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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
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.
They rely on the transformer to saturate at more or less the required charging current.
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
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.
this article belong to "Rick's
Workbench" in Tasmania
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