Basic electronics

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OK I'm missing something here...

I have wired two case fans together in series:
Black wire from fan 1 connected to red wire on fan 2. Red wire from fan1 and
black wire from fan2 wired into 1 socket and plugged into a 12v power

I expected them to run at half speed (6v each). However, it didn't work and
they both run at full speed. Clearly my basic school physics is rusty, but I
thought things in series would 'share' the voltage, just as batteries in
series combine the voltages. Where did I go wrong?

Re: Basic electronics

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I also would have expected the fans to run at reduced speed.
(Actually closer to 1/4 th the speed with half the voltage)

Try running a single fan at 12 volts...maybe they just seem to be running at
normal speed?

Re: Basic electronics

GT wrote:
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The part you're missing, is the impedance of a motor is not a constant.
A normal resistor on the other hand, is a fixed value.

If you account for the power in the circuit, one fan draws P, and
two fans in series (based on your observation they are at full speed),
are drawing 2P. Since V is fixed, the current through the series
circuit must be 2I. V * 2I = 2P. All the power is accounted for.

The impedance of each motor has magically cut itself in half.

If the fans did not achieve full speed, then not as much additional
current would be needed to account for the power used by the fans.
And the degree of impedance change would not be as great.

In any case, the impedance of the fan is a function of its operating
conditions, with a stalled motor representing the lowest impedance
condition. (Barring any funny behaviors caused by the commutation
function used in brushless motors.)

If you put enough fans in series, it is just possible that
one won't spin, and several of the others will do well. The
stalled fan would have the lowest impedance of the lot. I'd
try the experiment here, but I don't have enough identical fans.


Re: Basic electronics

Paul wrote:
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I tried my own experiment here, with a couple Sunon KDE1208PTB1-6
12V 2.6 watt fans. A 12V supply was used (my computer). I tried
series connecting the fans, then tried Fan #2 by itself. This is
what I got.

Test_Case            Fan #1   Fan #2    Current

Series Connected    2300RPM  1663RPM   0.11 amps
Single Fan            ---    3200RPM   0.16 amps (1.92W, less than the rated

So no violation of common sense was needed. The fans in my case, did
not run at full speed. Neither did the current do anything crazy.
Perhaps if you know the appropriate equation for power needed versus
fan speed, you can check whether the input power and output power
(air movement) agree or not.

The fan speed was measured with a home made optical tachometer. When the
tachometer is pointed at an incandescent light bulb, the readout said
"120" (and we're on 60Hz electricity here). I expect a light bulb uses
both "bumps" of the sine wave power, and that is why a 120Hz component
was detected.

The fans have seven blades, so I take the readout off the meter, divide
by the number of blades (seven), then multiply by 60 to get RPM. The
tachometer operates in transmission mode - a LED flashlight is held on
one side of the fan, and the phototransistor is held on the other
side, to get the chopped signal.

So the series connection did reduce the current flow. My fans didn't
run at full speed for me, and the current flow reflects that to some


Re: Basic electronics

On Thu, 28 Aug 2008 10:15:22 +0100, "GT"

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You can't do that, because these are not purely resistive
circuits, they're switched based on rotor position because
they are brushless DC fans.

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You're lucky they run at all.  The most optimal way to
reduce fan speed is to use current limiting per each fan.
Some will say, just reduce voltage, and that too will work,
but only up to a point at which there is pulsating because
of the surge current every time a coil is energized.

Basically, you have to treat each fan as an individual
circuit, not a series resistance circuit.  Parallel the fans
and put an appropriate current limiter on each to achieve
the desired current (vs: RPM).  Generally speaking the
ballpark for inducing the desired current drop would be a 2W
series resistor on each in a value of roughly 33 to 120
Ohms, depending on the fans' specs. A random average would
be to use a 68 Ohm resistor, being too larger a value for
some (higher RPM fans) and too small a value for other lower
RPM fans.

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