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Portable 12-volt SLA battery charger This simple project allows you to recharge 12V sealed lead acid batteries from your car. It uses only one low-cost IC and charges at a constant 13.BV. By DARREN YATES How many times have you needed to charge a sealed lead acid (SLA) battery while on the move from one place to another? This small project does just that. All you have to do is connect one set of leads from the charger to your car's battery and the other set to the SLA battery, and turn the charger on. What could be easier? If you wish; you can leave this battery charger permanently in circuit between the car's battery and the SLA battery, even when the latter has a load on it. That's because the output of the charger is kept at a constant 13.8V. In operation, the charger will supply over 300mA initially to the bat54 SILICON CHIP tery, with this current gradually decreasing as the battery voltage reaches 13.8V. This makes it suitable for use with SLA batteries with a rating of 1.2Ah or more. Circuit.theory The 12V SLA battery charger is a step-up voltage converter based on the MC34063A DC-DC controller IC. The major elements of the MC34063 are shown in Fig.1. This IC contains all the necessary circuitry to produce either a step-up, step-down or an inverting DC converter for any voltage from 3-40V. Its principal sections are a 1.25V reference, a comparator, an oscillator an RS flipflop and a Darling- ton transistor pair (Ql & Q2). The frequency of the oscillator is set by timing capacitor CT, connected between pin 3 and ground. A value of .00lµF gives a frequency somewhere between 24kHz and 42kHz but rarely is the exact frequency of the inverter important. As shown in Fig, 1, the oscillator drives the RS flipflop which in turn drives the Darlington transistor pair (Ql & Q2). Each time Ql & Q2 turn on, Ll 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, whereupon they both turn off. The energy in the inductor is then dumped into reservoir capacitor C0 via a diode. The Ipk sense line at pin 7 is used to monitor the peak current flow through the Rsc sense resistor. This is used to limit the peak current through the inductor to 0.3V /Rsc• The output voltage of the converter is set by two resistors which form a voltage divider across the output ea- 170 ~H Fig.I: this basic diagram of a step-up converter shows the major elements of the MC34063 controller IC. It uses an internal oscillator to drive an RS flipflop & this in turn drives a Darlington transistor pair which switches an external inductor. The IC also includes a comparator which compares a sample of the output voltage with an internal reference to derive a feedback signal. L r-------- --------, sl 180 ~4'_1_..,. I I I I I I I I I I 12 7I I I Rsc 0 .22 1N5819 I I 6I Vin 12 V roo I I I I I I icTJ 1.25 V Ref Reg I 1500 I I 5I pF 14 I .__ ______ - ------------..II R2 Vout - - ~ w . - - - - - - - - - - ~ - 0 2 e v n 1 5 mA R1 2.2 k •~ k pacitor. The formula is as follows: Vout = 1.25 x (1 + RZ/Rl) This voltage divider feeds the inverting input of the internal comparator, while the non-inverting input is connected to the internal 1.25V reference. The circuit relies on the comparator for voltage regulation. If the output of the circuit goes too high, the inverting input of the comparator will be higher than 1.25V and so the internal Darlington transistor will 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 transistor can be toggled by the RS flipflop to switch current through the inductor. S1 F1 The result is a form of pulse width modulation which effectively reduces the amount of inductor current when only light loads are connected to the output and thus increases the efficiency dramatically. More importantly, it regulates the output voltage so that, under most loads, the output voltage remains as set. Circuit diagram Fig. 2 shows the complete circuit of the Portable 12V S1A Battery Charger. Power is supplied from the car battery, which is assumed to be about 13.8V. If this voltage swings around, it doesn't matter because the voltage regulation characteristic just described keeps the output of the charger at 13.8V. D.4W / 2A 5W •o--<:f'o--a---~-------, TO CAR BATTERY 18011 01 BY229 Q1 B0679 I.Cl MC34063 T0 12V SLA BATTERY C 