Once again, the 555 PWM question...

Discussion in 'General Electronics Chat' started by iONic, Dec 29, 2010.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    I'm looking for a simple way to control the duty cycle of a 555 that will eventually drive a 12VDC, 400mA Computer fan based of the ambient temperature. I thought of the LM34, but the output is a temperature to voltage relationship. A thermistor might work, but I am a am stumbling on how to setup the 555 to accomplish a wide range of duty cycle. I want it to reach ~100% duty cycle but am not sure on how low the duty cycle will be needed to keep the fan spinning yet. I also want some sort of Hysteresis as I do not want the fan turning on/off all the time.

    Any idea's on how to setup the 555 for this sort of arrangement?


    Thanks
     
  2. Wendy

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    Mar 24, 2008
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    A simple thermistor making a variable voltage comes to mind. You could shape the temperature response curve (such as slope and endpoints) with op amps, or possibly use the thermistor by itself.
     
  3. iONic

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    Nov 16, 2007
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    In other words, replace the variable resistor with the thermistor. This in itself would not give a on/off delay (hysteresis) effect, would it? how might I achieve this effect?
     
  4. Wendy

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  5. SgtWookie

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    Jul 17, 2007
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    I used a thermistor as part of the voltage divider for the feedback input to an LT1171 switching regulator IC. I set it up so that the output of the switcher had a range of 7v-11v under the expected operating temperature range, which was within the fans' specifications.

    The fan is a 92mm x92mm 85cfm fan that draws around 800mA with 12v in, and it's mighty noisy when running full steam - but it moves a LOT of air. Running at 7v, it's barely audible; moves just enough air to keep it flowing over the thermistor. If the CPU starts heating up, the fan automagically compensates.

    I went that route rather than PWM because computer fans are BLDC (brushless DC) rather than brushed (they used to be, many years ago - and broke quickly.) Trying PWM on them probably won't work terribly well.

    Don't have my schematic handy; I've been away from home for a couple days.
     
  6. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Then the LM34, having a linear voltage output could be used??

    Sgt. I could probably do a similar thing as the fan does function even at 5V and PWM may not be necessary. Just a comparator with Hysteresis to vary the voltage maybe.

    I have bee out of town also and just had my best buddy (my dog) put down. He had cancer(sarcoma).
     
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