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c i t a m o t u 10-Amp A Battery Charger Have you tried to start your car or boat recently and found the battery wouldn’t do its job? Do you need a fast charger for the battery? This new automatic 10amp charger should fit the bill. By RICK WALTERS Although it’s possible to buy a commercial battery charger for just $30, it won’t get you out of trouble in a hurry if you have a flat battery. These chargers usually have a maximum cur­rent rating of just 4A continuous, which means that it can take many hours to bring a flat battery up to scratch. Often, it’s a case of connecting the charger and letting the battery charge 70  Silicon Chip for the rest of the day or overnight. Many low-cost commercial chargers also lack any useful indication of the charging rate. What’s more, they often only have a single fixed charging voltage and come with rather flimsy clamps and cables. When it comes to battery chargers, the old adage “you get what you pay for” is quite true. By contrast, this design is capable of pumping out a hefty 10A on a continuous basis and can automatically charge 6V, 12V or 24V batteries. You don’t have to manually set the charging vol­tage either. When connected to a battery, this unit measures its voltage and then automatically selects the correct charging rate. This scheme works because a “flat” battery is generally only a few volts below its nominal voltage. For example, a flat 12V battery generally would be at about 10V while a 24V battery would drop to about 20V. The only drawback of this scheme is that the charger will not automatically recognise a 12V battery that has gone below 8V or a 24V battery that has gone below 16V. That’s because the sensing circuit assumes that anything under 8V is a 6V battery, while anything between 8-16V is a 12V battery. Most of the compon­ents, including the main PC board, the power transformer, the electro­ly­tic capacitors and two bridge rectifiers, are mounted on an aluminium baseplate. This provides an excellent heatsink and simplifies mounting the various components. Note the heatshrink tubing covering the mains switch and fuseholder terminals. We don’t think this will happen very often but if it does, the solution lies in the “override” pushbutton switch on the back panel. All you have to do is hold this pushbutton down for short periods until the correct voltage indicator LED stays on. We’ll talk about this function later. Because of its high current rating, this battery charger is just the shot for quickly topping up a battery that’s not quite up to the job. It can really get you going again on those cold winter mornings. It’s also ideal for getting your boat or recrea­ t ional vehicle battery up to speed, if it’s been lying around neglected for a while. Seven front panel LED indicators give you a good idea as to what’s going on. First, the BATTERY CONNECTED LED lights whenever a battery is connected, even if the power is off. Three more LEDs indicate whether a 6V, 12V or 24V battery is being charged, while the remaining three LEDs indicate FEATURES ✔ Automatic selection for 6V, 12V or 24V batteries ✔ Manual override button for single voltage setting ✔ 10A maximum charging current ✔ Automatic change over from high through medium to trickle charge ✔ Battery voltage and charge status indicator LEDs ✔ Output short circuit protection ✔ Reverse polarity protection the charging rate (trickle, medium or high). How it works Refer now to Fig.1 for the circuit details. This circuit can be split into four blocks: (1) a battery voltage sensing and reference voltage summer (IC1); (2) a switching regulator (IC2 and associated circuitry) which regulates the battery charging voltage. This circuit block also senses and limits the battery charging current; (3) a power supply based on transformer T2, full-wave bridge rectifier BR1 and 3-terminal regulator REG1; and (4) charging and voltage indicators based on transistors Q5-Q7 and LEDs 1-7. Let’s take a closer look at each of the various circuit functions. The battery voltage sensing circuit consists of three com­parators: IC1a, IC1b and IC1c. As shown, pins 3 & 5 of IC1a & IC1b respectively are June 1996  71 Fig.1: comparators IC1a-IC1c provide the automatic battery voltage sensing function, while IC2 is the switching regulator. The latter generates