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Topic: control of fan speed with sensor (my plan, please give your openion) (Read 1 time) previous topic - next topic

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?
begginer Board Designer.

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.)

Quote
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?
begginer Board Designer.

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..

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