Electronic – Measuring current in 12V DC fan


I am trying to measure the current of a DC computer cooling fan (Intel E18764-001) at various voltages. Its rated specs are: 12V 0.20A. Check it out here.

So I am suspecting that this is telling me: "give it 12V and you should get 200mA of current" – Am I right?

Anyway, I supplied the fan with various voltages using my power supply starting from 5V all the way to 12V. I measured the current with a multimeter connected in series. Here are the results:

  • @5V –> 50mA (0.05A)
  • @6V –> 30mA
  • @7V –> 30mA
  • @8V –> 30mA
  • @9V –> 40mA
  • @10V –> 40mA
  • @11V –> 50mA
  • @12V –> 53mA

Interesting results. Here are my questions:

  1. Why isn't there a proportionate increase in current with every increase in voltage? In fact, after 10V, the current goes up as voltage goes up! Does it have anything to do with either the fan going all funny because it is not getting its designed voltage or the wires can't handle any current more than 50mA? (Last one unlikely as that still doesn't explain the drop to 40mA.)

Why are the current readings nowhere near the power rating of 0.2A?

Best Answer

Since DC motors are also generators, the reverse voltage raises proportional with the RPM. So the higher the RPM, the higher the reverse voltage -> less current. Also the ambient temperature is most probably not stable, which leads to fluctuating current measurement results.