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Build this advanced small-cell charger and step up to the newest generation of super-capacity rechargeable batteries Fast charging small batteries demands more smarts than you’ll find in typical ‘off-the-shelf’ chargers. We’ve packed what you need into a small, portable unit that makes rechargeable batteries almost as convenient to use as alkaline batteries. Pt.1: By PETER SMITH Recently, capacity ratings for ‘AA’ size Nickel-Metal Hydride (NiMH) batteries topped 1800mAh. Similarly, ‘AA’ Nickel-Cadmium (NiCd) batteries with ratings of 1000mAh or more have become commonplace. These new super-capacity rechargeables are ideal for use in a whole range of high-drain devices, including camcorders, digital cameras and portable music players. In fact, they can last 2-3 times longer on a single charge than alkaline batteries in some applications. 56  Silicon Chip However, their attractiveness begins to fade somewhat if you find you have to wait for half a day or more every time you want a recharge. Despite what the marketing hype might say, most off-the-shelf ‘fast’ chargers can’t fast charge these new small, high-capacity cells. In fact, the majority of so-called ‘fast’ chargers require at least three hours to recharge even the lower-capacity varieties. By contrast, the S ilicon C hip SuperCharger allows you to safely fast-charge small, high-capacity cells as well as all the usual lower-capacity varieties. You don’t have to wait around for half a day with this charger. Battery capacities & ‘C’ rate The capacity of small batteries is generally marked in milliamp-hours (mAh). This figure is usually arrived at by first charging for 16 hours at the 0.1C rate, followed by discharging at the 0.2C rate. The results are then “normalised” to mAh for comparative purposes. The ‘C’ value we’re referring to is simply a representation of some fraction of the (normalised) battery capacity. It’s a convenient way of expressing a particular charge or discharge rate, based on the stated mAh rating. For example, the 1C charge current for a 1600mAh battery is 1600mA. This is the current that’s required to charge the battery to 100% capacity over a one-hour period – at least, in www.siliconchip.com.au theory. Charging the same battery at a 0.5C rate, or 800mA, for two hours would also return the battery to full capacity. In reality, slightly more than the rated capacity must be applied to return full charge, due to losses in heat and the electrochemical exchange process. Note, however, that the mAh rating stamped on a battery does not imply any particular maximum charge or discharge rate. This information must be obtained from the manufacturer’s technical data sheets. Fast charging The most common rate used for charging NiCd and NiMH batteries is probably still the ‘standard’ 0.1C rate. This is the rate supported by most ‘supermarket’ cabinet-style chargers. Why? Simply because it is cheap and foolproof. No complex or high-power electronics are needed and if you forget to switch off the charger after 14-16 hours, nothing bad happens! In fact, 0.1C is still the recommended charge rate for mid-range cylindrical (‘C’ size and larger) NiMH batteries. The good news is that many smaller batteries, in particular the AA, AAA and 1/2AA sizes, do support fast charging. Typically, NiMH-chemistry types can be charged at up to their 1C rating, while high-capacity NiCds (eg, Sanyo’s Cadnica Ultra series) will accept a 1.5C charge rate. As a bonus, NiCd and NiMH-chemistry batteries can actually benefit from a fast charge regimen. Fast charging minimises an effect called “voltage depression”, a problem that can significantly reduce the output of a cell over time. By the way, ‘fast’ charging – as opposed to ‘standard’ (0.1C) and ‘quick’ (0.33C) charging – refers to any rate above 0.5C. The main aim of this new charger design is to allow you to charge all the popular format small cells in the shortest possible time – without exceeding their maximum allowable ‘C’ rate. This means that, in most cases, you can have your batteries back in action in about an hour! MAIN FEATURES • • • • • • • • • • • • Designed for charging high-capacity AA, AAA & ½AA NiCd & NiMH batteries Charges NiCd & NiMH batteries from 200mAh to 1800mAh, selectable in 200mAh steps Charges from 1 to 6 cells Supports rapid (1.5C & 1C), fast (0.5C) and standard (0.1C) charge rates Returns more than 90% battery capacity in the first hour Includes 2-hour top-up charge to return near 100% rated capacity Automatically switches to trickle charge at end of rapid/fast charge Includes intelligent charge termination to limit unnecessary overcharging Discharge before charge mode reconditions both chemistry types Can recover totally flat cells Small, portable design operates from a plugpack or cigarette lighter socket Optional high-current battery holder or utilises off-the-shelf holders Once a charge cycle has begun, battery condition must be closely monitored and the charge current cut off at just the right point. As a cell approaches 100% charge, its internal temperature and pressure rises rapidly. If left unchecked, venting of the gaseous electrolyte occurs, resulting in permanent cell degradation. Repeated overcharging greatly reduces cell life at best and at worst, can result in cell destruction (or even explosion). A number of different methods can be applied to detect the fully charged state. One of the simplest involves detection of the small drop in voltage that occurs as a cell moves from the charged to overcharged state. For NiCd batteries, this is about -20mV per cell, while for NiMH batteries, it is about -5mV to -10mV per cell. This method of charge termination is called ‘Negative Delta Voltage’ (-∆V). Another popular method involves detecting the sharp rise in cell temperature mentioned earlier. Typically, battery temperature is sensed by a thermistor placed in direct contact with one of the cells. When the temperature rises at a rate of about 1°C Fill ‘er up! Supplying the correct charging current is only part of what is required for successful fast charging. www.siliconchip.com.au Building the SuperCharger is easy, with virtually all the parts on two PC boards: a main board and a front panel board. November 2002  57 These high-capacity NiMH AA-cells are typical of the new-generation rechargeable batteries that are now available. per minute, the charge is terminated. This is called Delta Temperature/ Delta Time (∆T/∆t) termination and it results in slightly less overcharge than the -∆V method. The SuperCharger uses -∆V as its primary termination method. This is easier to implement in a “loose” cell charger, where repeated attachment and removal of a temperature-sensing device is awkward. To minimise overcharge, the SuperCharger terminates both NiCd and NiMH fast charges with a -∆V of only 6mV. Should the primary method fail for any reason, a timer terminates the charge at 120% of rated capacity. This minimises any risk of cell damage due to overcharge. Memory effect Most of our readers will have heard of the infamous NiCd battery ‘memory effect’, so we’re not going to ramble on about it again here. Suffice to say, this problem has been eliminated by changes made to cell materials and construction. However, both NiMH and NiCd batteries can suffer from a related problem called ‘voltage depression’. This is caused by repeated shallow charging and discharging at low ‘C’ rates. When this occurs, batteries can exhibit an apparent loss of capacity and low charge acceptance. Batteries left idle for long periods can also exhibit this problem. A number of NiMH batteries we purchased recently were perfect examples. Although essentially ‘flat’, they would accept at most only about 0.2C charge (when fast charged) before entering the overcharged state. Restoring full capacity Luckily, this condition is easily 58  Silicon Chip reversible by first charging to full capacity at the 0.1C rate and then cycling several times at the fast charge rate. By cycling, we mean discharging down to no less than 0.9V per cell, followed by a charge to 100% capacity. Why 0.9V per cell? Well, despite what you might have read, rechargeable batteries should never be totally discharged. In a typical battery stack, one or more cells will be slightly ‘weaker’ than their neighbours and will reach total discharge (0V) first. They will then be charged in reverse, causing similar life-reducing effects to those found in overcharging. It goes without saying that we’ve incorporated a safe ‘discharge-before-charge’ function into the Super­ Charger, as well as provision for charging at the standard (0.1C) rate to cater for the above scenarios. How it works The circuit diagram for the