8 22k 4.7k L1 : 60T, 0.633mm ENCU ON TOROIDAL CORE , NEOSID 17n32/22 2.2k PORTABLE 12V SLA BATTERY CHARGER Fig.2: the final circuit uses the MC34063 (ICl) to switch an external Darlington transistor (Ql) & this in turn switches inductor Ll. Each time Ql switches off, the energy stored in Ll is dumped into the 220µF capacitor via Dl. Zener diode ZDl protects the circuit against voltage spikes generated by the car's electrical system. It will also conduct heavily and blow the 2A fuse if the car's voltage rises above 15V. In addition, the 2A fuse provides protection against shorts in the output stage of the charger (eg, a short in Ql). The 0.4 70 5W resistor between pins 6 and 7 of IC1 sets the peak current through the inductor to about 650mA (ie, 0.3V/0.47Q = 650mA). Although Fig.2 works in the same manner as Fig.1, there is one important difference and that involves external transistor Ql. Unlike the circuit of Fig.1, Fig.2 uses the internal transistor pair to switch Darlington transistor Ql (BD679) and this then switches 11. This saves the IC from having to dissipate most of the heat and thus prevents possible damage. Each time Ql switches off, the collapsing magnetic field around the inductor tends to maintain the current flow in the same direction. This current is now diverted via Dl and charges the 220µF output capacitor. Ql then turns on again and the cycle repeats itself. The internal oscillator frequency is set by the .00lµF capacitor on pin 3 to somewhere above 24kHz, the exact figure varying somewhat for each IC. Because of this high switching frequency, a normal 1N400X rectifier diode is unsuitable for Dl - it would just get hot and eventually fail. The BY229 used instead is a fast recovery diode which is designed for switching currents at high frequency. The 220µF capacitor is used to store the energy from the inductor and also acts as a filter to smooth out the ringing waveform. Diode D2, a 1N4004, is necessary to increase the voltage difference between the input and output. If you look at the circuit, you will see that there is a direct low-resistance path from the input to the output; ie, via the switch, the 2A fuse, the 0.47Q resistor, inductor 11 , and diodes Dl and DZ. Because the circuit is a step-up converter, direct current would flow from the input to the output if the input voltage were to rise high enough to overcome the 1.2V drop across the diodes; ie, above 15V. Because car systems are regulated to 14.4V or less, this undesirable situation should JULY 1992 55 Fig.3: mount the parts on the PC board as shown in this wiring diagram. Take care when installing Qt & Dt. Qt is mounted with its metal face towards the 4. 7kQ resistor, while Dt is mounted with its metal tab towards the 220µF capacitor. PARTS LIST 1 PC board, code SC14107921, 60 x 45mm 1 plastic zippy case, 83 x 54 x 28mm 1 Dynamark front panel label, 78 x50mm 1 SPST toggle switch (S1) 1 toroid core, 14.8mm OD x 8mm ID x 6.35mm H (Altronics Cat.L-5110) 2 M205 fuse clips 1 2A M205 fuse 2 large alligator clips 1 cigarette lighter plug 1 metre of 0.63mm diameter enamelled copper wire 3 metres of heavy-duty figure-a red/black cable Semiconductors 1 MC34063A DC-DC controller (IC1) 1 BD679 NPN Darlington transistor {01) 1 BY229 fast recovery power diode (D1) 1 1N4004 diode (02) 1 15V 1W zener diode (ZD1) Capacitors 1 220µF 25VW electrolytic capacitor 1 .001 µF 63VW MKT polyester capacitor Resistors {0.25W, 1%) 1 22k0 1 1800 1 4.7k0 1 0.470 5W 1 2.2k0 The 13.8V output is set by the 22k0 and 2.2k0 voltage divider resistors. This provides the converter with negative feedback which regulates the voltage and stops the circuit from taking off. Construction of 0.63mm enamelled copper wire (ECW) wound on a small toroidal core. Begin with a 2-metre length of wire and thread it half-way through the centre of the toroid. Now, using one half of the wire, wind on 30 or so turns around the toroid until you have completed one layer. Make sure that you wind the turns as neatly as possible and keep them as close together as you can. The other half of the wire is then used to wind a second layer over the first. Both ends of the wire should exit from the same point on the toroid. Once the winding is completed, the leads can be trimmed and bent at right angles to fit the circuit