Molex undervolting trick - bad?

I put some new fans in my case hoping to make it quieter, but it didn't. I'm looking for a way to slow down all of my fans on-the-cheap.

I have a Switch 810 with this PCB on the back to power all my fans.

I'm thinking about rigging a 3" molex extender I have so that the PCB is receiving 12V from the PSU and 5V from the PSU (instead of 12V and ground) for a 7V difference (instead of 12V). Will this cause any long-term damage to my fans, PCB, or PSU?

Why not just use a fan controller?

I actually have one, but I'd have to buy 3-pin extensions because the cables are pretty short and my case is on the bigger side. If this method works it'll be quick and easy and free.

Brushless dc motors are pretty forgiving on undervolting, it just drops the rpm to ~ 60% of listed. If anything, it should extend the life of the fan. As for the PCB, same deal, as long as there's no weirdness with the two ground pins. I'd use an ohmmeter to doublecheck, personally, before connecting a modified molex to it. It *might* ground out the ground pins, which would cause problems if you hooked your 5V line to that point.

Ok cool. How about the PSU? This would be sending current back through the 5V line instead of ground...

It's been a few years, but I used to use molex-modded fans a lot, and never had a power supply issue. I think the only danger is if your fan motor shorts out or something else causes the 5V and 12V lines to short to each other. The PSU's overcurrent fuses are probably designed to block shorts from 5V to ground and from 12V to ground, but not necessarily between 5V and 12V.

There's an easy solution though, just put in something like this http://www.amazon.com/Manual-Variable-Speed-controller-connector/dp/B002D3DK1I between your power supply and your fan PCB, then you can easily dial it back up or down with changing seasons.

I'm running fans undervolted via Molex like this in all of my machines, and have been for years (in at least one case, literally five years with the same fans and PSU). Neat, tidy, and quiet. I have never had a single problem yet, either with the fans or with the PSUs.

-Tuthmose

Similar experience as Tuthmose. I've been doing it for years with no issues at all.

Although you could just use the 5v w/ ground to give the fans 5v instead of 7v if it bothers you. They'll run a little less than 50%.

Back with some of my older computers I had access to tons of 120mm by 38mm server fans that where loud loud loud and it was common practice to cut the molex connecters so i could run 5 volt. As for 7 volts i cant say.

On a side note its good to see you upgraded your computer.

Ok, thanks everyone. I chickened out and just swapped the 5V and 12V line on the molex connector so all my fans are running at 5V now. VERY quiet now, really happy with it. Temps suffered about 10degrees, but still at about 50C under full load so I'm good with that. (360 rad on CPU only)

Almost all 12V fans are rated to run at 6V, and below that the problem is the fan not starting reliably.

Don't be surprised if the RPM signal doesn't work when the fan is connected to +5V and +12V.

people have been sourcing ground for fans off the 5V rail for ever...its fine

jamsomito said:

Ok cool. How about the PSU? This would be sending current back through the 5V line instead of ground...

Click to expand...

You are correct that the current would flow out of the fan and in to the 5V line, but don't make the mistake of thinking that the current will flow all the way back to the PSU.

As long as you have other loads on your 5V line that exceed the current going through the fan, the power supply will still be sourcing current on the 5V rail.

Consider an example: Let's say you have 2A of total load on your 5V rail. Your fan consumes 0.1A @ 7V. This means 0.1A is sourced from the 12V rail and sunk in to the 5V rail. However, that's still far less than the 2A of load on the 5V rail, so your PSU will source an extra 1.9A to make up the difference.

The only scenario in which this doesn't work is when you have no real load on your 5V rail to sink the current flowing through the fan.

In practice, you should be fine.