I'm hoping to convert some leftover 140mm PWM case fans (standard 4-pin computer connector) to a quiet room air circulator, with proper pulse-width speed control instead of voltage based.
I haven't found much on exactly what the supply needs to be, i.e. ground, +12v, pwm, rpm sense, but what does the pwm signal need to look like? Best I have gathered so far is it's a 21 kHz 12v square wave signal where the ratio of how long the signal stays at +12 within the cycle determines the fan speed.
My thought was to use one side of a 556 to supply the 21 kHz to trigger the other side to hold output high for an adjustable time, feeding the output to a power transistor to drive the 4 fans.
Does this seem reasonable, or am I in the wrong ballpark?
I haven't found much on exactly what the supply needs to be, i.e. ground, +12v, pwm, rpm sense, but what does the pwm signal need to look like? Best I have gathered so far is it's a 21 kHz 12v square wave signal where the ratio of how long the signal stays at +12 within the cycle determines the fan speed.
My thought was to use one side of a 556 to supply the 21 kHz to trigger the other side to hold output high for an adjustable time, feeding the output to a power transistor to drive the 4 fans.
Does this seem reasonable, or am I in the wrong ballpark?