Exercise EE1-02.04 – Case fan schematic, symbols, calculation

Prof. Dr. Thorsten Jungmann (last updated 2025-10-21)

Relates to EE1-02 Electrical quantities and units, terms, symbols and models

Task

A case fan is designed for connection to $12\mathrm{V}$ DC. With the help of a series resistor, the operating voltage of the fan is reduced to $9.0\mathrm{V}$ so that the fan runs more slowly and more quietly. In low-noise operation, a current of $40\mathrm{mA}$ is measured.

a) Sketch the electric circuit and label all electrical quantities.

b) Calculate the resistance of the fan at this operating point, and also enter the value in the schematic. Here you only need to calculate the resistance of the fan in operation, without the series resistor.

c) Using the calculated fan resistance, calculate the current for operation without the series resistor (at $12\mathrm{V}$).

Solution

a) Sketch / equivalent circuit diagram
b) $230\Omega$
c) $53\mathrm{mA}$

▶️ Full worked solution