Inverted pwm with 555

Thread Starter

valepe69

Joined May 28, 2019
15
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.
 
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crutschow

Joined Mar 14, 2008
34,281
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.
 

Thread Starter

valepe69

Joined May 28, 2019
15
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:
C:
pwm = 255 - out * 255
Where out is a range from 0.0 to 1.0
 

Thread Starter

valepe69

Joined May 28, 2019
15
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?
 

crutschow

Joined Mar 14, 2008
34,281
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.
 
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