Connecting two tach signal wires in parallel

halfpower

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Mar 19, 2005
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I am planning to connect two fans in parallel. Specifically, two Nexus Real Silent 120mm fans to a single fan connector on an Asus A8V. I expect to have enough wattage to do this. I'd like to put the two tachometer wires in parallel to see how the board reports fan RPMs. However, I am unfamiliar with how the tachometer thing works. Has anyone tried this? Is it safe?

thanks,
halfpower
 

Buz2b

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Jun 2, 2001
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Two thoughts:
1) I wouldn't do it if you're talking about a fan header on the MB, which it seems you are. From the PSU, Ok.

2) Any rpm reading you got (if from the MB, while things were still ok) would be meaningless and/or bogus.

Look, it might be just fine, but why chance it? Those headers are designed for a single fan. They might have the power to run two but why do it when you can run one from the PSU? I know, no rpm reading, but you don't have to have that.

LOL! Now I know how you got your username! :laugh:
 

halfpower

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The motherboard I am using has three fan headers and only two are capable of changing the output voltage. The reason I am considering doing this is because I want to be able to throttle three fans. I'm not sure how the tach sensor works, but it is in the realm of possibility that this proposed setup will yield RPM readings that are a sum of the two fan RPMs.
LOL! Now I know how you got your username!
I especially like the user icon.
 

Buz2b

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Yeah, it's in the "realm of possibility" but unlikely in the realm of electronics. The signal will most likely be fubared and show (if anything) a totally whacked number. They are not built to take an "average" or "sum" of voltage/reading. The truth is though, you are playing with fire to do this. You could easily blow out the fan header on the MB. If that happens, you'll be lucky if that is all that goes wrong. Am I sure?? No! Just call it a good guess. Those are single fan headers; not multiple fan headers.
If you just want to throttle down a fan, either get an adjustable one like this, or get a controller like one of these. You and your MB will be much happier.
 

L00PY

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Sep 14, 2001
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Originally posted by: halfpower
The motherboard I am using has three fan headers and only two are capable of changing the output voltage. The reason I am considering doing this is because I want to be able to throttle three fans.
I'd suggest just disconnecting one of the tachs and joining them with a standard Y-connector. Some splitters will come wired this way to being with. Since they're the same fan their speeds probably won't differ by much. Just pretend they're going at the same speed. You don't need to know the exact speed of the second fan -- you just need to be able to control its speed via the mobo header. This of course assumes your mobo header can handle the load.
 

L00PY

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Originally posted by: Buz2b
This of course assumes your mobo header can handle the load.
Therein lies the problem; assuming, and electricity, just don't mix. :p
Sure they do. I mean unless you have some hard numbers, you're automatically making some assumptions that the header will handle any fan. Also, he said he was using "Silent" fans which tend to be low draw. Even in the larger size, I wouldn't be surprised if a pair of them fell right near the close to that often undocumented max draw allowable for mobo headers.
 

Buz2b

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Originally posted by: L00PY
Originally posted by: Buz2b
This of course assumes your mobo header can handle the load.
Therein lies the problem; assuming, and electricity, just don't mix. :p
Sure they do. I mean unless you have some hard numbers, you're automatically making some assumptions that the header will handle any fan. Also, he said he was using "Silent" fans which tend to be low draw. Even in the larger size, I wouldn't be surprised if a pair of them fell right near the close to that often undocumented max draw allowable for mobo headers.

I think we're saying the same thing; don't do it. I'd never try it on any MB I spent my hard earned cash for. The assumption that it might work could spell the end of, at minimum, the fan header.
 

halfpower

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Mar 19, 2005
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My motherboard manual says
6. CPU, Power and Chassis Fan Connectors
(3-pin CPU_FAN, PWR_FAN, CHA_FAN)

The fan connectors support cooling fans of 350mA-740mA(8.88W max.) or a total of 1A~2.22A(26.64W max.) at +12V.
The fan maker lists the fans specs as
Rated Voltage 12 Volts
Voltage Range 6.5 ~ 13.8 Volts
Input current 0.30 Amp Max
Input power 1.8 Watts Max
But because

13.8V * 0.30A = 4.4W > 1.8W = Fan's Max Input Power

I'm a little confused. Maybe the discrepancy in the fan specs has to do with impedance or something.

I suppose either way it is still under the motherboards limits.
 

halfpower

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Actually I'm not sure now. Max input power might different than max current draw. Maybe my motherboard would fry the fan motor.
 

Buz2b

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Originally posted by: halfpower
Actually I'm not sure now. Max input power might different than max current draw. Maybe my motherboard would fry the fan motor.

