PSU, 7805, 6.5V output

Discussion in 'Homework Help' started by bug13, Aug 1, 2012.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    as seen in the picture, I expect a small amount of current comes out of the 7805's ground pin, not goes in, but I get different, what's wrong?

    all the voltage resistors is measured value, and the load current is measured value, other three current are my calculated values.

    PS:
    this lab is get a 6.5v regulated output from a 7805
    AC from secondary of a transformer = 11.3V
    DC at 1000uF cap = 12.7V
    Vripple on regulated out put = 48mV
    (All measured value)

    [​IMG]
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    my target output is 6.5V, my real value is 6.49V, but that's not what confused me.

    What I don't know is, on the common pin/ground pin of the 7805(as seen in pic), I expect current coming out of the chip, not going in the chip.

    the datasheet didn't mention the normal current of the common pin, but it does mention the current should be coming out of the chip.

     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yes the constant current through the 220 Ohm resistor should add up to the standby current of the LM7805. Those two added currents would then flow through the 76 Ohms. So your output voltage should actually be 6.9V.

    That's how I understand it, never used it this way...

    EDIT: Ok, just tested it with a 7808 and the same resistor values of yours. The current through the76 Ohm resistor is indeed 3.5mA higher than through the 220 Ohms. So the current flows out of the ground pin, as it should... Please verify your measurements and resistor values. (tolerances)
     
    Last edited: Aug 1, 2012
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Guarantee the current flows OUT of the ground pin on a 7805. If it ever flows in, you must have turned on a substrate diode and that is a destruct mode.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    I will also say that 6.49 volt output is in range then all uncertainties are accounted for. The output from a 7805 is seldom (never) 5.0000 volt. And no resistor are exact. And the same for the voltmeter probably
     
  7. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    what do you mean by
    ?

     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    All IC devices are built on a substrate which has an inherent diode that turns on when a pin is forced to a voltage opposite of normal operation. The diode turns on, current flows and the chip melts quickly if the current is not controlled.

    It's the reason makers of the LM317 and similar linear regs recommend a protection diode from output back to input. It parallels the internal diode and keeps it from turning on.
     
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