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Recently I switched some case fans in my pc to find the horror that the new ones were even louder than the old ones. After some quick checks just by eyeing and monitoring on my pc I can confirm the new fans like to spin with the pc reporting about 1000 RPM whilst the old fans at best ran at around 600 RPM. These fans only have three pins so I though I could make some PWM controllers as a fun project, calculating the power drawn from the new fans would be in the range of about 2-2.5 watts per fan. So I'm thinking of using 5 watt components to drive it. However I came across no clear way to actually do this, for the tachometers to work I need straight DC into the fan.

I came across this circuit:
enter image description here
So would something like this work given I invert the signal just before sending it to the a power stage like the one shown in the upper quarter image? I think I remember PC fan PWM signals saying the fans should be off when the duty-cycle approaches 0 (until the built in shmitt trigger activates).

Would this work?

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  • \$\begingroup\$ No, because the PC fan is brushless and it has it's own driver. What you are going to do is turning on and off the circuit making him mad. The fans with 3 pins have an input PWM for controlling the speed. \$\endgroup\$
    – Marko Buršič
    Jan 16, 2016 at 21:40
  • \$\begingroup\$ Actually 3 pin fans only have monitoring, above image is supposed to be inverse-pwm to dc \$\endgroup\$
    – MetroidChild
    Jan 16, 2016 at 21:42
  • \$\begingroup\$ @MarkoBuršič, I believe it responds fine to PWM drive. I've used 2 PC fans - one PWM'ed fine at 1kHz, the other "went mad" as you mention, but was fine at 50Hz. \$\endgroup\$
    – Tahmid
    Jan 16, 2016 at 21:45
  • \$\begingroup\$ @MetroidChild You are right, the 3rd wire is tacho. You wont know until you'll try it. pcbheaven.com/wikipages/How_PC_Fans_Work \$\endgroup\$
    – Marko Buršič
    Jan 16, 2016 at 21:46

2 Answers 2

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The 100uF capacitor will cause the speed not to change instantly with duty-cycle. That does, however, cause the speed to change more smoothly and slowly and can contribute to a 'soft-start' effect. The time constant is about 0.1s so keep that in mind when thinking of fan speed response time. However, you can go ahead and use this circuit as a starting point and deduce if you need to change anything based on what you find.

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  • \$\begingroup\$ Thank you! I was able to tweak things enough to get it running in an online circuit simulator applet thing. I've decided to use a 22uF cap instead of the 100uF one presented as it worked perfectly down to about 30% duty cycle, hopefully I'm gonna have an easy time making this work \$\endgroup\$
    – MetroidChild
    Jan 16, 2016 at 22:05
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There are chips available that PWM-control 3-wire fans and can monitor the tach line, understanding to only check while the PWM is on. The ADT7470, for example, can control and monitor 4 fans and interface to a host device over I2C. Unfortunately, it's only available in surface mount so it's harder to solder for a novice.

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  • \$\begingroup\$ But that would take all the fun away! \$\endgroup\$
    – MetroidChild
    Jan 17, 2016 at 18:01

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