Needed: fan control circuit... then a MOSFET ? also

Discussion in 'The Projects Forum' started by The Rick, Oct 2, 2009.

  1. The Rick

    Thread Starter New Member

    Nov 29, 2008
    Hi. I am looking for a fan control circuit that will turn the fan on at 160 degree’s F and off when temperature drops to 130 degree’s F. I built one of my own and managed to get it to turn ON at 130 and OFF at 160. It was turning the fan on as it passed the 130 mark. That didn’t occur to me at 1st. Silly me. Then I tried various circuits I found on the web and even one from a Data Sheet for the LM56 by National Semi.

    I have now built 4 circuits, and not a one of them works as needed. Now I am out here asking if any of you guru’s might have such a circuit or be able to point me to such a circuit, or even dream one up that I can try.

    I have another circuit, a constant current PWM, that is working, but I now understand it is running the MOSFET in “linear mode”. For best functionality, at least as I now understand MOSFET’s, the MOSFET should be running in switched mode. I am not sure this is entirely correct. If this sounds like something you can guide me on, please, educate me. I’ll submit the PWM schematic (just have to get it prepared) if you need it to determine precisely what I mean.

    I’m just an old dog who has been around electronics many years and can usually piece something together when needed. But these 2 circuits are giving me fits. Any suggestions, designs, help of any sort will be greatly appreciated.
  2. Audioguru


    Dec 20, 2007
    I guess you know that a fan will not cool unless it can blow away the hot air and replace it with cool air.

    Because some NOOBs put a fan inside a closed hot box.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    You don't need PWM unless you want to vary the speed of the fan. You just want to turn it on at a higher temp and off at the lower one. All that takes is a window comparator and a FET, and a temp sensor like an LM35 or similar.
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Sounds like the idiots I worked for at Power Ten. We put a high velocity fan inside to blow through the heat sink assembly and out the rear of the chassis..... but Joe whined about the "extra cost" to punch rows of holes in the front side walls of the chassis to let cool air in. Good old <snip>...... always ready to save ten cents at the expense of thousands of dollars.
    Last edited by a moderator: Oct 2, 2009