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software PWM

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large_ghostman

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just checking i am on the right path, i need to control a number of small fans with pwm, so far the frequency looks like being around the 3kHz range. i am thinking software pwm with two timers, i dont need huge resolution probaly 10 settings between 70% and 100% should do it, am i correct in thinking i need to set first timer as the frequency, so as it rolls over i turn all fans on the second acts as duty cycle so i set it for a period that equals 1oth of the main clock and switch off any fans that match the number of times the timer has rolled over?
hmmm i think i have confused it all there but i kind of know what i mean :D
 

Pommie

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With anything mechanical like fans I would use a frequency that's not audible, either below 20Hz or above 20kHz. For resolution it would be easiest to do the full 100% range. Therefore, I'd choose something like 15Hz and 100 steps which gives an interrupt frequency of 1500 times per second. Assuming a 4MHz oscillator and a pic, you can use timer 2 with prescaler=4, PR2 = 166 and this will give you 1500 interrupt per second. In your interrupt have a counter that goes from 0 to 99 and when it wraps set all outputs that don't have PWM=0 high. Then each subsequent interrupt check all PWMs against the counter and set the ports of any that match low.

If you use a higher crystal frequency then prescaler=16 may help or use timer1 (16 bit) with the PWM special event trigger to generate the interrupt.

Have fun,

Mike.
 

large_ghostman

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thanks pommie i will try that. not 100% sure the clk yet as i have to check something first. i will go with 15Hz, i tried 1K upto 20K and anything above about 3.5 didnt seem to work too well, these are ex pc fans and i think they are BDLC with the controllers inside, they sure didnt like me winding them right up :D so i will try the lower frequency, i did try and make them scream or whine but none of them made much noise :(
 
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