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LM338 (x3) power supply cuts out on high loads

Discussion in 'Electronic Design' started by [email protected], Sep 12, 2006.

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  1. Guest

    I recently built an adjustable power supply with three LM338s
    essentially in parallel. The design was copied directly from National's
    tech notes PDF; all values are exact. I'm powering it with a 26v 15A
    transformer through a full wave bridge rectifier. I'm actually using
    this supply to charge a large capacitor bank (700,000uF charged up to
    12-20 volts). I realize this creates a large load for a short amount of
    time, so to help pad the load I put a 1 ohm power resistor in series
    with the capacitor bank. This worked for probably 20 charges or so,
    then something changed.

    I noticed the DC voltage at the bridge rectifier rose up to 40v (but
    only when the regulator circuit was connected, so I know nothing is
    wrong with the transformer). With just a small light bulb (less than
    1/2 amp) it works fine throughout the whole voltage range. However,
    when I add about 5A of light bulbs, the output drops to zero instantly
    until I remove the large load. This never happened before.

    I'm assuming it shuts off because it's now having to regulate from 40V
    to 15V which is a rather large spread. I guess my main question is what
    is causing the voltage at the bridge to rise from 26V to 40V when I
    connect the regulator circuit? (it never did this initially). Is it
    more than likely that the LM338s are toast? I find it hard to believe
    since they still work great at very low currents.

    Note: I've read the app notes pretty thoroughly, and therefore
    understand the basics of the LM338 (filter caps close to the
    regulators, protection diodes, reference wire connected directly to
    case for low resistance, etc).

    I'd appreciate any suggestions / ideas that you may have about this
    situation. If the LM338 is not the way to go for charging a large cap
    bank, what is? I'm trying to keep this design relatively simple and
    cheap.

    THANKS!
    Kevin
     
  2. Phil Allison

    Phil Allison Guest

    ** Groper Alert !!


    ** Is that the one using an LM307 with its + input tied to the + rail ??



    ** What - no filter caps ??


    ** No - it was always 40 volts.

    26 volts AC produces around 40 volts DC, when the AC rectified and
    filtered.

    This is much too high for LM338s operating at low output voltages.



    ** A 5 amp lamp load will initially be a **50 amp* load when the filaments
    are cold. With a 40 volt in-out differential, the LM338s will immediately
    self protect and shut down the output voltage.


    ** Fraid it did.


    ** No clear evidence of that so far.

    Try a *resistor* load of 2 ohms with the voltage set to 15 volts - ie a
    7.5 amp load.

    If the voltage holds steady, the circuit is probably OK.



    ....... Phil
     
  3. Guest

     
  4. Phil Allison

    Phil Allison Guest

    ** GROPER ALERT !!!




    ** What about supply filter caps - you have IGNORED my question on this
    issue ????

    Got any ???

    26 volts AC produces around 40 volts DC, when rectified and filtered.

    Do you know nothing about PUS circuits ?

    BTW:

    NO NOT cut my posts to little pieces and write idiotic answers.





    ** Try learning some basic electronics and posting manners too.




    ........ Phil
     
  5. Guest

    I have a decent filter cap (I want to say it's around 10,000uF)


    You might want to take a breather...in no way was I ever trying to cut
    your posts to little pieces. I appreciate the help, and I do not see
    why you were so offended.

    Try being a little leniant to new posters. That was literally my first
    post on this list (or any list for that matter). I have posted on
    message boards for years but mailing lists are new to me. Sorry if I
    didn't realize the effects of a greater than symbol on my first
    reply...

    Also, I've been messing with electronics since I was 5. It's only been
    a hobby and I've never fully mastered it. Do you think I would be
    asking these questions if I knew the answers?

    Try having a little tact...

    Again, I appreciate the help but please do not reply if you're just
    going to cut people down and get offended when the OP never had any
    malicious intent.

    Kevin
     
  6. Terry Given

    Terry Given Guest

    Uh-oh, you're in trouble now. Phil rants like that at everyone, so dont
    take it personally.

    Cheers
    Terry
     
  7. Phil Allison

    Phil Allison Guest

    ** ARROGANT OVER-SNIPPING GROPER ALERT !!


    ** Finally, he supplies crucial info I had to chase so hard to get out of
    him.




    ** But you HAVE done it EVERY TIME !!!

    DO NOT remove the prior context from view.

    That FORCES me to keep putting it back.



    ** LISTEN ASSHOLE:

    1. A " usenet group" is not some fucking "list" !!

    2. YOU are posting on Google Groups - which is NOT part of usenet.

    3. Google Groups is destroying usenet by pouring in a steady stream of half
    witted trolls.

    4. Go look up what "usenet " is and learn the etiquette.



    ** Grrrrrr - this is not a damn MAILING LIST !!


    ** No-one is making you post bullshit.


    ** How about YOU try not rudely contradicting and posting bullshit.

    YOU are in no position to judge the replies posted here.



    ** No-one gives a hoot about your alleged but unknowable "intent" !!

    STOP snipping posts to pieces when you reply.

    Do reply to questions as fully as possible.

    DON'T expect anyone here to believe impossible claims.

    Capice?



    ........ Phil
     
  8. Proper posting etiquette is to trim the original. Some people on these
    groups are blind and they have to listen all the way through piles and piles
    of crap to get to anything new. Have pity on them and trim your replies.
     
