Re: 18 volt charger

Thank you for your reply, Chris. Lots to digest. Let's go one piece at a time:

Chris wrote: 
approximation, you can assume that about 130mA will also be flowing
through the collector.  Since the transistor Q2 (BC547) is going to
work to keep its base-emitter voltage at no more than 0.7V, you can
size Rx so that about 0.7V will be impressed across Rx when the 130mA
is flowing through it.  Thus, Rx = 5 ohms.  (By the way, .13A * 0.7V =
.09W.  A 1/4 watt resistor should work fine for Rx.)

Before we get too far, I should mention that this circuit was found here: http://web.telia.com/~u85920178/begin/batt-00.htm#charg
It is a constant current charger circuit for AA batteries, but I wanted to adapt the circuit for my 18V pack. The 0.7 b-e voltage I referred to was just what the author had published for this circuit. Using E/I --> 0.7v/.13A=5.38, so I am with you so far. I also see where I went wrong on the wattage of the resistor. For some reason (brain fart) I thought this resistor was carrying the charging current, as it is connected to the emitter of the power transistor. Another look at the circuit, along with your explanation, pointed out my error.

can never be worse than 10.  That means that Q1 will never need more
than 13mA of base current to drive 130mA of collector current.  That
means that if Ry is less than 1700 ohms, there will always be enough
current for the base of Q1, and Q2 will shunt away the rest.


Understood.  I'll scratch up 1500-1600 ohms

By the way, if your regulated 24VDC power supply is a linear with a
tweaker adjustment, try to bump it up a little to 26V or more.


It is a linear, and has an adjustment.  I can get about 28 volts max.

also, if you want
your charger to last more than a few minutes, use a heat sink for Q1
that can dissipate several watts, along with a thin coating of thermal
grease.

Absolutely. We can't regulate that much current without making some heat. That heat has to go somewhere.

just put a 2700 ohm resistor in series with your LED across the
power supply terminals.


That is simple enough.

Also, putting the LED in series with Ry won't work
to show charging

Yes. That LED is for a power-on indicator only. I may skip the charge indicator just to keep it simple.

(By the way, most of the time, attachments are frowned upon in the
newsgroups.  If you want to post a picture, and ASCII art doesn't do it


I will look into that ASCII art program you used.  Thanks.

Nels
.

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