Potentiometers Control

ae804 · Post by **ae804** » Tue Aug 22, 2006 6:37 pm

Ok, i'm certain w/ the vast knowledge in here that the answer is already here, but i'll ask again anyway so someone can point me in the right direction.

If I wanted to make a fan controller using Potentiometers or Rheostats, what value should i be looking to use (ohms) or does it matter that much?

Thanks,
AE804

krusty · Post by **krusty** » Tue Aug 22, 2006 8:28 pm

It depends on the specific fan current and what the minimum fan startup voltage is.

250 ohm.. about 3 watt ...with a startup cap is a starting point

I found some 250 ohm 3 watt pots at a surplus store for $0.99,
and 3500uf caps for $1.

http://img.photobucket.com/albums/v235/ ... modao6.gif

thanks DrCR

You can parallel resistance across the potentiometer or add series resistance to tweak the voltages. Too many fans for a pat answer..

ae804 · Post by **ae804** » Tue Aug 22, 2006 8:31 pm

Ok, so the current i'm using in these fans is like .9 A (Deltas

). I'm going to try to use some slide pots, and the only ones i can find are like 1kohm at the smallest value. I'll continue to look around and see if i can find anything, but it'll be hard.

Le_Gritche · Post by **Le_Gritche** » Wed Aug 23, 2006 5:49 am

ae804 wrote:Ok, so the current i'm using in these fans is like .9 A (Deltas ). I'm going to try to use some slide pots, and the only ones i can find are like 1kohm at the smallest value. I'll continue to look around and see if i can find anything, but it'll be hard.

With such a powerful fan (between 8W and 10W I would say based on the 0.9A startup(?) current you provide) you need at least a 3W rated potentiometer as said krusty. They generally cost a couple bucks more, when 1W rated ones are around 1$ (surplus and garbage bin offers excepted)

1k ohm or even 250 ohm will be too much, at such a setting the fan would stop.
You still can use such a potentiometer, but with 2 drawbacks : the useful range of setting on the slide will be reduced to maybe a tenth of its full range (so no way to do precise tuning of the speed) and you will have to be aware than you could stop the fan by just moving the slider a little too far.
Insuring that nowhere on the setting range you will stop the fan is much more secure. To achieve that you will need a lower resistance.

If you can't find a 47 ohm potentiometer rated for 3W where you live, you will probably have some modding to do :

- you could add a startup cap like pointed to by krusty (ensuring that the fan will start reliably then spin as slow as possible, rather than having to tune up then down the potentiometer at each booting up of your computer)

- you could solder a resistor in line, if the maximum resistance of your potentiometer makes the fan spin too fast for your taste (unlikely in your particular case)

- you could solder a resistor in parallel with the potentiometer to reduce the total resistance (almost transforming the 100 ohm potentiometer in an equivalent 50 ohm potentiometer for exemple), that would be very useful in your case.

If you solder a 100 ohm resistor in parallel with a 100 ohm potentiometer, the resulting thing would be very close to a 50 ohm potentiometer for the fan. I think you should try that if you can't find better in an electronic store

ae804 · Post by **ae804** » Wed Aug 23, 2006 7:30 am

Ok... here's what i want it to look like:

. The rotational pot will be able to tweak all of the fans connected while the slide pots will effect single fans. The only slide pots i can find so far have been 5k+ so i'm going to look for some smaller slide pots... doubt i'll find them. Ideally i'd like to have 50ohm slide pots and then put that in series w/ 12 ohm fan... we'll see. Maybe i can do a something else to get it to work...

Le_Gritche · Post by **Le_Gritche** » Wed Aug 23, 2006 2:27 pm

ae804 wrote:doubt i'll find them.

Yes, I guess there is a reason why all these kind of racks are using rotational pot.
Besides, if your rotational pot has to be able to tweak all fans, it means all circuits will go through it. You will need a good rated pot and to cool it (we are talking about 10 or 15W dissipation here). Plus there's still the awful consequence of something going wrong with it and shutting all your fans off.

Maybe some kind of 3-position-switch will do the trick (0V/12V/slide regulated), switchs are sturdier than potentiometers.

Can you develop a little bit more what are the specs and use of the fans you want to regulate ?