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

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.....

Black and Yellow

Black and Red

Red and Yellow