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LM317 wrong output voltage

Discussion in 'Electronic Basics' started by Alessandro Mulloni, Sep 13, 2004.

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  1. Hi everyone,

    I must state first that I'm a total beginner in electronics.

    I have a problem with a simple Ni-Cd battery recharger circuit. The
    diagram is the sequent

    ---------------------- 1N4004 diode
    + o----|----------|input (LM317) output|----|----------->|----o +
    | | adj | > U
    input | ---------------------- > 47ohm U
    = 0.1uF | | battery
    | |---------------- U
    - o----|--------------------|---------------------------------o -

    and the input is between 4V and 12V DC.

    What I would like to get is 1.25V between the adj and output pins (as
    should be from the LM317 specs)

    What I get is a varying voltage depending on the input one (that is,
    3.3V if the input is 4V, 4.8V if the input is 5.5V, and so on..)

    Do you all know why this is possible? Shouldn't the LM317 in the above
    circuit guarantee 1.25V _always_ between the adj and the output pins?

    Thank you all in advance.

    Alessandro Mulloni
     
  2. Byron A Jeff

    Byron A Jeff Guest

    -Hi everyone,
    -
    -I must state first that I'm a total beginner in electronics.

    Well congratulations.

    -
    -I have a problem with a simple Ni-Cd battery recharger circuit. The
    -diagram is the sequent
    -
    - ---------------------- 1N4004 diode
    - + o----|----------|input (LM317) output|----|----------->|----o +
    - | | adj | > U
    -input | ---------------------- > 47ohm U
    - = 0.1uF | | battery
    - | |---------------- U
    - - o----|--------------------|---------------------------------o -
    -
    -and the input is between 4V and 12V DC.

    Looks like a standard constant current LM317 charger circuit.

    -
    -What I would like to get is 1.25V between the adj and output pins (as
    -should be from the LM317 specs)

    Not exactly. NiCads want constant current and are not too concerened about
    the voltage requirements. Hence the single 47 ohm resistor which will for
    a constant 1.25V/47 Ohm -> 26 mA of current across the battery.

    -
    -What I get is a varying voltage depending on the input one (that is,
    -3.3V if the input is 4V, 4.8V if the input is 5.5V, and so on..)

    That's exactly how it's supposed to work. Measure the current. You'll find that
    the current stays the same even as the voltage fluctuates.
    -

    -Do you all know why this is possible? Shouldn't the LM317 in the above
    -circuit guarantee 1.25V _always_ between the adj and the output pins?
    -

    Nope. The current is fixed, not the voltage.


    BAJ
     
  3. There are some aspects of your schematic that I'm unclear about. Is
    the lower end of your 47R connected direct to ground, as apparently
    drawn? Or to the wiper of a pot, as would be the case for variable
    control? If the latter, what is its value? And, although not strictly
    relevant to your question, what voltage battery are you charging?

    Anyway, begging answers to those, here are a couple of simulations
    that may help.

    http://www.terrypin.dial.pipex.com/Images/317Source.gif
     
  4. Nice diagrams, thanks :) the lower pin is connected directly to the
    ground, like in your first diagram.

    My circuit is indeed _exactly_ the one drawn in your first diagram,
    except from the voltage I get.

    The battery is a 1.2V AA or AAA. With the 47R I would like to get some
    27mA load so I can recharge even the 250mAh batteries, maybe I'll change
    the resistor when I'll realize I only have batteries with more mA so to
    recharge them in a shorter time. Without a constant voltage between the
    output and the adj pins I cannot precisely do this calculation (that's
    simply I = V / R).

    Alessandro
     
  5. Yes, I need 1/10 of the mAh of the battery to recharge it, but with a
    non-constant voltage between the output and adj pins how can I calculate
    properly the value of the resistor (currently set to 47ohm)?
    Ok, but what I really do not understand is that by specifications the
    LM317 should guarantee 1.25 of voltage between those pins. Is there
    maybe some wrong connection in my circuit?

    Alessandro
     
  6. In that case, I suspect that you have the 317 connected incorrectly.

    See http://vancouver-webpages.com/peter/lm317.gif for the correct
    pinout for a TO-220 package. That drawing shows a fixed-voltage
    regulator. For the fixed-current regulator that you want, the Adjust
    pin should not be grounded, and the output is taken from the junction
    of the adjust pin and the resistor.

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  7. CFoley1064

    CFoley1064 Guest

    Subject: LM317 wrong output voltage
    Hi, Alex. Are you sure you've got the pinout on the IC right? It's different
    than the LM78XX. This shows the TO-220 IC as viewed from the front of the
    package (view in fixed font or M$ Notepad):

    LM317 Front View
    .---------.
    | o |
    | |
    |---------|
    | |
    | |
    | |
    | |
    | |
    '---------'
    | | |
    | | |
    | | |
    | | |
    ADJ OUT IN

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

    One of the famous newbie problems with the LM317 -- check your wiring and
    pinout. I did it myself once many years back.

    Good luck
    Chris
     
  8. Ban

    Ban Guest

    http://www.national.com/ds.cgi/LM/LM117.pdf page 17 precision current
    limiter,
    look your schema is wrong, there is no connection to gnd, just inserted into
    the 12V line. Also the rsistor has to be in series with the load and the
    adj. pin is connected at the load side.
     
  9. Thanks for all your replies.

    The problem was not in the pinout, but in the circuit itself. I actually
    got it right at the first time (that is the Ban's circuit), but it
    wasn't working, so I changed it to the one posted to this newsgroup.

    When I went back to the original circuit, I noticed that something was
    wrong.. I didn't have 12V in input, but -12V :) I know it's a stupid
    mistake but I didn't even had a tester the first time I made the circuit.

    Anyway, the problem is gone, the input + and - are connected in the
    right way into the circuit, and I have some nice 1.25V between the
    output and the adj pins.

    Thanks everyone.
    Alessandro
     
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