Basic electronics
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Posted by GT on August 28, 2008, 5:15 am
Please log in for more thread options 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 connector. 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? | |||||||||||||
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Posted by philo on August 28, 2008, 6:40 am
Please log in for more thread options > black wire from fan2 wired into 1 socket and plugged into a 12v power
and
> connector. > > I expected them to run at half speed (6v each). However, it didn't work > 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? > > 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? | |||||||||||||
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Posted by Paul on August 28, 2008, 6:57 am
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GT wrote: > 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 > connector. > > 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? > 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. Paul | |||||||||||||
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Posted by Paul on August 28, 2008, 11:22 pm
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Paul wrote: > GT wrote:
>> 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 connector. >> >> 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? >
> 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. > > Paul 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 value) 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 degree. Paul | |||||||||||||
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Posted by kony on August 28, 2008, 11:19 am
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On Thu, 28 Aug 2008 10:15:22 +0100, "GT" >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 >connector. 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. >
>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? 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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>
> 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