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Zener diode help

Discussion in 'General Electronics Discussion' started by Shadow1976, Jul 15, 2014.

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  1. Shadow1976

    Shadow1976

    18
    1
    Dec 7, 2013
    Hi guys,
    I am building a proto board that requires 11vdc power supply and I want to use a zener setup but have not used one before and I am confused with calculating series resistor required. The input voltage fluctuates between 12 to 14 vdc. Load current (circuit) is 1 amp max but normally 150ma. At 150ma, load resistance is 10k. Zener diode is of the 1w type. I am confused as to what value the resistor should be to satisfy the load.
    Anyone with an answer would be great please.
    Regards,
    Shadow
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,397
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    Jan 21, 2010
    It's not a great solution for a zener diode.

    The resistor has to be sized for the minimum voltage and the maximum current -- it will be a 1 ohm resistor.

    When the voltage is 14V and the load is 150mA, the circuit will draw 3A, and the zener will be called on to dissipate a little over 31W.

    I would advise you use some form of low dropout linear regulator.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
  4. Shadow1976

    Shadow1976

    18
    1
    Dec 7, 2013
    Thanks guys.
    I have another query for ya. Lm7805 how much voltage can you apply before starting to think about heatsinking. Reason I am asking is I got an 7805 operating from input that varies from 12 to 14v and it gets hot of course. I put a heat sink on it and the heat sink builds up the same heat. It's got me a bit worried about 7805 not lasting long. What are your thoughts.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,397
    2,777
    Jan 21, 2010
    Voltage is not the issue. The issue is dissipated power and that comes from the product of the current and the differential between the input and output voltages.

    If this exceeds a couple of watts, you need a heatsink.

    Check out the resource n heatsinks for more information (it's not yet complete, but it's getting there)
     
    KrisBlueNZ likes this.
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