Serial connected fans?

Thomas-BP2 · Post by **Thomas-BP2** » Fri Sep 01, 2006 3:56 pm

Serial connected fans?

Thank you for a great site and a very interesting forum. I am in the process of planning my new rig, which I will build in a couple of months. Before I found your site I did not know anything about quieting techniques, but now I am getting ambitious! Basically, I always built my systems under the assumption that noise was a 'necessary evil', so instead of thinking about noise I always looked at getting the most bang for the bucks - literally, SCREECH! RATTLE! SWOOSH! SCREECH! RATTLE! BANG!

I am not fanatic about getting below 20dB total noise and it will actually be impossible to build a rig noisier than the Boeing 747 sitting under my desk at the moment. I have to work within a limited budget, so I don't want to spend money on a fan controller as that would mean buying some other component with more noise/lower performance as a trade-off.

Calling me 'a technical type' would be the exaggeration of the century, but I seem to remember something from my school days (almost two decades ago) about that serially connecting two identical devices on a DC line will make both devices run at half the voltage. As the motherboard I am looking at has a 'case fan socket' with three pins (ground, 12V, rpm sensor), I got the idea to connect two fans in serial and connect the rpm sensor wire to one of the fans (as they are of the same brand and model, logically, they should run at about the same speed). In that way I could run both fans at 6V without a fan controller and I could get the rpm feedback through the MB.

The two fans will be Scythe S-Flex SFF21F with the following manufacturer's specs:

1,600rpm @ 12V - 0.20A - 1.8W (63.7CFM)

I found the following test results at www.stillepc.dk:

12V: 1,620rpm @ 31dB(A) (manufacturer's specs: 28db(A))
9V: 1,215rpm @ 24dB(A)
7V: 922rpm @ 19dB(A)
6V: 750rpm @ 16 dB(A)
5V: 588 rpm @ 14dB(A)

They use some laser tachometer gadget for measuring rpm, so they also compare noise level at the same rpm (regardless of voltage). By coincidence, the SFF21F was measured to require 6V in order to produce 750rpm for the rpm based comparison, so that is why 6V gives such a 'neat' rpm number!

The two fans will have to fight quite a bit of back pressure as I need a better air filter than the thin foam thingy normally used, so I was looking for a quiet fan with high rpm when undervolted (I assume that means it has higher torque to fight the back pressure). I was hooked on the Yate Loon, but they are not represented in Hungary, so that is why I chose the Scythe SFF21F.

These are my questions:
1. Am I wrong about being able to run two serially connected fans at 6V?
2. Is there a reason why it would be a bad idea to use the motherboard connector for this (e.g. risk of damage to the MB)?
3. Do you think I would still be able to get rpm readings from one of the two fans?

Thanx, Thomas

kogi · Post by **kogi** » Fri Sep 01, 2006 5:11 pm

1. Am I wrong about being able to run two serially connected fans at 6V?

Your correct. two identical fans in serial will be 6v each

2. Is there a reason why it would be a bad idea to use the motherboard connector for this (e.g. risk of damage to the MB)?

I think that the MB fan headers have a max 1 amp. I

3. Do you think I would still be able to get rpm readings from one of the two fans?

If they are in serial I guess you will get rpm readings from the 2 fans combined

Post by **NeilBlanchard** » Fri Sep 01, 2006 6:13 pm

Welcome to SPCR, Thomas!

You might be better off running them at 5v (by swapping the pins around), or just get a Zalman FanMate2. And you would probably be fine with the 1200RPM model; rather than the 1600RPM.

Tibors · Post by **Tibors** » Fri Sep 01, 2006 8:06 pm

Easy 6V Fan Mod...DIY.
Now with tacho.

Thomas-BP2 · Post by **Thomas-BP2** » Sat Sep 02, 2006 1:47 am

Guys,

Thank you for the good answers (and the welcome).

If there is a 1 amp max on the board connector (I did not find any mention of this on the GigaByte website or in their manual), then I guess two fans rated for 0.2A @ 12V each will be fine. I think I remember, that 'if you half the voltage, you double the ampere', but that is for 'traditional' transformers. Would that also count when the fans are connected in series?

Would the two fans draw a total of 0.4A or a total of 0.8A?

In general, do you think it is safe to assume a 1 amp maximum for the MB fan headers or should I get in contact with GigaByte?

Tibors,

Thank you for the link, which is exactly what I was looking for. I guess I was not creative enough in my Internet search (e.g. "serial connect fan" and "connect fan series" give very different hits from the search engines!)

What is really comforting for me in your post is the "Then I soldered these ends together. (The first time I held a soldering iron in 20 years.)". I have not quite hit the 20 years, but I will buy my first soldering iron in a couple of weeks! 'My First Soldering Iron' sounds lika a Sony product, but I guess soldering irons are not targeted at kids

Tibors · Post by **Tibors** » Sat Sep 02, 2006 5:39 am

If you put two fans in series, they will draw less Amps then one of them (if the power source stays the same).

Two fans in series means twice the resistance. V=I*R -> I=V/R The voltage stays the same and the resistance doubles, so the current halves.

Bobfantastic · Post by **Bobfantastic** » Sat Sep 02, 2006 10:29 am

You should be wary about start-up though, there may not be enough current to get them spinning in the first place.
It sounds like a nice idea though, and would probably help with cablegami in a larger case.

Thomas-BP2 · Post by **Thomas-BP2** » Sat Sep 02, 2006 2:37 pm

In the test of the SFF21F they say that 3.7V is the lowest current where the fan will reliably start spinning and that, when it is spinning, 3.2V is the lowest current where it will continue spinning, so I guess I have plenty of 'safety margin'.

By the way, the MB that I am almost set on is the Gigabyte GA-M59SLI-S5, which is actually built for power computing, but with my recently acquired insight it will fit my purposes for a mid-power PC running (relatively) quietly. I am currently waiting for GigaByte tech support to tell me how big a performance drop I will get if I use the PCI-X #2 instead of PCI-X #1, which is where they recommend placing a single VGA card - if they say it is less than a 10-15% performance drop, then I guess I will go for the board. Here is the link to the GA-M59SLI-S5

I am happy that the serial fans part of my plan is solved, now I will go on to figure out my biggest challenge; building a ghetto HDD selector (that's why I will buy a soldering iron). I am a security freak, so currently I am using HDD rack drawers to physically separate my three primary HDDs (Work Drive, Test Drive, and Game/Multimedia Drive), but to avoid the rack drawers with their small aggressive fans in my next build I want the HDDs inside the case permanently and then being able to switch the power between the 3 primary HDDs with a rotary switch and have a flip switch to turn on/off one HDD for backups. I have already taken a huge leap forward, because now I know that I am looking for a 4-pole 3-way rotary switch, which I thought was better described as a turn knob gadget no more than two weeks ago