Potentiometers for custom fan controller?

[BryceAC]

G'day,

I'm looking at making myself a custom fan controller to go with a side project that is currently underway, as the fans run awfully slow on the cheap mobo, yet too fast straight off the PSU.

It's kinda scrapyard wars inspired, so I'm being cheap and scabby and just want to make one instead of spending more money.

With the info out of the way, I basically need to know what kind of Potentiometers I need to use.

Do they need to be rated to a certain amperage and voltage so they don't overheat/explode?

Also, what kind of resistance do I need to have, in order to have a fairly wide range of control over the speed/noise of the fans?

If you've done anything like this yourself, I wouldn't mind having a look.

Any advice would be greatly appreciated!

Cheers.

[Electronics Wizardy]

You need about a 100ohm 2w(ish) pot.

 

Your much better off using something else to do this.

 

If you have some electronics stuff laying around, you can use a micro to use pwm to control, so you don't have to worry about heat.

 

Or just connect it to the 7v power line, it moves enough air, and its almost silent on most fans.

 

[mariushm]

It depends on how you plan to use the potentiometer.

You can control fan's speed either by limiting the current going into the fan, or by limiting the voltage.

Limiting voltage is easier, as fans don't use a lot of current, so it's harder to do very fine control over how much current the fans would use.

You can buy an adjustable LDO ( linear regulator) like ROHM  BA00BC0WFP-E2  or INFINEON  IFX25401  or LM2941 and look in the datasheets for example circuits. All of them adjust the output voltage using two resistors, and you can replace one resistor with one potentiometer and that's how you control the output voltage. as the feedback uses very little current, you can use any potentiometer to adjust the voltage.

 

For example, here's how you would do it with a LM2941 :

IFX25401 is pretty much the same, you just have to connect the enable pin to the input voltage instead of ground to enable the chip (turn it on).

 

So that's how you make yourself a very cheap and simple (and inefficient) fan controller.

The only thing that's important to know is that linear regulators work by dissipating the difference between input voltage and output voltage as heat, so the lower you'll configure the output voltage the warmer these chips will be but since the fans usually consume little power the heat produce won't be much .. so your controller will actually produce some heat as you lower the fan speed (due to lower voltage going to the fan).  That's generally ok though, as you'll most likely have the chip near the fan anyway, which takes out the tiny amount heat right away.

Controlling the current can be done by limiting the current going into the base of a transistor.

 

A transistor is like a switch which limits the current going through it, it can be fully on letting energy flow between two of its pins, or partially on allowing just a limited amount of current through it, and how much open it is can be controlled through the amount of energy going into it through another pin.

 

This video explains how you would do it :

 

 

So just like with the linear regulator above, instead of using a fixed resistor at the base of the transistor, you'd use a smaller base resistor (like 1000 ohms) which basically guarantees a minimum resistance between your voltage and base pin (so you won't damage the npn transistor if the potentiometer will be set on 0 ohm), and connect this resistor in series with your potentiometer that will be probably 10kOhm or even higher.  The more you turn the potentiometer knob, its resistance will increase, so the total resistance at the base of the npn transistor will increase and therefore the transistor will receive less current, so in turn would let less current flow between the emitter and collector pins, effectively limiting the current going to the fan.

The trick is to have the resistance at the base at such a value so that the npn transistor won't be fully turned on, and you have in the datasheet for whatever npn transistor you choose the aproximate current amount from where it's fully turned on. That tells you what resistance you should use at the base.

 

However, it's important to remember from the video that npn transistors have a hFe (amplification factor) that varies within a range from chip to chip and also with temperature, and fans also use little current ( around 100 to 250 mA) so a very minute amount of current at the base of the transistor will change the output by a lot, which means even slightly adjusting the potentiometer at the base of the npn transistor would change the fan speed

 

 

[corrado33]

Guys, it's a fan. Sure, the above solutions are the RIGHT way to do it, but a simple voltage divider with a pot will work fine and most likely forever. 

 

I'll tell you what, I'll test this when I get home. I'll hook a 12V source up to a fan I have at home with a pot and see how hot the pot gets. 

 

Just grab yourself a 100 ohm potentiometer and it'll probably work fine. Hook one side of the fan to the center pin, and 12V to one of the other pins and ground do the other. 

 

Just make sure you don't use a tiny little trimmer pot (they're really... small and usually adjusted with a screwdriver or very small knob.) If the pot is bigger than say a nickel or quarter, you'll probably be fine. And in the end, it's a fan, even if it shorts it'll be fine... Computer power supplies shut off if they detect a short... ask me how I know.  

 

Larger value pots will severely limit the current going to the fan, so it would never spin fast at all. 

 

Honestly, the best way to do this would be to buy a cheap fan controller for $20. Unless you already HAVE the parts to make this, it'll be cheaper that way.

[mariushm]

Do the math @corrado33 :  V = I x R   so R = V / I  ... a 12v 150mA (0.15A) fan will have a R = 12 / 0.15 = 80 ohm

 

Now let's say we make a voltage divider between a 1 kohm resistor and a 100 ohm potentiometer.. here's a circuit simulating this:  http://goo.gl/uFqeQd  (it's long url, so made it short)

You can play with the resistor in place of a potentiometer, right click on the 1 ohm resistor and change the value. Hover mouse over to see the data.

 

At 1ohm, you'll send almost 12v to the fan and the potentiometer will be fine, with a Pd of only about 25 mW.

At 10 ohm, you have 10.5v to the fan but the potentiometer drops about 1.4v so about 200mW of power is wasted in the potentiometer.

