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

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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?

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icengBest Answer (author)2012-08-08

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.

a

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Mihaiu (author)iceng2015-03-22

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.

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frollard (author)iceng2012-08-08

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.

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WWC (author)frollard2012-08-08

frollard

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

Thanks

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frollard (author)WWC2012-08-08

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.

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WWC (author)frollard2012-08-09

Ah that would be great. I will check that out. How would it be controlled?

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WWC (author)2012-08-08

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.

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iceng (author)WWC2012-08-08

Glad I was correct about the pins.
Try replacing D3 with a 1N400X.

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WWC (author)iceng2012-08-09

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.

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iceng (author)WWC2012-08-09

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 ?

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WWC (author)iceng2012-08-09

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.

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