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Context: I'm wanting to tinker around with some mini solar panels to power some old PC case fans I have laying around. I know that case fans are generally 12v so I was thinking of running some 3v panels in series. I know some basics from college but I'm pretty much a noob. I decided some experimentation was order.

My fan connector has 3 pins, so I identified the center one as the 12V and one on the side as the ground (the other goes to the tach, which I don't care about in this case - the power supply connector doesn't connect to this pin anyway).

First I measured the resistance. I get a reading of 14.48 on the 20kOhm scale which should be 14,480 Ohms. That seems very high to me because that would mean the fan runs on 0.83mA which seems very low.

I had an old computer power supply laying around so I shorted the power on pin and hooked up the fan (so nothing else is hooked to the power supply besides the fan). I confirmed that the voltage is right around 12V and when I hooked up the multimeter in series I get a current reading of 0.18A which is much more inline with what I would expect, but that means I should've gotten a resistance reading of around 66 Ohms.

I know I'm missing something obvious here, but I'm not sure what it is. Any help? Thanks!

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  • \$\begingroup\$ Isn't the initial reading 14.48 ohm instead of 14.48 k\$\Omega\$? \$\endgroup\$
    – K. Rmth
    Commented Jul 15, 2015 at 9:58
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    \$\begingroup\$ You are missing that the fan isn't just a motor with low resistance coils, but a non-linear semiconductor device to control the motor. If you measure the forward resistance of a silicon diode, do you see a similar reading? Above 0.7V the diode will turn on ... above a couple of volts, the motor controller will start, and try to drive the fan. \$\endgroup\$
    – user16324
    Commented Jul 15, 2015 at 10:20
  • \$\begingroup\$ @K.Rmth:The initial reading is definitely 14.48 kΩ. If I crank down the scale to the 200Ω range I get no reading. \$\endgroup\$
    – Landon
    Commented Jul 16, 2015 at 18:17

2 Answers 2

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A fan isn't a passive device; you can't expect to measure its resistance and get a meaningful value, because its voltage/current curve is nonlinear. More than that, it will vary depending on external conditions, such as backpressure and whether or not something's blocking the fan blades.

tl;dr: A fan isn't a resistor, so you can't measure it like one.

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  • \$\begingroup\$ Understood! Thanks, Nick. I'll use the amp readings I took while the fan was in series as a baseline for which panels to try. Thanks. \$\endgroup\$
    – Landon
    Commented Jul 16, 2015 at 18:19
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Put a 1 Ohm resistor in series with the fan. Turn it on. Measure the the voltage across the resistor, use Ohms law to calculate the current I (current) = V/R

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    \$\begingroup\$ This is unnecessary. The OP has already used an ammeter to measure the running current. This will not tell him anything else. \$\endgroup\$
    – DoxyLover
    Commented Jul 24, 2018 at 8:37

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