jASjE

Hi, I'm planning on making a rheobus/stat/dunno how it's called for my fans..
At this moment, I'd like to control 4 Thermaltake 80mm fans.
On the fans is written:
DC-12V - 0,18 Amp
Which potmeters should I buy to control the fans (from 5v to 12v or sthing) (or it can start at 0v, but I think they only start spinning @ a certain voltage)

Anthro? Help me out

Anthroplast

okay

12v 0,18 amp -> P = U.I
so we get 2,16 watts/fan

of you want to control the fans with one knob, take a 10 watt pot (wich are expensive )

if you want 4 knobs ( 1 fan/knob) 2 watt modells will do.

now calculating resistance

ok here we go:

we will name the fan : R2
the pot we will name : R1
total current ( 0,18 Amps @ 12volt ) : I

now we make a scheme:
you wanted starting voltage of 5 volt ==> with pot at max resistance the voltage appllied to the fan must be 5 volt
wich gives :

internal resistance of the fan is : R = U/I
R2 = 12/0,18
R2 = 66,6 Ohms

now we can calculate the pot's max resistance

R = U/I

now we take U = 7 ( 12 - 7 = 5 (start voltage))
I remains the same ( 0,18)

R = 7/0,18
R = 38,8 ohms


so you need a 2 watt 38 ohm (or nearest value) pot



so you get a pot ( 38 ohm) in series with a 66,6 ohm fan.



but you can do it the easy and cheap way... buy yourself a chip regulated rheobus ~10 watts model you can get it at loveno.be ( 17 euro i think)
but i can make it also @ 12,5 euro

you can solder it yourself if you like

jASjE

ok, tx for the calculations..
I'm in a lack of money atm, but I'll c..
Btw is it possible to connect ccfls to potmeters so you can change their intensity? (on/off isn't enough for me, and my green ccfl in the bottom of my chieftec is way too bright, my hands are green at this moment )
edit: (686 BEF) € 17.00 for 1 rheobus..
I need 8 of them... too much money to handle

Anthroplast

you can hook up multiple fans on a rheobus...
yes ccfls are dimmable, take 5 watt pot's


17 € commercial @ loveno
or 12,5 € @ me

jASjE

So, let me get this straight, if I buy 4 "2 watt 38 ohm" (or nearest value) pots, then I'm settled for my fans?
Does that need heatsinks (as seen on some rheobusses?)
How much does that cost approximate?
And could you do a quick calculation on what pots I should use for ccfls? (or is it the same, but then 5 watt 38 ohm ?)

ModdiN MansoN

you can't place place heatsinks on pot resistors cuz' they are round
Voltage regulators on the other hand DO need heatsinks, but you don't need those for what you're doing.

jASjE

Ok, call me stupid, but what's the difference between a normal potmeter in between my fans and my molex, or a voltage regulator (as found here ?

ModdiN MansoN

with a potmeter, you simply make the resistance between your molex and your fan variable so it gets less/more voltage, with a voltage regulator, you can make an electrical scheme that does the same thing, not by changing the resistance, but by directly changing the voltage. You still need potmeters if you use voltageregulators though, otherwise you can't affect the voltage you get from the voltageregulator, you would just get the voltage the VR is built for
(eg. 7812 -> 12V).

jASjE

And the advantage of a voltage regulator compared to a potmeter id?

ModdiN MansoN

dunno, I've been out of the electronics-business since I left school , but I think you get a higher max-wattage