a PWM (pulse width modulated) waveform and drives Mosfet Q4 via a buffer stage (Q2 & Q3) and isolating transformer T1. IC1d monitors the voltage across the 0.01Ω current sensing resistor and drives the charge indicator LEDs. 72  Silicon Chip PARTS LIST 1 plastic instrument case with plastic front & rear pan­els, 260 x 180 x 65mm (Jaycar HB5984 or equivalent) 1 self-adhesive front panel label, 250 x 60mm 1 PC board, code 14105961, 145 x 83mm 1 PC board, code 14105962, 51 x 48mm 1 160VA toroidal power transformer with 18V secondaries (Jaycar MT2113 or equivalent) 1 mains lead with moulded 3-pin plug 1 mains switch with plastic rocker & neon indicator (S1) (Jaycar SK0985 or equivalent) 1 pushbutton momentary contact switch (S2) 2 3AG panel-mount fuseholders (Jaycar SZ2020 or equivalent) 1 2A 3AG slow-blow fuse (F1) 1 16A 3AG fuse (F2) 1 10A battery clip - red (DSE P-6420 or equivalent) 1 10A battery clip - black (DSE P-6422 or equivalent) 5 TO-3 insulating bushes 2 TO-220 insulating washers 1 E-type ferrite transformer complete with bobbin - Jaycar LF-1270 or equivalent (T1) 3 190mm cable ties 1 ETD29 transformer assembly (Philips 2 x 4312-020-37502 ferrite cores; 1 x 4322-02134381 former; 2 x 4322-02134371 clips) (L1) 1 1.5-metre 30A battery cable (red) 1 1.5-metre 30A battery cable (black) 7 5mm LED clips 1 3mm x 10mm tapped spacer 4 3mm x 6mm tapped spacers 2 3mm x 15mm bolts 5 3mm x 12mm bolts 8 3mm x 6mm bolts 16 3mm hex nuts 11 3mm flat washers 12 3mm spring washers 2 mains cable clamps (Jaycar HP0716 or equivalent) 3 6PK x 10mm screws 4 6mm female solder quick connectors (BR1) 1 260mm length 20 B&S enamelled copper wire (for .01Ω resistor) 1 6-metre length 21 B&S enamelled copper wire (for L1) 1 9-metre length 30 B&S enamelled copper wire (for T1) connected to the positive terminal of the battery via a 20kΩ resistor and to ground via two series 10kΩ resistors. This arrangement ensures that half the battery voltage appears on pins 3 & 5, while one quarter of the battery voltage appears on pin 10 of IC1c. A voltage divider string fed from the +15V output of REG1 is used to set the bias voltages on the inverting inputs of IC1a-IC1c. As shown, pin 6 of IC1b is biased to +2V, while pins 2 & 9 of IC1a & IC1c are biased at +4V. If a 6V battery is connected, the output of IC1b will switch high, turning on Q7 and lighting LED7 (the 6V indicator LED). Similarly, a 12V battery will cause the outputs of both IC1a and IC1b to switch high. Because pin 1 of IC1a is now high, LED7 turns off and LED6 turns on (via Q6), indicating that a 12V battery is being charged. Finally, a 24V battery causes all three comparator outputs to switch high. LEDs 6 & 7 will now both be off, while LED5 will be on to show that the battery is being charged to 24V. Semiconductors 1 LM324 quad op amp (IC1) 1 TL494 or TL594 switching regulator (IC2) 1 BS170, BS170P or VN10KM N-channel IGFET (Q1) 1 BD139 NPN transistor (Q2) 1 BD140 PNP transistor (Q3) 1 MTP75N05 N-channel IGFET (Q4) 3 BC548 NPN transistors (Q5-Q7) 1 7815 3-terminal regulator 5 1N914 switching diodes Switching regulator IC2 is a TL494 PWM switching regulator IC from Texas In­struments. (D1,D2,D5-D7) 1 BYV32-200 ultra-fast diode (D3) 1 1N4004 power diode (D4) 1 400V 35A bridge rectifier (BR1) 1 400V 6A bridge rectifier - P04 (BR2) 7 5mm red LEDs (LED1-LED7) 1 15V 400mW zener diode (ZD1) Capacitors 3 4000µF 63VW chassis mounting electrolytic 1 470µF 63VW PC electrolytic 1 220µF 25VW PC electrolytic 1 100µF 16VW PC electrolytic 2 22µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 1 4.7µF 16VW PC electrolytic 3 0.1µF 100VW monolithic ceramic 1 .0022µF 100VW MKT polyester Resistors (0.25W 1%) 1 10MΩ 7 4.7kΩ 1 1MΩ 1 2.2kΩ 2 220kΩ 1 1.8kΩ 1 100kΩ 2 1.5kΩ 3 56kΩ 6 1kΩ 2 27kΩ 1 910Ω 1 20kΩ 1 470Ω 1 15kΩ 1 330Ω 3 10kΩ 1W 1 100Ω 2 10kΩ 1 91Ω 2 8.2kΩ 1 0.01Ω 1 5.6kΩ Miscellaneous Red, orange & black hook-up wire; heatshrink tubing. This device contains an on-board oscillator, a refer­ence regulator, two error amplifiers, and a pair of output driver transistors. In operation, this device monitors the voltage between its pin 1 and pin 2 inputs and adjusts its output duty cycle accordingly, to give the correct charging voltage. In greater detail, pins 1 & 2 are the non-inverting and inverting inputs respectively of an internal error amplifier (designated