Hi, i would like to build a circuit generating an inverted 25 kHz pwm with 555 ic and a trigger to regolate the pwm.
Is it possible?
I must drive a pwm fan with inverted Logic (0V full Speed), it's a fan from a hp rack server.

 

Welcome to AAC, @valepe69

A circuit like the one below can generate PWM from 0% to 100% and will do your job.
https://circuitdigest.com/electronic-circuits/555-timer-pwm-generator-circuit

 

Welcome to AAC, @valepe69

A circuit like the one below can generate PWM from 0% to 100% and will do your job.
https://circuitdigest.com/electronic-circuits/555-timer-pwm-generator-circuit

Thanks but i forgot to add a trigger for generating the signal (now i edit my post).
A question: It seems a standard pwm not a inverted one (full speed at 0V and stopper at 5V). Am I wrong?

 

Is rotating a pot to change the PWM duty-cycle, as the 555 circuit does, sufficient for your purposes?

 

A question: It seems a standard pwm not a inverted one (full speed at 0V and stopper at 5V)

The PWM duty-cycle goes from 0% (0V average) to 100% (5V average).
What you want to call full speed is just an arbitrary definition for that duty-cycle.
It has no effect on the waveform.

 

Is rotating a pot to change the PWM duty-cycle, as the 555 circuit does, sufficient for your purposes?

I suppose yes. I saw a sketch for Arduino where the pwm is generated with this code:

pwm = 255 - out * 255

Where out is a range from 0.0 to 1.0

 

I suppose yes. I saw a sketch for Arduino where the pwm is generated with this code:

pwm = 255 - out * 255

Where out is a range from 0.0 to 1.0

That should work if you want to use the Arduino to generate the PWM.

 

That should work if you want to use the Arduino to generate the PWM.

Well, I'd like to build a discrete circuit so my question. The code is to illustrate a working solution. But re-reading the code i think that it doesn't change the duty cycle but the voltage. Am i wrong?

 

Well, I'd like to build a discrete circuit so my question. The code is to illustrate a working solution. But re-reading the code i think that it doesn't change the duty cycle but the voltage. Am i wrong?

Not that familiar with Arduino code, but the code line starts with PWM so I would assume it generates a PWM signal that varies in duty-cycle by 1/255 step size changes with an out value of 0.5 giving a 50% duty-cycle for example.