24/4 volt to 12 volt only

Discussion in 'The Projects Forum' started by bigjerm, Nov 18, 2014.

  1. bigjerm

    Thread Starter New Member

    Nov 18, 2014
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    i have a circuit with an output of 24v when active and 4v when in stand by. i need to power a 12v fan only when the circuit is in the 24v phase. how would i go about creating this additional circuit?

    if this has been covered before, please link me to the thread. i searched but couldn't find anything similar.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You could use a 5V zener followed by a transistor switch (to control the fan). This would handle the shutoff at 4V. You could use a 12V regulator to drop the voltage from 24 to 12V.

    Actually you could probably just use the 12V regulator. When it sees a 4V input, the output won't be enough to run the fan.
     
  3. bigjerm

    Thread Starter New Member

    Nov 18, 2014
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    thanks. is a 12v regulator a single part i can buy or would it be a circuit i need to build?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Look up 7812 voltage regulator. It needs a couple of components - capacitors to prevent oscillation.
     
  5. bigjerm

    Thread Starter New Member

    Nov 18, 2014
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    thanks. could you explain to me how it would be hooked up? or link me to some place where i could learn?
     
  6. goahead113

    New Member

    Dec 6, 2010
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    <SNIP>


    The fan wont run with this wired up at 4v as the vdrop across regulator it will only be out putting under 2v or a transistor can be added to turn it off completely.

    Moderators note: link contained information that violates the ToS
     
    Last edited by a moderator: Nov 19, 2014
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    For a small fan; all you need is a 12V zener diode connected in series. The dissipation of the zener will equal the power of the fan so make sure you use one with a suitable power rating.

    Also, Check the fan specification to see what its maximum voltage is. If the fan is ok at a voltage >12V you can use a lower voltage zener which would reduce the dissipation in the diode and improve the efficiency.
     
    Last edited: Nov 19, 2014
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is the fan going to be powered from the 4-24V signal, or is that just used to switch on/off a separate 12Vdc power supply that will power the fan?
     
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