Vmod Delta AFB's

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catawalks

n00b
Joined
May 18, 2008
Messages
11
Alright, so I got these two Delta AFB1212GHE 120x38mm fans that are fantastic, the only problem is that they can be a bit loud for invited guests and what not. So I was thinking of trying to make a switch that would switch between 12v and say 7v. I did some calculations for getting to the 7v via resistors and I came up with this:

I = 29.4w/12v = 2.45 Amp

R = 5v/2.45a = 2 Ohm

P = 2.45*5v = 12.25 Watt



2Ω/12.25W resistor x 2 (1 per fan, each fan on separate switch)

http://www.sidewindercomputers.com/deaf12recgrh.html

I think those numbers are correct but that seems like a huge resistor and the heat output of something like that worries me a bit.

I was wondering if you guys had any suggestions for other ways of doing this or if this way is safe?

Thanks
 
How about a double pole, double throw switch that has GND and 12V on one side and 12V and 5V on the other? One side gives 12V, the other 12-5=7V. The common poles go to the fan. The 12V connections are on one side, the GND and 5V on the other. No resistors and no power wasted.

Code: 
              12V  GND   
               |    |
Fan V ---------      ---------- Fan GND
               |    |
              12V  5V
 
Well, as long as that will work. I'm just afraid that since they have such high amperage compared to a regular fan that it will cause problems. If not then I'll try it out.

Thanks
 
It's not too hard to get a switch rated for 15A or more. The PSU should have no problem as long as you don't connect the fan to a line that already supplies power to a lot of other devices, putting its draw close to the line's rating. Some fan-only power lines might be rated for low power only, so watch which line you use.
 
Ok, so tomorrow I'm gonna stop by my friendly neighborhood Radio shack to get the switch and a molex extender that I can use to splice the switch into. But I was thinking, couldn't I just use a SPDT switch for the gnd and the +5v and leave the +12v constant? Or is there some negative effect that doing such will have on the system?
 
Alright, so I stopped by Radio shack and picked up the only SPDT switch they had over 3A. Its not the pretties but it'll do the job. I did a rough splice of the design to make sure it worked and everything was grand. Soldered the whole thing together and went to do another test, but unfortunately I didn't cover up the 12v line that I had soldered together and it grounded against my case. I'll never do that again. Luckily all it did was shutdown my computer. Rebooted and had to chkdsk my drive due to a corruption but everything is back up and running. I finished up the wrapping of the wires and everything, installed the new switch, and bam, thing works perfectly, only weird thing is that i no longer get a fan speed read out. I guess I can live with that.


Thanks a lot, so much quieter now.
 
The reason you don't get a fan speed readout (at least when you're running it at 7V), is that the motherboard is expecting the fan to pull the RPM sense line to ground, typically twice per fan revolution. Since the fan doesn't have access to a GND line, it's only pulling the RPM line down to 7V.

It's not harmful in any way, but you'll lose the RPM sense as you noticed.
 
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