board. Scrape away the enamel from the ends of leads before soldering them to the circuit board. The plastic case can now be drilled to accept the on/off switch (S1) and Because the circuit uses so few parts , we've managed to fit the whole project, including the inductor and fuse, inside the smallest available zippy box. It measures just 83 x 54 x 28mm. All the parts are mounted on a small PC board coded SC14107921. Fig.3 shows the parts layout. The parts can be installed in any order but take care with component orientation. In particular, note that D1 is mounted with its metal tab towards the adjacent 220µF output capacitor, while Q1 is mounted with its metal face towards the 4. 7k0 resistor (see also Fig.2). Push these parts down onto the board as far as they will comfortably go be-00-0 fore soldering their leads. Each M205 fuseclip has a retainer at one end and this should go to the outside position. If the fuseclips don't fit into the board, use a 1.2mm drill bit to enlarge y the hole. Make sure that the electrolytic capacitor goes in the right way around and don't forget the ~ -...- - 14107921 two wire links. Inductor 11 consists of60 turns Fig.4: this is the full size PC artwork. ff l never occur. However, if it does, ZDl and the 2A fuse provide added protection. Note that D2 does not have to be a fast recovery diode since it is not involved in switching fast waveforms. An ordinary 1N4004 is sufficient. r RESISTOR COLOUR CODES 0 0 0 0 0 No. 1 0 56 SILICON CHIP Value 4-Band Code (1%) 5-Band Code 1% 22kO 4.7kO 2.2kO 1800 0.47O5W red red orange brown yellow violet orange brown red red red brown brown grey brown brown not applicable red red black red brown yellow violet black red brown red red black brown brown brown grey black black brown not applicable PCB and SCHEMATIC CAD :-- --- - - . - ---- - ':::t ·= i! TV If AMPLIFIE I ,,_ •• ' : • L •''J ,...,., ~~ ~ !~ ! ,i: +---------------- ,__ __ _____ __________ .J C•WWfh-• This inside view shows how the board fits inside the case. Tie knots in the battery leads before they exit the case to prevent them from coming adrift. the two sets of battery leads. Once this has been done, feed the leads through the case, connect them to the PC board and complete the wiring to the switch. The PC board can then be pushed down into the case and the switch mounted in position. Fit the output leads with crocodile clips (or some other suitable connectors) to mate with the SLA battery. For in-car use, the input leads can be attached to a cigarette lighter plug. Testing To test the unit, you will need a 12V DC supply plus a multimeter. A car battery is suitable but don't use a 12V DC plugpack supply as its output voltage under no load will be about 17V DC [which is much too high). A 9V DC plugpack should be OK but check its output voltage first. Apply power and measure the output voltage. It should be around 13.8V, although this may vary by about 200m V or so. If you don't get the correct reading, switch off immedi- =ffliJJlll/Jllf =Ill/I#'===== PORTABLE 12V SLA BATTERY CHARGER CHARGE VOLTAGE: 13.8V EASY-PC • Runs on PC/XT/ AT/286/386 with Hercules, CGA, EGA or VGA. • Design Single sided, Double sided and Multilayer boards Connect the main battery leads to a cigarette lighter plug if you intend using the unit in your car. ately and check for incorrectly oriented parts ~nd for missed solder joints. If everything is OK, connect your multimeter in series with the SLA battery to be charged and re-apply power. Depending on the charge of the SLA battery, you should get a reading of about 300mA or less. The closer the SLA battery is to 13.8V, the smaller the charge current. SC Fig.5: this is the fullsize artwork for the front panel label. The holes for the screws that secure the lid to the case can be cut out using an artwork knife. • Provides Surface Mount support • Standard output includes Dot Matrix/Laser/Inkjet printers, Pen Plotters, Photo-plotters and NC Drill • Award winning EASY-PC is in use in over 12,000 installations in 70 Countries World-Wide • Superbly Easy to use • Not Copy Protected Options: • 1000 piece Schematic symbol library • Surface Mount symbol library • Gerber Import facility For fullJnfo 'phone, fax or write: BTC PO BOX432 GARBUTT 4814 QLD. PH (077) 21 5299 FAX (077) 21 5930 JULY1992 57