Correct circuit

Insofar as an electrical current strength and an electrical voltage are to be measured in an electrical measuring circuit , the measurement of one variable leads to a feedback deviation on the measurement of the other variable. It is only possible to measure one of the two quantities without being falsified by measuring the other quantity. There are accordingly two different circuits.

With the introduction of the digital multimeter, the problem has lost much of its importance in measurement practice , as their input resistance for voltage measurement is typically 10 MΩ and is therefore orders of magnitude larger than analog multimeters, depending on the measurement range.

Measuring circuit with current and voltage measuring device

options

The influence of the circuit should be explained when measuring a consumer (an ohmic resistor in the picture ): ${\ displaystyle R}$

In the picture above, the current intensity flowing through the consumer is measured directly (without branching). If a voltage drops on a real ammeter , the voltage drop on the ammeter is also measured when the voltage is measured. With regard to the consumer, this results in a correct current circuit and at the same time a voltage error circuit . ${\ displaystyle U_ {I}}$

If you want to avoid this systematic deviation , you can connect the voltage measuring device directly to the consumer as shown in the picture below. But then the ammeter also measures the current consumption of a real voltmeter . With regard to the consumer, this results in a voltage- correct circuit and at the same time a current fault circuit . ${\ displaystyle I_ {U}}$

The opposite is true if the voltage and current of the source (not the consumer) are to be measured.

Then the upper circuit is the voltage-correct circuit
and the circuit below the correct current.

You must therefore pay close attention to the point of view from which the statement "correct" or "incorrect" is to be made. A statement that applies jointly to the source and the consumer is not possible.

In the correct current circuit, the current intensity is measured correctly within the error limits as long as the circuit remains unchanged. The ammeter is often inserted into the circuit for a one-off measurement and then removed again. When feeding from a constant voltage source, the removal of the measuring device results in a higher voltage at the consumer , and the current intensity is inevitably higher than the "correctly" measured. For examples, see under feedback deviation . ${\ displaystyle U_ {I}}$

selection

If the amount of the relative systematic deviation of a measured value is significantly smaller than its relative error limit, no correction is necessary. In case of doubt, preference will be given to the circuit in which the correction can be omitted. In the case of the electronic measuring devices that are currently used, the voltage measuring device will predominantly cause a smaller relative deviation in the measured current value than the current measuring device, which causes a relative deviation in the measured voltage value. ${\ displaystyle {\ tfrac {I_ {U}} {I}}}$ ${\ displaystyle {\ tfrac {U_ {I}} {U}}}$

For example, the current through the digital voltmeter with 10 MΩ leads to a relative deviation of <1 ‰ if <10 kΩ. If a voltage drop of 200 mV is to be expected with the digital ammeter (at the end of the measuring range), this can lead to a relative deviation of> 1 ‰ at <200 V. In the case of low voltage and smaller loads, the voltage-correct circuit with regard to the consumer is to be preferred with these measuring devices. ${\ displaystyle R}$ ${\ displaystyle U}$

Selection and correction instructions are also given for power and resistance measurements.

literature

Horst Clausert, Gunther Wiesemann: Basic areas of electrical engineering 1 . Oldenbourg, 2005 ( limited preview in Google Book Search).
Wilfried Weißgerber: Electrical engineering for engineers 1 . Vieweg, 2005.
Kurt Bergmann: Electrical measurement technology . Vieweg, 1997.
Ulrich Dietmeier: Formula collection for electronic circuit technology . Oldenbourg, 2003 ( limited preview in Google Book Search).

Web links

Student Level Declaration ( LEIFI )