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Build this portable 6V SLA battery charger If you own one of the new 6V SLA batteries from Jaycar, this simple charger will keep them in top condition. It uses only a single IC and charges the battery to a fixed voltage of 6.9V at currents up to 500mA. By BRIAN DOVE Keeping batteries in top condition is not as easy as you may think. Many of the more popular battery chargers simply thump the battery with a rough DC current and hope for the best. Anoth­er problem is that very few chargers cater for the 6V variety. Whether you’re operating a video camera, a security torch or other equipment requiring a 6V supply, a 6V SLA battery has many advantages over the more traditional nicads. These include less critical charging parameters and 62  Silicon Chip much greater power capaci­ty. This Portable 6V SLA battery charger is specifically designed to mate with Jaycar’s range of 6V SLA batteries. What’s more, it uses only a handful of components and can be powered from your car battery or any 12V DC source. In operation, the charger will initially supply over 300mA to the battery, with this current gradually decreasing as the battery voltage reaches 6.9V. This makes it suitable for use with batteries with a rating of 2A.h or more. Note that because the output of the charger is fixed at 6.9V, no damage to the battery will occur if the unit is left on for an indefinite period of time. The circuit is based on the MC­ 34063A DC-DC convert­er IC. In this circuit, it’s connected as a “buck” or step-down converter which switches a 12V DC input down to 6.9V. The beauty of this circuit is that it is very efficient. Whereas a linear regulator would need to waste about half the input power, this circuit is about 80% efficient. Block diagram Fig.1 shows the internals of the MC34063A IC. It contains all the necessary circuitry to produce either a step-up, step-down or inverting DC converter for any voltage from 3-40V. Its principal sections are a 1.25V ref- PARTS LIST 88 1 S Q Q2 Q1 R 2 77 IPK CT OSC RSC 66 VIN D1 VCC 3 COMP 100 100 1 PC board, code 6VSLA, 61 x 41mm 1 plastic case, 83 x 54 x 28mm 1 toriodal core (Jaycar Cat. LF1240) 1 1.5-metre length x 0.5mm dia. enamelled copper wire 2 red alligator clips 2 black alligator clips 1 2-metre length medium-duty figure-8 cable (for input & output connections) 2 M205 PCB mounting fuse clips 1 2A M205 fuse 1 SPST or SPDT toggle switch 6 PC pins CT 1.25V REF 55 L 4 R2 Semiconductors 1 MC34063A DC-DC converter (IC1) 1 FR104 1A fast recovery diode (D1) 1 5mm red LED (LED 1) VOUT R1 CO Fig.1: this diagram shows the major internal elements of the MC34063 controller IC & shows how it is wired to function as a step-down converter. erence, a comparator, an oscil­lator, an RS flipflop and a Darlington transistor pair (Q1 & Q2). The frequency of the oscillator is set by timing capacitor CT, connected between pin 3 and ground. A value of .001µF gives a frequency somewhere between 24kHz and 42kHz (the exact frequency is not important). As shown in Fig.1, the oscillator drives the RS flipflop via a gate and this flipflop in turn drives Darlington pair Q1 & Q2. Each time Q1 & Q2 turn on, L1 is effectively placed across the supply voltage. These transistors stay on just long enough for the current through the inductor to build up to saturation, at which point they both Fig.2: the final circuit for the 6V SLA battery charger. The output of the internal Darlington pair appears at pin 2 and drives diode D1, inductor L1 and a 470µF capacitor which together form a standard step-down circuit. The 6.9V output is set by the 47kΩ and 10kΩ resistive divider across the output. turn off. The energy in the inductor is then dumped into reservoir capacitor CO via a diode (D1). The IPK sense line at pin 7 is used to monitor the peak current through the RSC sensing resistor – ie, it monitors the voltage across RSC and thereby limits the peak current through the inductor to a value of I = 0.3V/RSC. Voltage regulation is provided by the internal comparator. This compares the internal 1.25V reference with the output from a voltage divider consisting of resistors R1 & R2. These two resis­tors set the output voltage (VOUT) as follows: VOUT = 1.25 x (1 + R2/R1). The comparator works as follows. POWER S1 TO CAR BATTERY Capacitors 1 470µF 16VW electrolytic 1 .001µF MKT polyester Resistors (0.25W, 1%) 1 47kΩ 1 470Ω 1 10kΩ 1 0.33Ω 5W Where to buy parts A kit of parts for this project will be available from Jaycar Electronics Pty Ltd for $29.95 plus $4.50 p&p (Cat. KC-5164). Note: copy­ right of the PC board for this project is owned by Jaycar Electronics. If the output voltage goes too high, the inverting input of the comparator will be higher than 1.25V and so the output of the comparator will be low. 0.33  5W F1 2A ZD1 15V 1W 6 7 IC1 MC34063A 3 4 L1 : 2 LAYERS 0.5mm DIA ENCU WIRE ON NEOSID 17-732-22 TOROIDAL CORE 8 1 2 L1 A 5 D1 FR104 470 16VW 47k .001 .001 A  LED1 TO 6V SLA BATTERY K 470 K 10k 10k PORTABLE 6V SLA BATTERY CHARGER July 1994  63 LED1 TO 6V SLA BATTERY A K D1 470 .001 1 L1 IC1 TO CAR BATTERY 470uF 0. 33 5W 47k 10k ZD1 POWER S1 F1 Fig.3: the parts layout on the PC board. Inductor L1 consists of two layers of 0.5mm-dia. enamelled copper wire wound on a small toroidal core. As a result, the oscillator is effectively gated off and so Q2 & Q1 will both be off. Conversely, if the output goes too low, the inverting input of the comparator will be below 1.25V. The output of the compara­tor will thus be high and so the Darlington pair can now be toggled by the RS flipflop to switch current through the inductor. The result is a form of pulse width modulation which effec­tively reduc- es the amount of inductor current when only light loads are connected to the output and thus dramatically increases the efficiency. More importantly, it regulates the output voltage so that, under most loads, the output remains as set. Circuit diagram Fig.2 shows the final circuit diagram of the unit. The PC board sits in the bottom of the case, while the LED protrudes through a hole in the front panel. Tie knots in the power & output leads before they exit the case to prevent them from coming adrift. 