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MEGA BATTE A large finned heatsink is necessary to keep the main power transistor cool. A LED on the front panel lights when the batteries have charged. This Mego-Fast Nicad Charger will safely charge o 1300mAh nicod racing pock in 20 minutes. It's easy to build and can be powered from a 12V car battery or from a 12V battery charger. By JOHN CLARKE & GREG SWAIN Radio-controlled (R/C) model cars are a lot of fun, but the hobby comes with one in-built frustration - under normal use, the batteries go flat within about 30 minutes. It's even worse under race conditions, when heavy loads are placed on the battery pack. In these conditions, the batteries can go flat in less than 15 minutes. What's needed is a way of quickly recharging the battery pack so that it can be put back into service as soon as possible. According to the manufacturers, for correct charging of nicad batteries, each cell should be first discharged to its end point (usually around 1. 1V) and then recharged at the 10 hour rate for 14 hours. This 62 SILICON CHIP means that if the battery pack is rated at 1300mAh, it should ideally be recharged for 14 hours at a 130mA rate. But what R/C car enthusiast wants to wait for 14 hours while the battery pack is recharged? To overcome this problem, many enthusiasts resort to fast charging. In its crudest form, this simply involves connecting the 7. 2V nicad pack to a car battery via a pair of resistive leads. The resistive leads limit the current into the nicads to a safe value but with one proviso you must remember to disconnect the leads after a preset time to prevent overcharging, otherwise you'll damage the cells. Damaging a racing pack is expen- 1.8 1.7 - ~ / / I I 1.4 1.3 0 10 15 20 CHARGE TIME (MINUTES) Fig.1: charging curve for a single cell in a 7.2V 1300mAh racing pack. Note how the voltage falls at the end of the charging cycle. sive, since they cost around $60 to $70. Clearly, there has to be a better way. Automatic switch-off The Maga-Fast Nicad Charger charges at a high rate but stops overcharging by automatically switching itself off when the cells are fully charged. How does it do this? Well, when a nicad cell is fully charged, further charging leads to a slight drop in its output voltage. The charger has inbuilt circuitry to FAST NICAD RYCHARGER +12V BA NOMO-C:r--.r.~--------------------------------------------, 7.2V 1300mAH NICAD PACK1000 25VW 100 25VW + - - 02 1N414B 10M 2200 25VW 2200 25VW + 2200 -25VW - + _ 01 BYX9B A 10A VR2 20k 470k .01 0 / 04 1N414B CURRENT SOURCE 0 8 VIEWED FROM BELOW MEGA-FAST NICAD CHARGER SC14-1·05BB Fig.2: the circuit diagram. ICla, IClb, Qt and Q2 form the constant current source, IClc is the level detector, and ICld is the timer circuit. The circuit is reset at the end of the charging cycle by disconnecting the supply. detect this voltage drop and then turn itself off. Fig.1 shows the charging curve we plotted for a single cell in an Arista NCRP72 7.2V 1300mAh super racing pack. Note that the voltage across the cell rises steeply towards the end of the charging cycle and then levels off and begins to fall. The charger cuts off when the cell voltage just begins to fall. To ensure fast charging, the charger pumps a constant current of six amps into the battery pack until the end point voltage is deteded. As a further safety measure, an inbuilt timer is included in the circuit. If the end point voltage is not reached, the timer will automatically shut the charger down after a preset time. If you like, you can regard the timer as a "belts and braces" feature to ensure that the charger shuts down within a specified time. Without the timer, the circuit might not shut down if connected to faulty cells. The front panel of the charger features two LED indicators, one red and one green. The red LED lights when power is applied to the charger, while the green LED lights when the battery pack has charged. How it works Fig.2 shows the circuit diagram. IC1a, IC1b, Ql and Q2 form the 6A current source, IC1c is the level detector, and IC1d forms the timer circuit. Let's see how the circuit works. LED 1 is used for the power indicator and as a voltage reference. Trimpot VR1 feeds a portion of the LED voltage to pin 12 of IC1 b which, together with ICla, drives Darlington pair Ql and Q2. The current through Q2 develops a voltage across its 0. rn emitter resistor and this voltage is applied to pin 13 of IC1b via the 27kn resistor. So IC1 b compares the voltage developed across the 0. Hl resistor with the voltage