Super­ Charger is divided into two sections, corresponding to two separate PC boards. Most of the electronics resides on the ‘main’ PC board and its circuit is shown in Fig.1. The display electronics, including all the LEDs and switches, reside on the ‘front panel’ PC board, as shown in Fig.2. Basically, the circuits in Figs.1 & 2 can be divided into four main sections: microcontroller & front panel circuitry; battery management; constant current source; and power supply. Let’s look at each of these in turn. Power supply Power for the circuit can be supplied via either a 16VAC 1.5A plug­pack or a 13.8V DC car lighter socket. You’ll note that we’ve provided a separate input socket for each source. This minimises the voltage drop on the DC input (CON2) side. The current path for the DC input is via just one diode of the bridge (DB1), rather than two as would be the case if both sources connected via CON1. For operation in an automotive environment, TVS1 limits peak voltage transients to no more than 40V, while capacitors C1-C4 provide the necessary filtering when an AC supply is used. The resultant unregulated rail voltage (+VIN) when the circuit is idle (not charging) is about 21.5V with an AC input and just under 12V with a DC input. This unregulated voltage is used to charge the battery stack as well as supply two DC regulator circuits. The first of these is a 3-terminal regulator, in the form of an LM317 (REG2). It provides +5V for the microcontroller (IC2), op amp (IC4) and associated circuitry. Transistor Q1, diode D1 and zener diode ZD1 form a second series-pass regulator. This circuit provides power to the LTC1325 battery management IC (IC3). Its sole purpose is to ensure that the VDD supply to the chip never exceeds +16V. Microcontroller & front panel All elements of circuit operation are controlled by an Atmel AT90S2313 microcontroller (IC2). Its many tasks include detecting and responding to user switch presses, turning LEDs on and off and sounding the piezo buzzer. It is also responsible for charge control and monitoring the battery state (via IC3), which we’ll look at in detail shortly. Eight microcontroller port lines (PD0–PD6 & PB0) are routed to the front-panel circuitry via CON4. Referring now to Fig.2, the eight signals arrive on CON7, where they are used to control 14 LEDs and read four switches. The LEDs and switches are accessed in a matrix (row/column) format. Looking at the LEDs first, we can see that port bits PD4–PD6 & PB0 control the four columns. They drive transistors Q5-Q8, which in turn provide power to each of the four strings of LEDs. The rows are formed by port bits PD0-PD3 which, when driven low (0V), can switch on any LED in an active column. The columns are driven sequent­ ially, with each being active for only 5ms. Therefore, it takes 20ms to refresh the entire LED array. Although any LED is switched on for only 25% of the total time, it appears to the naked eye to be always on. The pushbutton switches are ar- Fig.1 (right): the circuit diagram for the main PC board. An AT90S2313 microcontroller handles all aspects of the charge cycle, with help from an LTC1325 Battery Management IC. www.siliconchip.com.au www.siliconchip.com.au November 2002  59 Fig.2: all LEDs and switches reside on the front panel PC board, as shown here. Resistors R30-R32 isolate the switches from the LED row drivers, so pressing a switch does not interfere with the LED display. ranged in a similar row-column format. Port bits PD2 and PD3 form the columns, while PD0 and PD1 form the rows. To begin a switch read cycle, the micro activates a column by writing a logic low (0V) to the associated port bit, leaving the alternate column bit high (+5V). Resistors internal to the micro pull up PD0 and PD1 to +5V. Now when the micro reads the two row bits, both will be high (+5V) – unless a switch is pressed. Pressing a switch in the active column pulls the connected row down to 0V, allowing the micro to determine which switch is depressed. The micro cycles between the two switch columns every 5ms, allowing it to quickly detect user input. All of these operations are made possible by the program running in 60  Silicon Chip the AT90S2313 microcontroller. The program code for the AT90S2313 is contained in 2KB of on-chip ‘Flash’ memory. This can be programmed in-circuit via CON3, the ISP (In-System Programming) header. We’ll