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Old 23rd February 2008, 07:35 PM   #1
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Default PWM fan control noise

My LM4766T surround amp will have the 3 IC's on one heatsink.The heatsink is from the DVD player I got the IC's from and was cooled by a 12v fan.
To keep the sound level of the fan down I was thinking of making a temp controlled pwm controller for it.
My question is,will my attempts at keeping the fan noise down cause audible elecrical noise in the signal path,and if so is there some way of minimising/eliminating it?
I have searched the forum but did not find anything about this.
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Old 23rd February 2008, 09:26 PM   #2
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Default Re: PWM fan control noise

Quote:
Originally posted by Aidan135711
My LM4766T surround amp will have the 3 IC's on one heatsink.The heatsink is from the DVD player I got the IC's from and was cooled by a 12v fan.
To keep the sound level of the fan down I was thinking of making a temp controlled pwm controller for it.
My question is,will my attempts at keeping the fan noise down cause audible elecrical noise in the signal path,and if so is there some way of minimising/eliminating it?
I have searched the forum but did not find anything about this.
Yes it will make an electrical noise and it will cause the fan motor to make sounds and wear out quickly.
A resistor will have the fan powered quietly.
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Old 23rd February 2008, 10:24 PM   #3
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Ok,so a resistor will slow down the fan permanently but I want the fan to run slow at low powers and when I turn up the volume the amp will heat up and I want the fan to speed up proportionally,automatically.
Maybe I will put some sort of temperature display and manual fan speed control knob on the front panel of amp,just like some pc fan controllers I have seen.
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Old 24th February 2008, 12:44 AM   #4
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The slowed fan just might work better than you think. I slowed this fan to about 1/3,the heatsinks never got hot.

Maybe a Thermistor?

http://www.diyaudio.com/forums/attac...amp=1114126181
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Old 24th February 2008, 05:23 AM   #5
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Howabout a slow fan and a simple bimetallic thermostat? Then the fan will last.
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Old 24th February 2008, 05:51 AM   #6
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Quote:
Originally posted by Aidan135711
Ok,so a resistor will slow down the fan permanently but I want the fan to run slow at low powers and when I turn up the volume the amp will heat up and I want the fan to speed up proportionally,automatically.
Maybe I will put some sort of temperature display and manual fan speed control knob on the front panel of amp,just like some pc fan controllers I have seen.
There are some nice designs for analog (as opposed to PWM) 12V fan controller based around MOSFETs on the net. An analog control will probably be best for what you are doing, but be aware that even without PWM, you will need to carefully filter out the electrical noise created by the fan motor.
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Old 24th February 2008, 11:21 AM   #7
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Ok,some good simple ideas there for me to think about.
How then do I filter any electrical noise from the fan and where does the circuit go eg on fan pwr supply(which be a seperate winding,rect,smoothing caps etc) or on the supply rails to the amp or what?
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Old 27th February 2008, 01:50 AM   #8
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You could build it and find out...............

I'm not sure about PWM, but regular linear works fine,

You just need a good heatsink for the regulator transistor, I used MJL4281 to run 6 fans.

I used a trimpot to set fan idle speed ~ 5.5V

Has up to 17V (12V unregulated) to pull from, so a lot of the voltage is dropped, but it's quiet and reliable. I used High Speed fans so when I ran them slow and quiet, they still pushed good air. Also that gives the option to have high speed fans if you give them some voltage.
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Old 27th February 2008, 01:57 AM   #9
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There's an electric fan circuit from Nelson Pass here somewhere.
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Old 27th February 2008, 03:04 AM   #10
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This is the one I use. Using a seperate 12v supply and only three parts. The speed varies with temperature.

http://www.heatsink-guide.com/conten...=control.shtml
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