Author Topic: pwm w/ 555  (Read 2248 times)

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Offline BANETopic starter

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pwm w/ 555
« on: May 08, 2009, 05:41:11 PM »
Hello,

I'm trying to make a circuit for my robot fan system.  I basically want the two micro fans to run at two speeds; 100% and 50%.   Running at 100% I will just apply 5v to a N channel mosfet from one pin (B) from my microcontroller.  At 50% i want to have a 555 timer provide PWM to the same N channel mosfet from another pin (A).  I think this will be simple.  Here is a circuit for the 555 w/ mosfet
I want the 555 to run on 5v and the fans to run on 9.6.  Also I don't want to have a pot in it, just a straight resistor for the end product.  The question I have is at what resistance do i need to have on the trigger to produce a 50% duty cycle?  Is there a formula?

being that the pot is 100k can i assume 50k will work?

thanks,

Bane
« Last Edit: May 08, 2009, 05:49:04 PM by BANE »
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Offline Soeren

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Re: pwm w/ 555
« Reply #1 on: May 09, 2009, 03:32:53 AM »
Hi,

Since you only want full speed and 50%, your circuit is quite overkill.
You can do this with one transistor for 100% and another transistor for 50% where you connect a resistor or a zenerdiode of a suitable value in series with the fan.
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Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline BANETopic starter

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Re: pwm w/ 555
« Reply #2 on: May 09, 2009, 09:41:13 AM »
Gosh, I'm always overdoing things :P
I'll draw something up hopefully more simpler

Bane
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