A1). Pin 1 monitors the battery voltage via a voltage divider (5.6kΩ and 910Ω), while pin 2 monitors the output of the reference June 1996  73 Fig.2: most of the parts are installed on these two PC boards. Make sure that transformer T1 is correctly oriented, as it’s easy to install it back-to-front. In addition, the two round plastic corner lugs on the base of this transformer must be cut off so that the pins go through the PC board. voltage summer formed by IC1a-IC1c. When a 6V battery is connected, pin 7 of IC1b goes high as we’ve already described. As well as driving Q7, this output is also applied to a voltage divider consisting of 56kΩ, 4.7kΩ and 330Ω resistors. As a result, +1V DC is applied to pin 2 of IC2. When a 12V battery is connected, the output of IC1a goes high as well and so the two 56kΩ resistors at the comparator outputs are effectively in parallel. This means that a signal of +2V is now applied to pin 2 of IC2 and this jumps to +4V for a 24V battery (all three comparator outputs high). Thus, depending on the voltage of the battery connected to the charger, the comparators apply a fixed DC voltage to pin 2 of IC2 (ie, to amplifier A1’s inverting input). This voltage is then compared with the divided battery voltage on pin 1 74  Silicon Chip of IC2 (the non-inverting input of the A1 error amplifier). As a result, IC2 adjusts its pulse width output accordingly so that the battery is charged to the correct voltage. This works out to be 7.2V for a 6V battery, 14.4V for a 12V battery, and 28.8V for a 24V battery. Note that these full-charge voltages respectively equate to 1V, 2V and 4V signal voltages on pin 1 of IC2. Override function Pushbutton switch S2 provides the override function. As explained previously, this is used in situations where the bat­tery is so flat that it is no longer automatically recognised by the charger. To simplify the circuit, however, S2 provides just one override voltage (either 6V, 12V or 24V). That’s because most individual users will only want to charge one type of battery (usually 12V). The actual override voltage is determined by the value of resistor R1 and this is selected when the unit is built. When S2 is pressed, it simulates the relevant comparator output(s) going high and applies the appropriate voltage to pin 2 of IC2. This forces switching regulator IC2 to charge the battery at the correct voltage, even though the automatic detection circuitry has failed to identify the battery. Assuming that the battery is OK, this will very quickly bring its voltage up to a level where the automatic detection circuit can take over. The battery will then charge to the cor­ rect voltage. In practice, it’s simply a matter of holding down S2 for short periods until the correct charge indicator LED remains on when the switch is released. Note, however, that it should rarely be necessary to use the override switch. Only batteries that have been severely dis­charged will have an output that’s so low that they will not be automatically recognised. And any battery that’s left in this state for too long will quickly deteriorate. Current limiting The need for current limiting is obvious – without it, a discharged battery could attempt to draw 30-40A or more. This would certainly be no good for the battery or for the charger itself. In this circuit, the maximum charging current has been limited to 10A. This is done by monitoring the voltage developed across a .01Ω current sensing resistor and applying it to the non-inverting input (pin 16) of a second error amplifier (A2) inside IC2. This voltage (ie, on pin 16) is then compared with a fixed 10mV reference on the inverting input of A2 (pin 15). As long as the charging current remains below 10A, the voltage across the .01Ω resistor remains below 10mV and no cur­rent limiting takes place. However, if the current attempts to rise above 10A, the voltage on pin 16 will rise above the voltage on pin 15. The A2 amplifier then generates an error signal and this in turn reduces the duty cycle of the pulse width modulated (PWM) output at pins 9 & 10. As a result, the maximum output current is effectively limited to 10A. If the current does try to rise above this, the error amplifier immediately reduces the PWM duty cycle to reduce the current again. Mosfet Q1 and its associated components provide a delayed start-up