What is voltage deviation

Measurement error circuits

In the case of indirect resistance measurement with a current and voltage measuring device, Ohm's law is used to calculate the unknown resistance from the measured current and voltage values.
Because the measurement result is falsified by the internal resistance of the current and voltage measuring device, a different measurement circuit is selected depending on the size of the unknown resistance.

Current fault circuit (ammeter before voltmeter)


In the current error circuit, a parallel circuit consists of the internal resistance of the voltmeter and the resistance to be measured. A current I flows through the voltmeterU. This falsifies the current I to be measuredR.flowing through the resistance to be measured.
The current Itotal is about the current I.Uflowing through the voltmeter is too big.

Measurement with R = 220 Ω
U in voltsI in mAR in Ω (calculated)
Current fault circuit
R in Ω (calculated)
Voltage fault circuit
2090222222
0,20,91220333

Typically the internal resistance of voltmeters is very large. The current error circuit is therefore only suitable for measuring resistance on small resistors, where the current through the internal resistance of the voltmeter has very little effect on the measurement. As soon as you want to measure a large resistance with this circuit, the parallel connection of the internal resistance of the voltmeter and the resistance to be measured falsifies the result.

Voltage error circuit (voltmeter before ammeter)


With the voltage fault circuit, a voltage divider is created from the internal resistance of the ammeter and the resistance to be measured. The voltage drop UI. on the ammeter falsifies the voltage measurement. The measured voltage Utotal is about the voltage UI. too large.

Measurement with R = 10 kΩ
U in voltsI in mAR in Ω (calculated)
Current fault circuit
R in Ω (calculated)
Voltage fault circuit
202,4833310000
0,20,024833310000

Typically, the internal resistance of ammeters is very small. The voltage error circuit is therefore only suitable for measurements on large resistors, where the voltage drop across the internal resistance of the ammeter has very little effect on the measurement. As soon as you want to measure a small resistance with this circuit, the series connection of the internal resistance of the ammeter and the resistance to be measured falsifies the result.

Electronic multimeters have a very high resistance when measuring voltage (1..10 MΩ). The current fault circuit is therefore only relevant if very small currents (µA range) are measured. The voltage error circuit is also used because the shunt resistor in the measuring device causes a relevant voltage drop.

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