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Author Topic: Powering 3 computer fans in series: options?  (Read 4191 times)
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Hello,

I have three small 5v computer fans I want the arduino to turn on at the same time when humidity drops below a predetermined point.

If wired in series, how do you recommend the arduino control them?  A SSR plus external power supply, a transistor, or just run them all off the arduino board directly.  I have a 9v 1mA power supply running the arduino now.

Thank you!
Jacob
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If you already have an ssr that will run them (with a suitable 5 VDC power supply), that is probably the easiest choice. The next would be to use one power transistor or mosfet (or 3 small transistors, one for each fan, hooked in parallel to a single i/o line on the arduino), plus the external supply. You probably can't run them off the Arduino's regulator as they likely pull more current together than the regulator can supply (iirc, its a 1 amp regulator on the standard Arduino).
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Thanks.  Does the arduino have enough juice to run a 5VDC SSR?
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Thanks.  Does the arduino have enough juice to run a 5VDC SSR?

The current requirements to switch an SSR by the Arduino is usually much less than what the pins can provide, so you should probably be safe; you typically don't even need a current limiting resistor from the pin to the SSR. With that said, however, look at the datasheet for your SSR to be sure.
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I have a 9v 1mA power supply running the arduino now.

1mA? That's a pretty pathetic power supply. Hopefully, there's a typo in there somewhere.
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Yes there is.  Meant 1000mA.
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What's 3 0s among friends...
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Will this little guy be all I need?

http://www.sparkfun.com/commerce/product_info.php?products_id=524

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Wiring them in series is not the way to do it - if they take different currents one of them will see most of the voltage and they'll run at different speeds.  Maybe not at first, but after the bearings wear this will eventually happen.  Wire them in parallel and run off 5V with a beefy enough rating (not the on-board Arduino, a 7805 or similar).  A bipolar transistor or logic-level MOSFET to switch them will be fine.  These devices have their own controller so free-wheeling diodes shouldn't be needed AFAIK.
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One of my fans is 12 v so I'll be going with three 12 v fans.  One is 0.08A and two are 0.18A.  The power supply for them is an unregulated 9v.  With this setup, I don't need any transistors or resistors, right?  Also, since all fans are 12, does parallel vs. series matter?

I just need to know what is required for the arduino to control the SSR which will turn the fans on and off.

Thanks!
« Last Edit: May 06, 2010, 11:31:10 am by .jacob » Logged

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Running 12V fans on 9V is not a terribly good idea.

Richard, most 12 VDC PC fans will run at 7 VDC, so 9 VDC shouldn't be a problem. Its done all the time by PC case-modders to allow the ability to have a quieter system (typically for media center boxes). They get the 7 volts from the power supply by using the 12 VDC rail with the 5 VDC rail as "ground".

While it may not be orthodox, I've never read anything about problems or fans dying; typically on these cheap fans the bearings die long before anything else. Regardless, they are cheap PC fans - nearly a dime-a-dozen...

 smiley
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At worst it is possible that it would reduce their life expectancy because they are struggling on insufficient power.

In fact running a fan at below its rated voltage actually increases it's life time not reduces it. On set top boxes with hard disc PVRs the fans are run at a lower voltage whenever possible to increase the lifetime. The 12V fans in my current projects (at work) will run down to 3V and we can slow the fan down if the temperature excess doesn't warrant it. This is endorsed by the fan manufacturer for increasing the reliability.

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typically on these cheap fans the bearings die long before anything else
Even the more expensive ones we are using with magnetic levitation bearings fail with thee bearings.
« Last Edit: May 06, 2010, 01:05:15 pm by Grumpy_Mike » Logged

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Well they fail with bearing problems because there is nothing else that can realistically fail with a brushless motor!
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Not true, there are components inside the fan that control the current flow. The manufacturers Sunon are in the process of introducing a more reliable fan. The improved MTBF figure is based on the fact that there are less components in the current steering circuit and so it is more reliable. Tests so far prove this to be true. We have been running the fans at elevated temperatures for about a year and a half.
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OK, I will wire the 3 fans in parallel.  That is no problem.

The same power source I have the fans is powering the arduino.  Currently I'm using a splitter to direct power the arduino's barrel plug and the fan.  I just need a way for the arduino to control the fans.   Since I'm splitting the unregulated 9v (at 0 pull it's outputting 12.5 v) power supply and the arduino only outputs 5v, I believe the easiest way would be to have arduino throw a switch to control the fans.  Some have said that a SSR is overkill for this application because the fans are DC and low voltage.  Would a transistor be a better option?  Is there a write-up explaining how they should be wired and what code to use?

Thank you,
Jacob
« Last Edit: May 06, 2010, 02:43:29 pm by .jacob » Logged

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