DeWalt 9116 charger R49

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

 

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

 

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

 

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

 

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.

 

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