setting from VR1. Thus IC1 b sets and controls the current through Q2. Now what is IC1a doing there, tacked on to the output of IC1 b? The problem is that IC la is not up to the task of driving Ql by itself. So IC1a is connected up as a "current follower". It monitors the voltage produced by IC1b between pin 14 and the base of Ql and it acts to produce the same voltage across its own 1000 output resistor. So for every milliamp delivered by IC1b, IC1a does the same. Voltage monitoring As mentioned above, the circuit automatically shuts off when the battery pack is fully charged. This condition is detected by IC1c. JUNE 1988 63 comparator IClc will normally be low, D4 will be reverse biased, and the charging cycle will continue. When the nicad battery pack nears the end of the charging cycle, the voltage across it will suddenly start to reduce instead of increasing as before. This will mean that the voltage at pin 5 will rise above that on pin 6 and so the output of IClc will go high. D4 will conduct, pin 13 of ICl b will be pulled high, and so Ql and Q2 will be turned off. At the same time, D2 conducts and latches up IClc so that the circuit remains off. LED 2 is also now forward biased and so it lights to indicate that the battery pack is fully charged. Timer circuit The printed circuit board is mounted in the case on 6mm standoffs and secured by machine screws and nuts. Use 4mm (or thicker) auto cable for all wiring connections and pass all external leads through grommetted holes. Both inputs of IClc monitor the voltage across the battery pack via separate time constant circuits. Pin 5 has a relatively short time constant (22k0 and lOOµF) while pin 6 has a long time constant (22k0 and 1000µF). So IClc is set up to detect the point . at which the voltage across the battery starts to reduce. During most of the charging cycle, the battery voltage will be increasing at a very slow rate. This means that the voltage at pins 5 and 6 will be steadily decreasing (with respect to the negative supply rail). Further, because of the short and long time constants, the voltage at pin 5 will always be slightly lower than at pin 6. Hence the output of r; Above is a full size reproduction of the printed circuit artwork. 64 SILICON CHIP Comparator stage ICld and its associated components form the timer circuit. A reference voltage derived from trimpot VR2 is applied to the pin 3 input, while pin 2 is connected across an RC timing circuit consisting of a 2200µF capacitor and a 470k0 resistor. When power is first applied to the circuit, the 2200µF capacitor is discharged. This means that pin 2 will be high, pin 1 low and D3 reverse biased. Thus the timer initially has no affect on the rest of the charger circuit. Now consider what happens as the capacitor charges towards the negative rail via the 470k0 resistor. Eventually, the voltage applied to pin 2 of ICld will drop below the pin 3 reference voltage. When this happens, pin 1 of ICld will switch high and pull pin 5 of IClc high via D3. Thus, pin 7 of IClc switches high and shuts down the charging circuit via D4 as described previously. Essentially, the timing circuit is included as cheap insurance against the circuit not shutting down via the level detection circuit. In most cases, by the time the timer operates, the output of IClc will have already switched high and the circuit will have shut down. Trimpot VR2 allows the timer to be set so that it operates shortly after the nominal charging period. It should be set to cut out at around 20 minutes Power for the circuit is derived from a nominal + 12V supply [eg, a Fig.3: here are the wiring details for your Mega-Fast Nicad Charger. Note that transistors Q1 and Q2 must be insulated from the metal case (see text and Fig.4 below). car battery or a battery charger). The two 2200µF 25VW electrolytics provide supply line filtering, while diode Dl and the BA fuse form a simple but effective "crowbar" protection circuit to guard against reversed battery connections. If the connections are reversed, Dl conducts and blows the fuse. The large power diode is installed directly on the PCB and secured from the copper side using a nut and star washer. When the nut is tightened, the star washer bites into the copper pattern and thus ensures good contact with the cathode (K) of the power diode. A short piece of tinned copper wire is used Construction Construction is straightforward with most of the parts mounted on a printed circuit board (PCB) coded SC14-1-588 and measuring 112 x 69mm. This is housed in an aluminium case measuring 133 x 76 x 54mm. A large finned heatsink mounted on the top of the case provides heat dissipation for the 2N3055 power transistor. Fig.3 shows how the parts are mounted on the PCB. You can start assembly by soldering in the small signal diodes, 0.25W resistors and the ICs. When these have been installed, you can mount the electrolytic capacitors, the trimpot and O. rn 5W power resistor. The latter should be mounted 1-2mm proud of the PCB to allow air circulation. -TRANSISTOR 0 HEATSINK 0 ~---CASELID ~ j_ <:$ ® © ~-INSULATING BUSH ~ - SOLDER <at>.....