control of fan speed with sensor (my plan, please give your openion)

[KobiAflalo]

i need to control the speed of a fan with control of a heat sensor, the fan will probably will change speed with PWM modulation.... right?
i found LTC6992 pulse width modulator

can i simply connect the fan voltage input into the PWM modulator (square wave) and get variable speed? or shuld i use something like integrator/capacitor in the output to get something like this:


also the MOD input will be on/off by a comparator circuit something like this:

the idea is that the sensor will probably give 2 voltage signals 1V for 120[celcius] above and 0V for 30[celcius] below (just a guess at this moment)

is my plan is ok, or am i simply make something simple more complex?
and can you suggest plans to make it better?

[DVDdoug]

If you're using an Arduino, it's simpler because the Arduino has PWM built-in.  But, if you  just need some simple logic and you don't need the Arudino, you can use something cheaper that doesn't require programming.  (i.e. A regular non-programmable thermostat doesn't have a "brain" or memory.
 

You just need a transistor or MOSFET to boost the voltage & current.  (I haven't looked at the datasheet, but I assume the LTC6992 also needs a transistor/MOSFET to drive the fan.)

or shuld i use something like integrator/capacitor in the output to get something like this:

No.  The inertia of the motor & fan take care of the "smoothing".   Plus, the transistor/MOSFET operates more efficiently in switching mode.

The tricky part is the transfer function between temperature and fan speed...   You can easily end-up with something that oscillates between fast & slow as the fan continuously adjusts and "hunts" for the right speed as the temperature changes follow.  (That's assuming a "feedback" system where the fan is cooling whatever you are measuring the temperature of...  Like the temperature of a computer...  If you just trying to make the room feel  cooler by turning on a fan when it gets warm, that's not an issue.)

It's if you just turn the fan on or off, or maybe have 2-speeds with a time delay before it goes back to slow or off.   (It will still tend to cycle like a heater or refrigerator.)

[Wawa]

Do you want to control the SPEED of the (unknown) fan or the POWER to the fan.

In the first case, you need to have/read a speed sensor attached/inside the fan.
That might give problems with a PWM-ed fan.

I think we need to know what sort of fan we are talking about.
Leo..

[KobiAflalo]

Do you want to control the SPEED of the (unknown) fan or the POWER to the fan.

In the first case, you need to have/read a speed sensor attached/inside the fan.
That might give problems with a PWM-ed fan.

I think we need to know what sort of fan we are talking about.
Leo..

if the fan have a tachometer, what can i do?

[Chagrin]

Going to assume that this is a DC, brushless, "muffin" fan of a type typically found in a computer.

It's not necessary to read the tachometer signal; if you simply PWM the power to the fan you'll get variable speed, and you can assume that it will always be the same speed for a given PWM signal.

You probably will want to add a capacitor in parallel with the fan's power leads. Feel free to experiment and try various sizes of electrolytic capacitors. Without the capacitor, fans have a tendency to make a high pitched noise due to the quickly switching current.

[Wawa]

Either forget about reading the speed and just PWM the negative lead of the fan with a NPN transistor or N-channel mosfet.

Or use a level translater and high side switching .
Then you can detect the speed by reading the (yellow) tach wire.

Let us know what you want to do.
Leo..