That won't happen. But again, just get an manually adjustable fan or a fan controller. It's just not worth taking chances with the heart of your system. Besides, even if it doesn't blow out the fan header, you will not get any sort of usable reading and/or meaningful throttle down/up of the fans using two on one header. For that reason alone you should just ditch the idea and go with a controller. And, the controller gives you far more fan control than the MB. That's a win-win situation.
 

L00PY

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I'm in disagreement with Buzz2b. The specs strongly suggest you'll be fine using two of those fans off a mobo header. Just run a single tach wire to the mobo header.

Running off the mobo header lets you automatically adjust fan speeds depending on temperature. For example, when the internal case sensors in my rig report temps under 30C, it automatically ramps down the fan speed to a minimum of 40% (the minimum speed required to keep the fans turning). When temps go higher, they rise up to 86% (the maximum noise I'm comfortable with). If for some reason I want them running at 100%, I'll open up the program and set them to run at 100%.

I have no idea what RPMs the fans are running at (they're 80mm Panaflo L1A's without RPM tach), but that doesn't matter to me. All that matters is the temperature, noise, and automatic adjustment.
 

Buz2b

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Originally posted by: L00PY
I'm in disagreement with Buzz2b. The specs strongly suggest you'll be fine using two of those fans off a mobo header. Just run a single tach wire to the mobo header.
If that is the case, then why don't MB manufacturers spec out the fans (plural) that can be used on one header. And why aren't there MB header fan splitters for this purpose, as common as there are with ones for the PSU. "Strongly suggest" is the same as saying "might". Yeah, it "might" work. But yeah, it "might" blow out a header on the MB. Silly thing to try when there are multiple other remedies and/or methods to accomplish the same thing with no worries about MB.
Running off the mobo header lets you automatically adjust fan speeds depending on temperature. For example, when the internal case sensors in my rig report temps under 30C, it automatically ramps down the fan speed to a minimum of 40% (the minimum speed required to keep the fans turning). When temps go higher, they rise up to 86% (the maximum noise I'm comfortable with). If for some reason I want them running at 100%, I'll open up the program and set them to run at 100%.
And........? This much is already known. The same thing can be manually adjusted via a simple fan controller. Temps can be monitored with any number of free apps out there. In fact the controllers will give you infinitely more "control" of the fan speeds than the pre-programmed MB settings.

I have no idea what RPMs the fans are running at (they're 80mm Panaflo L1A's without RPM tach), but that doesn't matter to me. All that matters is the temperature, noise, and automatic adjustment.
In this case you would want to add MB survival. Look, I'm not saying that without a doubt, the MB would be damaged. It might well survive. However, this is a silly damned thing to try when there are other, safer ways to do the same thing. Have you ever read a thread titled, "Look at the two/three panaflo's I have hooked up to one of my MB headers? No? You ever wonder why? /Rant. Even if he did get them to "run", running two tach wires to the single header will only provide garbage as a readout; thus eliminating the one thing he wants to do, gain control over the fan speeds. Running one tach wire as you seem to suggest might have a better chance of working but it only partially solves the dilemma. Ramping the speed up and down of two fans at once on one header will effectively double the draw. Unless they are both some mighty low speed fans, with small requirements, this is a recipe for trouble and not what the MB was designed to do. :)

 

halfpower

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Mar 19, 2005
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The thing is that those are Panaflos. A 120mm medium speed Panaflo will move 85 CFM. A 120mm high speed Panaflo will move 115 CFM. My fans are 120 Nexus Real Silent fans and can only move 37 CFM.
 

Buz2b

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Originally posted by: halfpower
The thing is that those are Panaflos. A 120mm medium speed Panaflo will move 85 CFM. A 120mm high speed Panaflo will move 115 CFM. My fans are 120 Nexus Real Silent fans and can only move 37 CFM.

The best thing to do would be to get one of these. Not much more than the cost of a decent fan.
 

L00PY

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Sep 14, 2001
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Originally posted by: Buz2b
Originally posted by: L00PY
I'm in disagreement with Buzz2b. The specs strongly suggest you'll be fine using two of those fans off a mobo header. Just run a single tach wire to the mobo header.
If that is the case, then why don't MB manufacturers spec out the fans (plural) that can be used on one header. And why aren't there MB header fan splitters for this purpose, as common as there are with ones for the PSU. "Strongly suggest" is the same as saying "might". Yeah, it "might" work. But yeah, it "might" blow out a header on the MB. Silly thing to try when there are multiple other remedies and/or methods to accomplish the same thing with no worries about MB.
Don't be silly. MB manufacturers can't possibly list all possible combinations of fans. They rarely give actual header information. When they list max amp draw, that's the max amp draw. If you'd like to explain how two fans combined drawing less than 1 Amp will blow out a header, when one fan drawing more than 1 Amp won't, I'd love to hear it.