refer to ISP programming in more detail in the construction section. Battery management Battery charging, discharging and front-line monitoring are carried out by IC3, an LTC1325 battery management IC from Linear Technology. This device features a programmable 111kHz PWM (pulse width modulated) constant current source, a 10-bit A-D converter, two voltage regulators, a discharge-before-charge controller, a programmable battery voltage attenuator and a serial interface. Unfortunately, a detailed descrip- tion of the internal workings of the LTC1325 is beyond the scope of this article (detailed information is available from the data sheets, which can be downloaded free from http:// www.linear.com/). Instead, let’s touch briefly on some of the more important features of this IC as they relate to our project. (1) PWM current regulator: charge current is delivered to the battery via Q2, L1, D3, R21 & R22, which together with IC3 form a PWM current regulator. The PWM signal from pin 17 of IC3 drives the gate of a P-channel MOSFET (Q2). When switched on, Q2 charges inductor L1 from the DC rail. When it switches off, L1 delivers its energy to the battery via D3 and R21/R22. The voltage dropped across R21/ R22 is fed back to the PWM control circuitry via pin 11. When it reaches 160mV, the loop is in regulation. Internal control circuits integrate the www.siliconchip.com.au feedback voltage such that it is directly equivalent to the average charge current through the battery. Therefore, a simple Ohm’s law calculation reveals the average regulated current as follows: Average battery current = 160mV/ (0.1R||0.68R) = 1.835A This is the maximum supported charge current. However, the Super-Charger boasts programmable charge currents from 200mA right up to 1.8A in 200mA steps. This is achieved by modifying the feedback voltage with a second programmable voltage from a D-A converter. The D-A converter consists of a microcontroller-generated PWM signal, an integrator and a buffer. The PWM signal appears on pin 15 of IC2 and is integrated by R13 & C12 to provide a DC voltage level. It is then applied to the input of op amp IC4, which acts as a non-inverting unity-gain buffer. Varying the duty cycle of the PWM signal varies the DC voltage level. As the PWM is programmed for 8-bit mode, a 1% change in duty cycle gives about a 19.5mV change in the DC level. The voltages from the op amp output and the current sense resistors (R21/ R22) are summed at pin 11 of IC3 via scaling resistors R14 and R18. Therefore, by varying the D-A converter’s output voltage, the microcontroller can ‘fool’ IC3 into reducing the current in the charging loop to the desired level. So far, we’ve neglected to mention PTC1 and D2, which are also situated in the charging circuit. PTC1 is a 3A ‘Polyswitch’, otherwise known as a resettable fuse. In normal operation, its low resistance has little effect on circuit operation. However, at current levels above 3A, such as might occur if a battery pack is connected in reverse, its resistance increases rapidly. This reduces circuit current to safe levels and prevents smoke & fire! D2 is included to prevent the battery from discharging back through the DC rail via the body drain diode of the MOSFET (Q2) when input power is disconnected. (2) Discharge-before-charge: battery discharge is performed by a lamp load, consisting of four parallel-connected 12V 120mA globes. We’ve elected to use globes rather than resistors to reduce heat generation inside the case. When the gate of Q4 is driven high by IC3 (pin 16), it switches on and www.siliconchip.com.au The two halves of the case need quite a bit of surgery before they’re ready to accept the completed PC boards. This photo shows about half of the work complete, with the posts removed but the circular and smaller rectangular sections yet to receive the treatment. connects the globes across the battery. Surge current through the MOSFET is limited by resistor R17. (3) A-D converter: the 10-bit A-D converter in the LTC1325 can be programmed to sample voltages from a number of different sources. In this design, it is used to read the battery voltage and the DC rail voltage. Battery voltage is picked off at the junction of D2 and PTC1, where it is filtered by R15 and C8 before being applied to the VBAT input (pin 15) of IC3. ZD3 and R16 provide over-voltage and reverse