Nexus Fans at 5V or 7V?

xenithon · Post by **xenithon** » Wed Jul 20, 2005 1:26 am

Hi All. I have a pair of Nexus 120mm case fans. I was thinking about running them slower but have two concerns.

Firstly to run them at 5V, I would use the multiconnector cable which came with my VF700 graphics card cooler to run one of the fans at 5V. My concern is that I have read that the Nexus doesn't start at 5V. Is this true?

Secondly I thought of running them at 7V by either modding the cable myself or getting one of those 12V-->7V cables. However I have read a bit about problems with PSU's, something to do with feedback while running at 7V. Does anyone know where I can get more info? I have tried searching but the search doesn't recognize short words like "7V"

Cheers,
Xa

RachelG · Post by **RachelG** » Wed Jul 20, 2005 1:46 am

I can only give you my experience of my Nexus plugged into the "fan only" connector on the Sonata PSU (according to the SPCR review, this supplies 5V). The fan only starts up 2/3 to 3/4 of the time. I now automatically check the fan whenever I turn the computer on, and give it a quick whirl to get it going if I need to. It's lovely and quiet at this speed though

Michael Sandstrom · Post by **Michael Sandstrom** » Wed Jul 20, 2005 3:29 am

My 120mm Nexus always starts on minimum fanmate voltage which I believe is 5 volts.

Tephras · Post by **Tephras** » Wed Jul 20, 2005 7:37 am

xenithon wrote:Secondly I thought of running them at 7V by either modding the cable myself or getting one of those 12V-->7V cables. However I have read a bit about problems with PSU's, something to do with feedback while running at 7V. Does anyone know where I can get more info? I have tried searching but the search doesn't recognize short words like "7V"

Cheers,
Xa

Read the caution note in this article for info.

xenithon · Post by **xenithon** » Wed Jul 20, 2005 9:23 am

Yup, read that as well as some other articles about it. I do not think I will risk it, even though most places say that in the real world 7V-ing fans works fine. I think I may just use ANOTHER fanmate in the system

because 5V won't be enough to kickstart the fans.

X

mathias · Post by **mathias** » Wed Jul 20, 2005 1:08 pm

Since nexus fans are so popular, it should be known what kind of resistor could be used with each one. A fan mate is probably better, but resistors can be more convenient to get. (BTW, a 33ohm one brings my 80mm nexus down to 1250rpm)

Orbit · Post by **Orbit** » Fri Jul 22, 2005 8:02 am

Dont quote me on this, but I believe a few months ago when I was reading some of the fan control articles on the site, it was suggested to use Zener diodes to drop the voltage of ur fans. They mentioned that with those diodes, it will still provide the full 12V when starting up, then drop it down. So your fans will always startup, and still run slower & quieter once they've started

Could be just what you're after, piece of mind. Have a look through the articles

alleycat · Post by **alleycat** » Fri Jul 22, 2005 10:36 am

I use 1N400x series rectifier diodes (use whatever is readily available, they are all specified way above this application) which drop the voltage by about 0.75V per diode. They are cheaper than Zener diodes and easier to find. I don't have a 120mm Nexus so I can't tell you how many to use, but at about AUD 0.10 apiece it's easy to experiment. I couldn't imagine you needing more than about 10. Don't worry about soldering, just twist them together. Remember that the end with the line on it goes towards negative.

wim · Post by **wim** » Fri Jul 22, 2005 12:44 pm

3 nexii here (thosee 120mm orange ones) none of which start at 5V
am surprised Michael Sandstrom's always starts, i thought this product line was reputed to have very tight variances?

jamesm · Post by **jamesm** » Fri Jul 22, 2005 1:42 pm

my 120 mm nexus real silent case fan starts at 5V always.

I've gotten it to start at 4V even (fanmate at highest setting on the 5V rail)

xenithon · Post by **xenithon** » Sun Jul 24, 2005 6:46 am

Does anyone know how these 120mm Nexus fans would fare being attached to the Antec PSU's "fan only" connectors?

X

Post by **Ralf Hutter** » Mon Jul 25, 2005 5:56 am

xenithon wrote:Does anyone know how these 120mm Nexus fans would fare being attached to the Antec PSU's "fan only" connectors?

