simple 24v fan control problem

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  • harp
    Badcaps Veteran
    • Jun 2022
    • 624
    • Planet Earth
    #1

    simple 24v fan control problem

    I am working on automated fan speed for lab psu... the fan is 24v 0.15A, and I am connected it to 820 ohm to ground, that provide minimum silent air flow on startup. It is powered around 4v at minimum point. Parallel to resistor I connected BD139, that drive a 10k NTC. Everything work like expected, but in extended test, where I isolate NTC, I have few issues, and like to ask here for help. 1. If ntc is not connected to heatsink, it go slowly to heat up and up and fan make noise than... At point "A" I measure voltage and put NTC in flame, everything is working ok till about 1.1v, and than, if pass some level it self latch and begin heating itself, pull huge current... I must power off to prevent damage. 2. I try to put 2 diode in series from "A" to ground, but it not work, not limit voltage to 1.2v... how to control max voltage on point A but not modified others resistor (it is perfectly set to what I want)?
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    • stj
      Great Sage 齊天大聖
      • Dec 2009
      • 31204
      • Albion
      #2
      icrease the value of the pulldown resistor,
      the ntc is shorting itself to ground!
      that said, something must be smoking hot!

        Comment

        • petehall347
          Badcaps Legend
          • Jan 2015
          • 4440
          • United Kingdom
          #3
          Originally posted by harp
          I try to put 2 diode in series from "A" to ground, but it not work, not limit voltage to 1.2v... how to control max voltage on point A but not modified others resistor (it is perfectly set to what I want)?
          would a zener work ?

            Comment

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