Need help on 78XX and 79XX based PSU

Discussion in 'The Projects Forum' started by 0karl02, Aug 8, 2012.

  1. 0karl02

    Thread Starter New Member

    Aug 8, 2012
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    Hi I need help on my project, I need to build a PSU that has outputs of +- 24v to +-5v dc by specifically using a 24v,2A center tapped, 78XX and 79XX series voltage regulators and a KBPC604 square bridge rectifier and the rest is up to us I did the schematic diagram in Circuit Wizard and when I simulate it, it works.. but whenever I transfer it on the PCB the negative values does not show the needed output. What am I doing wrong? If anyone could show me a schematic diagram with the similar set-up, could you please provide me with it so I could use it as reference. Thanks! :)
     
  2. Paul Kerry

    Member

    Jan 9, 2012
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    Please post a copy of your schematic, that will help us help you :)
     
  3. 0karl02

    Thread Starter New Member

    Aug 8, 2012
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    Here sir,

    [​IMG]
     
    Last edited: Aug 8, 2012
  4. Paul Kerry

    Member

    Jan 9, 2012
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    I think you may need to wait for one of the moderators to allow your diagram to show up:rolleyes:
     
  5. 0karl02

    Thread Starter New Member

    Aug 8, 2012
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    The diagram is showing up now. I posted the wrong link for the image.. my bad :rolleyes:
     
  6. Paul Kerry

    Member

    Jan 9, 2012
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    The pin-out of the 78xx and 79xx regulators are different as shown in the attachments.

    Can you measure the voltage between ground and the positive end of C1.

    Can you also measure the voltage between ground an the negative end of C2.
     
  7. 0karl02

    Thread Starter New Member

    Aug 8, 2012
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    It's just displayed like that in Circuit Wizard but the simulator somewhat makes it like the attachment (GND IN OUT) so its okay.

    For the voltages betweeen C1 and C2, it's +- 35.7 volts respectively
     
  8. Paul Kerry

    Member

    Jan 9, 2012
    36
    4
    What are your actual voltages on your PCB?
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Are you using electrolytic capacitors (2.2 μF and 1 μF) on the 79xx regulators?
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You verified the physical pinout on the layout for all the negative regs is G-I-O?

    If all the negs are not working, makes me wonder.
     
  11. 0karl02

    Thread Starter New Member

    Aug 8, 2012
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    The voltage across the positive input is 35.7v but for the negative inputs, first its -35.7v but when i measure them again it shows a different voltage reading (less than -35.7v)

    Yes all of them are eletrolytic capacitors

    yes sir, im sure of it :D
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I don't know if this is the problem or not, but the datasheet specifies that they should be tantalum, and if electrolytics are substituted, they should be at least 25μF.

    I am planning a similar supply and am particularly interested in your project. Even when it works, I wonder about the heat dissipation on the low voltage regulators; it seems that cascading the regulators might have been a better design. A 35 volt input on a 5 volt regulator is going to require a lot of heat dissipation unless the load is miniscule.
     
    Last edited: Aug 9, 2012
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