Lm7805

Discussion in 'General Electronics Chat' started by sidkap, Jul 12, 2014.

  1. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    hi guys,

    I am using LM7805 regulator IC, how to use it,please suggest,
    while working with it i got very confused,
    it is not regulating 5v when i am applying a variable load resistor across IC's out put, the out put across load resistor is 4.5v at 200mA load current not 5v, WHY ?
    it is said LM7805 is 5v voltage regulator,
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post the full schematic you are using?
    When the input voltage is to low, the regulator will not work.

    Bertus
     
  3. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    here it is,
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What are the voltage and current ratings of the transformer?
     
  5. sidkap

    Thread Starter New Member

    Jul 12, 2014
    8
    0
    Open 9.5Vs
    Imax 500mA
     
  6. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    check the V/I readouts,
     
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  7. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    I noticed one thing now, after the first smoothing capacitor in parallel, Voltage builds up to 12.8v across this capacitor, I think this is high enough to malfunction the IC regulation?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What is the voltage accross the first capacitor when the load is applied?
    If it is lower than 8 Volts you are in trouble.

    Bertus
     
  9. eetech00

    Active Member

    Jun 8, 2013
    649
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    Hi

    I think either the xfmr output is insufficient or DC load is too high.:confused:

    eT:)
     
  10. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    What types of capacitors are you using for filtering? What are the values of your capacitors? Are you sure your capacitors are good? Are you sure your regulator is good? What is the load resistor set to?
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
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    1. Check the datasheet for the LM7805 to be sure you have the pinouts correct.
    2. Get rid of the variable load resistor; the 7805 will regulate just fine with no load. After you get the 7805 to output 5V, then you can add the load up to about 500mA, but your heat sink may not be big enough for 1/2 A of output.
    3. Be sure you have the ceramic caps on the input and output of the regulator exactly as shown in the datasheet.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the voltage on the input capacitor is >11V, and the current is 0.2A, the LM7805 needs to be on a heatsink because the power dissipation in it is (11-5)*0.2 = 1.2W, which will cause the chip to go into thermally induced shut down to protect itself it it is not on a heatsink. You are using the TO220 version, are you not?
     
  13. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    Transformer:eek:pen V is 9.5
    bridge rectifier
    smoothing capacitor: polarised 50v 1000uf
    capacitors around LM7805 first 330pf ceramic,second 100pf ceramic
    max load current=200mA,
    Big heatsink on LM7805 TO220,

    The out put is around 4.6v at 200mA load current(max load current) and 5 at 0 load,

    i think Tracemon's 2 point will do, let me do it again,
     
    Last edited by a moderator: Jul 15, 2014
  14. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Your capacitor values are quite low, at best your output will probably be unstable. They should be 330nF and 100nF, not pF.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Check the voltage on your 1000µF cap while under load. This will tell us what the regulator has to work with.
     
  16. sidkap

    Thread Starter New Member

    Jul 12, 2014
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    0

    yes this is my conversion mistake, I will replace the capacitors,
     
  17. sidkap

    Thread Starter New Member

    Jul 12, 2014
    8
    0

    it is 6.9v - 7v at 200mA load current,
     
  18. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    That's quite low--the 7804 needs a minimum input voltage of 7.3v to give you the 5v output.
     
  19. BobTPH

    Active Member

    Jun 5, 2013
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    Which means the transformer's rated current is suspect.

    Bob
     
  20. wayneh

    Expert

    Sep 9, 2010
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    Bertus nailed it in #2, and again in #8.
     
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