chassis or zero volt line.

Precautions when measuring voltage with a DMM

When measuring voltage using a digital multimeter, DMM, there are a few pointers that can be employed:

  • Set multimeter range:   The meter must obviously be turned on. The range should be set, normally to one well above any expected voltage, and then reduced accordingly to provide the most accurate reading. Many multimeters, though can be auto-ranging and will automatically set the correct range.

  • Be aware of impedance:   Todays digital multimeters have a very high input impedance. This means that they will place a very negligible load onto the circuit. The main issue will be the levels of stray capacitance from the leads, etc may be an issue and where higher frequencies are used this may need to be taken into account.

  • Stray pick-up :   With very high levels of impedance, stray pickup can be an issue when using a DMM, if the source impedance of the circuit is also high. Care may be needed when using a digital multimeter under these circumstances, and the effects of pick-up will need to be born in mind

  • Beware high voltages:   It almost goes without saying when measuring voltage using a digital multimeter, but when dealing with any electronic equipment for repair, extreme care needs to be taken as high voltage points may be accessible.