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L7812 Voltage sag with 12V DC motor

Discussion in 'General Electronics Discussion' started by btaz, Sep 28, 2012.

  1. btaz

    btaz

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    Sep 28, 2012
    I'm experiencing a voltage drop across a pretty simple circuit and don't understand why.

    I have a L7812 setup to give my circuit a 12V DC power. The L7812 has bridge rectifier, caps, and transformer 120V to 15V. From the LM7812 I pass the current through a pnp transistor that is fully saturated. Looks like I get about 18-19V into the L7812 and when the motors (small pc fan motors) are off (pnp open) I see a stable ~12V.

    I have a small resistor in series (out of motors to ground) with the motors (3 in parallel) to limit the current. The motors are expected to draw 100mA. So I expect to drop a bit over 300mA across the circuit. My transformer and L7812 are all spec for up to 1A. When I turn the motors on (pnp saturated) the voltage sags to near 9V pulling about 300mA. So I'm not too sure why I'm seeing the sag as I should be able to drive plenty of current and with my resistor in series of the current path there should be current limited.

    Any ideas? Thank you.
     
    Last edited: Sep 28, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Measure the current, don't trust your expectations.
    Show us the schematic of the circuit, the textual explanation is a bit difficult to understand.
     
  3. MrEE

    MrEE

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    Apr 13, 2012
    Also measure the emitter collector voltage of that PNP. It should be 0.1 to 0.2V. Are you using it as a switch? Also you might want to add a smaller cap, about 100 uf @25V on the output of the regulator. Long wires and inductive loads can cause some instability.
     
  4. BobK

    BobK

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    Jan 5, 2010
    Are you measuring the voltage across the motor, or across the motor + resistor? If you are measuring across the motor, the resistor might be dropping the voltage. If they are 12V fans, you should not need a resistor anyway.

    Bob
     
  5. btaz

    btaz

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    Sep 28, 2012
    To BobK: I'm measuring from the LM7812 output to ground. So in a sense it is across the pnp, motor and resistor. As for the resistor need, I wasn't too sure how the motor would act and have thought it to be unnecessary too, but I'd rather be safe than sorry.
     
  6. btaz

    btaz

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    Sep 28, 2012
    I'll get a schematic out soon. But the values are measured. The current measurement is from the LM7812 output to the rest of the circuit.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    What package is the 7812 in? TO92, or TO-220 (or other)
     
  8. btaz

    btaz

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    Sep 28, 2012
    Thanks for all the help so far.

    Here is a sample diagram more of a hack job of the real circuit. Many of the switches don't exist and are here for me to test the sim. My real circuit doesn't have SW1, but uses some other things to mimic a switch, but I get the same effect where the voltage at QE2 drops from ~12V to closer to ~9V when SW1 goes to ground (lower position).

    [​IMG]


    Steve: it is a TO-220 package. I have a heat sink on it and it stays pretty cool
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Two issues:

    1) C2 is far too small. A rule of thumb is 1000µF per 1A of current, so instead of 33µF you should have at least 330µF at 300mA. I suggest 1000µF. If you measure the input voltage of the 7812 with an oscilloscope you will see lots of ripple with your current setup.

    2) Unrelated, but: Why does RB2_GND go to GND? It will always provide some base current to Q2. I'd have expected this resistor to go to +12V (emitter of Q2) so an open base will safely be turned off.
     
  10. btaz

    btaz

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    Sep 28, 2012
    1) thanks for the suggestion. I don't have an o-scope, but have increased the cap size. I imagine my voltage has less ripple now, but I still the the voltage sag.

    2) The short answer is the SW1 really isn't in the circuit and I have it there to ensure that some of the other components in the real circuit have a ground path (though quite limited).
     
  11. btaz

    btaz

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    Sep 28, 2012
    I'm starting to think it is the L7812 that is creating the problem. I bypassed the pnp transistor and see the same sag. The exact 7812 that I am using is L7812ABV
    from digitkey: http://www.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=L7812ABV

    Also, I did a few measurements around the 7812. The Vin is about 22V +/- 0.6V, where the 22V is a DC measurement and the 0.6V is an AC measurement. Vout when the motors are off is 11.75 +/- 0.005V. When the motors are on I see it drop to 10V +/- 2.5V.

    Any new ideas are greatly appreciated.
     
    Last edited: Sep 30, 2012
  12. btaz

    btaz

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    Sep 28, 2012
    A quick measurement shows just about 0.1V drop across the emitter and collector. You can see with the diagram now posted that I am using the transistor as a switch.
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    Have you changed the input and output caps on the regulator chip yet ?

    1000uF on the input 100uF on the output

    Dave
     
  14. btaz

    btaz

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    Sep 28, 2012
    I'm off to buy some larger caps now. The 100uF seem to have helped some.
     
  15. btaz

    btaz

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    Sep 28, 2012
    Just added the 1000uF on the input and the voltage sag is gone.

    Thank you (and everyone) for all the ideas and help.
     
  16. davenn

    davenn Moderator

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    Sep 5, 2009
    you're welcome :)

    D
     
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