for the switching regulator (IC2). This is necessary to give IC1a-IC1c sufficient time to apply the correct reference voltage to pin 2. When no battery is connected, Q1’s gate is at ground and so it is turned off. As a result, pin 4 (Inhibit) of IC2 is held at the pin 14 reference voltage (5V) via a 4.7kΩ resistor and diode D2 – ie, the 22µF capacitor between pins 4 & 14 will be dis­charged. This prevents the switching regulator from producing any output. If a battery is now connected, the output of IC1b (and perhaps IC1a & IC1c as well) will go high after a short delay, as set by the 22µF capacitor at pin 5. This high turns on Mosfet Q1 The LED indicator board is mounted on the front panel by pushing the six charge indicator LEDs into matching plastic bezels. Note the 10mm spacer attached to the middle of the board – this ensures correct spacing between the board and the front panel. (via D1) and so the 22µF capacitor on pin 4 of IC2 charges via the 100kΩ resistor in Q1’s drain. As a result, the voltage on the Inhibit pin slowly reduces as the capacitor charges. This allows the output pulse width at E1 and E2 to increase slowly from zero to a width which is con­ trolled by the battery voltage. Note that pressing the override switch (S2) also applies a high (+15V) to the gate of Q1 (via D7). This ensures that IC2 starts when S2 is pressed, even if the battery voltage is so low that none of the op amp outputs has gone high. Buffer stage The paralleled emitter outputs from IC2 drive a buffer stage based on complementary emitter followers Q2 & Q3. From there, the PWM signal is fed to transformer T1. The transformer secondary then drives Mosfet Q4 via a 0.1µF capacitor. ZD1 is included to protect Q4’s gate circuit from voltages in excess of 15V. T1 is necessary to isolate the switching regulator circui­try (IC2, Q2 & Q3) from the output circuitry. This is because Q4 operates as a source follower and its source is effectively at the battery voltage. In operation, Q4 is switched on and off by the waveform ap­plied to its gate. Each time it turns on, it applies a DC pulse to the positive battery terminal via inductor L1. When Q4 turns off, the field around L1 collapses and D3 conducts so that the energy stored in the inductor can continue charging the battery. Note that although Q4 switches a +55V rail, the average voltage applied to the battery is determined by the duty cycle of the PWM waveform from IC2. The pulse widths are at their narrow­est for 6V batteries and at their widest for 24V batteries. Bridge rectifier BR2 is there to protect the circuit against reverse polarity connection of the battery. Using a bridge rectifier may seem a little odd here but we are really only just connecting the top two diodes in parallel and with reverse polarity across the output. The bridge rectifier is simply a low-cost way of obtaining two diodes with adequate current ratings. If the battery is connected the wrong way around, the two top diodes inside the bridge become forward biased and conduct a heavy current. This blows 15A fuse F2, thereby disconnecting the battery from the charger before any damage can occur (other than to the fuse itself). Charge indicators Op amp IC1d, together with LEDs 2-4, provides the charge rate indication – either trickle, medium or high. It does this by monitoring the June 1996  75 Fig.4: the winding details for transformer T1. Wind the primary first, cover it with insulating tape, then wind on the secondary. Fig.3: the core halves in inductor L1 are separated using washers cut from TO3 mounting insulators. voltage developed across the .01Ω current sensing resistor. This voltage is applied to pin 12 of IC1d which oper­ates with a gain of 214, as set by the 1MΩ and 4.7kΩ feedback resistors on pin 13. The output from IC1d appears at pin 14 and is applied to the charge LED indicators. If the charging rate is greater than about 3.5A, then IC1d’s output will be above 7.5V and both the HIGH and MEDIUM LEDs will be lit. At the same time, the TRICKLE LED (LED4) will be reverse biased and so it will be out. As the battery charges, the output of IC1d gradually reduc­es. Because the cathode of the HIGH current LED is biased to about 5.5V, it will gradually dim and then extinguish as IC1d’s output falls. The MEDIUM LED now remains lit until the charging current drops to about 