From: kolstad@cae.wisc.edu (Joel Kolstad)
Newsgroups: alt.games.lynx
Subject: Re: How to make Lynx be a Nicad charger?
Date: 21 Aug 92 18:13:01 GMT
Organization: U of Wisconsin-Madison College of Engineering

In article <BLK.92Aug21115257@vanity.mitre.org> blk@mitre.org (Brian L. Kahn) writes:
>
>I've read that some people modify their lynxen to charge nicads when
>hooked up to the AC adapter.  I presume this is just a resistor and
>diode stuck in for trickle charging.  Can someone tell me a good value
>for the resistor, and maybe suggest a place to wire it in?  I have a
>classic lynx (two actually).

Well, sure, a standard SLOW charger does only consist of a resistor and a
diode.  What are the values?  Let's see...

First off, let's assume that the NiCads are 1.0V each, for a total of 6.0V.
(They're actually higher when they're charged -- 1.2-1.25 volts, but we want
to make sure we don't put too much current through them... But assuming a
low voltage, we'll initially have a resistor that charges more slowly than
need be).  The Lynx AC adapter supposedly puts out 9V at 1A.  The "real"
voltage will vary...  In fact, with the Lynx off (only charging batteries),
it'll be quite a bit higher.  We'll be conservative again and assume
it's putting out 12V.  So, between the adapter and the batteries there's a
6V drop.  Ok, next... your standard C and D cell NiCads are 1200 mAH (i.e.,
they'd last 1 hour if you drew a current of 1200 mA).  Run of the mill batteries
would have you change them at 1/10 the "C" (one hour discharge == 1200 mA) rate.
This is 120mA.  So, using Ohm's law, 6V/120mA=50 ohms.  If you're getting
parts from Radio Shack, the closest thing you'll find is 47 ohms.  Good enough.
The power rating?  P=V^2/R, so for a 47 ohm resistor we have 6*6/47=.76 watts.
Oops.  Radio Shack doesn't sell 1W resistors, do they?  Ok, so use 2 100 ohm
resistors in parallel.  Now, each one is dissipating .36W, so use 1/2 W
resistors.

If you have access to an ammeter, you'd be well advised to check the current
and make sure it's in the right ball park (90-150 mA should be fine).  I'm
guessing at some of the numbers above, after all.  (And I'm _almost_
positive that the Lynx's adapter is DC.  The paragraph above is total hogwash
if I'm wrong!)

Finally, here's how to hook it up:

          |---------------------------------------|
          |-------Resistor------|-------+Diode----| 
      ++++++++++           ++++++++++++         ++++ 
      AC Adapter           Battery Pack         Lynx
      ----------           ------------         ----
          |---------------------|-----------------|

The diode keeps the AC adapter from dumping large amounts of current into the
battery pack via the connection to the Lynx.  (You think there ought to be a
diode between the AC adapter and the resistor?  Nah, that's only if the
AC adapter puts out AC).  Oh yeah... make sure the diode can handle 1A, has
a PIV of at least 15V, and can dissipate 600mW.  This kind of diode is pretty
easy to come by.  Radio Shack most certainly has them.  Remember that the
cathode (minus) end of the diode has a band around it, and you must orient
it properly or it will be useless and you'll zap you NiCads!

I don't have the battery pack, so you'll have to work out the physical
implementation yourself.

Have fun.  And remember that I don't take any responsibility if you blow up
your Lynx. :-)

Personally, I'm going to work on the FAST charging version...

						---Joel Kolstad
Newsgroups: alt.games.lynx