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Need help with Fan Speed Control

Discussion in 'General Electronics Discussion' started by David Steed, Jul 29, 2014.

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  1. David Steed

    David Steed

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    Jul 29, 2014
    I'm new to the forum and new to electronics. I am using a 12V/80 Watt fan and want to use a switch to control high and low speeds. From what I've read it appears that using a diode (or multiple) would be the way to go but everything that I've read online has been in reference to CPU cooling fans, not larger fans drawing more amps. I need to mount the switch in a small plastic box so I think that heat might be an issue if I use resistors. Any ideas (written where a layman can understand) would be appreciated.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Whether you use diodes or resistors, the same amount of heat will be generated. Diodes just give you better control of the amount of voltage dropped.

    A 12V 80W fan will draw 6.7A at 12V. At lower voltages it will draw less current.

    What voltages do you need to provide to your fan? If you need, for example, 12V and 8V, then you could generate the 8V using a buck switching converter. These are fairly efficient so less heat will be generated.

    If the fan draws 4.5A at 8V, or 35.6W, and the buck regulator is 80% efficient, it will dissipate 20% of 35.6W which is 7W. This is still a significant amount of power, but a lot less than a linear voltage dropping solution.

    If you need multiple speeds, or if the low speed needs more than 8V, a switching supply may not be worthwhile.

    Another option is PWM (pulse width modulation), where the full voltage is applied to the fan, but not constantly. A circuit switches the voltage ON and OFF, and the ratio of ON to OFF determines the average voltage, and therefore the speed, of the fan. PWM does generate interference though; a switching converter is a cleaner option.

    Google PWM fan speed control and check out http://en.wikipedia.org/wiki/Fan_control

    Tell us more about your requirements and we will be able to recommend a design.

    P.S. Oh, and welcome to Electronics Point :)
     
  3. David Steed

    David Steed

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    Jul 29, 2014
    Thank you very much for the input. I've ordered but haven't yet received the fan so I'm not sure how much I can restrict the voltage and still start the fan. I understand that some fans need a minimum voltage to startup. Basically I'm just wanting to switch to a lower fan speed. The project is an ice-chest air conditioner to use in my airplane. Simple design - fan blows mass amounts of air into and over ice and cold air comes out of the tubes in the top of the ice chest. I also carry a 12V/7ah battery as a backup to run the unit before I start the engine, then switch to the aircraft power outlet. This is a new design and in the past I've used a much smaller motor that only draws 3-4 amps so switching wasn't necessary. This time I'd like to be able to run on low primarily when the small battery is powering the unit as well as for comfort control and to control the rate at which the ice melts (more air movement - faster ice melt). So, the desired result is less battery draw as well as slower fan speed. I'm open to any ideas.
    Thanks,
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
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