0.75A. It then dims and goes out, by which time LED4 has come on to indicate the trickle charge mode. Note that the output from IC1d must 76  Silicon Chip Fig.5: install the power switch on the front panel with the ring on the rocker oriented as shown here. drop to about 2.4V before LED4 begins to turn on. That’s because LED4’s anode is biased to about 4.8V using a voltage divider and diode D6. In summary, LEDs 2 & 3 both light when the charging current is above 3.5A; LED3 lights when the charging current is in the range 0.75-3.5A; and LED2 lights when the charging current is below about 1A. Note that there is a transition period when both LED3 and LED4 are on (ie, LED4 gradually turns on as LED3 dims). Power supply Power for the circuit is derived from the mains via T2, a 160VA toroidal transformer with 18V secondaries. This drives full-wave bridge rectifier BR1 which, together with three 4000µF filter capacitors, produces a +55V rail for the drain of Q4. The three 4000µF filter capacitors are required in order to provide an adequate ripple rating so that the charger can deliver 10A. A neon indicator wired across the primary of the trans­former provides power on/off indication, while fuse F1 provides overload protection. D4 and 3-terminal regulator REG1 provide a regulated +15V rail to power the rest of the circuitry. Construction Most of the parts for the Autocharger 10 are installed on two PC boards: (1) a main board coded 14105961 (145 x 83mm); and (2) an indicator board coded 14105962 (51 x 48mm). Fig.2 shows the parts layout on the two PC boards. Before installing any of the parts, carefully check both boards for etching defects (in most cases there will be none). If everything is OK, start the main board assembly by fitting PC stakes to the 12 external wiring points, then install the six wire links. The diodes and resistors can be installed next, followed by the ICs, capacitors and transistors. Be sure to orient transistors Q2 and Q3 with their metal tabs facing away from T1. Fig.7: the mains cord must be anchored securely and the wiring installed exactly as shown here. Be sure to cover the switch and fuseholder terminals with heatshrink tubing. The thick lines indicate heavy-duty (30A) cable. No heat­sinks are required for these two devices. As explained previously, resistor R1 is selected to set the desired override voltage. Use 56kΩ to provide a 6V override, 27kΩ for 12V override and 15kΩ for 24V override. Care is required when mounting Q4, D3 and REG1, since their metal tabs must later line up with matching holes in a metal baseplate. Note that these devices are all mounted on the June 1996  77 copper side of the board, as shown in Fig.7. The mounting procedure is as follows: (1) Bend the device leads upwards at a suitable distance from the bodies (note: the holes in the metal tabs must match the relevant baseplate holes if this has been pre-drilled); (2) Install the devices so that the bottom faces of their metal tabs are exactly 6mm below the PC board. This can be checked out by fitting 6mm spacers to the PC board and then placing the assembly on a flat surface. Make any adjustments as necessary before cutting the device leads off flush with the top of the board. Inductor L1 consists of six lengths of wire, all wound together on a Philips 4322-021-34381 former (as one winding). This is done to achieve a high current capacity using a small, manageable gauge of wire. The winding procedure is as follows: (1) cut the 21 B&S wire into six 1-metre lengths; (2) tin one end of each wire, form it into a hook and solder each hooked end to a separate pin on the 6-lug side of the former; (3) bundle the wires together and wind on 20 turns (the direction doesn’t matter); (4) check that the ferrite core halves fit the former, then terminate the six ends on separate pins on the other side of the transformer; (5) cover the windings with a couple of layers of insula­tion tape, then slip one of the ferrite core halves into the side of the former with the six lugs and secure it with one of the clips; (6) cut three TO-3 mounting insulators as shown in Fig.3 (these serve as Fig.8: the mounting details for D3 and Q4. Make sure that the area around their mounting holes is smooth and free of metal swarf, to avoid punching through the insulating washers. spacers between each leg of the two core