5V voltage regulator problem

Discussion in 'The Projects Forum' started by ksp1717, Mar 17, 2012.

  1. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    Hi

    I am not sure where to put this topic but here it goes.

    I am using a -5V and +5V voltage regulators.


    PTN78000AAZ for -5V,1.5A
    LD1086D2T50TR for 5V,1.5A

    I am using a 12V,1.5A supply to power both the devices. I have followed their datasheets PTN78000 and LD1086 to make the basic application circuits.

    Now here is the problem. When I power up the circuit, the PTN78000 functions as expected and gives me ~-5V, but the LD1086 also gives me negative voltage of ~-3.5V as output. The LD1086 also gets hot and soon if I measure the voltage at input it drops to ~2.8V instead of 12V and I am measuring 0.4V from PTN78000 and 1.7 from LD1086.

    Can some one explain me what is going on. I am also attaching the full schematic.

    The power supply directly is connected to only these two devices. All others are downstream.

    Thanks for your help
    ksp
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why is your schematic a hard-to-see negative image?
    Most schematics are black lines on a white background.
     
  3. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Is this better?
     
  4. mlog

    Member

    Feb 11, 2012
    276
    36
    Better, but the pin numbers are too small to be readable by a human eye. Can you read the pin numbers?
     
  5. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    No, and values for the components would also be useful.
     
  6. mlog

    Member

    Feb 11, 2012
    276
    36
    The schematic is difficult to read, but it looks like the +5v power supply ouput is connected to the collector of a PNP transistor (Q1), while the negative power supply output is connected to the collector of a NPN transistor (Q2).
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Why is there schematic drawing software available that makes the schematic a negative covered in dots and makes the numbers too small to see??

    EDIT: It also makes crossed lines all over the place.
     
  8. ksp1717

    Thread Starter Member

    Oct 30, 2008
    13
    0
    HI

    Thank You for your replies. The problem was not with the devices mentioned but with the transistor matching as mentioned by mlog. I have swapped the transistors and now it is working as it should. Thank you all

    Btw sorry for the schematic image. I made those pins small :( on purpose. I should have made them large.

    ksp
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Sometimes with dual polarity supplies, one side will yank the other output to the wrong polarity on turn-on and it will latch up (and get hot). Thats why they show diodes on the outputs with reverse polarity.
     
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