In article ,
David Nebenzahl wrote:
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.

Easy to make one. I'd go for a constant current 1/10th capacity charge
rate. This will charge the battery overnight (approx 14 hours) and it will
have a very long service life - much longer than a fast charged one. A
constant current charger may be made with a suitable DC supply, and one
power transistor with few extra components.

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.)

Site's not available at the moment so can't comment.

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).

An LED is current driven so needs the series resistor. You'll find tables
for this resistor value from the voltage if you Google. Or it can be
worked out by a formula. If you emit the resistor the LED will fail.

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.)

Some makers use the same battery with different chargers. The third
terminal is usually a temperature sensor, and this connection is omitted
with nasty or low output chargers.

--
*Taxation WITH representation ain't much fun, either.

Dave Plowman London SW
To e-mail, change noise into sound.