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AMATEUR RADIO By GARRY CHATT, VK2YBX DC-to-DC battery charger for 12V gel batteries Want to recharge a 12V gel cell from your car battery? This simple circuit will do the job. Based on an LM3524 switching regulator and a BUZ71 Fet power transistor, it provides a 14.4V output at currents up to two amps. A significant problem encountered by amateur operators in the field is that of battery life. If equipment is operated from sealed lead acid cells, there's usually no way to recharge a flat battery without access to a 240 volt generator and a suitable charger. Attempting to charge a spent 12 volt gel battery by connecting it directly to a 12 volt car battery is not a good idea. If the car battery is fully charged, it may deliver very high currents to the gel battery and if it is not fully charged, it will be a futile exercise. But there is a way of charging a 12V gel battery from a source of lower potential. It can be done by using a step-up switching regulator. The circuit described here is capable of supplying 14.4V DC at 2 amps, which is quite ample for charging sealed gel cells. It is capable of operating at around 80% efficiency from input voltages as low as lOV. How it works Basically, the circuit comprises a pulse width modulator (PWM) which drives inductor Ll via switching transistor Ql, as in Fig.1. In this circuit, Ql is used to switch Vin across inductor L1 at a pulse rate determined by the PWM generator. For the duration of each pulse, Ql is turned on and energy is drawn from the supply and stored in Ll. Diode D1 is reverse biased, and output current is supplied by the charge stored in capacitor Cout• When Qt turns off, Vl will rise to the point where D1 turns on. The Vout -JONr Lfl --1 OFF 1-- + PWM Cout+ -= RL i lout .,. Fig.1: basic scheme for the switchmode charger. It uses a pulse width modulator (PWM) to drive inductor L1 via switching transistor Qt. When Ql is on, current is supplied to the load by Cout; when Ql is off, the current is supplied via L1 and D1. 78 SILICON CHIP output current is then supplied through L1 and D1 to the load and any charge lost from Cout during the previous cycle is replenished. The complete circuit is shown in Fig.2 and uses an LM3524 Pulse Width Modulator chip which is available from several manufacturers. An RC network connected from pins 6 and 7 of the LM3524 sets the frequency of operation to about 50kHz. Resistors Rl and R2 set the voltage on pin 2 to 2.5V by dividing down the internallygenerated 5V reference (VREF) at pin 16. R3 and R7 form a second voltage divider. Under no load conditions, this also sets the voltage at pin 1 of the chip to 2.5V. This divider acts as a voltage sensing circuit connected to the output of the regulator. As the output of the circuit is loaded, the output voltage drops, as does the voltage at pin 1 of the chip. When this happens, ICl automatically increases its output pulse width, thus allowing higher output current to be supplied at the preset output voltage. In our circuit the output voltage is set by R3 and R7 to about 14.4 volts, which is sufficient to safely charge a typical sealed gel battery. If you wish, you can increase the output voltage to about 15.2V by increasing R7 to 1 lkO. The pulsed output from IC1 appears at pin 12 and drives NPN transistor Ql via diode D1. Qt, in turn, drives Q2 which is a BUZ71 Fet power transistor. Each time pin 12 ofICl goes high, Ql and Q2 turn 87 10k +v~o------+-------....------.....- - + - - - - - - t - - - - - - - - , ea + 4.7 16VWJ 89 1k 15 1&ao 16VW + - C2 IC1 LM3524 .Ol C6 .01 R10 1k 16 81 4.7k VR1 83 2.2k 86 120k C3 .0022 10k C4 470pF ).. LED1 CHARGE ... ~ ~ B EQc SWITCHMODE CHARGER VIEWED FROM BELOW GOS K A Fig.2: the PWM output from the LM3524 IC appears on pin 12 and drives Fet power transistor Q2 via D1 and Qt. R7 and R3 set the output voltage to about 14.4V but this can be increased by increasing R7 to 11kn. PARTS LIST 1 PCB, code SC 141-0988-1 , 88 x 52mm 1 small heatsink, DSE Cat. H-3490 4 alligator clips to suit (for battery connections) 1 panel-mount fuseholder 1 3A fuse 1 T0-220 mica washer and insulating bush A small heatsink must be fitted to the BUZ71 power Fet to aid cooling. Isolate the Fet from the heatsink using a mica washer and insulating