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Please help with this parallel LM317 circuit

Discussion in 'General Electronics' started by Mike, Nov 14, 2003.

  1. Mike

    Mike Guest

    I searched heavily on how to parallel 2 LM317 voltage regulators and I
    think I found the answer but I need a little help :). I'm new with
    working with some of these components so please bear with me.

    Here are the circuits I found, note that they have 3 LM317's but I'll
    want to use 2 so just ignore one of them.

    _______
    | | 0.025 to
    o--+--| LM317 |----- 0.25 ----------,
    | | ___|___ |
    | |___| | |
    +----|-| LM317 |----- 0.25 ------+
    | | | ___|___ |
    | | |___| | | 5.0V
    +----|---|-| LM317 |--+-- 0.25 --+--+---o 0 to 4A
    | | | | | | |+
    | | | |_______| | === 4.7uF
    === | | | R1 47 |
    | | | | | gnd
    | '---+---+--------+
    gnd R2 | Vo = 1.25 (!+ R2/R1)
    174 or 180 + n (R2 * 50uA)
    |
    gnd

    Or to increase the accuracy and stiffen the output, add a jelly-
    bean TL431 reference chip to servo the output voltage,
    _____
    | LM |
    o--+-| 317 |--- 0.025 -----------,
    | |_____| to 1.0, etc. |
    | | _____ |
    | | | | |
    +----|-| 317 |--- 0.25 -------+ Vo = 2.5 (1 + R2/R1)
    | | |_____| |
    | | | _____ | 5.0V
    +----|----|-| |--+- 0.25 -+-+----+---o 0 to 4A
    | | | | 317 | | | |+
    === | | |_____| 470 | === 10uF
    | | | | | 10nF | |
    | '----+----+---+-+--||-, | gnd
    gnd | | R2
    __|__ 2.2k | 1.00k
    | TL | | |
    | 431 |----+----+
    |_____| |
    | R1 1.00k
    gnd |
    gnd

    In Circuit#1 I don't understand the equation, how come it differs from
    Circuit#2? Circuit#2 sounds better but I have no idea what a Jelly
    Bean TL431 is. I searched and found different designes of the TL431
    but I have no idea which one is this Jelly Bean :). Most importantly
    though I have no idea which pins of the TL431 to connect to. I have
    no problems with the LM317, but the TL431 is a completely differnt
    story. Looking at the circuit above, which pins are being connected?

    By the way if you are wondering what I'm doing, I'm trying to convert
    12->5V or 3.3V. I'm not quite sure which one yet, but it doesn't
    matter as long as I use the equation to find the resistor values.
    Although the equation of Circuit#1 makes no sense to me ;). I want to
    combine both LM317's because they should give me a nice 3A, sure I can
    use some other voltage regulator but I'm having problems locating some
    with more ampage :(. Might aswell make use of what I got.

    Thanks for he help, I really appreciated it.
     
  2. Robert Baer

    Robert Baer Guest

    I would tie the top end of R1 (47 ohm) to the load, not to the output
    of one of the regulators, like you show in the second diagram.
    You may have problems with diag #2; cannot say without researching it.
    Download the PDF datasheet for the TL431 from National Semi.

    Perhaps a more elegant way is to use a power transistor as the pass
    element, and have the LM317 drive its base..i vaguely remember that a
    datasheet showed this scheme in the applications section (may need to
    find an old databook).
     
  3. Ted Wilson

    Ted Wilson Guest

    Hi Mike

    In cct-1, the 1.25V reference appears between the Vout and the ADJ
    pin, so across R1 + R2, at a first approximation, you see 1.25 * (R1 +
    R2)/R1 = 1.25(1 + R2/R1). Things are then complicated a bit by the
    current required by the ADJ pin which, since the current through R1 is
    defined as constant, (= Vref/R1), has nowhere else to go other than
    through R2, elevating the the voltage at the junction or R1 and R2,
    (and hence Vout), by Iadj*R2. If there are more than one references
    connected to the junction of R1/R2, you have to allow for more than
    one Iadj - hence the n(R2 * 50uA) term in the first equation.

    The term "jelly bean" I take to be an Americanism for "ten a penny",
    and in the second cct you connect the TL431's Anode to 0V, the
    Reference to the junction of R1/R2 and its Cathode to the ADJ pins of
    the LM317s. Now you have a reference of 2.5V between the TL431's
    Anode and Reference pins, with the Iadj currents of the LM317s taken
    care of by the TL431's cathode, so output voltage is now 2.5(1 +
    R2/R1).

    Be warned however: you can't achieve a regulated output less than
    around 3.75V with this arrangement due to the minimum recommended
    output for the TL431 of Vref, (2.5V). You could use the TL device in
    conjunction with discrete transistor(s) to do what you want, but you'd
    have take care of current limit etc. yourself.

    Hope some of this helps

    Ted Wilson

    "If the players want to make it hard for me, I am happy to make it
    twice as hard for them." - Wendy Toms, the first female referee to
    officiate in a professional game.
     
