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Can two cooling fans be wired in series?

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dknguyen

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I have some 12V brushless cooling fans here (the kind to cool computers) and a 24V battery. I don't have enough money to dedicate a switching DC-DC converter just for the fans, but I lose a ton of power by hooking each fan in series with a resistor to step down the voltage.

I was thinking about connecting the fans in pairs to get 12V across each fan to reduce parts and conversion losses. But I am not entirely sure how a brushless fan works. I think they have built in electronics to commutate the brushless motor (that are so integrated I can't tell where they are) because I can just plug the motor into a DC source and it will run.

Will the electronics in the high-side fan function properly? The high-side fan's ground becomes the low-side fan's voltage input and I can't guess what will happen.
 
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Give it a try, I think that you can put 2 x 12 Volts dc brushless fans in series on a 24 V dc supply allright, provided both motors have the same current draw. Put a capacitor in parallel with each fan to stabilise its 12 Volts section across the motor.
Do a voltage check when motors are running and check for correct voltage shareing across each motor.
Bear in mind that the volume of cooling air is very dependant upon the speed of the motor, say ½ speed equates to 1/8 th air flow. as compared to normal speed.
 
Why don't you just use 24V fans?
 
People do this in computer cases to undervolt 12v fans to 6v and it works fine. I don't see a problem.
 
DirtyLude said:
People do this in computer cases to undervolt 12v fans to 6v and it works fine. I don't see a problem.

or running them off of 7 volts by connecting one lead to +12 and the other to +5.
 
24v

Hero999 said:
Why don't you just use 24V fans?

The website I bought the fans from only had the 12V version of this particular fan. I guess I assumed they were carrying the 24V version. I didn't notice it until a week after I bought them...so...yeah...hehe
 
dknguyen said:
The website I bought the fans from only had the 12V version of this particular fan. I guess I assumed they were carrying the 24V version. I didn't notice it until a week after I bought them...so...yeah...hehe

As long as the fans are identical there should be no problem running them in series.
 
philba said:
or running them off of 7 volts by connecting one lead to +12 and the other to +5.
Yes, that's the most popular way to undervolt, other than actually buying a controller.

There are people on the silent boards that want lower than 7v but higher than 5v. I know that's not all that much, but there are still many people that do it this way to get the fan speed they want without a controller.
 
Wiring them in series will work until the fans become unbalanced. If one fan decides to stall (due to bearing failure, electrical issue, or fingers), there's a good chance that both fans will shortly nuke themselves. A pair of beefy 14V zener diodes, one paralleling each fan would be the paranoid solution.
 
Fan

So, each fan should be in parallel with a zener diode and a cap for voltage protection and noise? I'm not sure how much noise matters to a brushless fan controller.
 
It's not the noise, the problem is that since they are in series, they will draw the same current, and split the voltage in half - until one of them stops. When that happens, one of those fans will start trying to draw more current - or equivalently decrease it's impedance, making the voltage divider not so equal - and probably exceeding one of the fans' voltage rating and releasing the magic smoke.

Worst case, this will happen when power is first applied, and one fan decides to wait a fraction of a second before starting. Zeners would limit the voltage to the fans to something reasonable, and allow it to startup properly.

Like I said - paranoid. It'll probably work without it, but I wouldn't want to put money on it , especially if there is a chance of something halting one of the fans for any reasonable (couple seconds) period of time.
 
Not a problem, I gotta make a tiny PCB anyways to plug in these tiny fan connectors. MIght as well add some zeners alongside.
 
I had no problem with serial wireing, even when one of them stalled, the other one didn't even change it's speed. I use them to cool my hi-fi amplifier, and I didn't want to use a 7812, so I serialised them and it works just fine. It's old P4 heatsinks and fans, 150mA.
 
according to the series connection pattern total voltage=voltage in motor1+ voltage in motor2 ,so one motor and especially the one closest to the supply will get all the adequate power while the other will get negligible voltage. Therefore use parellel connection where the total voltage=voltage in motor1=voltage in motor2 just like with street lights!
 
Things in series have exactly the same current. None are "closer" to the power source.
If one tries to draw more current than the other then it will have less voltage.
 
Resistance

ADELRESTA said:
according to the series connection pattern total voltage=voltage in motor1+ voltage in motor2 ,so one motor and especially the one closest to the supply will get all the adequate power while the other will get negligible voltage. Therefore use parellel connection where the total voltage=voltage in motor1=voltage in motor2 just like with street lights!

Doesn't that go against resistor theory? It's two identical fans that presumably have identical resistances
 
dknguyen said:
Doesn't that go against resistor theory? It's two identical fans that presumably have identical resistances
Connect a 1 ohm resistor in series with a 1M resistor and with a voltage source. Both resistors will have exactly the same current.
 
dknguyen said:
Doesn't that go against resistor theory? It's two identical fans that presumably have identical resistances

static yes, but dynamic?

imagine two identical fans in series and one expreriences a higher load it will have to draw more current to match the load BUT to do that the low fan would have to draw that current BUT it has no extra load to counter the new magnetic force that is developed so it speeds up, speeding up causes more voltage to be required thus taking voltage from the other fan
 
Yes..but..

audioguru said:
Connect a 1 ohm resistor in series with a 1M resistor and with a voltage source. Both resistors will have exactly the same current.

You forget that, they will have different voltages across them, and therefore consume different amounts of power.
P=VI.

EDIT:. Nvm Audioguru, we were looking at it the same way from two different perspectives, plus, I misread your first the quoted post. I'm thinking about the static case, while you are talking about the dynamic case.

Styx said:
static yes, but dynamic?

imagine two identical fans in series and one expreriences a higher load it will have to draw more current to match the load BUT to do that the low fan would have to draw that current BUT it has no extra load to counter the new magnetic force that is developed so it speeds up, speeding up causes more voltage to be required thus taking voltage
from the other fan

I see your point. What you are saying is that one fan can start to act like a current source for the other. I'm modelling the motors with fixed resistors but that's not quite right so yeah, I see.
 
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In series both fans of equal power rating will draw exactly the same current.
When conditions change, e.g. one fan stops the stopped fan may draw more current,and voltage drops across it. The running fan will run faster because it's voltage is increased.
Some fans may have tachogenerators on them to provide feed back to the control cct.

To be sure do tests first before making it a permanent arrangement.
From a 24 Volt DC supply, do tests on the two fans and measure voltage across, current through, under different conditions with one fan stalled, running slow etc. Check for heat built up in the fan motors. If results are satisfactory, connect it up. There is nothing to stop you to put a mA meter in series permanently, knowing the running current and keeping an eye on it once in a while.

I use two 110 Volts AC fans in series on 230 Volts AC on my old Philips TV for the last 14 years and they have run now 24182 hours with no problems.
About once or twice a year I vacuum out all the dust built up at the ventilation slots of the TV.
 
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