Disconnect DC fan when voltage drops below

Discussion in 'The Projects Forum' started by phil w, Oct 14, 2014.

  1. phil w

    Thread Starter New Member

    Oct 14, 2014
    3
    0
    Im looking at an example of a DC fan circuit that regulates voltage according to temperature. Its an online example that i have modified slightly.

    R1 is a 10k thermistor.
    Q1 and Q2 are NPN and PNP transistors.

    Id like to modify the circuit so that the fan will only come on when at least 3v is present. The fan is not shown on the diagram but i would put in it parallel with R7/D2. The fans I have have a minimum operating voltage of 6v and so running them at anything less will probably stall them.

    thus if the output is less than 3 volts then the output should be 0
    if the output is >=3 then the output passed should be 6
    else the output should be passed unadjusted.

    any help is appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    The base of Q1 will be ~0.5V when the V1 voltage drops to 3V. Doesn't that shut off the fan?
     
  3. phil w

    Thread Starter New Member

    Oct 14, 2014
    3
    0
    it would for a battery but here V1 is 12dc from a PSU
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Yeah, sorry, I misunderstood the problem. You're talking about the situation when the controller sends, say, 2.9V to the fan.

    There are plenty of ways to accomplish the cutoff (such as comparator, voltage reference, MOSFET switch), but by the time you implement all that you might as well switch to using PWM of fan speed in the first place. It'll give you smoother control and allow the fan to run to a lower rpm if needed.
     
  5. phil w

    Thread Starter New Member

    Oct 14, 2014
    3
    0
    Thanks for your suggestions.

    Ive added a MOSFET with a gate voltage of 4v. Do you think this would work?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Not predictably. You're way off the chart for that MOSFET, so only an experiment will tell you how much current it will pass with 3V on the gate.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    I only see one voltage source in your schematic. What am I missing?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    He wants the fan to cut out when its being supplied <3V. Or is it 6V?
    Slows down under temp control, then stops completely.
     
Loading...