Multimeters – measuring voltages, currents and resistances.

Multimeters - measuring and diagnostic tools.

In the daily work of an electronics technician or electrician, quick measurements of electrical values have to be made time and again. To measure small voltages, control currents, resistances, frequencies or simply for a quick check of electronic systems and the status of operating states, the specialist requires a large number of measured variables from the electronic measuring devices.

A multimeter can be used to carry out basic measurements such as AC / DC voltage, AC / DC current, resistance, capacitance, continuity tests and diode tests, to name but a few. This is why the multimeter is one of the most important measuring and diagnostic tools in electronics and electrical engineering alongside the oscilloscope.

The current clamp

With the exception of current clamps, which, as the name suggests, can only measure currents, all other types of multimeter (see also the section on multimeter designs) are able to measure electrophysical variables. However, there are also devices that can record and measure a number of other measured variables, such as temperature using a positive temperature coefficient (PTC), negative temperature coefficient (NTC) or thermocouple (type K, J, S, A, T, etc.). Other measured variables include frequency, event counting, power factor, phase angle, active power, reactive power and apparent power and others.

The resolution.

The resolution is closely related to the number of digits and the measuring range used. In the above case of the number sequence +1999, this would mean a maximum display of 19.99 V in the 10 V measuring range. The last digit in this measuring range has a 10 mV value and therefore a 10 mV resolution.

If the voltage is increased, the measuring range must be switched, e.g. to the 100 V range, whereby the display would now show 200.0 V, for example. The last digit therefore has a value of 100 mV, i.e. a resolution of 100 mV. You must be aware that the last digit is subject to inaccuracy, as it "jumps" and you do not know whether it jumps at the lower edge of the threshold or at the upper edge of the threshold – and that is 100 mV in the latter case.

The following table shows how many bits are required to "generate" a 4½ -digit display, for example.

For 4 digits, 19999 different digital levels plus one for the "0" are required. This corresponds to 20000 values, for which X bits are required. 20000 values = 2x bits which is converted to X bits = log 20000 values / log2=14.29 bits. If you add a bit for the "+/-" sign, this is the value that is shown in the table under "Bits" as 15.29, for example.

For example, Keysight gives the accuracy over one year for a 7 ½-digit benchtop / system multimeter for the DC-V measuring range 100 mV as ± 0.0040% of the display value and +0.0035% of the measuring range. If, among other things, the boundary conditions, a warm-up phase of at least 1 hour for the device to reach operating temperature and a zero adjustment no longer than two days ago are taken into account, an inaccuracy of ±7.5 μV can be calculated for this digital multimeter when these inaccuracies are added together.

Another manufacturer, such as Chauvin Arnoux, quotes the accuracy as e.g. 0.8% (display value) + 4D:

This means 0.8% of the value read from the display + 4 digits. These 4 digits refer to the least significant digit in the measuring range. In the 100 mV measuring range, the least significant digit has a value of 0.1 mV. If, for example, a voltage of 100 mV is read, the accuracy in this example is calculated as (0.8 mV + 0.4 mV) = 1.2 mV.

It is immediately apparent that the resistance of the cable in the current-carrying path would cause an error in the voltage measurement if the measuring cable were not connected. Four-wire measurements are always necessary when a large current has to be conducted to and from the measurement object. For example, this is important for diode measurements (characteristic curve) or for precise resistance measurements. A current is impressed which flows through the component to be measured, resulting in a voltage drop at the supply wires. In order not to measure this voltage increase, but only the voltage that drops directly across the component, an additional pair of wires is inserted that is connected directly to the test object on one side.

A small example: Distance between measurement object and measuring device 1 m, wire connection 22 AWG (American Wire Gauge = American standard for wire cross-sections) with 0.0547 Ω/m, an impressed measuring current of 1 mA and a nominal resistance of 1 kΩ to be measured would result in a voltage measurement of 1.0001094 V if there is only a 2-wire measurement. The error in this case would be 0.01094%.

Different designs for different areas of application.

Multimeters are available in a wide variety of designs and for a wide range of applications. They are therefore designed for these applications and offer the corresponding accuracy.

Each device is accompanied by a more or less extensive range of accessories such as measuring cables, thermocouples, test probes, etc.