halves); (7) fit the second ferrite core half to the former, along with one of these insulating washers as a spacer between the two centre legs; (8) push the other two spacers into the gap between the outer legs of the core halves, then secure the assembly using a 190mm plastic cable tie. Fig.4 shows the winding details for driver transformer T1. This is wound on a plastic bobbin using 30 B&S enamelled copper wire. Be sure to wind the turns in the direction shown in Fig.4, as the phasing of this transformer is critical. The primary is wound first. To do this, terminate the start of the wire on pin 2, wind on 100 turns and ter- NOTE: THE OVERRIDE SWITCH ON THE REAR PANEL IS FOR USE WITH _____ VOLT BATTERIES ONLY. PRESS THIS SWITCH IF . . . (1) No charging voltage is indicated; or (2) The indicated charging voltage is too low. Release override switch every 10 seconds until the correct charging voltage is indicated. WARNING! – MAKE SURE THAT THE BATTERY IS BEING CHARGED AT THE CORRECT VOLTAGE BEFORE LEAVING THE CHARGER UNATTENDED & ALWAYS CHARGE IN A WELL-VENTILATED AREA. Fig.9: this label should be attached to the top of the charger. Be sure to fill in the value for the override voltage in the space indicated (either 6V, 12V or 24V). 78  Silicon Chip minate the finish on pin 1. Cover this winding with a layer of insulating tape, then wind on the 110-turn secondary, starting at pin 5 and fin­ishing on pin 7. Note that the secondary must be wound in the same direction as the primary. The last item to make is the 0.01Ω resistor, as follows: (1) take a piece of 20 B&S enamel wire and cut it to 260 mm; (2) clean each end with a knife or emery paper and tin for about 5mm; (3) wind the wire into a coil (we used a pencil as a former and ended up with nine turns). T1, L1 and the .01Ω resistor can now be installed on the PC board, as shown on Fig.2. Be sure to match the start and finish windings of T1 to their designated locations. It will be necessary to cut off the two plastic lugs on the botton of T1, so that it can be pushed all the way down onto the board. LED indicator board The LED indicator board will only take a few minutes to assemble. Begin by installing PC stakes on the copper side of the board at the external wiring points, or if you wish just solder flying leads into the holes as shown in one of the photo­graphs. This done, fit the resistors, transistors Q5-Q7, diode D1 and the six indicator LEDs. Note that the LEDs must be mount­ed so that the bottom of each LED is 6mm above the board. The easiest way to do this is to cut a 6mm-wide cardboard jig. This jig is then insert­ed between the LED leads as they are being pushed down on the board. Finally, a 10mm spacer is fitted to the top of the board – see photo. Case assembly A standard plastic instrument case with plastic front and rear panels is used to house the circuitry. Most of the compon­ents, including the main PC board, power transformer, electro­ly­tic capacitors and the two bridge rectifiers, are mounted on an aluminium baseplate. This provides an excellent heatsink and simplifies mounting the various components. Begin by attaching the label to the front panel, then use this as a drilling template for the LED indicators (6.57mm), the fuseholder and the battery cable clamp. Note that larger holes are best made by first drilling a small pilot hole and then carefully enlarging Fig.10: this full-size artwork can be used as a drilling template for the front panel. them using a tapered reamer or, for the battery cable clamp hole, a small file. The cutout for the mains switch is made by drilling a series of small holes around the inside circumference, then knocking out the centre piece and carefully filing the hole to shape. Don’t make this hole too big – the mains switch must be a tight fit so that it is held securely. The LED bezels, fuseholder F2 and the mains switch (see Fig.5) can now be fitted to the front panel. The battery cables consist of 1.5-metre lengths of 30A cable (red for positive and black for negative). These are each fitted with a large battery clip at one end. Secure them using a cordgrip clamp, leaving a length of about 250mm for each cable at the back of the panel. The LED indicator board is now fitted by pushing the LEDs into the bezels, until the spacer contacts the front panel. Once the front panel assembly