View Full Version : Potentiometer resistance confusion

26-07-2004, 01:32 AM
Heres one for the elecotechies here...

I wanna hook all my fans (+ LED for each fan) to a potentiometer so I can control the speeds. Cold_fu5ion has informed me that if I want the fan to turn off when the pots at zero then the resistances need to match.

So I tested the resistance on a dse 120mm fan with a multimeter and it said like... 5.7 ohms or something. So far all the pots i've seen have been like 10k ohms or 150k ohms. I also remember hearing somewhere you have to have the mutimeter in the circuit... not simply hanging off... ?:|

I think there was something else but dope makes you forgetful....

26-07-2004, 03:07 AM
It would be nice if it was that simple, but life seldom is.
Consider watts as well as ohms, and you may well find yourself favouring a 3 position switch and a 3 terminal regulator

With a dpdt 3 pos switch and a 7808 or 7810 regulator, you could have the :
Up position giving full 12 volts for Fast/Loud
Mid position switches fan(s) off, for Stop/Silent
Down position via regulator for Medium/Quiet

If you wished to replace the 78xx regulator with an LM317 and a few other bits, the down position could switch you to a variable speed/noise option, then your potentiometer would be useful AND inexpensive

Billy T
26-07-2004, 09:47 AM
Given the dope factor :D and your possible shortage of electronic design expertise, you may find that This Kitset (http://www.dse.co.nz/cgi-bin/dse.storefront/41041a9d036b16982740c0a87f9906af/Product/View/K3072) can provide exactly the control you want at a modest cost. You could use a stepped switch for preset levels, or fully variable control if desired.

There is a severe risk of overheating your computer with manual fan control though, and it would be a good idea to add a thermostatically-controlled override to restore full cooling capacity if temperatures rise too high. You will probably find a suitable kit for that at DSE or JayCar, and technical assistance to interface the two circuits should not be hard to find.


Billy 8-{)

Terry Porritt
26-07-2004, 11:07 AM
There are also lots of websites that give fan control circuits of varying sophistication, but this one (http://www.procooling.com/articles/html/quiet_fans_-_power_control_met.php) is quite good for a read.

Graham L
26-07-2004, 04:36 PM
The simplest, and probably cheapest, way would be to get a rotary switch and a few low value 5W or 10W resistors to be connected in series with the fans (all in parallel). You can have an infinite resistor (none ;-)) if you want an Off position.

A potentiometer is very good for varying a voltage ("potential"). However, it's a terrible speed control.

The DC resistance of a fan motor is not a useful indication of the current it pulls when running. (Especially since many of them are not DC motors ... they are multiphase AC motors, which have an electronic driver. :D).

The name plates usually show the full load current and/or power in watts. If you use the R=V/I or R = W/I*I relations, you can work out the "resistance" for one or all (just add the currents or powers) at the normal 12V. Then a resistor equal to that will cause the fan to get rouighly 6V. Its power consumption will vary non-linearly with the speed but experiment a bit. The resistors cost about 70C each. ;-)

You won't be able to smoothlyu change the speed from normal down to zero anyway, so why use a complicated speed controller? I'd guess they would stop at about 5 or 6 volts. Three or four steps should be plenty.

Pete O\'Neil
26-07-2004, 06:45 PM
Have a look at coolcases they have a solid selection of fan controllers, there not the cheapest but come assembled and have fancy LED's etc. The majority have individual channels so you can monitor each fan, some of the better controllers can handle up to 20w per channel. You easily hook up a couple of current sucking Deltas to a single channel channel.


28-07-2004, 11:32 PM
Well can't afford one of those cool ass fan controllers and can't be bothered with a rotary switch.... I would've like a potentiometer but a rotary switch is just plain boring.

I decided on using just plain switches. Oh and an LED below each one to telll whats on. :D

Cheers for your help.