Re: Help needed w/battery charger circuit


"David Nebenzahl" <nobody@xxxxxxxxxxxxxxx> wrote in message
news:445ec34e$0$3701$822641b3@xxxxxxxxxxxxxxxxxxxxxxxxxxx
lsmartino spake thus:

David Nebenzahl ha escrito:

The problem is finding a battery charger for a cordless drill. I
inherited a Skil 12-volt drill and battery, but no charger. The only
hope of finding one seems to be getting a used one, which I haven't been
able to find.

I opened my friend's charger and drew the circuit, which I've attached a
link to. (PDF, and a big one, sorry; somehow a little bitty 35 KB JPG
made the file bloat up over a megabyte.)

Can someone comment on this? I've got all the values except for the one
resistor in series with the LED, which I can probably figure out (or
maybe just omit altogether, as it seems only to be an indicator lamp).

What's the function of the capacitor? (I believe it's a non-polarized
electrolytic in the original unit.)

Would this work if I constructed it?

The other problem is that the battery has 3 terminals instead of just 2.
Two are marked "+" and "-", but there's an unmarked middle terminal. But
the charger only had connections to the two terminals. What's the 3rd
terminal for? (Picture of bottom in the PDF.)

Oh, yeah, the PDF is here:
http://www.geocities.com/bonezphoto/misc/BatteryCharger.pdf

That charger seems horrendous to me. If for any reason you touch any of
the exposed battery terminals of the charger, without the battery in
place, while the charger connected to the AC, you will get instantly
shocked by 120 VAC. I can´t believe that such a charger, without any
interlock switches can be manufactured, and approved to be sold.

Look, save your horror and outrage, OK? The thing is mass-produced by a
well-known reputable company, is UL, SA and who knows what else approved
up the wazoo, so apparently the regulators think it's safe. The whole
thing is safely encased in plastic, so there's hardly a chance someone's
gonna get their fingers zapped by line voltage.

Or maybe you prefer the version of the world where everything is covered
with soft foam rubber bumpers, with flashing lights, sirens and klaxons
going off every time you attempt to plug something in?


the circuit makes little sense to me as Imax through the 22K resistor would
only be 5 mA the bulk of the current would have to go through the series RC
circuit 2 ohm + whatever Xc is (at operating frequency). finding the
capacitor might be a trick in itself :) the 120 V rating (of C) is
suspicious because when charging a depleted battery nearly all the 120 V
will be dropped across it. that's not much of a safety factor.

it was probably done this way to partly avoid the heating you get with
series resistors.

lets assume for now its a 1 AH battery. to charge it in 10 hours would
require 100 mA (or a wee bit more) all you need is to come up with a
charger that will pump 100 mA into a 12V load that wont fail or burn up or
set you shop on fire. there is probably 50 or do ways to do it but I'd not
go the transformerless rout.

a cheap way might be to get a car battery trickle charger and put flashlight
lamp bulbs in series to limit the current or calculate the needed voltage
drop by measuring Vno load minus 12 and sizing the appropriate resistor .

another approach might be to use a "wall wart" supply. i'm a little hesitant
to recommend that as a buddy of mine clams that one failed and burnt his
house down.

i'm in the same situation as you with an old flashlight that someone gave
me. it's not worth finding a charger for it. i just pull the battery pac and
tape gator clips to the contacts and hook to a bench supply with an ammeter
in series. set it to 50 mA and let it charge for a day.




.

Quantcast