Analog multimeter

An analog multimeter , also an analog multiple measuring device, is an analog measuring device for measuring various electrical quantities such as voltage , current strength or resistance . There are usually several measuring ranges for each measured variable . Among the multimeters , devices that work according to an analog measuring principle with a dial display can often measure passively, while digital multimeters with a numeric display always require a power supply (e.g. from a battery).

A simple analog multimeter

functionality

A moving- coil measuring mechanism or (more historically) a moving iron measuring mechanism is used to display the measured value . In terms of their physics, these devices are current measuring devices , as they can only display current with their magnetic measuring mechanisms. This article deals with multimeters with moving coil movement. This records the equivalent value of a current. This means that only the direct current or ohmic component of the coil resistance needs to be taken into account (in contrast to the moving iron measuring mechanism). For currents that are greater than the coil can handle, different current measuring ranges are used with current dividers formed from measuring resistors .

Circuit in the analog multimeter for direct current and voltage, simplified to a few measuring ranges

Scales of a multimeter for
- alternating quantities (marked in red),
- constant quantities (linearly divided),
- resistance (from ∞ falling to 0)

For use as a voltage measuring device, current is measured which flows through the internal resistance of the device. Voltage dividers are formed accordingly for the different voltage measuring ranges. The circuit diagram shows the internal circuit of an analog multimeter that works without measuring electronics, simplified to a few measuring ranges.

Two scales together for direct and alternating voltage and current,
including resistance and level

The measurement of alternating current and alternating voltage by means of a moving-coil measuring mechanism is possible when using a measuring rectifier, preferably a germanium diode . With passive measuring devices, the scales for direct and alternating current are different because of the non-linear characteristic of the rectifier. Electronically assisted analog multimeters are often equipped with a precision rectifier . In this case, the non-linearity of a diode has no influence on the display; common scales for constant and variable quantities are possible. Even voltages that are lower than the diode forward voltage can be measured with a linearly divided scale. These multimeters form the rectification value ; however, they are set so that they display the rms value for sinusoidal input variables . (For the considerable measurement deviation possible with other waveforms, see under rectified value.)

Increasingly multimeters are available with the effective value forming circuit . In the case of a mixed variable , a distinction must be made as to whether the effective value of the entire input variable or that of its alternating component is measured. These devices may offer a switchover option.

For use as a resistance measuring device , a current is formed, to which is always a built-in battery is necessary. The display range of the moving-coil measuring mechanism zero… maximum covers the resistance range ∞… 0. This means that only a single measuring range is required. Since reading at the edges of the scale is very uncertain, a measuring range switch is often built in; this brings different display areas into the middle of the scale.

When making this measurement, it must be remembered that a current flows through the measurement object . This must have the behavior of an ohmic resistor . It must also not contain any voltage or current sources .

Due to the supply from a non-stabilized voltage source (battery with age-related drop in voltage), the device must be adjusted before the resistance measurement. The same applies after switching the measuring range. To do this, a short circuit is made between the measuring terminals and an externally accessible potentiometer is set so that is displayed. ${\displaystyle R=0}$

Measuring ranges, indicators of self-consumption

Reference values ​​for realized measuring ranges ( measuring range end values ) are

• Current measuring devices: 10 μA ... 10 A,
• Voltage measuring devices: 100 mV… 1000 V.

If a measuring amplifier is installed, measurements can be made down to the μV range and down to the pA range, especially when using an FET input stage.
The frequency range for the measurement of alternating quantities includes approximately

• 10 Hz ... 10 kHz (... 10 MHz).

When grading the measuring ranges, it has proven useful in high-quality devices to provide two ranges per power of ten, e.g. B. with a gradation of 1: 3: 10. In this way you can avoid measuring in the lower third of the measuring range, because the relative error limits increase sharply with a small deflection . Steps such as 1: 5: 20: 100 with three measuring ranges over two powers of ten can also be found repeatedly.

With an amplifierless multimeter it can be assumed that it has a different internal resistance in each measuring range . The information requires a longer table. A general specification that applies approximately to all current measuring ranges is the voltage drop at the end of the measuring range . ${\displaystyle R_{i}}$ ${\displaystyle U_{I{\text{(MB) }}}}$

• Guide value 100… 1000 mV (rather less for small measuring ranges).
• Depending on the construction, 10 mV is also possible for direct current.

For the voltage measuring ranges, the corresponding parameter is the current consumption at the measuring range end value . However, its reciprocal value is rather given as voltage-related resistance : ${\displaystyle I_{U{\text{(MB) }}}}$ ${\displaystyle \varrho }$

${\displaystyle \varrho ={\frac {1}{I_{U{\text{(MB)}}}}}={\frac {{\text{Innenwiderstand}}\ R_{i}}{\text{Spannungsmessbereich}}}}$.

• Guide value 1… 20 kΩ / V (the same for all measuring ranges, but limited to = 10 MΩ), ${\displaystyle R_{i}}$
• for devices with measuring amplifier 100 kΩ / V or

${\displaystyle R_{i}}$ = 1… 10 MΩ in all ranges.

Self-consumption is often the cause of a feedback deviation , which systematically means that too little is measured.

service

Analog multimeters often have very different operating concepts

If the type of measured variable is unknown , switch to "alternating voltage". If the value of the measured variable is not known approximately , you switch to the largest measuring range. Next, you connect the measuring lines to the measuring device sockets, which are labeled with the desired measured variable, and finally to the measuring object. Starting from this, the measuring range is switched down to the level before the measuring range is exceeded. Particular care should be taken to avoid the following operating errors :

1. Under no circumstances may an alternating quantity in the same quantity range be connected; the display is always zero. A possible overload cannot be seen, only smelled.
2. In the event that a voltage source with low internal resistance is connected to a current measuring range, some multimeters are protected by a fuse. One should not trust that it is present in every measuring device.
3. If, for example, two measuring ranges differ by a ratio of 10: 1, no further downshifting is permitted if more than 1/10 of the full scale value is displayed. The overload capacity of the measuring mechanism is not great.

The measured variable can be read on the scale . In order to avoid the parallax error caused by looking at the pointer at an angle , the device often contains a mirror scale, as can be seen on several of the measuring device or scale photos shown. You should look at the scale from the direction from which the pointer is aligned with its mirror image. Correspondingly, with the knife pointer, one should look at the scale so that the pointer appears as narrow as possible.

application

For a long time, analog multimeters were the only simple way of determining electrical quantities with acceptable error limits . These are described by a class symbol. Since the introduction of digital multimeters , however, analog measuring devices have been pushed into the margins. Digital multimeters generally have smaller error limits and are read without a measurement deviation due to estimation uncertainty. How far these advantages can be exploited, however, is at least questionable because it is all too easy to forget to take into account the error limits also present here as well as the internal consumption of the input circuit and the sources of error due to external circumstances.

The advantage of the analog multimeter lies in

• the quick visual recording of the measured value,
• the easy recognition of tendencies (movement of the pointer) (with time constants above 10 s),
• the averaged display with rapid fluctuations in the measured variable (at frequencies above 10 Hz),
• the independence of batteries (except for resistance measurement, or if measuring electronics are installed), therefore can be used as a permanent display in the laboratory.

To compare the analog measurement method with the digital, see also digital measurement technology .

Measurement errors

See also accuracy class , measuring device deviation , feedback deviation

literature

• Reinhard Lerch: Electrical measurement technology: analog, digital and computer-aided processes . 6th edition. Springer, 2013, ISBN 978-3-642-22608-3 , chapter 6.

Web links

Commons : Analog multimeters  - collection of images, videos and audio files