Loading Effect (1)

The degree to which a measuring device affects the electrical properties of the electrical circuit to which it is connected is called the loading effect. This content covers the topic of the loading effect of the voltmeter.

Loading Effect of The Voltmeter

Before talking about the loading effect of the voltmeter, let's briefly explain what a voltmeter is and what its features are.

The voltmeter is the measuring instrument used to measure voltage, that is, voltage. It is connected in parallel to the circuit while measuring. The internal resistance of the voltmeter is too high. In ideal voltmeters, the internal resistance is considered infinite. Therefore, it cannot be connected in series with the circuit. In cases where the voltmeter is connected in series with the circuit, a very small current flows over it (in ideal voltmeters, it is considered as current is not flowing) and cannot measure. It cannot only measure when connected in series. But the measuring instrument is not damaged.

The loading effect of the voltmeter can be explained as follows. It can be defined as the difference between the voltage value of the circuit before the voltmeter connection is established and the voltage value it has after the voltmeter connection is established.

In order to better understand the loading effect of the voltmeter, let's give an example through a circuit.

Example:

• First, let's build a circuit and then calculate the voltage V₂ falling on the resistor R₂ in the circuit. The circuit is shown below.

The voltage-divider rule is applied to calculate the V₂ value.

Voltage-divider rule:

V_in = V_out. (R₂/R₁ + R₂)

In this circuit, V_in is the source of V₁ that powers the circuit. V_out is the value we want to measure, V₂.

V₂ = V₁. (R₂/R₁+R₂)

V₂= 10V . (5MΩ + 10 MΩ)

V₂ = 6.6 V

• In the second step, let's connect a voltmeter over the resistor R₂. Let's assume the internal resistance of the voltmeter we connected is 10 MΩ. It is shown in the figure.

Similarly, the voltage-divider rule is applied to calculate the voltage V₂. The point to be considered in this circuit is that the resistor R₂ and the internal resistance of the voltmeter are parallel.

              Voltage-Divider Rule:

V₂ = V₁. [(R₂ || 10 MΩ) / R₁ + (R₂ ||10MΩ)]

V₂ = 6V . [(10 || 10)/ 5 + (10 || 10)]

V₂ = 5V

As can be seen, the circuit voltage was calculated as 6.6 V before the voltmeter was connected. However, after the voltmeter connection was established, the circuit voltage was calculated as 5 V. This difference between the two calculations is called the loading effect of the voltmeter.

>>> In the next content, the loading effect of the ammeter will be explained.