Baffled by simple linear regulator

Discussion in 'General Electronics Chat' started by PickyBiker, Dec 8, 2015.

  1. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I am making a + & - 12V power supply using L7812 and L7912. The former is a +12V regulator and the later is a -12V regulator. The Pos regulator is supplied with 25.2 VDC and the output is +12.1 V. However, the Neg regulator is supplied with -24.8 VDC and the output is -17.0 V.

    At first I thought I had a bad L7912 so I replaced it and to my amazement, I still have -17 volts instead of -12 V.

    I am aware that the pin functions are different on these devices.

    Looking at the front, the L7812 pins are Input, Ground, Output
    The L7912 pins are arranged as: Ground, Input, Output.

    This has me baffled. Any ideas?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Please post a schematic.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    Please post a schematic.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    Wow, sick minds think alike.
     
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  5. dl324

    Distinguished Member

    Mar 30, 2015
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    [comment self censored]
     
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  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    It acts like ground on the 7912 isn't grounded. You get voltage out but it changes with the load?
     
  7. PickyBiker

    Thread Starter Member

    Aug 18, 2015
    56
    1
    Here is the schematic. The voltages are with no load and the -12V is actually at -17V.
    Capture.JPG
     
    Last edited: Dec 8, 2015
  8. PickyBiker

    Thread Starter Member

    Aug 18, 2015
    56
    1
    The ground is good on both devices.

    With no load on the Neg regulator, output is -17V
    With a 3ma load on the Neg regulator, output is -12.1V.
    The pos regulator works fine with no load so I don't know why the Neg regulator needs a small load to regulate correctly, but it does.

    Problem solved. Thank you HP1729
     
  9. #12

    Expert

    Nov 30, 2010
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    Because the datasheet for that chip says it needs a load (and it wasn't just dreamed up by a bunch of nerds trying to annoy you).
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    The TI datasheet states minimum load is 5mA.
    upload_2015-12-8_16-25-50.png
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    Same for 7812:
    upload_2015-12-8_16-28-43.png
    Parts will sometimes/often operate outside of their specs, but it isn't guaranteed.

    I was breadboarding a circuit with an LM317 which has a minimum load current of 10mA. I was operating at 20mA and it still wasn't regulating. Worked at 30mA. It's out of spec, but who can I complain to??
     
  12. PickyBiker

    Thread Starter Member

    Aug 18, 2015
    56
    1
    Thanks, That clears it up.
     
  13. SLK001

    Well-Known Member

    Nov 29, 2011
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    You are dissipating a lot of power in your regulators due to your high input voltage. You should try a smaller transformer, say +/-16V to operate your circuits. If not possible, then provide big enough heatsinks to extend the life of your regulators. At 1 amp output current, your regulators will dissipate 13 watts of power.
     
  14. PickyBiker

    Thread Starter Member

    Aug 18, 2015
    56
    1
    The transformer is actually rated at 15V-CT-15V. I suspect the voltage is so high because there is no load. I will need to see what it looks like when there is a load. If it's still too high, I might try a 12V-CT-12V.
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    Or he could turn his regulators into switchers with the addition of an external pass transistor, inductor, diode, and some resistors.

    Or he could use this self oscillating, switching, tracking, pregulator that I suggested in another recent thread:
    upload_2015-12-8_19-10-29.png
     
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  16. dl324

    Distinguished Member

    Mar 30, 2015
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    15VRMS will give you about 20V. 12VAC would give you about 4V less.
     
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