Step down voltage

Discussion in 'General Electronics Chat' started by 1-3-2-4, Mar 7, 2013.

  1. 1-3-2-4

    Thread Starter Active Member

    Dec 26, 2008
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    I have a question.. I have a 18.6V supply and a 12V fan..

    I have a LM2940T and a IRF510.. now what I want to make is this

    http://www.radiolocman.com/shem/schematics.html?di=47949

    So the trimmer would be acting as a voltage regulator? I want to make sure this is good because last time I went to turn the trimmer my NTC thermistor just pretty much went up in smoke.

    I assume the TO-220 is going to be giving off a lot of heat as the fans pull .62A
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The pot will be acting like half of a voltage divider with the thermistor as the other half.

    If you turned it down to zero ohms, and the thermistor was low resistance, too much current would flow and blow up thermistor.

    The MOSFET will get warm due to acting in ohmic region rather than as a switch.

    There are much better designs around, usually including a minimum resistance in series with the potentiometer to prevent a zero ohm pot from destroying anything. PWM is the best speed control method for tasks like this, but it is more complicated.
     
  3. 1-3-2-4

    Thread Starter Active Member

    Dec 26, 2008
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    If you did PWM could I use a 555 and keep the thermistor? How much more complicated could it be? It's going in on a heatsink so it's going to be set and forget.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I would suggest an 8 pin PICAXE. Thermistor read by ADC, PWM output to MOSFET for motor control. Total of maybe 5 parts. However, I'm biased toward using a digital hammer for analog projects. :D

    555 Method
     
  5. 1-3-2-4

    Thread Starter Active Member

    Dec 26, 2008
    194
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    Yeah I just found that site.. but wont I have issues being it's a brushless fan.. It's a Delta FFB0412SHN-FOO
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Brushless fans, especially those for PCs, are designed to be "adjusted" by PWM.

    If you have a BLDC motor connected to a fan, with a separate BLDC Motor Driver board, then that's a different problem. If the controller is integrated to the fan, it's usually not an issue.
     
  7. 1-3-2-4

    Thread Starter Active Member

    Dec 26, 2008
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  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Looks like a standard 40mm fan such as the small video card fans. The speed control on the motherboard is also PWM, so those type of fans are designed to work with PWM.
     
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