24V DC computer fan speed control - diyAudio
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Old 24th March 2012, 07:10 AM   #1
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Default 24V DC computer fan speed control

Gooday all,
Can somebody plse help me with a diagram for a 24V dc motor speed control. I currently use a 24v transformer with 2 x 12 V DC computer fans connected in series. If it would be better then I will use a 12 V transformer with the motors in parrallel. A simple control using one transistor with a pot just to adjust the speed slower to reduce noise. Used for cooling AVR.

Thanks in advance.
Regards
Jan
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Old 24th March 2012, 07:23 AM   #2
wwenze is offline wwenze  Singapore
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Don't do that! (connecting 2 12V computer fans in series) You'll expose the components to 24V when one of the fan is "on" and the other is "off" (in order to reverse the direction of the current flowing through the windings).

12V power source, connect motors in parallel. You'll need a regulator too because the voltage of the power supply, after rectification and smoothing by capacitors, will be roughly ~16V, which you need to reduce to 12V.

Click the image to open in full size.

You'll need to know how much current your fan is drawing too, and get the correct package of LM317 and the appropriate heatsink.

Last edited by wwenze; 24th March 2012 at 07:48 AM.
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Old 24th March 2012, 08:09 AM   #3
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Default Fan spped control

Thanks for the reply WWENZE,
The motors are 12v / 0,28A brushless type, so I don't need a lot of current and only turns one way, so there is no direction change. I suppose your diagram will be sufficient ?

Thanks in advance.
Jan
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Old 24th March 2012, 09:16 AM   #4
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There's a very nice simple circuit from PAss labs to do this, allows you to adjust the speed via a 10k pot, and theres a mosfet that guarantees startup even at low fan speeds - I think it gives shot of current to get the fan running.

F5 power amplifier
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Old 24th March 2012, 09:26 AM   #5
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Soory why as you have 2 fans any way complicate the design whit voltagge change
why not keep in series?

I have made simple themperature controlled fan cooler I use for matching mosfets and hope it helps.

with 2 fans in series you wont need the LM317 part
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Old 24th March 2012, 09:59 AM   #6
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Default Fan speed control

Woodturner-fran,
I have a couple of IFRP 460 mosfet devices, will it be sufficient ?

Regards
Jan
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Old 24th March 2012, 10:35 AM   #7
wwenze is offline wwenze  Singapore
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Quote:
Originally Posted by Vrystaat View Post
Thanks for the reply WWENZE,
The motors are 12v / 0,28A brushless type, so I don't need a lot of current and only turns one way, so there is no direction change. I suppose your diagram will be sufficient ?

Thanks in advance.
Jan
Yupz, adjustable output voltage from 1.25V to almost 12V.

The LM317 is needed anyway because a 24V transformer gives ~32.5V peak after diode bridge drop.

When I said reverse the direction of current, it's about a DC brushless fan that spins in one direction. That's how they work - the magnets spin to align themselves to the magnetic field produced by the windings, then the current and hence field is reversed to force the magnets to continue moving. There will be a short duration where the output is "off" as the current's direction is switched.

Last edited by wwenze; 24th March 2012 at 10:41 AM.
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Old 24th March 2012, 10:38 AM   #8
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Default Fan speed control

WWenze,
So what you say is that I can still use the 24V transformer ? No need for 12V transformer ?

Regards
Jan
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Old 24th March 2012, 10:51 AM   #9
wwenze is offline wwenze  Singapore
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(If using my suggestion) 24V transformer is possible but not recommended... too much voltage drop = too much heat, you'd need a resistor before the regulator to eat some of the voltage and heat.
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Old 24th March 2012, 11:25 AM   #10
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Default Fan speed control

wwenze,
Would a 10 Ohm 25 Watt resistor mounted on heatsink between the transformer and LM 317 be okay ?

Thanks
Jan
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