Molex under 5v

[Nino.-]

Can I go lower than 5v on the Molex fan to lower RPM (noise level)

[LukeSavenije]

no, molex is meant to be 5v and 12v

 

if you want to control your fans, connect them to a fan header

[mariushm]

To add: some fans will NOT start with voltage lower than 5-6v ... they'll operate at 5v or less but may not start. So you have to be careful.

 

You can control fan speed without changing the voltage by limiting the current going into the fan... a simple method would be placing a resistor in series with the fan (cut one of the wires, place a resistor between the wires)

That's how those  low-medium-high speed switches work... you can switch between multiple resistors to get different levels of speed.

 

All you need to know is Ohm's law : Voltage = Current x Resistance.

 

Let me give you an example with these CoolerMaster fans - first random result on Google search : http://us.coolermaster.com/product/Detail/cooling/case-fan/silent-fan-120-si2.html

 

Determine your fan's internal resistance

 

Look at the label on the fan... it will say something like 12v 0.35A or 12v 1w  ( you can divide watts by voltage to get the current).

For the Cooler Master fans in the link above, on the specs tab you can see 12v  0.15A  and 1.8w  (12v x 0.15a = 1.8w)

 

So I can determine the own's fan resistance : Voltage  = Current x Resistance => Resistance = Voltage / Current = 12v / 0.15A = 80 ohm

 

Now, if I add a resistor in series with the fan, the total resistance between positive and negative of the power supply changes. Across each device connected in series, there will be some voltage drop ... The voltage drop across the resistor, plus the voltage drop across the fan, will add up to the voltage used to power the devices, which is 12v in this example.

So let's say I want the fan to "see" only 7v ... this means I'll have to pick the resistor value in such a way that there's gonna be the remainder (12v-7v = 5v) wasted by the resistor.

 

I know the fan resistance is 80 ohm (I calculated above), so going back to Ohm's law Voltage = Current x Resistance , now I know I want the fan to work at 7v but its internal resistance remains 80 ohm, so I can determine how much current will pass through the fan : Current  = Voltage / Resistance = 7v / 80 ohm = 0.0875A

 

Now I need to pick the other resistor so that it will drop 5v (12v - 7v) at 0.0875  ... Voltage = Current x Resistance so Resistance = Voltage / Current = 5v / 0.0875 = 57.14 ohm.  This is not a standard R24 value, so I'd pick the closest to a standard easy to find resistor of 56 ohm, or I'd use a 47 ohm + 10 ohm resistor in series.

The resistor must be rated for at least  Power = Current²xResistance = 0.0875²x56 = ~0.42w so while a 0.5w rated resistor would work, it's fairly close to its maximum rating so I'd use a 1w resistor.

 

You can repeat the math with 5v and you end up with 0.0625 current and 7v/0.0625 = 112 ohm (r24 standard value would be 110 ohm)

Let's do the math for 4v as well .. 4v/80ohm = 0.05a  and 8v/ 0.05a = 160 ohm

So you could have a "super low" and "low" speed, 160 ohm and 110 ohm, and have a switch that switches between them... start the fan with the "low" speed which gives at least 5v to the fan, and then switch to the "super low" 4v option.

 

 

 

 

 

 

 

 

[Pauleft]

On 9/10/2019 at 5:39 AM, Nino.- said:

Can I go lower than 5v on the Molex fan to lower RPM (noise level)

No you can't but molex to Sata was a good way to start a fire if you needed one