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LM340: Series zener connection

zabbza
Prodigy 60 points
Part Number: LM340

Hello;

May I use three zener dioe(Vth:5.1V 5W) to drop input voltage nearly 8V.

What is inrush current of LM7805?

If I use an inductor may I prevent inrush current problem?

Are there any situation which must be pay attention?

Best Regards.

  • 0
    TI__Mastermind 40885 points

    Hello,

    The way you have drawn it, this will not work; the input capacitors need to be directly at the input of the device. If you switch the location of the capacitors and diodes it will be better but I'm still not sure that it will work. You would need to test this on the bench. 

    Inrush current is not normally specified because it depends on the amount of output capacitance. With that said, with no output capacitor there will be very little inrush current. Note, however, that with no output capacitor the transient performance will be very poor.

    An inductor is not a good way to limit inrush current. When the current demand is high the inductor voltage can spike to high voltages and damage components. 

    I think the solution you need for this is a buck converter. It will be much more reliable than using diodes to get the voltage down.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 60 points

    Hello;

    The circuit is testted, and It works.

    There is some caps at the input of the device and  the output of device. (4.7uF 100nF)

    The circuit draw 500mA.

    But When I investigated on web, I can not find enough explanation.

    When this circuit works some of years, is it couse problems?

    Best Regards,

    Omer.

  • 0 in reply to zabbza
    Prodigy 60 points

    Hello;

    There are three 5.1V 5 Watt zener to drop voltage nearly 8V.

    The circuit draw 500mA.

    When this circuit works some of years, is it couse problems?

    Best Regards,

    Omer.

  • 0 in reply to zabbza
    TI__Expert 6825 points

    Hi Omer,

    This circuit will work the main concern i have is at light loads when the Zener diodes will not necessarily be in zener breakdown, which could cause some issues for you. otherwise i think that it would be ok. You have already had it working for years correct?

    Regards,

    Mark

  • 0 in reply to Mark Sellers
    Prodigy 60 points

    Hi Mark,

    It was tested only one day,

    I asked for that Is this circuit work on long period.

     is it couse problems?

    Best Regards

    Omer

  • 0 in reply to zabbza
    TI__Mastermind 40885 points

    Hi Omer,

    This is not a normal use-case so we don't have any documentation for this, specifically for long-term reliability. With that said, I don't see a reason why this would present long-term reliability issues except for the point that Mark made about light-load conditions. 

    Regards,

    Nick