PWM Cut off adapter.

[paulca]

How simple a circuit could you make to...

Take a 4 pin PWM fan header input.

Provide a 4 pin PWM fan header output.

Pass through power, PWM and Tach UNLESS the incoming PWM duty cycle is less than (say 20%).  Then cut  the power entirely to the output.

Spec would be 12V, 25Khz PWM frequency, max 200mA.

A microcontroller would be overkill.  I am wondering if a mosfet with a pulled down cap charged by the PWM pulse would cause the mosfet to switch off when the PWM duty cycle stopped charging that cap fast enough for the pull down to over come it.  But, how then do I stop that cap interfering with the PWM signal pass through?

An op-amp buffer, again sounds overkill.  The mosfet gate take sod all current, so... I could make the pull down cap, really small and put a large(ish) resistor like a 4.7K between the PWM line and the pulled down cap.

Making the mosfet "snap" ON/OFF and not ramp through the resistive phase would be an additional challenge... I'm sure there are ways, such as doubling the mosfet in a pair config or using hysteresis feedback on the gate.

The ultimate aim is noise control.  PC fans are all moving in a direction where they will not stop.  Mostly this is, sadly, to stop idiots RMI'ing their fans because they connected them to a DC header, so they got 0% PWM and so didn't start.  Most fans now force run below 20% PWM.  Not so surprisingly other brands have not started selling expensive "0db" fans as a "new" thing. 

So, I'm wondering can I solder something together for a few quid, wrap it in shrink wrap and modify any PWM for complete stop.... or should I just spend the extra £10! per fan, all 9 of them and buy the new "suckers" 0db fan.

[paulca]

I suppose making a PCB that can support 8-16 fans simultaneously would make luxuries like micro-controllers, mosfet drivers and op-amps more practical.  I even suppose if I use an MCU with built in hardware serial... and a USB Serial adapter I could even write a command line for disabling, enabling fans and forcing some not to cut out while others to do so.

[Terry Bites]

Why not just open circuit the fans- say a turn off mosfet in the common ground of the fans?

[ledtester]

How about: a PWM -> DC converter (just a simple RC low-pass filter) to a comparator which controls a MOSFET at the ground of the fan as Terry Bites suggested.

[Le_Bassiste]

some food for thought.

[Ian.M]

Caution: The motherboard PWM output is *supposed* to be open drain^#, so shouldn't be overloaded driving a powered down fan, but do you want to bet on that?  You may need to gate the PWM signal to disconnect it when the fan is powered down.

Also, as its PWM output is probably open drain with a pullup of uncertain value, in Le_Bassiste's circuit you may need to either use a rather large R1 to avoid excessive loading on the PWM signal, or buffer it before filtering.

# See Intel's 4 wire PWM fan spec: https://www.glkinst.com/cables/cable_pics/4_Wire_PWM_Spec.pdf

[Le_Bassiste]

Caution: The motherboard PWM output is *supposed* to be open drain^#, so shouldn't be overloaded driving a powered down fan, but do you want to bet on that?  You may need to gate the PWM signal to disconnect it when the fan is powered down.

Also, as its PWM output is probably open drain with a pullup of uncertain value, in Le_Bassiste's circuit you may need to either use a rather large R1 to avoid excessive loading on the PWM signal, or buffer it before filtering.

# See Intel's 4 wire PWM fan spec: https://www.glkinst.com/cables/cable_pics/4_Wire_PWM_Spec.pdf

thx ian, good catch about disconnecting the PWM when fan supply is down. this wouldn't be too difficult, though:
(as for the load on the PWM by the filter, high values of R1 (say, 1MOhms) should be possible, as the input of the opamp is high impedance anyway.)

[Ian.M]

Too many MOSFETs - M3 is redundant as the fan's PWM input cant source current when the 12v supply to it is off.  Otherwise that's a reasonable solution, though you'll have to change the R5:R6 divider depending on whether PWM is pulled up to a 5V level or a 3.3V one.