18 volt charger

 

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The charger for my 18 volt rechargeable drill met with an untimely and
violent demise this week, and I am trying to build a charger circuit for
my battery pack.  I am starting with this basic circuit (attached -- I
hope attachments are ok on the group).  The cells in the 18 volt Ni Cad
pack are rated at 1300 mah.  If I use the 10 percent capacity rule for
charging, I will need to build a charger which outputs 130 ma, constant
current.  To do this, I will need to make a few changes to the basic
circuit.

[I am somewhat low on the experience ladder, so feel free to jump in and
correct my thought process if some alarm bells go off.]

To start with, I will need a higher voltage supply.  Lacking a suitable
transformer, I dug in the closet and found a 24 volt, 2.5 amp regulated
and filtered supply that was a take-out from a machine control.  It's
overkill, but it's what I have.  That will take care of the supply side
of the circuit, as long as I control the current.

I do not have the specified BC547 or the TIP41.  Since this is a general
purpose NPN transistor driving an NPN power transistor, I plan to use a
2N3904 to drive the base of a TIP3055 from our friends at Radio Shack.
How am I doing so far?

Now how do I calculate the resistance values for Rx and Ry?  The
information I got with this circuit diagram says to calculate Rx as 0.7
divided by the charge rate.  If I want 130 ma, I use 0.7/.13=5.38 ohms.
  Correct?  That resistor will also need to be a beefy one to carry the
wattage.  How do I calculate Ry to give the proper voltage output?

I will also want to add an LED and limiting resistor ahead of this
circuit for a power-on indicator.  That's easily done.  Maybe I can put
it in series with Ry?

I would also like to add another LED in the charging output end to
indicate that the batteries are drawing current.  Either the LED stays
on as long as the battery is drawing current, or it lights up when the
pack is full, whichever is easier.  I'm not quite sure how to do this.
It would seem that I would have to shunt off the output somehow.  Any
suggestions?

Thank you for reading this long post.  Feel free to tell me where I've
gone wrong.  I will appreciate any help someone is willing to offer.  I
am trying to use what is on hand, or can be had very cheap.  If I have
to invest too much in this project, I will simply buy a new drill/charger.

Nels

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Nelson Johnsrud wrote:

The 2n3055 conducts 130 mA.  Divide that by the gain of the transistor,
and you have the base current.  Ry has to allow at least that much
current to flow into the base.  Size Ry to allow twice that much.  To
do the arithmetic you need to know the voltage across Ry.  It is supply
voltage minus two diode drops (the base-emitter junctions of the
transistors).  So if the base of the 2n3055 needs say 13 mA, and the
voltage across Ry is 22 or 23 volts, Ry is in the 15 or 20 kilohm
ballpark.

 



It's a tiny attachment, so yes it's fine.  But you will no doubt get silly
perfectionists calling it wrong because it's an attachment, even though it's
smaller than a large text post.


If it was my drill, I'd insist on a fast charger, which is what they come with.
The drill is always flat at the point where you need it most.


--
http://www.petersparrots.com     http://www.insanevideoclips.com    
http://www.petersphotos.com

FREE ONE DOLLAR BILLS!
(Please send $4.95 to cover posting and handling)

 

wrote:


perfectionists calling it wrong because it's an attachment, even though it's
smaller than a large text post.

---
Regardless of the size of the attachment, its definitely _not_ fine,
the reason being that since this is a text-only newsgroup, most if
not all, usenet providers won't post the attachment.

You, being a newbie and all, wouldn't know that but you should
refrain from issuing edicts as to what's allowed and what isn't
until you find out what's going on.  And that includes making
judgements about what's silly and what isn't.

 
--
John Fields
Professional Circuit Designer

 

wrote:


perfectionists calling it wrong because it's an attachment, even though it's
smaller than a large text post.

Incorrect.  Perhaps some won't allow them to SEND it.  But that obviously
worked.  I've never known a news feed that filters to that extent stuff that's
already on the news network.  If I did find one, I'd close the account promptly!


I've been on the internet (and newsgroups) since 1994.  Try getting your facts
straight before making an even bigger ass of yourself.


Allowed indeed.  This is a public group, not an over-policed admin-ridden forum.

--
http://www.petersparrots.com     http://www.insanevideoclips.com    
http://www.petersphotos.com

"I have left orders to be awakened at any time in case of national emergency,
even if I'm in a cabinet meeting." - Ronald Reagan