X

Probably won't fare too well. All of the "Fan-only" connectors that I've measured start off around 4.7-5V and don't start to ramp up until the PSU is under a lot of load.

Bluefront · Post by **Bluefront** » Mon Jul 25, 2005 6:06 am

Look.....save yourself a bunch of trouble. Wire the two fans in series.....then you'll be running at 6V, which should start without problems.

PSU 12v> fan1 red wire> fan1 black wire> fan2 red wire> fan2 black wire> PSU ground= fans running at 6V. The fans have to be exactly the same for this to work correctly.

mathias · Post by **mathias** » Mon Jul 25, 2005 8:41 am

Ralf Hutter wrote: Probably won't fare too well. All of the "Fan-only" connectors that I've measured start off around 4.7-5V and don't start to ramp up until the PSU is under a lot of load.

Do you know how those work? I'm wondering if it would be possible to connect a fan to a fan only connector and to a 12v line through a ~200-300 ohm resistor, without any possibility of it sending current backwards through the fan only connector.

Post by **Ralf Hutter** » Tue Jul 26, 2005 5:04 am

mathias wrote:
Ralf Hutter wrote: Probably won't fare too well. All of the "Fan-only" connectors that I've measured start off around 4.7-5V and don't start to ramp up until the PSU is under a lot of load.
Do you know how those work?

Nope, absolutely no clue. Sorry.

E M F · Post by **E M F** » Tue Jul 26, 2005 9:19 am

Bluefront wrote:Look.....save yourself a bunch of trouble. Wire the two fans in series.....then you'll be running at 6V, which should start without problems.

PSU 12v> fan1 red wire> fan1 black wire> fan2 red wire> fan2 black wire> PSU ground= fans running at 6V. The fans have to be exactly the same for this to work correctly.

::Frowns:: Maybe I'm mis-remembering my electrical theory, but that doesn't sound right. You've put the resistances in line with one another, but that doesn't drop the voltage on the first fan... does it? ((I'll have to look this up when I get home.))

jamesm · Post by **jamesm** » Tue Jul 26, 2005 10:32 am

^No, that would work.

ForumCkn · Post by **ForumCkn** » Tue Jul 26, 2005 10:35 am

Well... according to KVL (Kirchhoff's voltage law) the voltage around a circuit loop must be equal to zero. (assuming a fan behaves like a resistor)

So if you have two fans in a loop connected to a single power supply of 12V then 12V will need to be spread across the two fans. If we assume there was no voltage drop across the first fan then the voltage across the second fan would be 0V, but then we know that second fan is running so there has to be voltage across the first fan and a voltage drop across the first fan.

Since the fans the same they roughly should have the same resistance/impedance and will divide the voltage evenly across them. The circuit would end up looking something like this:

+12V -- <---- 12 Volts
|
# Fan 1
| <----6 Volts
|
# Fan 2
|
Gnd---- <----0 Volts
Edit: it would appear my ASCII diagram isn't board friendly

The voltage across both fans is 6V
(12V-6V=6V for the first fan and 6V-0V for the second)

Bluefront · Post by **Bluefront** » Tue Jul 26, 2005 11:30 am

I've done this 6V trick a bunch of times. Believe me...it works perfectly. You do have to use the same kind of fan.

If you try it with different fans......one will not run, or will run much slower than the other. But with the same fans, it'll work for sure.

Aleksi · Post by **Aleksi** » Tue Jul 26, 2005 11:35 am

Hmm... doesn't electricity and current go the other way around as you're on the other side of the planet?

But seriously, I just wanted to point out, that running your fans on 5V may not be a good option, especially if the fan just barely starts spinning. With time all ball and sleeve bearings fans have their startup voltage drift, so one day you may notice they don't start at the same voltage they used to start at.

Just a reminder if you're placing 5V running fans inside PSUs.

Badger · Post by **Badger** » Tue Jul 26, 2005 1:18 pm

Aleksi wrote:Hmm... doesn't electricity and current go the other way around as you're on the other side of the planet?

