Shedding Voltage?

[Omega]

So today I got some Aerocool Shark fans and they have advertised two running modes, a 12v "performance" mode @ full speed/volume and a 7v "silent" mode that is supposedly much quieter. If you buy the fans new they come with a little cable with a resistor or whatever so you can run them at 7v but I bought them second-hand and all I got was the fans themselves, plus the pigtails off of them.

The fans have the regular 3-pin connectors on them. If anybody has or knows of a pinout for the 3-pin that would be great. I am assuming red is power and black is ground and I have no idea what yellow does but I'm assuming it's maybe PWM control. I won't need to retain PWM function for what I'm doing.

What I want to do is wire in all the fans on the door of my CM Stacker 830 to a single switch that toggles between 7v and 12v. I can do the rest of the wiring (splicing everything together, running wires, etc.) but the part I'm confused about is what would be the best way to get that 7v. I know molex power has 12v and 5v, but with the fairly steep decline in airflow from 12v to 7v I wouldn't want to go any lower than that.

Basically I want to run the three fans off of the one switch with common power/grounds for all of them so whatever solution I use will need to be able to handle the current flow for four fans. The sharks say 1.65W for silent mode so about 5W of power is going to be going through it.

Would I be able to do that with a simple resistor or would I need to get a voltage regulator? If so, what voltage regulator would I need?

[TLHarrell]

Simple resistor(s) can do it. Just use Google to search for an online resistor calculator for voltage drop. Then make sure you size the resistors to be able to take the current. If your fan is pulling 1.65w, don't toss a 1/4w resistor in there and assume it's going to work. You'll need a couple 1w resistors in parallel before each fan, or you're going to need to stack a large mess of them in parallel if you're putting them at the switch. Total wattage of the resistors needs to be more than the wattage your fans will draw. I'd toss a multimeter inline with the fans at 12v and see how much current they're drawing.

[Omega]

Simple resistor(s) can do it. Just use Google to search for an online resistor calculator for voltage drop. Then make sure you size the resistors to be able to take the current. If your fan is pulling 1.65w, don't toss a 1/4w resistor in there and assume it's going to work. You'll need a couple 1w resistors in parallel before each fan, or you're going to need to stack a large mess of them in parallel if you're putting them at the switch. Total wattage of the resistors needs to be more than the wattage your fans will draw. I'd toss a multimeter inline with the fans at 12v and see how much current they're drawing.

actually what am I thinking, I should be able to do the math just fine on my own. I know Ohm's law :P

The resistors are going to be at the switch so it drops the voltage on all the fans at the same time. That way I only have one switch rather than three.

[slaveofconvention]

there is a simpler way, without resistors... if you wire the fans up directly to the 12v AND 5v lines, you'll end up with 7... Course then the switching thing would need a little rethink - basically you'd just have a permanent 12v on the positive, and have the switch flip between ground and 5v on the negative....

[Aldersan]

That'd be easy enough I'd think, just have the middle prong go to the fans, and then 5v on one side and ground on the other side.

[Omega]

there is a simpler way, without resistors... if you wire the fans up directly to the 12v AND 5v lines, you'll end up with 7... Course then the switching thing would need a little rethink - basically you'd just have a permanent 12v on the positive, and have the switch flip between ground and 5v on the negative....

wait what

you have completely lost me here. are you saying have +12V on one side and +5V on the other side of the switch? How would that get 7V to the fans if the only two power feeds are 12 and 5 respectively?

edit\\



That'd be easy enough I'd think, just have the middle prong go to the fans, and then 5v on one side and ground on the other side.

you. if you have any of the "silent mode" cables please measure the resistance on one for me

[Fuganater]

I have 7x 7v cables. Let me know if you want them.

[Omega]

I have 7x 7v cables. Let me know if you want them.

If you could just tell me what the resistor value is that'd be ace, then I should be able to just get an appropriate resistor and toss it in one of the power leads.

[Fuganater]

online calculators say 51 Ohms

Its big and gray. Green/Brown/Black/Gold

[slaveofconvention]

wait what

you have completely lost me here. are you saying have +12V on one side and +5V on the other side of the switch? How would that get 7V to the fans if the only two power feeds are 12 and 5 respectively?

edit\\




you. if you have any of the "silent mode" cables please measure the resistance on one for me

Heh - I know - it's odd and tbh I don't know the science behind it, but if you put +12v thru the positive and +5v thru the negative, you end up with +7v - if you doubt me, get a multimeter and stick the prongs into the red and yellow connectors on a molex.....


http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/12v.jpgBlack and Yellow

http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/5v.jpgBlack and Red

http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/7v.jpgRed and Yellow

[AmEv]

Because voltage is relative.


Physics class is teaching me that right now.
(need to pay more attention as to why...)


And resistance is futile :whistler:

But, yes, 12-5=7.

Still hasn't explained how they get the -12v rail working...

[Omega]

Heh - I know - it's odd and tbh I don't know the science behind it, but if you put +12v thru the positive and +5v thru the negative, you end up with +7v - if you doubt me, get a multimeter and stick the prongs into the red and yellow connectors on a molex.....


http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/12v.jpgBlack and Yellow

http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/5v.jpgBlack and Red

http://www.thebestcasescenario.com/slaveofconvention/imagestore/voltages/7v.jpgRed and Yellow

You have just seriously confused the **** out of me, though I suppose with AmEv's explaination that voltage is relative, it makes sense that with 12V on one side and 5V on the other you would get 7V across the component, 'cause that's how voltage drop works. Then the problem becomes how to wire that...

