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DeWalt 9116 charger R49

Discussion in 'Electronic Repair' started by legg, Oct 6, 2012.

  1. legg

    legg Guest

    This DeWalt 9116 charger has had battery packs forced into it
    repeatedly and strongly enough to crease-open board foil in the area
    and crack R49 ntc thermistor in half. The thermistor chip fragment
    remaining in the R49 half-package still measures ~77K, but standard
    values suggest this should be sloser to either 13K, 25K, 37K or 120K
    at room temperature.

    So what's the correct normal R49 value for replacement?

    As a result of the physical damage, other parts are also damaged, but
    I believe these have been fairly easy to track down, identify and
    replace.

    RL
     
  2. N_Cook

    N_Cook Guest


    Remove remnant and solder on a pair of wires , lead out to a 100K pot and
    "suck it and see" and post back the result to wwwland
     
  3. legg

    legg Guest

    I've already accessed the images - this version is a later revision
    with slightly different artwork and a completely different controller
    (higher pin count, wider body). The area around R49 and other
    temperature sensors is the same. The charger model has been recalled
    and revised a couple of times - has different voltage versions with
    the same part number, it seems.

    An unreadable schematic off the web assists in identifying functions
    of most parts in the power train and drive cctry. Have been working on
    switchers for some decades, so this section offers few surprises. I
    can probably get function with a fixed value here, but am looking for
    confirmation for a correct repair.

    This thing and it's exploded electrolytic picked off the floor at a
    work site. Cracked board looks like original fault, but much damage
    resulting.....no working unit for comparison. Just hope the controller
    section is as carefully partitioned as schematic suggests - no reason
    to fail unless chip supply(and program memory) also got scrambled.

    RL
     
  4. legg

    legg Guest

    Of course, when a repair is made, I'll let you know. I suspect that R
    values similar to those exhibited by the part epoxied onto the battery
    terminal may be expected. That would have been the simplest design
    approach.

    RL
     
  5. legg

    legg Guest

    It looks like the controller is well enough to control it's indicator,
    giving line fault and battery fault signals. R49 is currently adjusted
    to equal the epoxied thermistor value (77K at 20C still not easily
    replacable with off-the-shelf ntc values).

    Without a DeWalt battery to charge, sequencing a simulated battery
    insertion becomes an issue. For a start - I don't know what
    characteristics the third terminal is supposed to have - is this an
    internal ntc or ptc? There's no problem coming up with a
    representative NiCd string. I need more evidence of function from the
    controller - at least a charging cycle.

    The reason for the cracked board is finally suggested to be a piece of
    1/8th long wire insulation that has been stripped off a wire, then
    jammed sideways between the negative and signal terminals - preventing
    total battery insertion and possibly pushing the battery back out of
    contact when insertion pressure was removed. This was the same colour
    as the housing and could easily have been mistaken for an intentional
    physical detail of the assembly.
     
  6. legg

    legg Guest

    The unreadable schematic proves to contain a number of basic
    connection errors. This is kind of reassuring; as drawn it made no
    sense in certain areas. You see a capacitor in series with an emitter,
    you know somebody's playing with you.

    Extra PIC pins are generally just given pull-ups and are left alone,
    the later mod obviously just shooting for better program memory space.

    Funny construction. This thing should have been able to go through a
    single wave solder, with it's smd parts adhered to the underside. The
    only orphan in the process, now, is the PIC, which has to have been
    placed and soldered manually to the under side, post-wave. No plated
    through holes, but a pretty good supply of autotest points on the
    solder side, so not everything is left to luck in production.
    Sort of a warning to all designers - the product recall of this model
    number in ~Y2K was also about the connectors - they were detatching
    and rattling around 'presenting a potential shock hazard' when lodging
    in the air holes. There's no internal isolation barrier present, just
    the plastic of the charger frame and battery housing providing
    reinforced isolation. Makes sense if you're going to pay 'all that
    money' for the tooling and material anyways, just for a charger.

    Anyways, connectors need care.

    RL
     
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