Running off the mobo header lets you automatically adjust fan speeds depending on temperature.
And........? This much is already known. The same thing can be manually adjusted via a simple fan controller.
Manual does not equal automatic. If I'm out of the room and mobo temperature readings go up, fans speeds will automatically increase in my rig. How can a manually adjustable controller do that? If I'm watching a TV in fullscreen mode or in the middle of an intense fullscreen gaming session and the temperature rises, fan speeds will automatically increase in my rig. With a manual controller, you'll have to 1) notice that temps increased, and 2) turn the knob. Do you always insist upon buying something extra to do something that's already built into the mobo?


I have no idea what RPMs the fans are running at (they're 80mm Panaflo L1A's without RPM tach), but that doesn't matter to me. All that matters is the temperature, noise, and automatic adjustment.
In this case you would want to add MB survival. Look, I'm not saying that without a doubt, the MB would be damaged. It might well survive. However, this is a silly damned thing to try when there are other, safer ways to do the same thing. Have you ever read a thread titled, "Look at the two/three panaflo's I have hooked up to one of my MB headers? No? You ever wonder why?. Even if he did get them to "run", running two tach wires to the single header will only provide garbage as a readout; thus eliminating the one thing he wants to do, gain control over the fan speeds.
Tach wires don't control speeds, they just report them. If he wants to gain control over fan speeds, he needs to control the voltage powering the fans. To answer one of his original questions, all the tach wire does is carry a voltage blip when the fan makes one rotation. The mobo counts the blips and reports that number. (Of course there are different fans, different multipliers, and different divisors to those blips, but that's the gist of it.) It's possible that you might even be able to pass both tachs to the mobo header, double the divisor, and have an average fan speed reported. This gains you very little when using the same model fan though and probably shouldn't be done.

Again, please explain how using a properly wired splitter to power two 0.3A fans will draw more than .74A his headers can support? It should do exactly what he's looking to do.

I'm as certain that that setup won't blow a header as you must be that adding an additional piece of hardware won't introduce a short, will be properly wired and grounded, will mount in his rig, and will be able to support any fans he throws at it.

 

Buz2b

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Jun 2, 2001
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Originally posted by: L00PY
[Double post]

Yeah, it happens to all of us sometimes. ;)

Look, we can go round and round about this, over and over. I didn't say it would never work; I just strongly recommend against it. Period, end of story. I have my own time and experience to form the basis for that recommendation. It's just not a good idea. The splitters you link to are three wire, which would suggest sharing the tach wire. That will not give you the kind of control he seeks, as it will report garbage because of the two fans on one lead. Be that as it may, if you have to worry every time you leave your system unattended that it might overheat somehow, there are lots of other things you need to worry about than putting two fans on one header. :shocked: Personally, when I build systems, I make sure there is either adequate airflow or use liquid cooling so that whether someone plays "intense games" or watches full screen TV, they shouldn't have to worry about temps. There are also free utilities that you can use to provide temperature alarms on your system, no matter what you are doing.
As to buying something "extra" that the MB can do, let's look at it this way: I can spend $5.99 or $2.99. For $3 more, I can control twice as many fans with one device and monitor temps (with alarms) with a free utility. OR, I can save the three bucks, have it "maybe" work like I want it to and possibly cause damage to the MB. To me, that's a no brainer every time. Spend the $3 halfpower; don't "half" do things. ;) :beer:
 

halfpower

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Mar 19, 2005
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I dunno. If I cut one tach wire(maybe run it over to an unused fan header) and power two fans from a single header, I will be fine, IF the fan and motherboard specs mean what I think they mean.

As far as fan controllers go, I need one that use almost zero system resources (neglecting power). I ran SpeedFan today while trying to use an E-mu 1212M recording card in ASIO mode, and the audio system became totally unstable. Even Firefox can destabilize this card when low latency monitoring is used. I also need a fan controller that won't let my system cook accidently put the computer is some loop, or very long and drawn out process, and then decide to go visit another state. The big issue though, is being able to stop the noise when I want silence.
 

Buz2b

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Jun 2, 2001
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Originally posted by: halfpower
I dunno. If I cut one tach wire(maybe run it over to an unused fan header) and power two fans from a single header, I will be fine, IF the fan and motherboard specs mean what I think they mean.

