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Tale of interest
no.2
The plug-pack saga and the cordless
drill:
Some time ago, a
good friend of mine appeared at the front door clutching a plastic
bag.
Before I could
even open my mouth to greet him, he offered the
bag to
me.
Inside was a
cordless drill, two battery packs, a manual, and a plug
pack.
"More work for
me." I thought. And so it turned out to
be!
My friend's almost
new GMC cordless drill wouldn't charge up
anymore.
He was way ahead of
me..... He had
practically torn his house apart looking for the purchase
docket.
He could not find
it.
"On with the job." I thought. It
should only take me a couple of
minutes!
Well,,, it did
only take a few minutes
to determine the plug pack was
stuffed.
The primary of the transformer was open
circuit.
Fixing my friend's drill would only require me to design
and build a new battery
charger!
It's just as well that I like building
things.
Most sane people would just tell my friend
to.. "Throw
it away and buy a new one"; wouldn't
they?
Not yours truly! I must have
Scottish ancestry..... Everything can be
fixed!
Also..... Everthing should last forever
anyway!
When they go open circuit,, it is a sure sign
that the transformer overheated at some
stage.
This plug pack was no
exception.
The only problem in finding a replacement plug
pack, was the output
voltage.
The 24 Volt battery packs needed around 30 Volts
from the transformer to charge the NiCad
cells.
Not the sort of plug pack easily purchased at your
local electronics
shop!
Inside was a timing circuit to terminate the
charging after a given
time.
No current limiting cicuitry was evident in this
unit.
This practice is common with these style of plug
pack
chargers.
My old 7.2 Volt drill charger does this without even
the luxury of a cut out
timer!
My next step was to
set up the "charging sled" with a dummy load, to determine the
charge
time.
Once I knew the time, I could then determine
the correct current
needed.
I have found a cheap battery clock a useful tool for
timing longish
periods.
The clock is started at 12 o'clock and its power
controlled by the timing device under
test.
While the "charging sled" was timing, the
battery pack was opened to find out the capacity of the
cells.
The plug pack was rated at 300mA and, as it
turned
out, was the charge current
needed.
I hacked open the blown plug pack to see if anything
was salvageable
inside.
Four diodes and a 1000uF capacitor were taken out
and put
aside.
Out of
curiosity, the transformer was examined to see if it was
repairable.
As I suspected. It was fitted with a thermal
fuse.
I found the other primary connection, checked
it with the multimeter, and found the winding was still
intact.
I couldn't repair it on a permanent basis but wanted
to test it to find out what was wrong with
it.
I have built a handy gadget consisting of a 240V to
240V isolation transformer and two light bulbs for jobs like
this.
The light bulbs limit the current that can
flow, and the isolation transformer makes it possible to use
earthed
test
equipment.
The low wattage bulb was selected for this
test.
As the bulb didn't glow at all, I knew the
transformer was working properly (ie No shorted
turns).
I then checked the secondary voltage and at 36
Volts, thought it was a little
high.
Suddenly it dawned on me! ......
The stupid twits had used a transformer wound
for 220
Volts.
The timer switched off just after four
hours.
This made sense if the plug pack supplied
300mA.
I then tried to charge the cells with the original
transformer...... It was supplying nearly double the
required current.
Those poor little NiCad cells!
No wonder the transformer
overheated.
I could now work out the best way of contriving a
new battery charger for my
friend.
A transformer was needed with a 30 Volt
winding, capable of supplying 300mA or
more.
The local electronics store had just what I wanted,,
plus,, a suitable metal box to put it
in.
As an added bonus, both were "on
special", so my wallet wasn't drained too
severely!
The rest of the required components I had to hand or
were salvaged from the old plug
pack.
I designed a proper stabilized and current limited
charger. An LM317 makes an ideal constant
current
source.
I designed and made a circuit board and built the
thing
up.
Worked like a charm! Heh! Heh! My
friend's drill was now going
again.
I then charged both battery
packs.
The metal box may have not looked pretty, but
I am
positive that it will not
fail.
All this mucking about because of a cheap Chinese
plug
pack!
I wonder how many cordless drills there are out
there with dodgy plug packs, merrily over-charging their battery packs?
Till next time.
All
rights for
this article belong to "Rick's
Workbench" in Tasmania
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