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PWM signal from 555 timer

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fsleeman:
I have been trying to make a PWM generator from a 555 to control a fan and started with this circuit: http://www.kpsec.freeuk.com/555timer.htm#astable. I have been able to make an LED flash at the specified frequency but haven't had luck correctly setting the duty cycle.

The formula is:
Duty cycle  = (R1 + R2)/(R1 + 2R2)

which I have seen in other places as well. When I try to make that circuit, I seem only get a duty cycle close to 50% when I inspect it with an oscilloscope. When I change the R1 and R2 resistors in the shown circuit, I seem to get a lot closer to the duty cycle I am trying to get.

Can anybody verify if this formula or circuit is correct? If I am trying to get a 75% duty cycle, what values should actually work? If I solve the equation, it seems that if R2 is 10k then R1 should be 20K. Is that correct or am I doing this wrong?

Mechatrommer:
i strongly believe 555 only produce 50-100% duty cycle, thats the reason i abandoned it :P. you will sometime have to cope with resistor inaccuracy (5%? 10%?) so my advice is to use potentiometer to adjust to desired PWM.

Zero999:
It's possible to go down to near 0% by adding a diode, see the datasheet.

Add a couple of diodes, a potentiometer and you can get PWM from 1% to 99%.

Mechatrommer:
great!

qno:
Hi,

If you want a simple 0 to 100% pwm with an 8 pin dil look at this circuit.

http://www.extremecircuits.net/2010/07/luxury-car-interior-light.html

Replace the switch with a potmeter on pin 6 of opamp B and you are in business.