But seriously, I just wanted to point out, that running your fans on 5V may not be a good option, especially if the fan just barely starts spinning. With time all ball and sleeve bearings fans have their startup voltage drift, so one day you may notice they don't start at the same voltage they used to start at.

Just a reminder if you're placing 5V running fans inside PSUs.

But sleeve bearings last longer at a lower voltage

~El~Jefe~ · Post by **~El~Jefe~** » Wed Jul 27, 2005 2:43 pm

at 12 volts this thing is incredibly quiet

cant see 5 volts being a reality unless theres something odd about the case that is just noisy.

frankgehry · Post by **frankgehry** » Wed Jul 27, 2005 3:45 pm

I don't think I would use the 7volt trick, but I'm not sure why other than some users have expressed concern about using it. It is described here as well as how to use diodes. Some of the circuits have a kickstart feature that starts a fan at 12v and after a second or two the voltage drops to a lower setting.

http://www.cpemma.co.uk

E M F · Post by **E M F** » Thu Jul 28, 2005 9:43 am

My memory was faulty, and ForumCKM is correct. Yes, the first fan does actually get a 12V in, but provided the resistance is perfectly even between the two fans, it will only "see" half that voltage. (+12V in, +6V out = +6V used.)

Just a quick word of warning, though... if the first fan's resistance increases drastically, then the voltage that will reach the second fan will be lowered equally as drastically... perhaps even causing it to stop.

Use at your own caution. (IE, solder yourself up a set of leads that will put the monitoring lines into different inputs.)

xenithon · Post by **xenithon** » Thu Jul 28, 2005 9:56 am

Hey guys. I remembered that my Zalman VF700 graphics cooler has that multi-connector cable with 2x12V and 2x5V connectors. One 5V is used for the Zalman itself of course, and I use one 12V for the nexus fan as it makes cable management easier. As an experiment I connected the Nexus to the 5V and it started up without problems!

I only checked it twice, so perhaps they were lucky attempts? I have two questions really:

1. although they started without hassles this time, could they fail to start on other occassions (at random)?
2. I stopped the fan while it was running using a straw, then moved the straw away, and the fan didn't move! I then gave it a quick tap on the blades and it started running again. Is there anything wrong with this which maybe harms the fan? What I am thinking is to keep it connected to the 5V connector, and if it fails to start, just give it a small tap. Bear in mind that my PC is on almost 24/7 so it's only for major restarts, power failures etc. that the fan actually switches off.

X

mathias · Post by **mathias** » Thu Jul 28, 2005 10:16 am

xenithon wrote: 1. although they started without hassles this time, could they fail to start on other occassions (at random)?

Only a few ways to find out, try it a few dozen times or so at 5v, or try it a few times a slightly lower voltage, ~4.6V(5v volts with a small resistor in line should also work). Or try it at 5v with a heavy load of dust or something like that on it.

Bluefront · Post by **Bluefront** » Thu Jul 28, 2005 10:17 am

I would suggest checking fans a number of times under different conditions, to be certain of a reliable 5V start. You just never know at that low voltage. Few fans are designed to start at 5V.

If you stop the fan with a straw, and it won't restart at 5V.......don't count on it starting at 5V at all, every time.

xenithon · Post by **xenithon** » Thu Jul 28, 2005 10:20 am

Ok, thanks. Two add on questions:

1. if they don't start on occasion, does it cause harm to just tap them to start them up?
2. if they started, is there any chance they could stop for some reason?

Bluefront · Post by **Bluefront** » Thu Jul 28, 2005 10:38 am

Tapping or giving the blades a spin to start is probably harmless, although not reccommended.

Once running at a low voltage, fans don't usually stop, unless there is a big voltage drop in the AC line......or maybe a PSU fluctuation.

wim · Post by **wim** » Fri Jul 29, 2005 5:41 am

you will find that whether or not they start will depend on the rest angle of the blades. no harm in spinning them up with a finger, but don't you think that would be very annoying eventually? (unless, i suppose, it's an always-on computer)

Nexus Fans at 5V or 7V?

Nexus Fans at 5V or 7V?

Re: Nexus Fans at 5V or 7V?

KVL and my memory