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