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Zener shunt regulators: Leakage Currents?

Discussion in 'General Electronics Chat' started by DigiTan, May 3, 2005.

  1. DigiTan

    DigiTan New Member

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    I'm working a miniature battery-powered project this summer and one of my biggest constraints is power consumption. Part of the circuit requires dropping the battery's 12 Volts down to a regulated 5 Volts to power a microcontroller. Since space is also a limit, I'm looking for the smallest (non-surface mount) 5-Volt regulator possible.

    At first, I wanted to try a zener shunt regulator like the kind pictured here, but a friend of mine talked me out of this because of leakage currents, so I chose the LM2931 regulator instead. But, I'm still curious to know how big this zener leakage would have been.

    What is the typical leakage current for these types of regulators when there is no load? Are there any good strategies I can use to limit it?
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Digitan,
    In order to regulate voltage, a shunt zener diode regulator must have current through it to ground. The load also has current through it when it is working. Both currents flow through the current-limiting resistor from the unregulated supply. When there is no load the same current flows through the current-limiting resistor by the zener conducting to ground its own current plus the current that was used by the load. A huge waste of current.

    A series voltage regulator uses a small operating current and passes only the load's required current. When there is no load the operating current is very small. Much better efficiency than a zener. :)
     
  3. Electrix

    Electrix Member

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    "When there is no load the same current flows through the current-limiting resistor by the zener conducting to ground its own current plus the current that was used by the load. A huge waste of current. "

    Hi audioguru,

    When there is no load connected the zener should carry it's own current only, one that is going to maintain the reqd O/P voltage. Why should it carry additional current ??
    plz explain..
    Thx
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Electrix,
    With a fairly stable unregulated input voltage, the current through the current-limiting resistor from it will remain the same with or without a load. Without a load and with the the current remaining the same, it has to go somewhere: through the zener to ground. 100% wasted current.

    A shunt regulator regulates by shunting to ground any extra current that is not used by the load. If it didn't, then the output voltage would be equal to the unregulated input voltage because the current-limiting resistor wouldn't have a voltage drop.
     

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