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troubleshooting method for micro-controller board ?

Discussion in 'Electronic Repair' started by robb, Nov 5, 2007.

  1. Guest

    Oh. 5mA then, at least if it's the same as the L387A:
    It's a fixed 5V regulator, so no Vset resistors.

  2. robb

    robb Guest

  3. robb

    robb Guest

    well i am glad to have this advanced second nature diagnostics

    but it is Zero nature foreign to me though i am trying to
    understand it before i blow more components.

    i am very happy to have the help from everyone that know so much
    as i surely would have fried it worst if possible than it already

    i suppose i should read back through the data sheet on L387 again
    before powering up

    thanks again for help,
  4. robb

    robb Guest

  5. Guest

  6. robb

    robb Guest

    thanks TM,

    i'll try that tonight.

  7. Jamie

    Jamie Guest

    Have you looked at the bridge rectifier and filer cap before your regulator?
  8. robb

    robb Guest

    yes, that was the first thing i considered just because it was
    there next to the fuses

    i desoldered the bridge and then

    i tested the bridge by using diode tester on the DMM so i chose
    the combination of pin checks that tests each of the diodes AC1
    to (+) .,AC2 to (+), (-) to AC1, (-) to AC2
    and each gets ~.596 (+- .007) volts till turn on

    and there is no "continuity" between aany combo of bridge pins

    thanks for help and reply,
  9. Arfa Daily

    Arfa Daily Guest

  10. robb

    robb Guest

    hi TM,
    I tested the L387 and i think it must be bad as i get more than
    5v from the Vout ?

    i had already desoldered it from some previous suggestions about
    isolating the different supply rails

    so i tested by connecting 5V/1A DC source between Vin and
    GND-Pin3 and i checked the voltage with DVM between Vout and
    GND-Pin3 and i was getting 3.47 V

    i changed to a 18 v/3.5A DC source and i was measuring 8.67 V
    between Vout and GND-Pin3

    L387 is cool
    and when i connected an LED load to Vout the L387 got very hot
    very quickly
    so i suppose i at least need to get an L387 ?

    thanks again for help,
  11. Arfa Daily

    Arfa Daily Guest

    From that, it sounds as though the '387 is not doing its job correctly, so
    replacing it would probably be a good move.

  12. robb

    robb Guest

    Hi Arfa,
    thanks for all the useful help and iinformatin i really do
    appreciate your's and everyones helpful advice and information. I
    started this project as a learning exercise guess its more a

    Regarding the L387, since i had already removed it hat was asy
    to test.

    I am pretty sure it is bad from what you explained. that is 5V
    regulators only output 5V .

    when i applied a 5V/1A DC source to Vin/GND i measured 3.47V
    between Vout/GND.
    when i applied a 18 V / 3.5A DC source to the Vin/GND i measured
    8.37V between Vout/GND

    an LED load between Vout/GND caused the L387 to get very hot

    so a new L387 is in order so that i can complete the other tests
    that you TM and others have suggested

    thanks again for help,
  13. Eeyore

    Eeyore Guest

    How much more ? A couple of hundred millivolts, says 5.25 V is OK.

    Pointless. It won't operate correctly with only 5V in. 3.47V is sort of what I'd
    expect under those conditions. A linear voltage regulator has to have several
    volts more on the input than the output to function correctly.

    What value resistor did you have in series with the LED ?

    He didn't say if he had a resistor in series with the LED ! Could explain a lot.

  14. Eeyore

    Eeyore Guest

    With a load or no load ? If there was no load on the regulator output then
    that's quite possible even for a good L387. You must have a load resistor of
    around 470 ohms on the 5V output for testing for the chip to regulate properly.

    An LED with no series current limiting resistor might well do that ! You should
    have a resistor of ~ 150 ohms in series with the LED.

    It may not be necessary. Your testing used flawed methods.

  15. robb

    robb Guest

    hi graham,
    just being cautious and i do not have much in the way of handy
    power sources, various wall warts.
    you need to use a resistor ???

    just joking it was 100 Ohm 1/2 watt (Brwn Blk Brwn Gold)

    would you expect 8.67 Vout with 18V DC Vinput ?

    thanks for help,
  16. robb

    robb Guest

    ok, i set it up as you suggest a 470 Ohms between Vout and GND
    (Pin3) with 16v/3A DC source connected to Vin and GRD. The
    measurement between Vout and GND was 8.03 V when i put a LED in
    series with 470Ohms i get 7.92 V measured accross Vout and GND.
    that is why i am seeking help from "the group", "the fountain"
    of electronic knowledge :(

    thanks for help graham,
  17. Eeyore

    Eeyore Guest

    Not under those conditions.

    You do need a new L387.

  18. Guest

    So while you're waiting for the new L387 (or better yet, before you
    order it), replace the 30V and 12V fuses, connect your (5.2V) lantern
    battery to the 5V rail, and power up. Does the board work? Fuses
    30V and 12V rails measure OK?

  19. Guest

    Some need 1-2V more, others around 1/2V (I think I've seen
    LDOs down to 0.2V dropout [i.e. Vout-Vin]). I didn't find a spec
    sheet for the L387, but the L387A has a max dropout of 0.8V
    (and that's from 4.9V, so Vin[min] is 5.7V). Don't know whether
    dropout is one of the improvements in the A version.

    In most cases it's safe to test with Vin=Vout+2V, but with an
    adjustable LDO operating near the top of the Vout range, that
    may result in Vin exceeding spec. Checking the data sheet
    first is always a good idea.

    (LDO = Low DropOut, a category of linear regulator)

  20. Eeyore

    Eeyore Guest

    Sure, I was generalising somewhat for brevity.

    As you say the value may vavy from several hundred millivolts for true LDO types
    to as much as 3V in rare cases. 2V is typically a safe value to ensure correct

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