Lowering fan voltage = bad?

[xboxhaxorz]

How bad is it really though, i understand that a 12v if operating at say 7v or 9 will work fine but over time it may cause issues

Since the fan requires more power to get the blades spinning, if the fan is older it may not spin with the lower voltage being sent to it

Am i correct in this assumption?

[mariushm]

Re: Lowering fan voltage = bad?

The fan uses more power than normal only at the start, when it has to overcome inertia and friction and other physical issues. After that, it can work 24/7 with no change in power consumption (well, as long as it doesn't get dusty and there's lubrication)

Fan won't care about operating voltage as long as the voltage is high enough to start spinning.

[c_hegge]

Re: Lowering fan voltage = bad?

Most fans will start and run even at 5V with no problem. If anything, a fan will last longer at lower voltages, as it means a lower speed and thus less wear on the bearings.

[xboxhaxorz]

Re: Lowering fan voltage = bad?

I will lower the voltage then, probably keep it at 7v

From research it seems that when using a resistor it will produce excess heat

Is there a long term reliable solution for lowering fan voltage?

[Mad_Professor]

Re: Lowering fan voltage = bad?

A darlington transistor and a 555 and few components to make a simple voltage base fan controller. I built one with the help of badcaps members, but instead of 555, I used an atmel atmega168 20pu avr chip and use Arduino C langauage to program and set the fuses on it using a simple parallel port programmer made from scratch. Damn thing still going strong, I haven't had a problem yet.

You can use my thread for reference.
https://www.badcaps.net/forum/showthread.php?t=18774

[momaka]

Re: Lowering fan voltage = bad?

From research it seems that when using a resistor it will produce excess heat

Is there a long term reliable solution for lowering fan voltage?

The resistor IS one of them - simple and safe.

As far as "excess heat" - my guess is you are perhaps thinking that it will somehow produce a lot of heat in your PC or device... but NO, it won't. The "excess heat" is usually miniscule and much less than 1 Watt, depending on your fan, of course. In general, standard 12V, 80-160 mm fans are usually rated for 0.1 to 0.4 Amps current draw. So a resistor between 10 to 100 Ohms and rated for 2W to 3W will work for this application. But you will need to run a few calculations to find what resistance value you need.

Good news is, there is an even more simple option to make your fans run at 7V: connect the fan (+) wire to 12V and connect the (-) wire to 5V. The difference between the (+) and (-) wires is then 7V. Only downside of this method: motherboard won't read the RPM of the fan properly or at all if the fan is a 3-wire tacho. Also, 4-pin PWM fans may not work either.
This option is pretty much the same to the fan mods that people do with Xbox 360s (except, many usually do it between 12V rail and 3.3V rail on the DVD drive to get 8.7V on the fan).

[goontron]

Re: Lowering fan voltage = bad?

The resistor IS one of them - simple and safe.

As far as "excess heat" - my guess is you are perhaps thinking that it will somehow produce a lot of heat in your PC or device... but NO, it won't. The "excess heat" is usually miniscule and much less than 1 Watt, depending on your fan, of course. In general, standard 12V, 80-160 mm fans are usually rated for 0.1 to 0.4 Amps current draw. So a resistor between 10 to 100 Ohms and rated for 2W to 3W will work for this application. But you will need to run a few calculations to find what resistance value you need.

Good news is, there is an even more simple option to make your fans run at 7V: connect the fan (+) wire to 12V and connect the (-) wire to 5V. The difference between the (+) and (-) wires is then 7V. Only downside of this method: motherboard won't read the RPM of the fan properly or at all if the fan is a 3-wire tacho. Also, 4-pin PWM fans may not work either.
This option is pretty much the same to the fan mods that people do with Xbox 360s (except, many usually do it between 12V rail and 3.3V rail on the DVD drive to get 8.7V on the fan).

i've been burned by the 12 to 5v rail method. The motor driver shorted to "ground" A.K.A 5v. 12v on the 5v rail makes for some nice fireworks!

[momaka]

Re: Lowering fan voltage = bad?

i've been burned by the 12 to 5v rail method. The motor driver shorted to "ground" A.K.A 5v. 12v on the 5v rail makes for some nice fireworks!

I've seen that on on fans connected normally to 12V as well (in particular, small fans on older MSI and XFX video cards). Usually happens to crappy fans.

This has nothing to do with the 12V to 5V rail method, though. If you are using a crappy power supply that isn't sensitive to over/under-voltage... yeah, you will get nice fireworks. Heck, I got an amazing "fog show" with a crappy L&C PSU that didn't pick up a short circuit between the pins on its berg/floppy connector. Melted a whole strand of output wires before I caught it and shut it down. Would have likely kept going. I've seen videos of people on YouTube before where they shorted the output of various ATX power supplies, and a good handful of the ones that weren't designed well melted their wires.

[shovenose]

Re: Lowering fan voltage = bad?

5v is just fine - I do it all the time, if it's a molex fan just swap the yellow and red wire (I don't know the "right" way to do it but i use a small screwdriver to push the pins out) then if you ever need to make the fan 12v again it takes minutes with no soldering or splicing. only thing then is to make sure if the fan has a pass through molex that you swap it there as well or else you could get quite interesting results if you connect a hard drive or something! But really, who connects their hard drive to the molex extension in a fan lol.

[Pentium4]

Re: Lowering fan voltage = bad?

