POT switches?

**Mrfreddio55** · 01-24-2003, 06:07 PM

Hey all, I've got some cooling ideas in my mind, one of which is to wire my fans up to a POT switch (or some kind of turn switch) so that I can change the speed of the fans.. Now what I am wondering is what type of POT switches do you use for this if theyre running on the 12v channel? (im looking for the Ohms, etc)

Thanks

**bluesfusion** · 01-25-2003, 01:34 AM

Depends on the current drawn by your fan and the minimum voltage at which your fan will start spinning.

(Some people use the minimum voltage at which your fan will stall but I think thats stupid because if you keep the pot turned all the way, you have to mess with the knobs and jiggle them every time you boot. Who wants that stupid headache?)

So just find out the current draw and minimum starting voltage for your fan and go from there.

Pots have 2 ratings which must match - Power and Resistance.

Power = (12V * Current) --->(Pot wattage rating should be higher than this number.

Resistance = (12 - Starting voltage)/Current --->(Pot resistance should be approx equal or slightly lower than this number)

**oXyCoNTiN** · 01-25-2003, 05:15 PM

i can't remember who did this, but i asked a guy how had all his fans hooked up pot's, he told me 100 ohm, 5 watts should work fine (standard fans)

**bluesfusion** · 01-25-2003, 05:25 PM

Originally posted by oXyCoNTiN
i can't remember who did this, but i asked a guy how had all his fans hooked up pot's, he told me 100 ohm, 5 watts should work fine (standard fans)

Thats dangerous advice. For example you can get a Sunon 80mm high speed fan and it will draw 9.12 Watts according to specs. If you put a 5W pot on it, it will burn up in no time.

Thats why you should ALWAYS look up the fan specs before doing something like this and never trust generic advice like 100 ohm, 5W. It might work for some fans, but if it doesn't you could end up with damaged equipment.

**bluesfusion** · 01-25-2003, 05:41 PM

As an example -
Take a Sunon high speed fan with
Max voltage = 12V
Starting voltage = 6V
Current = 0.75 Amp

Then,
Power = 12*0.75 = 9Watts
Resistance = (12-6)/0.75 = 8 ohms

You should get a POT rated at 9W, 8 ohms.

If you get a pot with less than 8 ohms, you will still be able to reduce speed but your control range will be smaller, that is you will not be able to reduce the speed to the full extent.

If you get a pot with higher then 8 ohms, you stand the risk of the fan not starting up at all if you leave the pot turned at the minimum position (max resistance) when you start the system up and to start the fan you will have to jiggle the pot (i.e. turn it up and when the fan starts, then turn it down to get whatever speed you want.)

For set-and-forget operation, you really want a pot in the right range. Putting a 100ohm (linear) pot on this fan would basically mean that if the dial goes from 0-10, you can only control the fan from between 9 and 10. If the knob points to anything lower than about 9, the fan wont spin at all.

If you get a 8ohm pot on this fan, you will have gradual control all the way from 0 to 10.

**CrawlingEye** · 01-25-2003, 05:57 PM

Originally posted by bluesfusion

Thats dangerous advice. For example you can get a Sunon 80mm high speed fan and it will draw 9.12 Watts according to specs. If you put a 5W pot on it, it will burn up in no time.

Thats why you should ALWAYS look up the fan specs before doing something like this and never trust generic advice like 100 ohm, 5W. It might work for some fans, but if it doesn't you could end up with damaged equipment.

Not really, depending on the fans.

I'm sure it could be handled by something like a 15w though. Headroom's usually ideal.

As for ohms, that's what a POT is, a variable resistor. You could always use a 100 Ohm VR and just turn it down to a reasonable rating. I'd start by checking the resistence of the psu on the 12v connectors with a multimeter then set my VR at that and just work from there.

Generic advice with impendence isn't bad but the wattage I would agree with. You can never have too much headroom. Better safe than sorry.

**Mrfreddio55** · 01-25-2003, 07:15 PM

Hmm okay thanks for all the help guys... ill post back when i get some info on the particular fan ill be using to make sure i have the correct POT picked out

**bluesfusion** · 01-30-2003, 01:39 PM

Originally posted by Mrfreddio55
Hmm okay thanks for all the help guys... ill post back when i get some info on the particular fan ill be using to make sure i have the correct POT picked out

If you cant find the starting voltage specs for your fan, you can do trial and error at 5V and 7V.

For 5V -
connect the fan leads to a molex connector from your PSU (fan positive to molex red wire, fan negative to molex black) and see if it starts.

For 7V -
Fan positive to molex yellow, fan negative to molex red) and see if it starts.

Whichever one it starts at, use that as your 'starting voltage'.
Most fans have starting voltage of about 6-7 V. Some may start lower than that.

**bluesfusion** · 01-30-2003, 03:14 PM

Originally posted by CrawlingEye
Not really, depending on the fans.
I'm sure it could be handled by something like a 15w though. Headroom's usually ideal.

Actually the calculation I gave already has more than enough headroom for power. 12V, 0.75A fan with starting voltage of 6V. Requires ~ 8 ohm resistance (actually it requires 16ohms but this is a back-of-the-envelope calculation so lets assume that increasing the pot resistance doesnt decrease the current) to drop the voltage to 6V at the fan. The actual power disspiated through the pot is way lower than 9W (which is the fan dissipation.)