64  Silicon Chip Power is applied to the circuit from a car battery (either directly from the battery terminals or from the cigarette lighter socket), or from some other suitable 12V DC source. This passes via switch S1 and is fed to pin 6 of IC1 and to a 0.33Ω resistor (RSC) via a 2A fuse (F1). Zener diode ZD1 protects the circuit against high voltage spikes (eg, from an automotive electrical system). It will also conduct heavily and blow the fuse if the input voltage rises above 15V. In addition, the 2A fuse protects the circuit if the output is inadvertently short circuited. The 0.33Ω 5W resistor between pins 6 & 7 sets the current limiting, in this case to about 900mA (ie, 0.3V/0.33Ω = 900mA). Pins 8 and 1 are the collectors of the two transistors inside IC1 and these are connected to the output side of the 0.33Ω resistor. This internal Darlington transistor pair is capable of switching a maximum of 1.5A, so it is more than capable of han­dling the job. The output of the Darlington pair appears at pin 2 of IC1 and drives diode D1, inductor L1 and a 470µF capacitor which together form a standard stepdown circuit. When pin 2 of IC1 goes high (ie, when the internal Darlington transistor turns on), current flows through the inductor to the load – in this case, the battery being charged. During this time, D1 is reverse biased and the inductor stores energy. When the internal Darlington transistor turns off, the collapsing magnetic field around the inductor tends to maintain the current flow through it in the same direction. D1, an FR104 fast recovery type, acts as a flywheel diode. It now provides the return current path from the load and prevents the IC side of the inductor from going below -0.7V. The 470µF capacitor is used to store the energy from the inductor and also acts as a filter to smooth out the ringing waveform. The 6.9V output is set by a voltage divider consisting of 47kΩ and 10kΩ resistors which are strung across the output. These provide a feedback voltage to pin 5 of IC1. The resistor values are chosen so that when the output reaches 6.9V, the feedback voltage equals 1.25V. LED 1 provides a visual indication that the circuit is working correctly. In operation, the circuit has a quiescent current of about 20mA and will consume about 250-300mA when charging a battery. It will typically provide 400-500mA of charging current, this cur­rent gradually tapering off as the battery voltage approaches 6.9V. Construction Most of the parts for the Portable 6V SLA battery Charger are installed on a small PC board coded 6VSLA and measur­ing 61 x 41mm (see Fig.3). Before installing any of the parts, make sure that there are no errors such as breaks or shorts in the copper tracks. If you find any, use a small artwork knife or your soldering iron to fix the problem. Once you are sure that the board is OK, you can start by installing PC pins at the external wiring points. The resistors, diodes and capacitors can then be installed, followed by the IC and the fuse clips. Note that each M205 fuseclip has a small retainer at one end and this should go towards the outside position. If the fuseclips don’t fit into the board, use a 1.2mm drill bit to enlarge the holes. Make sure that the semiconductors and the electrolytic capacitor are oriented correctly. The next task is to wind the inductor (L1). This is a fairly simple job, since all you have to do is wind two layers of 0.5mm-diameter enamelled copper wire onto a small toroidal core. Begin with a 1.5-metre length of wire and just keep winding on the turns, nice and close together, until you have made two complete layers. Make sure that each turn is tightly wound, as loose turns will reduce the circuit’s efficiency. When all the turns are wound, clean and tin the wire ends, then mount the coil on the board. The completed PC board sits in the bottom of a small plastic case. Drill a hole in one end of the case to accept the power switch and another in the lid for the LED. You will also have to drill holes in either end of the case for the input and output leads. Once these holes have been drilled, complete the wiring as shown in Fig.3. Use red and black alligator clips to terminate the input and output leads (red for positive; black for negative). Alternatively, the input leads can be attached to a cigarette lighter plug. You can use either a bezel to mount the LED on the top of the case or you can use a dab of superglue. INDUSTRIAL STRENGTH COMPUTER ELECTRONICS Introducing everything you will ever need for your comput­ing needs from industrial automation control to product develop­ment. 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Don’t use a 12V DC plugpack supply, however. Its output voltage under no-load conditions will be generally be about 17V DC, which is much too high. A 9V DC plugpack supply should be OK but check its no-load output voltage first. Connect the supply, switch on and measure the voltage across the output. It should be about 6.9V but this may vary by 100mV or so. If you don’t get the correct reading, switch off immediately. If everything is OK, set your multimeter to the 1A range, connect it in series with the battery to be charged and reapply power. Assuming that the battery is discharged, you should get a reading of about 300-500mA but this will taper off as the battery charges. Once everything is working, you can fasten the lid to SC the case and get to work on those flat batteries! Phone No: _________________Fax No:_________________ Title: _____________________________________________ Your Address:_______________________________________ State: ________________________P/Code: _____________ Nature of Business ❏ Computer Service ❏ Computer Systems Retailer ❏ Distributor of our Products ❏ End User ❏ Industrial Control and Data Acquisition ❏ Manu­facturer ❏ Product Development ❏ Other For detailed information tick the boxes provided and Fax or Mail this form back NOW! NUCLEUS COMPUTER SERVICES Pty Ltd 9 Morton Avenue, Carnegie, Vic 3163 Ph: (03) 569 1388 Fax: (03) 569 1540 July 1994  65