As far as fan controllers go, I need one that use almost zero system resources (neglecting power). I ran SpeedFan today while trying to use an E-mu 1212M recording card in ASIO mode, and the audio system became totally unstable. Even Firefox can destabilize this card when low latency monitoring is used. I also need a fan controller that won't let my system cook accidently put the computer is some loop, or very long and drawn out process, and then decide to go visit another state. The big issue though, is being able to stop the noise when I want silence.

Sorry. I was going to leave my last post as THE last for me (after a while you just give up trying) but I had to pop back in on this one.
Halfpower, if you've got system resouce and/or power problems that bad, you don't need more fans. You need to solve the other problems first. What kind of overhead are you running there? What size/brand of PSU?
If I cut one tach wire(maybe run it over to an unused fan header) and power two fans from a single header, I will be fine, IF the fan and motherboard specs mean what I think they mean.
Uhhh, why not just use that "unused" fan header for the second fan? You can certainly splice/extend the wiring a few inches with no problems, if you need it.
 

L00PY

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Sep 14, 2001
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Originally posted by: Buz2b
The splitters you link to are three wire, which would suggest sharing the tach wire. That will not give you the kind of control he seeks, as it will report garbage because of the two fans on one lead.
Take a closer look at the splitter I linked. One of the leads has three wires, the other only has two. The one with the yellow wire will report a tach reading. The one without will report nothing. Remember, control via the header doesn't require a tach wire at all -- it's typically just PWM from a chip on the mobo along the 12V wire. You can have control with a tach wire; you can have it without a tach wire; you can have it with a locked rotor wire; you can have it with garbage on your tach wire. When you tell the header to run at 50%, it runs at 50% by cutting power to the header half the time -- it never looks at the tach wire.

I agree that in most cases you don't want to run two fans off a header. You also should shy away from using 120mm fans or even some standard case fans off a header. But once you know the specifications of the fan and the header, there's a lot less to worry about. I'm guessing you can't change voltages on the PWR_FAN header, but that's almost to be expected. You don't want to be able to turn off the PSU fan, just monitor RPMs. Running the extra tach wire over there should work fine -- it'll be no different running a RPM wire of a 3 to 4 pin cable.

Anyways, it sounds like halfpower has a system that's being used to process audio and maybe do some DSP. It's a high end card he's got in there and I can see the system needing to be silent. The PCI card may be a bandwidth hog or the software may touchy. I'd look into drivers / software before looking at the PSU. (Not everyone's been stuck with a K7S5A Buz2b.) If you're looking to silence your pc, I'd suggest going over to silentpcreview. I suspect you'll hear a recomendation for the mcubed t-balancer. I haven't had any experience with them myself though.
 

Buz2b

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Originally posted by: L00PY
Originally posted by: Buz2b
The splitters you link to are three wire, which would suggest sharing the tach wire. That will not give you the kind of control he seeks, as it will report garbage because of the two fans on one lead.
Take a closer look at the splitter I linked. One of the leads has three wires, the other only has two.
You're right, I didn't see that one was a 2 wire.

I agree that in most cases you don't want to run two fans off a header. You also should shy away from using 120mm fans or even some standard case fans off a header.
PROGRESS!!
But once you know the specifications of the fan and the header, there's a lot less to worry about. I'm guessing you can't change voltages on the PWR_FAN header, but that's almost to be expected. You don't want to be able to turn off the PSU fan, just monitor RPMs. Running the extra tach wire over there should work fine -- it'll be no different running a RPM wire of a 3 to 4 pin cable.
Oh well, that didn't last long! ;)

Anyways, it sounds like halfpower has a system that's being used to process audio and maybe do some DSP. It's a high end card he's got in there and I can see the system needing to be silent. The PCI card may be a bandwidth hog or the software may touchy. I'd look into drivers / software before looking at the PSU.
I made two points; what kind of overhead (like the card and other items that could possibly cause resource troubles) AND the PSU; not just the PSU. It's always good to get as much info as possible, in my book.
(Not everyone's been stuck with a K7S5A Buz2b.)
LOL! :) Neither am I but I sure built a lot of systems with them. K7S5A aside, the PSU is one place I've found most people go "cheap" on and then scratch their heads when "for some reason", they can't get their system stable running all their hardware.
If you're looking to silence your pc, I'd suggest going over to silentpcreview. I suspect you'll hear a recomendation for the mcubed t-balancer. I haven't had any experience with them myself though.
Same here. Going back to the fan controller (don't I always?) wouldn't that be easier to "dial in" when he wanted that extra bit of silence, as opposed to leaving it up to the MB controllers (if what you suggest even works)? He could run the system nice and frosty and then manually turn things down exactly when and how he wants to and start the processing/recordings. He could also target the cooling exactly where he wants it, when he wants it.