battery protection. The input range of the A-D converter is just 0-3V, so the voltage applied to the VBAT pin must be divided down to suit. This is handled internally by a programmable attenuator, which supports division ratios of 1 to 16. The second A-D input is used to sample the DC rail voltage. Resistors R19 & R20 first divide the rail voltage by eight before applying it to the general-purpose A-D input on pin 12 of IC3. (4) Voltage comparators: the LTC1325 includes a number of comparators for monitoring minimum and maximum temperatures and cell voltages. The reference (trip) levels for these comparators are supplied on pins 6 (LTF), 7 (MCV) and 8 (HTF). As temperature sensing is not used in this design, the low (LTF) and high (HTF) temperature comparators are disabled by tying them to fixed voltage levels. The same applies to the temperature sensor inputs on pins 13 (TAMB) and 14 (TBAT). The necessary voltages are generated by a voltage divider string (R9-R12) which is supplied from IC3’s internal +3V regulator (pin 1). Capacitor C8 provides filtering for the 3V supply. (5) Serial interface: to orchestrate this myriad of functions, the microcontroller communicates with the LTC­1325 over a 4-wire synchronous serial interface. Four port pins of the microcontroller (PB4-PB7) are dedicated to serial bus operation. The micro acts as the serial bus master, clocking data into the LTC1325 (DIN) on the rising edge of the clock (CLK) signal and clocking data out (DOUT) on the trailing edge. During each serial transfer, the LTC1325 receives a 22-bit command word and transmits back an 8-bit status word and a 10-bit A-D conversion word. Constant current source At the beginning of every charge cycle, the microcontroller tests for a short-circuit or reverse-charged battery. If such a condition is detected, a separate constant current source is used to bring the battery voltage up to a minimum of 850mV before switching over to the main PWM current regulator. An LM317 3-terminal regulator (REG1), together with R4, R5 and Q3 make up the constant current source. The short circuit current is equal to November 2002  61 Parts List For The SuperCharger 1 PC board, code 14111021, 72mm x 107mm (main) 1 PC board, code 14111022, 40mm x 78mm (front panel) 3 Mini ‘U’ TO-220 heatsinks (19°C/W thermal resistance) (Altronics H-0637) 1 TO-220 silicon or mica insulating washer and bush 1 inductor, 22µH 3.6A (L1) (Sumida CDRH127-220MC) (www. digikey.com) 1 miniature PC-mount Piezo sounder (PZ1) (Altronics S-6104) 1 PTC resettable fuse (polyswitch) 3A 30V (PTC1) (Farnell 608956, Altronics R-4561A) 1 3A M205 anti-surge fuse (F1) 2 M205 PC-mount fuse clips 1 20-pin IC socket (machined-pin type, for IC2) 1 red PC-mount pushbutton switch (S1) (Altronics S-1095) 3 grey PC-mount pushbutton switches (S2 - S4) (Altronics S-1094) 4 low-voltage bezels (Jaycar SL2620, DSE P-8050) 4 12V/120mA LES (Lilliput) globes to suit above (Jaycar SL-2652) 1 rubber grommet to suit figure 8 cable 1 16VAC 1.5A plugpack (Altronics M-9325, DSE M-9668) 1 small cable tie Hardware 1 135 x 94 x 47mm (L x W x H) instrument case (Altronics H-0470) 4 10mm (diameter) adhesive rubber feet 2 M3 x 16mm countersunk head screws 4 M3 x 9mm tapped spacers 3 M3 x 6mm nylon screws 3 M3 x 6mm nylon nuts 2 M3 x 6mm spacers 8 M3 x 6mm screws 7 M3 flat washers 4 M3 nuts the ADJ pin reference voltage (1.25V) divided by R5 (22Ω) – ie, about 57mA. The micro controls the current source via port pin PB1 and Q3, an N-channel MOSFET. Diode D4 pre62  Silicon Chip Semiconductors 1 MC34064P-5 under-voltage sensor (IC1) (Altronics Z-7252) 1 AT90S2313-4 or -10 microcontroller (IC2) programmed with SCHG. HEX & SCHG.EEP 1 LTC1325CN battery management IC (IC3) (www.linear-tech.com) 1 TS952IN dual op amp (IC4) (Farnell 332-6378) 2 LM317T adjustable voltage regulators (REG1, REG2) 1 BC337-25 NPN transistor (Q1) 1 MTP23P06V P-channel MOSFET (Q2) (Farnell 259-639) 1 2N7000 N-channel MOSFET (Q3) 1 MTP3055E N-channel MOSFET (Q4) 4 BC327 PNP transistors (Q5-Q8) 1 KBL404 diode bridge, 4A 400V (DB1) 1 1N4148 diode, 150mA 75V (D1) 1 1N5245B zener diode, 15V 0.5W (ZD1) 1 1N4740A zener diode, 10V 1W (ZD2) 1 1N4744A zener diode, 15V 1W (ZD3) 14 red LEDs, 3mm high efficiency (LED1-LED14) 1 4MHz crystal, parallel resonant, HC49/4H package (X1) (Farnell 221-569) 2 MBRS340T3 Schottky diodes, 3A 40V (SMD) (D2, D3) (Farnell 