-- LUG WASHER <at>....---SPRING WASHER <at>----NUT Fig.4: mounting details for transistor Q2. Smear all mating surfaces with heatsink compound and check the final assembly with a multimeter. for the anode connection. Once the diode is in place, the two LEDs and the BD139 transistor can be installed. Before mounting the two LEDs, be sure to correctly identify their anode (A) and cathode (K) leads (see circuit diagram). The leads are then bent at right angles about 2mm from the LED body and the LEDs installed so that their centres sit about 4mm proud of the PCB. If you've followed the above procedure correctly, you will find that the LEDs overhang the front of the PCB by about 3mm. This is important, since the LEDs later have to protrude through the front panel of the case. The BD139 transistor should be mounted using a 12mm lead length. Note that the metal side of the transistor body faces away from the PCB. With the PCB assembly completed, attention can be turned to drilling the metal case. First, mark out and drill mounting holes for the PCB in the bottom of the case, then temporarily mount the PCB on 6mm standoffs (bend the LEDs back) and JUNE 1988 65 Close-up view of the printed circuit board assembly. The o.rn resistor is mounted 1-2mm proud of the PCB to allow air circulation for cooling. mark the mounting hole for the BO139 transistor (Ql). The PCB can now be removed and holes drilled to accept Ql 's mounting bolt and an adjacent rubber grommet (see Fig.3). This done, carefully affix the front panel artwork to the case and drill clearance holes for the two LEDs. Note that the front panel artwork is not centred top and bottom on the case. Instead, it must be positioned so that the bottom edge is 4mm from the bottom of the case (see photo). Additional holes can now be drilled in the lid to accept the fuseholder, power lead grommet, and heatsink mounting screws (see photo). Be sure to position the fuse so that it will clear the capacitors mounted at the end of the PCB when everything is later assembled in' the case. The 2N3055 power transistor is mounted using a mica washer .and insulating bushes to electrically isolate it from the heatsink and case. The method of assembly is shown in Fig.4. You can mark the holes for mounting the transistor using the TO-3 mica washer as a template. After drilling, remove any burrs using a larger diameter drill. Check that the contact area is free of metal swarf and grit, then smear a thin layer of heatsink compound on the transistor mounting base and on both sides of the mica washer before screwing the assembly together. After the transistor has been screwed down, use your multimeter (switched to a high "Ohms" range) to check that it is completely isolated from the case. If the reading shows a short, remove the transistor and check the mating MEGA-FAST NICAD CHARGER 7 UJ CJ) => u. L 0 POWER 0 CHARGED <( _J Here is an actual-size reproduction of the front panel artwork. 66 SILICON CHIP surfaces carefully for small pieces of metal. The PCB and off-board components should be wired using 4mm auto cable (don't use thinner wire). This won't fit into normal PCB holes so we suggest that you use PC stakes at all external wiring points on the PCB. Use 1-metre or longer lengths of wire for the power leads and terminate them in alligator clips to allow rapid connection to the car battery. The charging leads for the nicad pack should be 100mm or longer. They can be terminated in a suitable socket, to accept the plug from the battery pack. To avoid confusion, use red cable for the positive leads and black for the negative leads. Once the wiring has been completed, the PCB can be mounted in the case and secured on 6mm standoffs using machine screws and nuts. Push the LEDs through the front panel holes and secure the BD139 transistor to the side of the case using a screw and nut. Note that the metal face of the BD139 must be isolated from the case using a TO-220 mica washer. An insulating bush is not required since