For instance, the capacitance accross the fan could be increased from 0.047 microfarads to a higher value in order to filter out more high frequencies introduced by the square wave in order to minimize high frequency buzzing traditionally associated with PWM at low fan speed. I am sure that there are things which could be tweaked in order to provide better operating characteristics. Also by me stopping the fan, you can visually see the effects of the electrical pulses going through the fan.Īs I said, this was only a proof of concept and not too much thought was put into it.
#Smc fan control wont change fan speed now series#
This shows off why the PWM design is cool because varying load doesn't affect the output voltage nearly as much as if you just placed the load in series with a potentiometer. Notice how me stopping the fan didn't cause the voltage to change much. This square wave output is fed to a NPN transistor which provides low impedance current to the fan. Two transistors connected to RC circuits cause a flip-flop action producting a square wave of varying duty cycle. I don't know if there is any truth to that though. He also believed that a fan rated at 12v would operate for a longer period of time without problems if provided with pulses of its rated voltage potential versus running the fan at lower voltage potential than it was rated/designed for. What I mean is that he believed the relation to potentiometer adjustment to the observed fan speed behavior would be more linear and predictable by adjusting pulse width instead of adjusting voltage. Regardless, he seemed to believe that adjusting speed via pulse width would provide more reliable fan operation versus adjusting speed via voltage. He graduated as an electrical engineer but never made it his profession. On another note, I asked my dad about this stuff a bit. The 35mA fan would only be operating at 8.9mA and would be spinning very slow. As you see, this wouldn't do good things for your fan control.
#Smc fan control wont change fan speed now full#
Given that information, I was able to conclude that with a 1K Ohm resistor in series with the fan such as you have drawn above, the voltage drop accross that resistor would be roughly 8.94v at full fan speed setting and thus the fan would only be provided with 3.065v for its operation. Given that the fan consumes 35mA at 12v I did some calculations and concluded that when the fan is operating at full speed, its internal resistance is around 343 Ohms. You would have to supply 5v from the power supply to run the PWM circuit and have the modulated output run to another final transistor connected to 12v would switch on and off accordingly and provide power to the fan. Adjusting the potentiometer would change the ratio between them so that all the way down would be off most the time with infrequent pulses of on while turning the potentiometer up would do the opposite and the duty cycle ratio would change accordingly for any speed in between.Īs you see above, the chip in the circuit can only handle between 2 and 6 volts. Duty cycle pretty much means the ratio of off time compared to on time. Turning the potentiometer changes the duty cycle of the power output. Turning the potentiometer all the way up would make the output to the fan effectively almost 100 since there would only be very infrequent breaks in power to the fan. Turning the potentiometer all the way down would make the output be effectively almost zero where there would be infrequent pusles of voltage to the motor.
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