878-390) 1 GS1G diode, 1A 400V (SMD) (D4) (Altronics Y-0174, Farnell 547529) 1 SMCJ30A Transient Voltage Suppressor, 30V 1500W (SMD) (TVS1) (Farnell 421-3580) 1 220nF 63V MKT polyester (C1) 1 220nF 25V multilayer ceramic (SMD 0805) (C5) (Altronics R-8641) 2 100nF 63V MKT polyester (C7, C13) 2 100nF 50V multilayer ceramic (SMD 0805) (C16,C17) (Altronics R-8638) 1 470pF 50V ceramic disc (C14) 2 27pF 50V multilayer ceramic (SMD 0805) (C11,C12) (Altronics R-8539) Capacitors 3 1000µF 50V PC electrolytics (C2C4), 26mm (H) x 16mm (Dia.) 1 33µF 16V tantalum (C18) (Jaycar RZ-6665) 1 22µF 25V tantalum (C9) 2 10µF 25V tantalum (C6,C15) 1 4.7µF 16V tantalum (C8) 1 1µF 50V monolithic ceramic (C10) SMD Resistors (1W, 5%) 1 0.68Ω thick film power (SMD 2512) (R22) (Farnell 310-4692) 1 0.1Ω thick film power (SMD 2512) (R21) (Farnell 310-4590) vents the battery discharging back through REG1 when power is removed. is straightforward, with all the parts mounted on the two PC boards referred to earlier. A separate PC board is used for the battery holders (to be described next month). Construction Construction of the SuperCharger Resistors (0.25W, 1%) 1 100kΩ (R2) 2 47kΩ (R3,R7) Note: when 3 15kΩ (R8,R9) charging six 16001 12kΩ (R12) 1800mAh cells 1 10kΩ (R13) in high ambient 1 2.7kΩ (R19) temperature, the 1 6.8kΩ (R10) unit might overheat. 1 5.6kΩ (R11) To help reduce the 1 4.7kΩ (R6) temperature of the 1 3kΩ (R14) 4 2.7kΩ (R30-R33) bridge, replace KBL404 (DB1) with 1 1.8kΩ (R25) 4 1.5kΩ (R26-R29) a GBU4D (Farnell 330-7256). For 1 300Ω (R20) installation details 1 1.2kΩ (R23) 1 1kΩ (R4) refer to the errata 4 330Ω (R34-R37) for this issue. 1 240Ω (R24) 1 220Ω (R18) 1 100Ω (R15) Resistors (0.5W, 1% & 5%) 1 470Ω 0.5W 1% (R16) 1 560Ω 0.5W 1% (R1) 1 22Ω 0.5W 1% (R5) 1 1Ω 0.5W 5% (R17) (Farnell 333189) Connectors & cable 2 2.5mm PC-mount DC sockets (CON1, CON2) (Altronics P-0621) www.siliconchip.com.au to the next. We’ll begin with the case preparation. 1 10-pin dual-row shrouded PCmount header (CON3) (optional, see text) 1 10-pin 2.54mm pitch single-row PC-mount header (CON4) (Altronics P-5500) 2 2-way 5mm pitch terminal blocks (CON5, CON6) (Altronics P-2034) 1 10-pin 2.54mm pitch single-row 90° PC-mount header (CON7) (Altronics P-5520) 2 10-way header sockets to suit CON4 & CON7 (Altronics P-5480) 600mm light-duty hookup wire 400mm medium-duty (5A) figure 8 cable 170mm 10-way rainbow cable 300mm (approx.) 0.71mm tinned copper wire for links Additional items for in-car use 1 2.5mm DC line plug 1 cigarette lighter plug 1.5m medium-duty (5A) figure-8 cable High-current battery holder (optional) PC board, code 14111023, 134mm x 74mm AAA PC-mount single-cell holders (Farnell 301-061) -and/orAA PC-mount single-cell holders (Farnell 301-073) -and/or1/2 AA PC-mount single-cell holders (Farnell 174-725) 1 2-way 5.08mm pitch cable-mount terminal block plug (Altronics P-2512) 6 2-way 5.08mm pitch 90° PCmount terminal block sockets (Altronics P-2592) 4 10mm (diameter) adhesive rubber feet Miscellaneous Neutral cure (non-acetic) silicone sealant, heatsink compound, battery holders to suit application (see text). The following instructions are presented in a specific order, designed to make construction a little easier. We suggest that you complete each step in the order given before proceeding www.siliconchip.com.au Case preparation To prepare the case, first split the case halves apart and place them side by side on your bench. You’ll notice that both halves are identical. Each has six mounting posts, a small rectangular-shaped area for a 9V battery, and a circular area for a loudspeaker. All of these protrusions must be removed, so that no more than about 0.5mm of material remains proud of the surface. The posts can be removed quickly and efficiently with an oversized drill. Choose the largest size that will fit in your drill chuck. Alternatively, you can cut them off with a sharp knife. If you do use a knife, cut off the post a few millimetres above the surface first, then gradually trim it away until you’ve removed the remaining stump. The same advice applies to all the other sections; remove small slivers at a time, rather than trying to remove large sections right at the base with the first cut. By the way, we’ve found that the best knife for the job is one that has a flexible blade. Stanley make a suitable utility knife (the type