  9. Phil Allison

    Phil Allison Guest

    "Homer J Simpson"

    ** Only to enhance clarity of meaning - not DESTROY IT by removing the
    original context from view and substituting an alternative that has to be
    guessed at by implication.

    Plenty for YOU to go learn.





    ....... Phil
     
  10. But not from you clearly.
     
  11. Phil Allison

    Phil Allison Guest

    "Homer J Simpson"

    ** Anonymous ASD fucked assholes learn nothing from anyone.

    EVER.




    .......... Phil
     
  12. colin

    colin Guest

    My gues is something went pop,..
    but 26vac will likely give 40vdc when rectified and smoothed with no load,
    so this isnt indicative of whats gone pop.
    measure all resistors, and check all devices with a diode meter etc.

    As said elsewhere light bulbs arnt constant loads, the resistance is very
    much lower when cold.

    I found an old toaster makes quite a good load, at considerably reduced
    power ofc, you can put the sets of elements in series or parellel to make a
    moderate choice of resistances.
    lots of surplus 2N3055s or similar around, ive got a big box of them
    Use a few in parallel with suitably large emitter resistors for current
    balance current sense and to take some of the brunt of the surge energy.
    Another 2n3055 to drive them and dual op amp for voltage and current limit.

    Colin =^.^=
     
  13. ehsjr

    ehsjr Guest

    You won't get steady 15 amps out of that setup. Your
    transformer is undersized and your Vin-Vout too great.

    You want a filter cap installed across the bridge,
    so that you don't need to connect the regulator
    to see the voltage across the bridge go up to ~ 40.

    If you are going to use a 1 ohm resistor, put it between
    the bridge and the Vin to the regulator, not between
    Vout and the cap bank. You could use a bigger value
    and switch it out with a 12 V relay when the voltage
    climbs to ~ 9 volts. Charging will be slower, but it
    will lower the surge current.

    Add a diode between Vout and the bank to keep the caps
    from discharging back through the regulator, and add a
    1K resistor from Vout to ground to assure some minimum
    current.

    Ed
     
  14. Guest

    Would you happen to have / know of a sample schematic of what you're
    talking about? I've seen a small (LM317) regulator drive a bank of
    3055's before, but not quite what you mentioned. Would that accomplish
    what you mentioned?

    Thanks so much for your helpful response!

    Kevin
     
  15. Guest

    I'm not so certain I need a steady 15 amps. The cap bank will draw a
    quick surge well over 15 amps. What I'm hoping for is a circuit that
    can handle this surge (even if it takes longer to charge than it
    normally would with a steady 15 amp output) without burning up.
    I'll try a larger cap yet and see if that helps.
    Excellent idea -- I'll try that out when I get to that point.
    I have a diode in place for that. I also have a small lamp that is
    always connected to make sure the 100mA minimum current requirement is
    met.
    Thanks so much for your help!
    Kevin
     
  16. Bob Scott

    Bob Scott Guest

    Why do you go out of your way to post off-topic rants? What has your
    encessant juvenile crap got to do with the LM338 and the user's circuit
    problem?

    Bob
     
  17. Phil Allison

    Phil Allison Guest

    "Bob Scott"


    ** Got get fucked - you asinine TROLL.







    ....... Phil
     
  18. Phil Allison

    Phil Allison Guest

    "ehsjr"


    ** The OP has clearly been lying about that missing filter cap.


    ** Huh ??

    The regulator circuit concerned has no input filter cap.

    It cannot work from raw, rectified AC.



    ....... Phil
     
  19. Bob Scott

    Bob Scott Guest

    I'm powering it with a 26v 15A
    Ooooh, I have a big case of DEJA VU. Twenty two years ago the chief engineer
    at CKVU TV needed a high current power supply to drive several hundred
    tri-color LEDs. These LEDS were used as lit pixels on a few dozen signs that
    spelled all of the Video sources in the news studio control room, like CAM
    1, Cam 2, VTR 5, VTR 6,etc. He showed me an article about power supply
    design in the magazine Broadcast Engineering. The article author was a
    condescending anal P ENG type who scorned the use of rules of thumb.
    Everything had to be calculated down to the last millivolt. The article had
    a schematic of a high current circuit utilizing 3 National Semiconductor
    voltage regulator chips operating in parallel. The Chief Engineer told me to
    build it as a supply to drive the hundreds of LEDS.

    I built it, exactly as per the magazine diagram with the 3 National Semi
    LMwhatevers. The circuit did not work. I consulted the National data book
    (We had REAL data books in 1984!) and came across an application note with a
    circuit diagram using 3 of the same LMwhatevers. Apparently, Mr.
    stickler-for-proper-engineering anal P ENG author had plagiarized the app
    note's diagram and taken credit.

    I noticed one wire from the app note diagram was missing from the magazine
    diagram. (Probably a mistake by the magazine art department.) I installed
    the wire to my built-up circuit.

    The power supply still didn't work. It was then I realized (again) that
    example circuitry schematics found in data books are often just used as
    illustrations, never left the desk, are not tested and may not work.

    I ended up designing my own power supply. It worked. I think the whole
    control room is still in operation 2 decades later because once in a while
    they broadcast scenes from the room and you can see the LEDS working..

    Have you considered the fact that all those hundreds of microfarads in
    filtering are supposed to connect to the rectifier output, not the regulator
    output?
    That the voltage on your secondary will be too high? (1.414*26)?

    Regards,
    Bob
     
  20. Guest

    Please tell me why I would lie about whether or not I have a filter
    cap. I don't lying would help solve the problem I am having.
     
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