  4. Ted Wilson

    Ted Wilson Guest

    [snip]
    Not a good idea Robert. If the LM317's reference were w.r.t. 0V I'd
    agree with you, but it's between Vout and ADJ, so doing as you suggest
    degrades load regulation rather than improves it.

    Regards

    Ted Wilson

    "If the players want to make it hard for me, I am happy to make it
    twice as hard for them." - Wendy Toms, the first female referee to
    officiate in a professional game.
     
  5. GPG

    GPG Guest

    Amped reg:








    --------------------------- --------------------|
    | v / |
    | --- |
    | | |
    | | |
    | | |
    | | |
    | ___ | |-----------| |
    |---------|___|--------|----- -----------------
    | reg |
    | |
    |-----|------
    |
    |
    |
    |
    |
    |
    |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
     
  6. JeffM

    JeffM Guest

    use a power transistor as the pass element
    Yup. Hook your sense resistor to the final outut.
    As the load current from the regulator approaches 0.6 amp,
    the drop across the resistor is enough to start turning on the transistor.

    .. ___________________________ _______________
    | C\ NPN __/E |
    | \ /| |
    | __________ \_____/ |
    | | \ | |
    | | 1 AMP \ | |
    +---| REGULATOR >-----+---+-----\/\/\--+
    | / | 1R0
    |__________/ ------- 1 ohm
    | _____
    | / ³ \
    | |
    +------------+ 1uF
    |
     
  7. Mike

    Mike Guest

    Thank you for the replies. I have completed and tested Circuit#2,
    unfortunatley with bad results. I spent 3 hours making sure it was
    just like in the shematic but something ain't right, although I did
    make a couple changes. First instead of a 0.025ohm resistors I used a
    0.5ohm. I couldn't get anything less but from what I understand it
    should sitll be ok. Second, I think I was suppose to but didn't show
    in the schematic, I put a 0.1uf capacitor between 12v+ and gnd at the
    begining of the circuit. I think that symbol means capacitor, and all
    the single LM317's circuits use a 0.1uf cap.

    Anyway onto the problem. I'm testing this with and OLD power supply,
    so old that it only outputs 10.8V on the 12V line. Well in the test
    it should matter as it's being converted to 5V. Unfortunately the end
    result is 9.5V. How I'm getting 9.5V from 10.8V is anyones guess :).

    The LM317's are cold to the touch so they eiher don't work or the
    little conversion from 10.8 to 9.5V doesn't make any heat. I'm a bit
    tired right now, but tomorrow I will try swtiching one of the 1K
    resistors to decrease voltage and see if it will make any effect.
    Also I'll take out the 0.1uf cap and see if that will make any
    difference.

    If you guys want to see my crummy work (hehe), I took a picture here

    http://www.angelfire.com/art/vmu/mod.jpg

    Now I know you can't tell much from the picture, but anyone knows what
    could be causing this?
     
  8. Mike

    Mike Guest

    Well I thought what the heck, let's try switching R1 today. Changed
    it to 4.7k which would lower the voltage and guess what, NO
    difference. Well I'm beat for tonight, I'm gonna go take a rest hehe.

    Just to double check, can you guys just tell me which way the
    capacitors should be connected. I mean where does the negative side
    go. I think I got it right, but heck the results speak for
    themself...I messed up somewhere :).
     
  9. Ban

    Ban Guest

    Mike wrote:
    || Well I thought what the heck, let's try switching R1 today. Changed
    || it to 4.7k which would lower the voltage and guess what, NO
    || difference. Well I'm beat for tonight, I'm gonna go take a rest
    || hehe.
    ||
    || Just to double check, can you guys just tell me which way the
    || capacitors should be connected. I mean where does the negative side
    || go. I think I got it right, but heck the results speak for
    || themself...I messed up somewhere :).

    Hey mike,
    you did just the wrong thing. R1 has to be as small as 120 Ohms for 2
    regulators, otherwise the supply current for the regulators cannot be
    delivered. The internal reference gets its supply out of the output, to have
    a more stable condition.
    Starting with that value you can calculate the bottom resistor accordingly.
    The ground is where the -side of the caps goes, don't turn them around
    unless you want to have a nice explosion (might take some time). Good luck

    ciao Ban
     
  10. Rich Grise

    Rich Grise Guest

    I've seen it done kind of the other way around, with a PNP on the
    input side of the regulator:

    PNP
    Vin ---+----------E C----------------+
    | B |
    | | |
    +---R--------+----in REG out----+---out
    |
    0

    You size R to determine the output current capability.

    The advantage is that you don't have a Vbe drop between the
    regulator out and the output. :)

    Good Luck!
    Rich
     
  11. Robert Baer

    Robert Baer Guest

    *THAT* was the configuration that i did not remember well enough to
    describe!
    The PNP is the way to go.
     
  12. Mike

    Mike Guest

    Thank you for bearing with this somewhat newb, thanks.

    I finally did it. I don't know why but Circuit#2 was bad, Circuit#2
    works. I can get a nice 5V now. Maybe the TL431 was busted or
    something, well anyways as long as this circuit works and is stable
    I'm happy :).

    Thank you everyone again.
     
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