with ‘snap off’ blades). But be careful – very careful. It’s so easy to slip with these and if you’re applying a lot of pressure, you could easily remove more than a post! The front-most section of the rectangular battery area is perhaps the most challenging. Note how it also forms part of the panel-retaining groove. Do not remove all of this section; leave about 2mm or so proud of the surface. Obviously, the panel-retaining grooves must not be damaged, as they’re the only means of securing the panels when the case is finally reassembled. We placed a layer of masking tape over the front edge of the groove (the part that’s visible when the case is closed) so as not to mark it with the knife. OK, with that job out of the way, choose one half of the case as the bottom. Orient it so that the cooling slots are closest to you; this will be the front end. Slip one of the panels into place in the rear end grooves and position the main PC board on the bottom, right side up (copper side down). What we’re going to do now is use the PC board as a template to mark out the four mounting holes. Referring to the overlay diagram in Fig.3, make sure that you have the rear of the PC board towards the rear of the case (CON1, CON2 & CON4 go to the rear). Push the board hard up against the rear panel and then centre it exactly between the left and right sides of the case. Now mark out the four PC board mounting holes with a pencil or metal scribe. Remove the PC board, gently centre-punch your marks and drill the case to take 3mm screws. As with any drilling in soft plastic, we strongly recommend that you start with a small drill size and work up to the correct size in a number of steps. Next, we’ll prepare the front and rear panels. Begin by placing one of the panels ‘rough’ side down and position the front panel PC board exactly centred on the panel, with the copper side facing up. You should find that the PC board is marginally smaller in height than the panel. In addition, the corners of the board should not overhang the curved corners of the panel. If the board is larger in any of these dimensions, then you will have to file it down to size. The next job is to mark out the two mounting holes, four switch positions and 14 LED positions. There are two ways this can be achieved. First, you can choose to photocopy the template shown in Fig.6, cut it out and tape it to the panel. It’s then just a matter of centre-punching through the template to mark out the drilling positions. Alternatively, you can mark directly through the PC board with a pin or other sharp instrument, and use a scribe and straight-edge to locate hole centres for each component. This method may be slightly more accurate but has a higher degree of difficulty. Finally, drill the holes to size, finishing off the switch holes with a tapered reamer. Note that the switch holes will need to be slightly larger that the switches themselves to prevent jamming. The rear panel can now be prepared using the photocopied template method described above. Main PC board assembly Before mounting any components on the main board, it’s important to check that the holes for the heatsink tabs have been properly formed. To do this, select one of the heatsinks and loosely affix an LM317 regulator November 2002  63 ing all of the surface-mount (SMD) components on the copper side of the board. Referring to the overlay diagram of Fig.4, first identify the mounting positions for each of the SMD components. Now prepare the pads/tracks as necessary, ensuring that they are tinned and free of excess solder. Mount the five capacitors first and follow up with diode D4. You’ll need a fine tip on your iron (eg, 0.8mm), fine solder and probably some 0.76mm desoldering braid as well. To prevent overheating these miniature components, apply your iron to the pad/ track first, not the component. You’ll need a third hand (who said genetic engineering is pointless?) to hold the parts in position while soldering. For the remaining (larger) components, step up a couple of tip sizes. Note that diodes D2, D3, D4 and TVS1 are all polarised components, so they must be oriented as shown on the overlay diagram. Once all the SMD parts have been installed, inspect your work with a magnifying glass. Check particularly for fine solder bridges between the tracks and pads. Top side assembly Fig.3: the overlay diagram