Perhaps I could have constant +12V going to the fan, then have the switch in the ground connector going between a -12V rail and a +5V rail. I really need to buy a cheapo PSU to use as a bench unit...



online calculators say 51Ohms

Its big and gray. Green/Brown/Black/Gold

Thanks. I'll look in to this too.

[Stonerboy779]

Green, brown, black, gold

5, 1, x1, 5%

51ohm resistor with plus or minus 5% error.




12v - 5v = 7v

It's the difference here and for 3 fans on a decent PSU you should have no issue with wiring it like this, however if you wanted to do more fans like this you would want to grab resistors.

Oh and do not use the -12v when wiring up.

Wiring up should be 12v (yellow from molex) to positive (red on 3pin fan connector) and 5v (red molex) to ground (black 3pin)

[slaveofconvention]

The wiring aspect would be pretty simple - you'd wire the 12v from the molex directly to the 12v on the fan(s) then get a SPDT switch and wire the common terminal (which one that is depends on the type and make of switch) to the ground on the fan - the two other poles on the switch are where you'd wire the molex ground, and the molex 5v - this would mean that the switch would be selecting either ground or 5v to go to the fans while the 12 is constant - in essence you'd end up choosing between 12v-0v=12v (Ground) or 12v-5v=7v

I think I have all the parts needed for this here at home so if I get time tonight I might breadboard it up to try to make it a lil clearer if you like....

[diluzio91]

Yeah. lol... way way way easier than using resistors. Just think of it mathematically, if you have 12v pushing one direction, and 5v pushing against it, then 7v will spill over, although i'm not sure of the actual physics, but resistors just complicate what you're trying to do. and the yellow/white wire is to report the speed of the fans.

[slaveofconvention]

OK so here goes - I hope this makes a little more sense....


http://www.thebestcasescenario.com/slaveofconvention/imagestore/fanwiring.jpg

This assumes you use a Single Pole, Double Throw (SPDT) switch, and the centre connector is the common connector with the two outer ones being the switchable ones (this is usually the case). (NOTE: Corrected from DPST switch - thanks xr4man)

The yellow wire coming out of the fan is for RPM monitoring and doesn't need to be connected to anything.

The two black wires on the molex are both ground and it doesn't really matter which one you use (9 times out of 10, they're actually wired together inside the PSU anyway).

As for more than one fan - that's not a problem - you just connect them in parallel - wire all of the red wires from all of the fans together and connect to the yellow from the molex - similarly wire all of the black wires coming out of the fans together and connect to the centre connector on the switch...

Clear as mud?

[xr4man]

wouldn't that actually be a single pole double throw switch? otherwise that circuit is correct.

[Fuganater]

Double throw sounds right. Middle is off, Up is 12v and down is 7v/5v right?

[xr4man]

pretty much except there wouldn't be an off position. i think to have an off position in the middle, you'd need a triple throw switch. (is that even a real term?)

[slaveofconvention]

wouldn't that actually be a single pole double throw switch? otherwise that circuit is correct.

I freely admit I'm not 100% sure but I was always under the impression that the number of poles represents the number of options, while the number of throws represents the number of circuits the switch can control. The switch in the circuit only has two options, so it goes from 12v to 7v without any central "off" position.

I was going to actually build this but when I went thru my parts bins, I only have SPST (on or off) switches, or DPDT switches which have six terminals - now I could have used that and ignored half of the contacts but I didn't want to confuse things, and I figured the diagram would probably be clearer than a messy breadboard lol

EDIT: Turns out you're correct - see here - http://en.wikipedia.org/wiki/Switch - so it IS a SPDT switch

[Omega]

Okay, now the question becomes: How safe is it really to run fans like this, in terms of the PSU? I won't be upgrading my PSU any time soon (850W cooler master PSU FTW) and I don't want to do anything that's going to make it bug out or blow capacitors or anything.

edit\\

and also i'm only going to have a 2 position switch. up for 12v, down for 7. no need to turn the fans off completely, they're going to be quiet enough at 7v that it won't bother me and the whole rig goes on standby after an hour being idle so it's not like i would need to shut off the fans independently or anything.

[Omega]

Got it all soldered, heatshrinked, hooked up and mounted.

With the sharks at 7v, the system core temps idle at 35c, GPU idle temp at 43c.
With the sharks at 12v, the system core temps idle at 29c, gpu idle temp at 40c.

It's heaps louder at 12V but the improvement in the cooling is definitely noticeable.

[slaveofconvention]

Cool - glad it all worked - I have been thinking about doing something similar myself (which is why I had the 7v answer all ready as soon as I saw the thread) just never got around to it yet - my server runs 4 120mm intakes, 3 120mm exhausts and a pair of 92mm exhausts so getting that fan noise down should be good

[diluzio91]

i do the 7v mod on almost all my fans that i don't have a controller for. :D

[Omega]

Having it switched is extra nice. the Sharks have crazy airflow which is awesome when I'm gaming but not so desirable when I'm watching TV shows or trying to get to sleep.

Plus having my little setup kicks the crap out of having a fan controller. No having to worry about fiddling with each fan individually or trying to figure out how to wire them in all right proper way or whatever. It was pretty no-nonsense

[Aldersan]

Lol my fan controller is one knob that controls 4 fans and lets me have more precise control for not much more effort :P

[diluzio91]

mines the same just for 5. lol