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How do you slow a 12V fan down?

DeanS

By DeanS
in Boat Equipment

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DeanS Veteran II

DeanS

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Evening all.

 

Do any of you who are intellectually superior to myself, know how to slow down a 12V computer fan.....by inserting a resistor or something in series..I would imagine? Do you know what value? I'd basically like to drop the speed of the fan to as slow as possible. It's rated at 3000rpm ...I wouldnt mind it going at 300rpm....

 

Thanks:)

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Chalky Experienced

Chalky

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Computer fans use brushless motors that use electronic control to control their speed. This is usually done by switching power between a number of coils on the stator in rapid succession - this attracting the rotor which may contain magnets or an iron core. Altering the voltage fed to this type of motor will have little or no effect on its speed (if the controller is correctly designed), but it will alter the motor torque. Speed is controlled by changing the switching speed. There are computer fans that have a 3rd lead that allows you to do this. You need one of these.

 

If it's a brushed motor fan then you need to drive it off a series of pulses - called pulse width modulation or PWM. A resistor would be wasteful and dissipate a reasonable amount of heat - depending on motor size.

Edited by Chalky
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Gibbo Veteran II

Gibbo

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Computer fans use brushless motors that use electronic control to control their speed. This is usually done by switching power between a number of coils on the stator in rapid succession - this attracting the rotor which may contain magnets or an iron core. Altering the voltage fed to this type of motor will have little or no effect on its speed (if the controller is correctly designed), but it will alter the motor torque. Speed is controlled by changing the switching speed. There are computer fans that have a 3rd lead that allows you to do this. You need one of these.

 

If it's a brushed motor fan then you need to drive it off a series of pulses - called pulse width modulation or PWM. A resistor would be wasteful and dissipate a reasonable amount of heat - depending on motor size.

 

Technically spot on. However in practice these fan controllers are terribly designed and do indeed slow down with the introduction of a resistor. However they sometimes dont fire up properly because of the resistor so you have to also stick a big cap across the resistor to get them going on initial power up.

 

If you let us know the rating on the motor we can work out the value of resistor and cap.

 

But chalky's right, it's not the correct way to do it.

Edited by Gibbo
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DeanS Veteran II

DeanS

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Technically spot on. However in practice these fan controllers are terribly designed and do indeed slow down with the introduction of a resistor. However they sometimes dont fire up properly because of the resistor so you have to also stick a big cap across the resistor to get them going on initial power up.

 

If you let us know the rating on the motor we can work out the value of resistor and cap.

 

But chalky's right, it's not the correct way to do it.

 

Hi Gibbo. Thanks for this. I was thinking of bypassing the resistor with a switch(short) and once it's turning, flip the switch to bring the resistor in?

The fan is a Bisonic, model SP802512H , DC12V, 0.2A, Brushless, speed 3000rpm, starting voltage 7V,

 

I'd like the fan to turn as slowly as possible...just to create a slow movement of air from A to B.

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DeanS Veteran II

DeanS

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Surely a computer fan is designed to dissipate heat from PCB and processors. If you reduce speed these may overheat and cause other problems.

Mike

 

Hi Mike.

People dont use computer fans for PCB and processors on narrowboats...they're handy for all sorts of air-movement projects.

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Robin2 Rising Star

Robin2

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I have an old computer fan and have never had any problem slowing it with resistors - maybe ignorance helps at times. I have 1 x 68ohm and 1 x 47ohm wired on a 3 position switch so I can have 0, 47 or 68 ohms for different speeds. I can't remember if I bought 7 Watt or 3 Watt resistors in Maplin. They get slightly warm to the touch.

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smileypete Veteran II

smileypete

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Hi Mike.

People dont use computer fans for PCB and processors on narrowboats...they're handy for all sorts of air-movement projects.

Try a car USB charger (£1 off Ebay).

 

I have a couple of standard 120mm PC fans right here that start and run fine from 5V.

 

cheers,

Pete.

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Robbo Veteran II

Robbo

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Surely a computer fan is designed to dissipate heat from PCB and processors. If you reduce speed these may overheat and cause other problems.

Mike

 

CPU's, etc. don't run full pelt all the time, many of the better fans change their speed with the temperature.

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Chalky Experienced

Chalky

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I've got a fan in one of the pcs that has a small speed controller atached to it. It was a commercial item so they are available.

 

Bit small for me - most of the brushless motors I deal with are 50 - 200kw!

Edited by Chalky
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DeanS Veteran II

DeanS

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I bought a 68ohm resistor (as mentioned by Robin2) , from Maplin, shoved it in series with the fan, and it drastically slows the fan down, without affecting the startup torque. All good. Thanks for all the advice. Sorted :)

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NBMike Rising Star

NBMike

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Hi Mike.

People dont use computer fans for PCB and processors on narrowboats...they're handy for all sorts of air-movement projects.

Doh! Of course they don't, and in fact I have 3 such fans sitting on a shelf in front of me for just such a use. Blinkered tunnel vision or something!!!

Mike

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Pretty Funked Up Rising Star

Pretty Funked Up

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As stated before they are very cheaply made these fans.

When I was using them I had a 12v regulator that had different voltage outputs (3v - 12v) works perfectly well for varying the speed.

In the summer I fed them 19v for what felt like double the air movement.

probably not ideal but works perfectly.

Edited by Pretty Funked Up
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Andy Woods Engager

Andy Woods

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Been running three computer fans mounted to the bottom of mushroom vents to aid moisture extraction in winter....works a treat by the way..

They all have 68ohm 1 watt resistors fitted with no problems at all. With this value resistor the fan runs virtually silently as one is mounted above the bed. The fan in the kitchen has been running constantly for nearly two years now!!

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