Quick wiring question: 12->7volt mod
 

Everyone speaks of running fans undervolted...
I know you can use a rheostat/ baybus to vary the voltage...(ps... what rheostat is needed to slow down a 120mm fan? How do you determine which to buy?)
I've also heard of hardwiring the fans at 7 or 5 volts. How is this accomplished, and which wire is which?? :)
(this probably belongs in a different forum, but this one has the highest traffic, and I'm going to use the fan on a radiator & perhaps PSU, so it kindof fits... hehe.)

Here is a link to the 12 - 7 volt mod thing. I use this switch on my two 120 MM Panaflo's on my heater core.

I have not had time to build the variable speed control circuit yet.
There is too much current consumed to simply connect a rheostat to these fans.

Go here for 12- 7 volt

Jim

ANd here is a link for a variable speed control.



Click here

Jim

I think Brian's article really sums up things nicely:

http://www.procooling.com/articles/h...trol_met.shtml

Although it doesn't mention the possibility of just using a switch with resistors to get 2-3 desired voltages (like Thermalteke does on their Volcano 7+).

I am not a big fan of the "7V trick" myself but I have done it with no negative effects.

assembling a 5/12v or 7/12v is really easy, and will work with just about any fan, but resistors will be limited to the wattage they can handle. fan buses will be limited by this too

Regarding SPDT & DPDT switches... What do those letters stand for? Is the only difference between them that the dpdt switch has two sets of connectors on the back?
And what about the On-off-on, on-none-on, on-none-momentary... what does it all mean?
Which switch do I want for a 7/off/12 volt mod?

Sorry for the silly questions.

SPDT = Single pole double throw (connectors look like - - -)
DPDT = Double pole double throw (connectors look like = = =)

On-Off-On means, that there are 3 positions for the switch, with the middle one being completely off. (left is an "On" setting as well as right) Same as On-none-On

Momentary is like, umm, a remote control button. It only works when you push it and hold it down. You DONT want this.

you want a SPDT with NO center off (only 2 positions for the switch, UNLESS, you want to be able to completely shut off the fan (Middle position))

I like DPDT for that reason myself, you can just turn them all off ;)

Thanks for clearing that up. Yes, I'd like to shut them off, 7/off/12 volts, so on/off/on is the way I'm going. I'd like to put in a rheo baybus for variable control. Caseetc.com had some components for sale, but I'm thinking it might be easier/cheaper to buy the whole unit preassembled rather than a bucket of parts, unless someone knows of a dirt cheap components vendor.

that maelstrom for $40ish is actually a good buy. and would probably cost the same.. or more if you went with a piece-by-piece-build-your-own.

it has 4 channels thatll go from 0v-12v(actually probably 11.5v)
and each channels will handle 1.5 amps (watts/volts = amps)

so itll handle most 120mm just fine as long as they are under 1.5 amps or 18 watts.

Here's a question for ya...

I ordered a comair/rotron fan... 24v .75A or 1A . Since the PSU only puts out 12v, I'll run it at 12v. :) It would be nice to run it lower... So, 24 x .75 = 18 watts = too much wattage. 12 x .75 = 9 watts, which should be fine. Which number is right? I'm guessing 9. I also bought a DPDT switch from mpja.com. Will that switch handle that wattage?

The Rotron can't run at 24V on a 12V PSU. It will be a 12W fan at 12V and I don't know how low it will run. I couldn't get it to run at 7V on a PSU that wasn't hooked up, but that might be because the 120mm I had wired to 5V wasn't pulling enough amps...

I knew that it would run at 12v max. I didn't know if it would spin up at 7v, but I thought you might could start it up @ 12v and then spin it down to 7v. It ought to be pretty quiet at 12v or 7v. Is it?
I don't claim to be an electrician... :) Does running a fan underpowered shorten its life?

Also, at 12v x 1A, and assuming the fan will run, could a rheobus handle it?

if it is 24v 1A, it will be 12v .5A. Running a fan undervolted is just fine

Why does the amperage change? If you have a 12v fan connected to a rheostat, and you run it at 7v, does it draw less current also?

V=IR

Volts = Current x Resistance

in a fan the resistance is always the same. Now your job is to work out how much current it will pull at 7v

Good spot for an Ohm's Law lesson. lol

As long as you know two values, you can calculate the third.

E
------
I x R

E=Voltage
I=Amperage
R=Resistance (Ohms)

E = I x R
I = E divided by R
R= E divided by I

I have a question to add.

I'm told watts can be calculated from the same formula, but I've never seen how to do it/don't know the relation. Anybody throw a quick wattage lesson in for me?

W = VI

yeah.. I'm familiar w/ both formulas... I just forgot to combine them ;)
So if a fan is made to run at 24v and pulls 1A = 24w, at 12v it only pulls .5A=6w, and at 6v it only pulls .25A=1.5w???

Also, if the equation is 24v=1A x ?R , clearly ?x = 24 something (ohms?) Is the answer 24 ohms? How would that be stated?

yeah, thats 24 ohms

phaestus, what did you mean when you said "with no negative effects" for the 7v/off/12v trick? What could happen? Seems like a perfectly feasible idea to me. Am I missing something?

cause it uses the 5v rail as ground, some people don't like it

but is it actually dangerous?

--MAtt

if you have nothing else connected to your power supply it is, but if your computer is connected to it, there is no problem