At 50 ohm, you have about 7v on the fan but the potentiometer drops around 4.8v and about 460mW of power is wasted in the potentiometer.

At 100 ohm, you have 5v on the fan, about 7 drop on the potentiomer and about 475mW dissipated on the potentiometer.

 

So you'd have to buy a potentiometer with a power rating of at least 0.75w - 1w .. good luck with that... they're not rare, but they're expensive. The cheaper ones are rated for maximum 250-500mW. Here's some examples of >500 mW 100 ohm potentiometers and their price.

 

Basically, with voltage dividers, the whole current has to flow through the potentiometer and on cheaper potentiometers, the material on the wiper of the potentiometer is simply not strong enough to let so much current flow without being damaged

 

When using potentiometers on the feedback of linear regulators or to control the current going into the base of a transistors, you're letting only a few mA of current flow through the potentiometer (up to around 5-15mA for linear regulators), which basically translates in less than 100mW of power in the potentiometer, so you can use cheap tiny potentiometers.

 

 

[corrado33]

24 minutes ago, mariushm said:

Do the math @corrado33 :  V = I x R   so R = V / I  ... a 12v 150mA (0.15A) fan will have a R = 12 / 0.15 = 80 ohm

 

Now let's say we make a voltage divider between a 1 kohm resistor and a 100 ohm potentiometer.. here's a circuit simulating this:  http://goo.gl/uFqeQd  (it's long url, so made it short)

You can play with the resistor in place of a potentiometer, right click on the 1 ohm resistor and change the value. Hover mouse over to see the data.

 

At 1ohm, you'll send almost 12v to the fan and the potentiometer will be fine, with a Pd of only about 25 mW.

At 10 ohm, you have 10.5v to the fan but the potentiometer drops about 1.4v so about 200mW of power is wasted in the potentiometer.

At 50 ohm, you have about 7v on the fan but the potentiometer drops around 4.8v and about 460mW of power is wasted in the potentiometer.

At 100 ohm, you have 5v on the fan, about 7 drop on the potentiomer and about 475mW dissipated on the potentiometer.

 

So you'd have to buy a potentiometer with a power rating of at least 0.75w - 1w .. good luck with that... they're not rare, but they're expensive. The cheaper ones are rated for maximum 250-500mW. Here's some examples of >500 mW 100 ohm potentiometers and their price.

 

Basically, with voltage dividers, the whole current has to flow through the potentiometer and on cheaper potentiometers, the material on the wiper of the potentiometer is simply not strong enough to let so much current flow without being damaged

 

When using potentiometers on the feedback of linear regulators or to control the current going into the base of a transistors, you're letting only a few mA of current flow through the potentiometer (up to around 5-15mA for linear regulators), which basically translates in less than 100mW of power in the potentiometer, so you can use cheap tiny potentiometers.

 

 

Adjustable power resistors at 100 Ohms (25W dissipation) are only $5. Sure, much more expensive than trimmer pots, but not what I would consider... expensive... Yes, you could build an entire voltage regulating circuit for less than that, but do you really think the OP is up for that? 

 

Your are, of course, correct. I, personally, would build a voltage regulating circuit as I have many adjustable voltage regulators at home, but sometimes you have to look at the big picture. Give the OP the schematic in the above posts and he'll get scared away. Is a voltage divider circuit wrong? Yes, would it work? Yes. 

[BryceAC]

Everyone's answers made sense (some not as much) and were interesting to read, but by the sounds of things, as much as I'd like to build the controller, it seems it'd be easier and cheaper to just buy one rather than working out which types, for which fans drawing which currents and the rest..

 

Anyone got any suggestions on a relatively cheap and straightforward 5.25 bay controller? No LEDs or screens or anything, literally a panel with some dials.

Thanks  

[r4tch3t]

Check eBay, just do a search for 5.25" fan controller. There will be a few.

[corrado33]

18 hours ago, BryceAC said:

Everyone's answers made sense (some not as much) and were interesting to read, but by the sounds of things, as much as I'd like to build the controller, it seems it'd be easier and cheaper to just buy one rather than working out which types, for which fans drawing which currents and the rest..

 

Anyone got any suggestions on a relatively cheap and straightforward 5.25 bay controller? No LEDs or screens or anything, literally a panel with some dials.

Thanks  

http://www.newegg.com/Product/Product.aspx?Item=N82E16811992012

 

NZXT is fine for a fan controller. 

 

EDIT: (I personally bought their digital flashy one when it was on sale and it's... meh, would have preferred the one I suggested above.) 

 

EDIT2: Note, most cheap fan controllers don't use PWM, they simply vary the voltage. 

 

EDIT3: In all honestly, if you know how to program microcontrollers, you can build a pwm controller for a few dollars and a few hours of your time, but that requires a lot of skills.

 

And the newegg search that shows what you're looking for.

 

http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007585 600022846&IsNodeId=1&bop=And&SrchInDesc=fan&Page=1&PageSize=60&order=BESTMATCH

[BryceAC]

Would it be possible to control the fans with a raspberry pi?

I've got one as a little media box, but I'd be able to repurpose it.

[corrado33]

2 hours ago, BryceAC said:

Would it be possible to control the fans with a raspberry pi?

I've got one as a little media box, but I'd be able to repurpose it.

Abso-freaking-lutely.

 

http://circuitdigest.com/microcontroller-projects/raspberry-pi-pwm-tutorial

 

Use a transistor to drive the fan and the pwm to drive the transistor.