Even though I prefer running the fans at a fixed speed (usually 5V or 7V), I have had it not work a few times. For whatever reason, I've had some PSUs not start using the 5V to ground method. However, I have had a few fans that would run even at 3.9V. Some would run forever, and some would eventually come to a slow stop

[xboxhaxorz]

Re: Lowering fan voltage = bad?

Searching on google, some forums had mentioned that putting a resistor would cause excess heat

This is an example, but there were a few more http://www.xtremesystems.org/forums/...=1#post2582757

I am actually not using the fan in a pc so there is only 12v available

[mariushm]

Re: Lowering fan voltage = bad?

Well, of course, if you want the fan to "see" a lower voltage, the difference between 12v and that lower voltage will be lost inside that resistor, in the form of heat.

This is a very simple and obviously not efficient method of reducing the speed. There are other much better methods, but more complex, like using a switching DC-DC converter to convert that 12v to any lower voltage, or building a circuit which sends those pulses on the 4th wire of a PC fan to tell the IC in the fan to adjust the speed without modifying the voltage.

[xboxhaxorz]

Re: Lowering fan voltage = bad?

I understand, so while it isnt an efficient method, will it last a while or require an occasional resistor change

[momaka]

Re: Lowering fan voltage = bad?

It will last forever if you pick a resistors with the right size or could burn within a second if you don't.

Here's how to find what you need:
Hook the fan to a variable voltage power supply and increase the voltage until the fan spins at a speed you like. Note the voltage and measure the current that the fan is pulling. Usually, the current will be less than the maximum stated on the sticker if the voltage is lowered.

So let's say you find that the fan runs great a 7.5V and it is drawing 0.1A (I'm making up these numbers here just as an example). Your power source is 12V and you want to put a resistor in series with the fan to lower the voltage.
Step 1: 12V - 7.5V = 4.5V <- this is the voltage the resistor has to drop across it.
Step 2: 4.5V / 0.1A = 45 Ohms <- this is the required resistance of the resistor. However, resistors come in standard values, so you will just have to pick something close to that (look up standard resistance values). Try to stay within 20% of that, though.

The closest to the above resistance is 47 Ohms or 39 Ohms (both of which are within 20%). Let's say you use the 47 Ohm resistor (since it is a bit closer).

Step 3: we will assume the fan still draws 0.1A even with the 47 Ohm resistor here, since its value is reasonable close.
The resistor will then drop 47 Ohms x 0.1A = 4.7V

Step 4: 12V - 4.7V = 7.3V <- this is approximately the actual voltage the fan will see with the 47 Ohm resistor

Step 5: 4.7V x 0.1A = 0.47 Watts <- this is the power the resistor will be dissipating
To make sure you resistor doesn't run too hot, multiply that by two.

Step 6: 0.47 Watts x 2 = 0.94 Watts <- this is the power rating of the resistor you should use. That said, watt ratings on resistors are standard too, so you won't find that particular value. So you want to use the same or higher watt rating. In this case, you have 1W, 2W, 3W, and 5W -rated resistors as options. Any of these will work the same. The higher the watt rating, the cooler the resistor will run.

Let's say you pick 1W. Then the resistor to look for: 47 Ohms, 1 Watt

There are different types of resistors: carbon film, metal-oxide, wire-wound, etc... but none of that matters in this application. So just pick the cheapest.

[mariushm]

Re: Lowering fan voltage = bad?

There's only two formulas you need:

Voltage (V) = Current (I) * Resistance (R)

Power (P) = Current (I) * Current (I) * Resistance (R)


You look at the fan and you see on the lab the voltage and the current rating. You could trust the current rating on the fan, or you could put a multimeter in series with the fan and measure the current.

But let's say your fan works at 12v and the current rating is 0.2A and you want to reduce the voltage to 7v.

Since you know the voltage and current, you can figure out the resistance of the fan :

V = I x R , 12v = 0.2A x R , therefore R = 60 ohm so this is the fan's resistance, from now on you can imagine the fan as a resistor valued at 60 ohm.

Now, if the fan will see only 7v, then the current going through the fan will be different and we need to figure out this current because we need this value to figure out what resistor to choose to put in series with the fan:

V = I x R , 7v = I x 60 ohm , I = 0.116 A

In a circuit, resistors in series will carry the same current, so if we want our resistor to let 0.116A go to the fan, we need to pick the resistor value in such a way to also drop 12v-7v = 5v on it.

V = I x R , 5v = 0.116A x R , R = 43 ohms. This is a standard resistor value, so it's easy to find, but it's ok to chose something close to this value, there's simply going to be a slightly different voltage.

Now you can figure out how much power is dissipated in the resistor :

P = I x I x R , P = 0.116A x 0.116A x 43ohm = 0.578 watts. So, you must choose a resistor that can handle more than 0.578 watts, the most common value would be 1watt.
Even so, the resistor will be hot (think 60-80c maybe), because it's a very small surface that has to throw those 0.578 watts into the air.

[ivtec]

Re: Lowering fan voltage = bad?

The fan uses more power than normal only at the start, when it has to overcome inertia and friction and other physical issues. After that, it can work 24/7 with no change in power consumption (well, as long as it doesn't get dusty and there's lubrication)

Fan won't care about operating voltage as long as the voltage is high enough to start spinning.

mariushm, since you're talking about cooling fans,i have this HDNfusion Satellite that has a 5 v Fan cooling the power supply on the back,i added another fan on top of the box where the filtering caps are,my question can i connect the other 5v fan to the same input on the board that goes to the existing fan?at the moment i have it connected to the USB port but it's always on,Thanks