has been completed, the rear panel can be drilled to accept the mains cord clamp, fuseholder F2 and pushbutton switch S2. The locations of these holes can be gauged from the photographs and from the wiring diagram (Fig.6). Note that the mains cord hole should be carefully profiled to match the cordgrip grommet. The next step is to drill the baseplate. This will need to be drilled for the transformer mounting bolt, the two bridge rectifiers, three filter capacitors, the PC board mounting screws, the three TO-220 devices (Q4, D3 & REG1), and the three fixing points to secure the baseplate into the base of the case. The latter three holes take self-tapping screws into inte­gral pillars in the base of the case. One of these is adjacent to the front-panel power switch, while the other two are just in front of the three filter capacitors. When the drilling is done, all the hardware is mounted on the baseplate before it is mounted into the case. Transformer T2 is secured using a large bolt, two rubber washers and a large metal washer. One of the rubber washers sits under the transformer, while the second sits under the metal washer at the top. The main PC board, the bridge rectifiers and the electroly­tic capacitors can now be installed on the baseplate. The board is secured at the front and rear using the 6mm spacers and 12mm long bolts. Note that Q4 and D3 must be isolated from the baseplate using standard TO-220 mounting kits – see Fig.8. After mounting, check that the device tabs are indeed isolated using a multimeter switched to a high ohms range. REG1 can be bolted directly to the baseplate, since its metal tab is at earth potential. Final wiring Fig.6 shows the final wiring details. Exercise extreme care when installing the mains wiring, as your safety depends on it. In particular, make sure that the mains cord is securely anchored by the cordgrip grommet on the rear panel and that it cannot be pulled out. The Active (brown) and Neutral (blue) wires from the mains cord go directly to the mains switch, while the Earth (yellow/ green) wire is soldered to an earth lug which is bolted securely to the baseplate. Use a star washer and an additional lock nut to ensure that the earth lug cannot come loose. The terminals of the fuseholder and mains switch should be covered with heatshrink tubing to prevent accidental contact with the mains. This involves slipping a length of June 1996  79 Fig.11: the full-size etching patterns for the two PC boards are shown here. Check your boards carefully for etching defects by comparing them with these patterns, before installing any of the parts. heatshrink tubing over all the leads before they are soldered to the terminals. After soldering, the heatshrink tubing is pushed over the fuse­holder and mains switch bodies and shrunk using a hot-air gun. The two thin orange wires from the transformer are the primary leads and these go to the mains switch and the fusehold­ er, as shown. The low voltage secondary leads are much thicker. Twist the ends of the pink and yellow leads together (to form the centre tap) and solder a short length of hook-up wire to them. The resulting joint should then be sleeved using heat­shrink tubing. The red and white transformer leads go to the AC terminals of the bridge rectifier via spade terminals, while the lead connected to the transformer centre-tap goes to D4 on the PC board. All leads between BR2, the fuse­ WARNING! Lead-acid batteries generate hydrogen gas which is explo­sive. This charger should only be used in a well-ventilated area and you should always connect the battery to the charger before turning the mains switch on. This is done to prevent sparks from being generated. If the BATTERY LED does not light when the battery is con­nected, check the 15A fuse and the battery polarity. This fuse will blow if the battery is connected the wrong way around and is there to protect the internal circuitry. Finally, always turn the charger off before disconnecting the battery leads. Again, this is done to prevent sparks from causing an explosion. 