How to eliminate PWM noise at slow speeds when using a PC fan?

My power supply has a cooling fan inside. I wanted to slow it down to reduce the fan noise. When i use this circuit in the picture the PWM works great except for the low speed buzzing. Turns the fan into a speaker. Is there some way to eliminate the buzzing?

Picture of How to eliminate PWM noise at slow speeds when using a PC fan?
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iceng5 years ago
First pins 3 and 7 should be exchanged..
I don't think you need the iK resistor or the 470 uF electrolytic Cap..

If you want a permanent fan slow down
Just put 3 or 4 1N4001 diodes in series to your fan,
each diode slows the fan more without PWM noise.

Mihaiu iceng2 years ago

Good point!

The cap. at pin 5 should be at least 10nF. A limiting resistor(say, 1kΩ) in series with wiper won't hurt either.

frollard iceng5 years ago
Another thought -- does the psu have built in fan speed control (controlled by the internal temperature) - if so, then the pwm might be fighting with the internal pwm - as one goes down the other goes up making some crazy interference patterns and making the motor choke.
WWC (author)  frollard5 years ago

This fan is not in a computer. It is a fan from a computer that is inside my power supply.

frollard WWC5 years ago
Right, just assuming it was the fan already built into the power supply, and many power supplies have built in speed control - independent of motherboard control.
WWC (author)  frollard5 years ago
Ah that would be great. I will check that out. How would it be controlled?
WWC (author) 5 years ago
I must apologize to both of you who took your time to reply, I had uploaded the incorrect sketch. If i didn't mess up again then the sketch i loaded now is the correct one.

Thanks my friend A for the diode suggestion, but that would just be to simple. You know we need to over design / build its in our nature. ha.
iceng WWC5 years ago
Glad I was correct about the pins.
Try replacing D3 with a 1N400X.
WWC (author)  iceng5 years ago
I had previously tried that but still the same. I think that circuit must be better as a dimmer. I put together the one with a LM324, what a nice operating PWM circuit. Has a " kick start " of a 4.7uF cap between the wiper of the pot and ground. It will start at slow speed nicely. 0 to 100% speed. No noise other that air movement.
iceng WWC5 years ago
Like your LM324 circuit and am interested if you have a high impedance
scope probe what the waveform on pin 7 looks like.
Otherwise a peek on pin 8 at 50 % will do ?
WWC (author)  iceng5 years ago
No i don't. That kind of stuff is expensive here, but i am planing to back to Oregon maybe February and i see them on Ebay US reasonable at times. Hope to pick one up.