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Quick newbie question. I am still getting 6.72 volts out of a 5 V regulator.

Discussion in 'Electronic Basics' started by Chris, Jan 1, 2004.

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  1. Chris

    Chris Guest

    Hi,


    I am getting 6.5 volts out of a 5v ragulator output leg. The total
    input voltage is via a 6 VDC volts AC adapter. it goes into a bridge
    regulator, a 100 UF cap, then into the 5 volts regulator. At its
    input jack - I am reading 10.72 voltage ( WTF, the adapter its rated
    as 6 vdc output, ? ) , then at output side of the voltage regulator, I
    am getting 6.72 volts.


    I dont quite understand it as why?



    The voltage is indeed dropping to 6.72 from 10.74. I thought the
    voltage regulator should drop it to at least 5 volts?

    I have tried two voltage regulator, and both reads the same output.

    I am wondering if my volt meter is defective, or if both regulators
    are defective.


    I cannot see wiring the regulator wrong. It has three legs. the
    middle is ground. Checking the inbound first leg, it reads the full
    10.72, the outbound *( last leg ) reads at 6.72


    What could be going wrong here guys,?


    Sorry if this question is very basic, I am just a newbie playing
    around with basic tools., and a bread board. Trying to learn :)

    Thanks to everyone in advance.
     
  2. Harrie

    Harrie Guest

    Are you using a 7905 maybe? Or somthing else?

    Harrie
     
  3. Two possibilities:

    You have in and out reversed.
    You have not added the correct bypassing capacitors. (Oscillations)


    --
    *
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

    http://hamradio.cmcorp.com/inventory/Inventory.html
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  4. CFoley1064

    CFoley1064 Guest

    Hi,
    You really need to give more information if you expect a useful answer. If you
    have a TO-220 LM7805/LM340T-5.0 or an LM317, these are the pinouts (view in
    fixed font as Courier or in M$ Notepad):



    .---------.
    | |
    | o |
    | |
    |---------|
    | |
    | |
    | |
    | |
    | |
    '---------'
    | | |
    | | |
    | | |

    1 2 3

    (Pinouts with plastic front of TO-220 facing viewer)

    LM317
    1. ADJ.
    2. V(out)
    3. V(in)

    LM7805/LM340T-5.9
    1. V(in)
    2. COM
    3. V(out)

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    When checking a breadboard circuit that doesn't work, always double check the
    wiring and pinouts. If that isn't it, check 'em again. Then look at other
    possibilities for three minutes. Then check your wiring and pinouts a third
    time. This simple procedure solves the problem about 80 to 90% of the time,
    and 5% of the time, the problem still ends up being the wiring and pinouts.

    (By the way, you almost certainly need a bigger input filter cap, too).

    Good luck.
    Chris
     
  5. Chris

    Chris Guest

    follow up.



    Once again many thanks to everyone!!!!!




    It was a darn battery on a digital meter. I am a newbie, and It was an
    honest mistake. Never thought the battery being the problem.

    I apologize for such a dumb msitake :*(
     
  6. Nells

    Nells Guest

    Check your bridge with an ohm meter, sounds like you have alot of
    ripple, you didn't indicate if it was full or half wave, in any case
    increase the capacitor to 470uf, also replace the cap, because the ESR
    may be too high, and it could be dried out.

    if it's 7805 1 amp regulator, you need to have at least 2 volts rms
    higher than the output, if it is less than the regulator won't function
    properly. In your case it should be 8.6 volts RMS, and not 6, you have
    to account for the diode drops, you can get the information out any good
    electronics text book. it's a good idea to place a 0.1uf cap on both
    input and output to get rid of any noise that adapter may spill into the
    line voltage.

    if all fails, replace the regulator, but only after you scope the cap
    and bridge network, to make sure you getting rectified DC from the
    input.

    nells
    Electronics Eng.
     
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