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PWM fan speed controller

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makr

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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?
 

Tony Stewart

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Most Helpful Member
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?
556's are not quite enough Umph for Hi or low side. since they drop 2~3V at 200mA

Whatever you use to drive the Fans should be < 0.5V @ 350mA or RdsOn=< 1 Ohm preferred low side (Nch) or high side (PCh.)

You can also bias a Schmitt inverter oscillator with a pot to and a few R's and Cap to input ground to shift the input waveform and output dutycycle to drive a FET.at 20kHz
 

dr pepper

Well-Known Member
Most Helpful Member
Frequency probably isnt important, the line has a pullup so default is 100%.
As Ian says voltage probably isnt important too 5 or 12v.
 
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