for the top side of the main PC board. Note that there are several small differences between this diagram and the prototype shown in the photograph directly above. As explained in the text, the three heatsinks need to be trial-fitted before assembly begins. using an M3 screw and nut. Using the overlay diagram (Fig.3) as a guide, temporarily insert the assembly into the REG2 position. If all is well, the heatsink tabs should be a firm fit in their respective PC board holes. In addition, the three regulator leads should slip easily into their holes. If not, this suggests that the tab holes are misaligned and/or incorrectly sized. Use a fine jeweller’s file to adjust the heatsink tab holes as you see fit, being careful not to file into any of the adjacent copper. Ideally, the holes for the tabs should be shaped exactly like 64  Silicon Chip the tabs themselves; ie, slotted rather than circular. You’ll need to repeat the procedure for the other heatsink positions (REG1 & Q2) as well. In addition, the heatsink for Q2 requires a small modification. The left side tab (as shown in Fig.3) must be filed down so that it is level with the underside surface of the PC board when installed. Do not remove the entire tab, just enough to achieve the desired result! Cloning gets the nod That job complete, let’s get on with the assembly. We’ll begin by mount- OK, let’s turn our attention to the top side of the board. Referring to Fig.3, begin by installing the 13 wire links, followed by all the resistors. Diode D1 and zener diodes ZD1-ZD3 can go in next, taking care to align the banded ends as shown. Next, install the socket for IC2, followed by all of the connectors. The keyed (pin 1) side of CON3 should face towards IC2. Also, note that the two terminal blocks (CON5 & CON6) must have their cable entry sides facing towards the middle of the board, which is the reverse to what you might expect. Before soldering each connector, ensure that it is seated firmly against the PC board surface. All of the capacitors, with the exception of the three 1000µF electrolytics, should be installed next. Make sure you have the marked (positive) sides of the tantalum capacitors oriented as shown. Follow with the three TO-92 packages (IC1, Q1 & Q3), the M205 fuse clips, crystal (X1), the piezo buzzer, polyswitch PTC1 and diode bridge DB1. Next, loosely assemble the three TO-220-packaged devices (REG1, www.siliconchip.com.au Fig.5: this diagram shows how REG1 is isolated from its heatsink using a TO-220 insulating washer and bush. By contrast, REG2 & Q2 are bolted directly to their heatsinks, without insulation. Fig.4: the copper side of the board, showing the placement for each of the surface-mount components. Again, there are some minor differences between this and the prototype photo shown above. Note that the cathodes of the MBRS340T3 diodes (D2, D3) are marked with a semi-circular ‘notch’ rather than the usual white band. REG2 & Q2) onto their heatsinks using M3 nylon screws, nylon nuts and steel washers. Place a thin smear of heatsink compound on both the heatsink contact area and rear of the devices before assembly. REG1 must be electrically isolated from its heatsink using a TO-220 insulating washer and bush (see Fig.5). The other two devices (REG2 & Q2) should not have insulators fitted. Mount REG2 first, making sure that the heatsink is perfectly square on the PC board surface. Push the LM317 regulator all the way down the heatsink slot until it can go no further and then solder it into position and tighten up the mounting screw. Repeat this procedure for REG1 and Q2, making sure that the tab of Q2’s heatsink (shortened earlier) does not interfere with the SMD inductor (L1) or short out to either of the inductor’s terminals. Now install the three 1000µF electrolytic capacitors. They must be seat­ed firmly on the PC board surface before soldering. Finally, fit 9mm tapped spacers to the four mounting positions. At this point, IC2, IC3 and IC4 should be the only components not installed. Do not install them until after you have performed the power supply checks detailed next month. Next month, we will describe the front panel PC board and battery holder assemblies and show you how SC to use the new charger. Fig.6: these are the full-size artworks for the front and rear panels of the SuperCharger. www.siliconchip.com.au November 2002  65