Fan undervolted (12V->7V); now no rpm readout?

KlaymenDK · Post by **KlaymenDK** » Wed Dec 17, 2008 3:41 am

Hi,
I have a slight problem that I find is not covered by any of the how-to guides (here or across the net).

I have:
* motherboard with PWM headers (Gigabyte GA-EP45-DS3R)
* two 12V case exhaust fans (Nexus 120mm Real Silent D12SL-12) (these have both standard Molex plugs and 3-pin fan header plugs)
* A home-built 12V->7V converter (like this)

I would like to:
* undervolt fans to 7V
* get rpm readouts via PWM headers

Now, if I connect the fans directly to the PWM headers on the motherboard, they of course run at 12V and the bios reports 1050rpms -- I know they're not going to be speed controlled, but so far so good.

But, in order to undervolt the fans, I've removed the 12V leads from the 3-pin plug, and instead supplied 7V through the fan's Molex plug via my Molex 12/7 converter. The fans run just fine, and extremely quiet.

Then, I connected the 3-pin header back into the PWM header (now, only actually connecting the yellow wire to PWM pin 3), intending to supply rpm signal. However, the bios reports no rpm data!

I know that the electrical connection is good, so I'm thinking the reason may be that the rpm signal is now no longer at +12V and so too weak to be picked up by the PWM header? If so, how can I bump that voltage back up without making a short somewhere?

I have not tried anything more than stated above, because I don't want to risk frying brand-new components...!

Strid · Post by **Strid** » Wed Dec 17, 2008 4:42 am

That is weird! I'm doing the exact same thing on an GA-EP35-DS3 and I get a RPM readout on both three pins MB headers and four pins header.

LodeHacker · Post by **LodeHacker** » Wed Dec 17, 2008 9:10 am

Have you tried to connect the fans to the PSU directly using Molex connectors WITHOUT your 7V mod? At the same time, connect the fan's RPM data pin to the PWM connector just like you did in your previous test.

I doubt it will work, because I think once the fan gets power from Molex it will disable the 3-pin plug to prevent the problem of having 12V coming from both connectors overvolting to 24V. Ok I don't know much about electricity and stuff either, but I just have the feeling you are doing something wrong.

You need 3-pin to 3-pin noise reduction cables. I have bought some ZALMAN fans which come with this type of adapter: http://www.quietpcusa.com/Zalman-RC56-7 ... 102C0.aspx

It works, no mods and RPM data is sent as well. I thought 7V is still too much for my taste so I bought a pack of ZALMAN RC-100 adapters (12V -> 5.5V)

Vicotnik · Post by **Vicotnik** » Wed Dec 17, 2008 9:24 am

By using +5 instead of ground on the fan, the mobo RPM sensor and the fan no longer share a common ground. So I would be very surprised if would work.

LodeHacker wrote:I doubt it will work, because I think once the fan gets power from Molex it will disable the 3-pin plug to prevent the problem of having 12V coming from both connectors overvolting to 24V. Ok I don't know much about electricity and stuff either, but I just have the feeling you are doing something wrong.

That's not how it works. The fan doesn't disable anything, "Double" +12v sources would still give +12v since the sources are in parallel.

LodeHacker · Post by **LodeHacker** » Wed Dec 17, 2008 10:13 am

Vicotnik wrote:That's not how it works. The fan doesn't disable anything, "Double" +12v sources would still give +12v since the sources are in parallel.

I knew I was wrong. So it's like with batteries, I can have two 1.5V dry alkaline batteries give 3V or 1.5V, but last longer. So the same applies here, ok good to know. Thank you!

Trav1s · Post by **Trav1s** » Wed Dec 17, 2008 10:18 am

LodeHacker wrote:
You need 3-pin to 3-pin noise reduction cables. I have bought some ZALMAN fans which come with this type of adapter: http://www.quietpcusa.com/Zalman-RC56-7 ... 102C0.aspx

It works, no mods and RPM data is sent as well. I thought 7V is still too much for my taste so I bought a pack of ZALMAN RC-100 adapters (12V -> 5.5V)

I have used similar connectors from www.coolerguys.com on both of my k45 systems. The have one that has three connectors (12, 7, 5.5v) which fit the perfect.

KlaymenDK · Post by **KlaymenDK** » Thu Dec 18, 2008 3:10 am

Okay so apparently, this is what I should be looking for:

1) It seems to be out of stock in a lot of places (not this specific item, but the general doodad).

2) Heck, I can solder in a resistor myself, and that's basically all it is. But I can't tell from any of the online images what size it is ... brown-blue-black? green-blue-brown? It seems I'm even too dumb to do my own resistance calculation...

Help?

maf718 · Post by **maf718** » Thu Dec 18, 2008 6:49 am

Resistors are a bit unpredictable because the size of resistor needed depends on the resistance of the fan. As I have found out there is not a "one-size-fits-all" resistor to give 7V. See this thread for more discussion of this issue and related links: viewtopic.php?t=51287

KlaymenDK · Post by **KlaymenDK** » Thu Dec 18, 2008 7:18 am

Right, zeners. I can remember reading about those. I'll look into it some more, and then post back if need be (or with a success story).

Thanks for now!

victorhortalives · Post by **victorhortalives** » Thu Dec 18, 2008 7:43 am

KlaymenDK wrote:Okay so apparently, this is what I should be looking for:

1) It seems to be out of stock in a lot of places (not this specific item, but the general doodad).

2) Heck, I can solder in a resistor myself, and that's basically all it is. But I can't tell from any of the online images what size it is ... brown-blue-black? green-blue-brown? It seems I'm even too dumb to do my own resistance calculation...

Help?

Stores on Ebay have this sort of stuff, postage may be a problem

e.g.

Ebay # 270209521395 - Belgium
Ebay # 400017346919 - UK
Ebay # 250337223205 - UK

cmthomson · Post by **cmthomson** » Thu Dec 18, 2008 6:53 pm

Your sensor doesn't work because the 7V mod you built reverses the + and - pins of the fan with respect to the motherboard. The sensor on the motherboard is seeing a signal that has a negative voltage with respect to ground instead of a positive voltage, so it doesn't work.

An inline resistor like the ones shown will work fine, but the fan speed will vary based on the fan type. Most inline resistors are either 56 Ohm or 100 Ohm.

Since you said you were willing to experiment, I'd suggest trying a few resistances and measuring the effective voltage.

Alternatively, you can use either Zener or standard diodes for a more predictable voltage independent of the fan's current draw (effective resistance).

Then again, for about $10 you can get a Zalman controller with a dial...

LodeHacker · Post by **LodeHacker** » Fri Dec 19, 2008 12:05 am

These are fairly cheap, couldn't they work as an alternative? http://www.akasa.co.uk/akasa_english/sp ... c_06bk.htm

KlaymenDK · Post by **KlaymenDK** » Fri Dec 19, 2008 12:46 am

I am not getting a fan control, not one with a front panel anyway. Right now, I'm pretty happy with the hardwired 7V (and no readings) setup, but in the new year I will use the fan header's 12V along with a zener of some size.

Thanks for the help and info, everyone! This forum is priceless.