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