80  Silicon Chip holder and the PC board must be run using 30A cable. The only exception is the lead between the fuseholder and the battery sense terminal on the PC board. The connection between the positive terminal of BR2 and the fuseholder is made using the bridge rectifier lead – it’s simply bent over to contact the fuseholder terminal. Again because of the currents involved, three separate leads are run from the +55V terminal on the PC board to the positive terminals of the 4000µF capacitors. Three more leads are run from the GND point to the negative terminals. Similarly, separate leads are run from the plus and minus terminals of the capacitors to the corresponding terminals on bridge rectifier BR1 (see Fig.6). Note the 10kΩ resistors across the capacitors – they’re there to discharge the capacitors after switch off. Warning: don’t touch the capacitor terminals as they can give you a shock. The remainder of the wiring be- tween the LED indicator board, LED1 and the main PC board can be run using light-duty hook-up wire. Complete the construction by fitting the fuses in the fuseholders. The 2A fuse goes in fuseholder F1, while the 15A fuse goes in fuseholder F2. Testing Before plugging the unit in and switching it on, it is a good idea to check the mains wiring using an ohmmeter. To do this, first check that there is an open circuit bet­ween the Active and Neutral pins of the mains plug when switch S1 is off and a resistance of about 13Ω when it is on. If this is OK, check that there is an open circuit between each of these two pins (Active & Neutral) and the earth pin. Finally, check that the meter reads zero ohms when connect­ed between the Earth pin on the plug and the metal baseplate. If everything checks out, plug the charger into the mains and turn it on. Both the mains switch neon and the TRICKLE LED (LED4) should light. If they don’t, switch off immediately, pull the mains plug and locate the problem before proceeding further. Now turn the mains switch off and connect a 6V or 12V DC battery to the charger leads (positive to positive, negative to negative). Check that the BATTERY CONNECTED LED lights. Next, disconnect the battery, switch on the mains and (carefully) measure the voltage across the 4000µF electrolytic capacitors (warning: do not touch or short any of the termi­nals). You should get a reading of about 55V. The voltage on pin 8 of IC2 should be around 15V, while pin 14 should read around 5V. If everything is OK so far, the unit is ready for its first trial. To do this, turn the charger off and connect it to a car battery (disconnect the battery from the car’s electrical system first). The BATTERY CONNECTED LED should immediately light. Now switch on the mains and check that the 12V LED lights (assuming that a 12V battery is connected). Depending on the state of the battery, one of the charge indicator LEDs should also illuminate. If the HIGH LED lights it will probably only be for a short period of time, then the charg­er will switch to MEDIUM. Eventually, depending on the condition of the The wiring connections to the LED indicator board can either be run directly to the copper pads on the back of the board, as shown here, or to PC stakes. Use cable ties to keep the wiring neat and tidy. Heatsinking is provided for REG1 (left), D3 and Q4 by attaching them to the baseplate. After mounting these devices, use a multimeter to confirm that the metal tabs of D3 and Q4 are correctly isolated from the heatsink. battery, the charger should switch to TRICKLE. Using the override button Before concluding, here are a few tips on using the over­ride pushbutton. First, remember that you have only one override voltage available. So if you selected a 27kΩ resistor for R1, the over­ride function is only available for 12V batteries. Of course, you can easily get around this if by adding a 3-way switch to select between the three possible resistor values. That way, you can provide an override function for all three battery types. The override function is easy to use. If the battery does not start charging at the correct voltage, hold the pushbutton down for 10 seconds, then release it and check to see if the correct charge indicator LED stays alight. If it doesn’t, repeat this procedure until it does. The battery should then charge to the correct voltage. Finally, note that the power transformer specified for the charger is rated at 160VA. While it is suitable for topping up 24V batteries, if prolonged high current charging of these bat­teries is envisaged, a 300VA transformer should be used. This will necessitate SC using a bigger case. June 1996  81