bush. (Note: the PCB was modified slightly after this photo was taken). off and current is supplied to the load and to C5 via 11 and D2. When pin 12 is low, Ql and Q2 are on and the output current is supplied by C5. D1, R4, R5, RB and Ql ensure that the Fet is turned on hard, thus ensuring clean switching and minimising power dissipation in the Fet. The main advantage of the Fet over bipolar transistors is its small dynamic resistance, typically less than rn. This allows the device to switch relatively high currents without the need for substantial heatsinking. Diode D2 is a high speed fast recovery diode capable of handling 7 amps, which is more than enough for this application. R6 presents a light load to the circuit when no external load is connected, thus ensuring good regulation from no-load to full load conditions. Semiconductors 1 LM3524 voltage regulator IC (Geoff Wood Electronics) 1 BUZ71 power FET transistor (Geoff Wood Electronics) 1 BC54 7 NPN transistor 1 BY229 or MUR1550 fast recovery diode 1 1N914 silicon diode 1 3mm red LED Capacitors 2 2 2 1 1 1OOOµF 16VW electrolytic 4. 7.µF 16VW electrolytic .01µF ceramic .0022µF greencap 470pF ceramic ~ Resistors (0.25W, 5%) 1 x 120k0, 1 x 1OkO, 3 x 4 .7k0, 1 X 2.2k0, 2 X 1k0, 1 X 2700, 1 x 4 70, 1 x 1OkO 5mm trimpot (horizontal mount) SEPTEMBER 1988 79 Fig.3: install the parts on the PCB as shown here. The spare pad adjacent to pin 14 of ICl can be used to terminate the anode lead for the optional charge indicator LED. Charge indication is provided by LED 1 which is wired between pin 14 of IC1 and ground. Note that the LM3524 has two output devices (see Fig.4). In this circuit, we are using one to drive Qt and the Fet, and the other to drive the charge indicator LED. As the pulse width narrows, as Fig.4: you can use this pattern to etch your own printed circuit board or you can buy a ready-etched board from the usual suppliers (see address panel, page 104). occurs when the circuit is lightly loaded, the brilliance of the LED drops. Under high load conditions, the pulse width will be longer and so the LED will be driven harder. Construction A small printed circuit board (PCB) has been designed to simplify Further notes on the RF sniffer probe & preamplifier FERRITE 1~!1 ,on .Di OUTPUT l--:-0 The circuit diagram for the RF sniffer probe published in the June issue was for an initial prototype, rather than tor the final version which was shown in the coded photograph. The above diagram shows the correct version, the main differences being the connections for the 4 7 on base bias resistors on transistor stages Q 1 , 02 and Q3. 80 SILICON CHIP The supply feedpoint was also shown incorrectly - it should go to the midpoint of the two 1on resistors as shown above. The coded photograph published on page 7 3 of the June issue is correct as are other constructional details. Our thanks to those readers who took the trouble to point out the above discrepancies. construction. Fig.3 shows where all the parts go. You can install the parts in any order but watch the polarity of the IC, transistors and electrolytic capacitors. Inductor 11 is made from 20 turns of 1mm enamelled copper wire wound on a 30mm length of ferrite rod (available from retailers as "antenna rod"). A small heatsink must be fitted to the power Fet. The type recommended is from Dick Smith Electronics (Cat. H-3490) and comes with two small pegs which allow it to be soldered directly to the PCB. Use an insulating bush and mica washer to isolate the metal tab of the Fet from the heatsink. The PCB can be housed in any suitable enclosure, either plastic or metal, as the RFI (radio frequency interference) generated by the regulator is minimal. The unit should include fuse protection to guard against output shorts, and this can easily be implemented using a chassis mount fuse holder on the enclosure. A 3A fuse should do the job. Before using the switching regulator, VR1 should be adjusted to give optimum efficiency. To do this, connect a 12V 18-watt (or thereabouts) lamp across the output and adjust VR1 for minimum current drain from the battery. Finally, don't charge a battery that is still connected to other equipment. The circuit has no high voltage, spike or current limiting protection, so damage could result if you try. ~