R(fan) = 12/0.75 = 16 ohms
If you add 8 ohm in series, R(total) = 8(pot) + 16(fan) = 24 ohms
V = 12 Volts
I(new) = 12/24 = 0.5 amp

Power dissipated by POT = I^2 * R(pot) = 0.5^2 * 8 = 2W
Power dissipated by fan = I^2 * R(fan) = 0.5^2 * 16 = 4W

So you can see that there is a safety factor of ~ 4.5 on Pot wattage using the rough estimate calculation.

If you want to do an exact calculation then you consider the decrease in current as the pot resistance increases (fan resistance = constant). In which case the max power dissipation by the POT is going to be about 2.25W. Still a good safety factor of 4.

Originally posted by CrawlingEye

As for ohms, that's what a POT is, a variable resistor. You could always use a 100 Ohm VR and just turn it down to a reasonable rating. I'd start by checking the resistence of the psu on the 12v connectors with a multimeter then set my VR at that and just work from there.

You reduce your control range on the pot if you get too high a resistance. Whats the point of having a pot for fan control if the fan stays off even after you turen the dial over 90% of the way and then the fan starts and you got only maybe 10-12 degrees of turn to adjust the fan speed from min to max? If you want poor control like that, why not just put a fixed resistor on the fan or even better - run it off a lower voltage rail. The whole idea of having a pot is to get good degree of control over fan speeds.

**Mrfreddio55** · 01-30-2003, 06:33 PM

Originally posted by bluesfusion

If you cant find the starting voltage specs for your fan, you can do trial and error at 5V and 7V.

For 5V -
connect the fan leads to a molex connector from your PSU (fan positive to molex red wire, fan negative to molex black) and see if it starts.

For 7V -
Fan positive to molex yellow, fan negative to molex red) and see if it starts.

Whichever one it starts at, use that as your 'starting voltage'.
Most fans have starting voltage of about 6-7 V. Some may start lower than that.

Well, i dont have the fan yet.. however the specs it gives it are rated at 12 VDC... and isnt that what the yellow wire is on the molex? So, are there any POT's to handle this?

**bluesfusion** · 01-31-2003, 03:16 AM

Originally posted by Mrfreddio55
Well, i dont have the fan yet.. however the specs it gives it are rated at 12 VDC... and isnt that what the yellow wire is on the molex? So, are there any POT's to handle this?

Well, I tested all the fans I have today and they all seem to start at 5V. The wattage of the pot is unlikely to exceed 2-3W, so get a 5W pot to be on the safe side.

The optimum resistance depends on the fan current. How many amps does the fan draw? I made an excel spreadsheet to calculate the resistance given the fan current. (If fan wattage is given, just divide the number by 12 volts to give fan current.)

Tell me what current the fan draws and I'll tell you what resistance pot you'll need. Otherwise I calculated some values so you can look at the table below and get a Pot somewhere close to what the table says it should be for whatever your fan current is. It doesnt have to be exactly the same resistance pot... just get the closest one you can find.

Fan Current -- Pot Resistance
0.15 amps -- 115 ohms
0.2 amps -- 85 ohms
0.25 amps -- 70 ohms
0.3 amps -- 55 ohms
0.35 amps -- 50 ohms
0.4 amps -- 45 ohms
0.45 amps -- 38 ohms
0.5 amps -- 35 ohms
0.6 amps -- 28 ohms
0.7 amps -- 25 ohms
0.8 amps -- 22 ohms

**Mrfreddio55** · 01-31-2003, 03:25 AM

12 Vdc @ 1.1 Amp. Thermistor heat sensor.

That's all the info that it has.. thank you for all the help so far..

**bluesfusion** · 01-31-2003, 03:31 AM

Ok. 12V, 1.1 Amps. Plugging this into my exact calculations, I get -

Pot resistance around 15 ohms and Pot wattage about 3.5 Watts.

You definitely need a safety margin on the wattage. Find the highest wattage pot you can find which has max resistance around 15ohms.

At a minimum I'll recommend a Pot rated at - 15 ohms, 5 watts.

**CrawlingEye** · 01-31-2003, 03:40 AM

Originally posted by bluesfusion
You reduce your control range on the pot if you get too high a resistance. Whats the point of having a pot for fan control if the fan stays off even after you turen the dial over 90% of the way and then the fan starts and you got only maybe 10-12 degrees of turn to adjust the fan speed from min to max? If you want poor control like that, why not just put a fixed resistor on the fan or even better - run it off a lower voltage rail. The whole idea of having a pot is to get good degree of control over fan speeds.

Just so you know, there is no '10-12 degrees of turn to adjust' ever. In all the VR's I've used, they'll keep turning even after you've reached their maximum limit.

**bluesfusion** · 01-31-2003, 03:47 AM

Originally posted by CrawlingEye
Just so you know, there is no '10-12 degrees of turn to adjust' ever. In all the VR's I've used, they'll keep turning even after you've reached their maximum limit.

So? If the pots you've used keep turning after you've reached the max limit, does the resistance also keep on increasing forever? lol. :rolleyes:

What I said was perfectly correct.

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