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2 wire fan control

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andy257

Member
Hi All,

I need to use a 2 wire Fan in a circuit designed for a pwm fan. I am having some problems with the control of the 2 wire fan. The fan is 24VDC.

I have attached the circuit below.

The attached circuit shows a mod i have integrated to allow the fan to start up under low mark/space ratios. Without the cap the fan simply wasnt getting enough kick to start up.

However i would still like the original circuit to be able to control the speed of the fan (using an analogue voltage control rather than PWM).

The purpose of the circuit is temperature control. Using 10 levels of mark/space pwm the fan should have 10 different speed settings. The pwm is adjusted according to the pre defined temperature settings.

Can anyone see how i can modify this circuit further or if you see any problems already with the current design.


Thanks
 

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  • pwm mod.jpg
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Last edited:

Boncuk

New Member
Hi,

I can't see the reason why to have 10 different speed settings for a small fan.

At 1/2 nominal rpm the air flow is also 1/2, but the fan total static pressure is decreased to 1/4.

At low fan pressure the fan won't do what it is meant for - cooling. It will just rotate without the desired effect.

I suggest to set the lowest speed to have the fan run without adding a cap parallel to it. A cap might be helpful to reduce fan noise caused by PWM control.

On the other hand using a big cap will almost null PWM.

Boncuk
 
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