drop in 5v dc

Discussion in 'General Electronics Chat' started by Roque, Oct 26, 2014.

  1. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    hello people....got a 5v regulator (7805) input pin 8v....output is 5v without load....once i connect load input then drops to 5.3v and output to 3.9 ....could the regulator be faulty? or leaky caps ..any ideas would be much appreciated thank you..cheers
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    Hi.

    How about some schematics, with cap values? What is your load?
     
  3. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    This seems obvious, but the local capacitors are necessary to prevent oscillation. Leaving those out always results in unexplainable output voltages.

    and specify the exact part number you have. They all require different local capacitor sizes.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,507
    2,367
    Also how far is the load removed from the regulator?
    Max.
     
  5. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    its an audio equipment all that it drives is a prom IC and two 7 segment displays..
     
  6. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    Hi AAA....its seems to be a pretty straight forward circuit all it drives is an prom IC and two 7 segment displays...
     
  7. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    Hi Max...it is connected by a foot long cable..and all its driving is a Prom IC and two 7 segment displays
     
  8. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    will try the caps tomorrow..probably also mite put in a new 7805...will share the results with you guy....thanks for your input....cheers
     
  9. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,226
    If the input drops to 5.3V then the input supply voltage is apparently being current limited. What is supplying the 8V?
     
  10. Shehwar Hussain

    New Member

    Oct 26, 2014
    21
    2
    You may check the current ratings of the supply you are using....or otherwise try changing the supply.
     
  11. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
     
  12. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    will post a schematic this evening ...thanks guys.
     
  13. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,226
    Then your transformer apparently is not large enough to supply the load current.
     
  14. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    hello everyone...the problem i had with 5v droping was finally fixed...i had a series bulb connected to the mains of the mixer ...guessing that was the reason the moment i plugged the equipment directly into mains it all came good....was the current insufficient ??
     
  15. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    thanks to you all who gave me clues...ta.
     
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