I opened and took out some components of an electric source of a desktop computer that doesn't work anymore (the source not the computer, I bought another source). One of them is a fan DC 12V: HA1225M12S-Z.

When I supply 12V to the welded terminals of the other side of the PCB, where it is plugged, the fan works normally.

My doubt is the electrical resistance of the fan. When I measure it with the \$\Omega\$ function of my multimeter, I get \$20k\Omega\$ what doesn't make sense, because it is written in the fan the current of \$0.45A\$.

Probably the measure is wrong, but on the other hand the multimeter seems to give good values for other resistances.

The function of measuring continuous current is not working, so I can not check the resistance by applying a voltage and checking the current with the fan locked.

I could not find the specification of the resistance of this fan in the web. Am I right to suppose that it must be \$26.7 \Omega \left(\frac{12}{0.45}\right)\$?

  • \$\begingroup\$ If your current function isn't working, you just need to re fuse your multimeter. \$\endgroup\$
    – K H
    Jan 25 '21 at 1:29
  • \$\begingroup\$ @K H only for DC. It works for AC. \$\endgroup\$ Jan 25 '21 at 1:30
  • \$\begingroup\$ A fan has impedance rather than resistance. Resistance and Inductive reactance form the short sides of a square triangle and impedance is the length of the hypotenuse. Because motors are inductive they have inductive reactance and do not behave as their resistance alone would indicate. \$\endgroup\$
    – K H
    Jan 25 '21 at 1:31
  • \$\begingroup\$ The use of the true resistance with an inductive load is to calculate watt losses, but to evaluate starting or running current curves you need to use the more complicated impedance. \$\endgroup\$
    – K H
    Jan 25 '21 at 1:32
  • 7
    \$\begingroup\$ The fan probably has some electronics, and your ohmmeter does not apply enough votlage to get the thing into an operating state where it will draw the normal current. \$\endgroup\$ Jan 25 '21 at 1:33

Fans often found in computers are DC brushless fans. They have drive electronics. So as an electronic load, they are not pure resistances so measuring the resistance is meaningless, except for determining if they have a short or open connection.


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