MiniSport Laptop Hacker (TM) - Vol #25.  November 1994 
To discourage pecuniary interests, Copyright (c) 1994 Brian Mork 
 
>>> ADMIN 
Any votes for an index issue yet?  Compendiums of all issues are 
available from ftp.cs.buffalo.edu as \pub\ham-radio\mlhackXX.zip, or on 
SimTel archives (eg. oak.oakland.edu) as 
\SimTel\msdos\packet\mlhackXX.zip.  The XX in the filename indicates the 
last issue number, and replaces the previous convention of using 
mlhacker.zip for all compendiums. 
 
>>> CUSTOM BATTERY CHARGER. 
Last issue, I concluded Jim Harvey's description of how he made a custom 
battery pack using a power drill battery.  Now he's built a dedicated 
charger unit and found a replacement for the expensive Li batteries. 
What a job, Jim!  Others, read on! 
 
"My ZL-2's heritage is Garage Sale.  It came with neither battery or 
charger.  I was able to obtain batteries for a reasonable price but 
Surplus Communications in St. Joseph wanted $70 for a Zenith charger, 
too much for a Garage Sale computer. For a year I have been 
unscientifically trickle charging the batteries with a CB radio power 
pack connected through a 47 ohm resistor. 
 
"I found several state of the art NiCad charger circuits in QST and 
other magazines.  Most depended on a tiny droop in battery voltage near 
the end of the charge cycle.  I wanted to leave the battery in the 
computer, though, and the 600 milliamp current surge when turning on the 
machine would surely confuse the chip.  One of the references I checked 
showed simple constant current charge circuits based on the LM317 
adjustable voltage regulator(1).  The schematic could not be simpler. 
Two parts!  Figure 1 has added a recommended protection diode and a 
bypass capacitor to bring the parts count up to four. 
 
The way this circuit works is:  The LM317 will do it's level best to 
keep the voltage difference between its output and adjust terminals at 
exactly 1.25 volts.  This will be the voltage dropped across R1 by a 
current of 1.25/R1 amps.  The 2.3 ohm value was obtained by paralleling 
two 4.7 ohm half watt resistors (at least one watt is needed). This 
gives a regulated current of about 530 ma, sufficient to operate the 
Minisport with a little left over to charge the battery. 
 
Fig. 1  Basic Constant Current Regulator    I = 1.25/R1 = 530 ma. 
 
          1N4004                          R1 
 From      |\ |       In+---------+Out +-----+           To Main 
Supply >---| >|----o----|  LM317  |----| 2.3 |---o-----> Battery 
+10 to     |/ |    |    |         |    +-----+   | 
18 VDC             |    |         |              | 
                   |    +---------+              | 
            0.33 -----       |Adj.               | 
            ufd  -----       |                   | 
                   |         |                   | 
                 -----       +-------------------+ 
                  --- 
                   - 
 
What was really needed was a circuit that would pass enough current to 
run the computer when it was operating, but would cut back to a 
traditional 0.1C charge rate when the machine was off. I added the two 
transistor circuit shown below in figure 2. 
 
The switch lead is connected to the Minisport's internal 5 volt power 
supply; I obtained this from pin 1 of the internal modem connector (see 
MLH #8).  When the computer is off, this is at a few tenths of a volt 
above ground.  Q1 is cut off, which allows Q2 to conduct.  Current flow 
through the 270 ohm resistor increases the voltage seen by the LM317 
adjust terminal and the regulator decreases the current.  The 2000 ohm 
value of R2 allows an output of about 160 milliamps. 
 
When the Minisport powers up, the 5 volts supplied to the switch sense 
lead causes Q1 to conduct which in turn cuts off Q2.  The LM317 then 
operates as in Figure 1. 
 
Fig. 2  Modified Current Regulator With Trickle Charge Switch 
 
          1N4004                          R1 
 From      |\ |       In+---------+Out +-----+           To Main 
Supply >---| >|----o----|  LM317  |----| 2.3 |---o-----> Battery 
           |/ |    |    |         |    +-----+   | 
                   |    |         |             +-+ 
                   |    +---------+             | | 
            0.33 -----       |Adj.              | | 270 
            ufd  -----       |                  | | 
                   |         |                  +-+ 
                 -----       |                   | 
                  ---        o--------o----------+ 
                   -         |        | 
                            +-+      +-+ 
                         R2 | |      | | 
                            | |      | | 27K 
                         2K | |      | | 
                            +-+      +-+ 
                             |        | 
                              C |     | 
                           Q2  \|     | 
                         2N3904 |B----o--C |                  Switch 
                               /|     Q1  \|     +-----+      From 
                              E |   2N3904 |B--o-| 33K |---< +5 volt 
                             |            /|   | +-----+      Supply 
                           -----         E |  +-+             Pin 1 
                            ---         |     | |             of COM2 
                             -        -----   | | 22K         Connector 
                                       ---    | | 
                                        -     +-+ 
                                               | 
                                             ----- 
                                              --- 
                                               - 
 
"I assembled the components on a piece of perf board which slides into 
the internal modem slot of the Minisport. It's wired directly into the 
main battery terminals at the motherboard.  The LM317, for heat sink 
purposes, is bolted to the metal bottom plate with a mica insulating 
washer underneath and a nylon screw (the tab of the LM317 is connected 
to the output terminal.)  I epoxied a 2.5 millimeter barrel type power 
socket into the phone jack hole of the modem slot cover to connect the 
external DC charger supply (a 14 volt 2 amp switcher, another Garage 
Sale acquisition). The charger could just as easily be built in an 
external box.  You would need a 3 wire connector to bring out the 5 volt 
switch sense lead though. 
 
"The LM317 current limited charger will happily run the computer from an 
auto cigarette lighter socket.  You should put an inductor in series 
with the lead and use an MOV surge suppressor to protect the machine 
from the relatively dirty auto voltage as discussed in MLH #12 and #21. 
 
"Another improvement made while I had the Minisport disassembled was to 
discard the expensive coin cell backup batteries and functionally 
replace them with a set of small NiCads tucked in the modem compartment. 
There are several cordless phone batteries that can be fitted into the 
space.  You need five NiCad cells for the necessary 6 volts, use thin 
wire wrap wire to carry the current over to the terminals on the edge of 
the motherboard adjacent to the coin cell compartment.  To keep the new 
backup battery topped off I connected the positive side to the new 
charger input jack through a 1000 ohm resistor.  This will allow 
somewhat less than 10 milliamps to trickle in which is reasonable 
considering the limited duty cycle of the backup system.  It is only in 
use while the main battery is being changed or is completely discharged. 
 
"(1) Engineer's Notebook - Integrated Circuit Applications, 1980 
Edition, Forest H. Mims, (C) Radio Shack.  [BjM: I use a similar, simple 
circuit to drive my laser diode experiments.  It works well.] 
 
Please provide feedback!      * Direct data 1-509-244-9260 
                              * ARO Net KA9SNF@ka7fvv.#ewa.wa.usa 
                              * Internet bmork@opus-ovh.spk.wa.us 
73, Brian                     * 6006-B Eaker, Fairchild, WA 99011 