the hole in the transistor body is already insulated. As before, smear the mating surfaces with heatsink compound before bolting the transistor to the case. Finally, use your multimeter to check that the metal tab is correctly isolated. You can do this by connecting the multimeter between the collector lead of the transistor and the case. If everything is correct, the meter will indicate an open circuit condition. Setting up At this stage, it's a good idea to go back over your work and check carefully for wiring errors. If everything checks out, set both VR1 and VR2 fully anticlockwise, connect a discharged nicad battery pack to the charger output, and connect your multimeter across the o.rn 5W power resistor. Set the multimeter to a low voltage range. Now connect the supply leads to a 12V car battery and slowly adjust VR1 for a reading of 0.6V on the meter. This corresponds to a current of 6A through the nicad bat- z DAVID REID ELECTRONICS Aust. Pty. Ltd . PORTASOL SCIENTIFIC CALCULATOR "AA" Nicads "PROFESSIONAL" BUTANE GAS IRON KIT EL-506H 10 DIGIT L.C.D. DISPLAY Four arithmetic calculations, constant calculation, memory calculation, degree/minute second ,_. decimal degrees conversion , trigonometric function, inverse trigonometric function , logarithmic function , exponential, square and power, Xth root of Y (\/y) , square root, reciprocal , factorial , co-ordinates con version, statistical calculation , hyperbolic and inverse hyperbolic functions , percent change , etc . A MUST FOR EVERY STUDENT, TECHNICIAN. COMPLETE w1TH WALLET AND INSTRUCTION MANUAL. 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TELEPHONE DOUBLE ADAPTOR NS D LINEAR DATABOOKS Vol 1. $2.95 67 PARTS LIST 1 PCB, .code SC14-1-588, 11-2 x 69mm 1 Scotchcal label, 1 1 0 x 40mm 1 folded aluminium case, 133 x 76 x 54mm 1 finned heatsink, 7 5 x 11 0 x 33mm 1 panel mount 3AG fuse holder 1 SA fuse 2 6mm grommets 4 6mm standoffs 2 metres red automotive cable (4mm dia). 2 metres black automotive cable (4mm dia.) 4 3mm dia. x 1 5mm screws 4 3mm nuts 3 2.5mm dia. x 10mm screws 3 2.5mm nuts 1 solder lug 1 T0126 mica washer 1 T0-3 mica washer plus insulating bushes 1 socket to suit plug on 7. 2V Nicad battery pack 2 automotive battery clips 4 rubber feet tery pack. Since the timer is also going to be adjusted during this procedure, you should also note the precise time when the 12V source is connected. Assuming that the battery pack Semiconductors 1 2N3055 NPN power transistor 1 BD139 NPN transistor 1 BYX98-300(R) 1 OA 300V diode 2 5mm LEDs (1 red , 1 green) 3 1 N4148, 1 N914 diodes 1 LM324 quad op amp Capacitors 3 2200,uF 25VW PC electrolytic 1 1000,uF 25VW PC electrolytic 1 100,uF 25VW PC electrolytic 1 0.0 1,uF metallised polyester Resistors (0 .25W, 5%) 1 x 1 OMO, 1 x 470k0, 1 x 27k0, 2 X 22kQ, 1 X 2.2k0, 1 X 6800, 3 X 1000, 1 X Q. 10 5W, 2 X 20k0 miniature vertical trimpots Miscellaneous Solder, heatsink compound, tinned copper wire, etc. was flat to begin with, it should take about 20 minutes for the pack to recharge. During this period, you should carefully monitor the temperature of the battery pack. If the battery becomes hot, disconnect Automatic light controller The accompanying photographs show the general layout inside the case. As can be seen, the PIR movement detector is mounted on the lid of the case, supported on 18mmlong pillars. Before mounting the detector, you will have to make a cutout in the lid to clear the lens assembly. A 7mm hole will also have to be drilled in the lid to accept the LDR. The control board is mounted on the bottom of the case and secured using machine screws and nuts. Drill holes to accept the mounting screws plus an extra hole ih the bottom left corner (looking from inside the case) for the mains cord entry. You will also have to drill a hole in the adjacent end for the mains cord clamp, plus additional holes in 68 SILICON CHIP continued from page 60 the sides of the case to accept the lamp holders (or to pass wiring to external lamps, depending on requirements). It's best to complete the wiring to the control board before mounting it in the case. Light duty hookup wire can be used for connections between the two PCBs and to the LDR but note that the wiring between the control PCBs and the lamps must be run using 240V AC cable. Lace up the cables or use cable ties to keep the wiring tidy. The control PCB can now be mounted in the case and the mains cord secured using a suitable clamp. The prototype used a clamp fashioned from scrap aluminium and secured with a screw and nut. This same screw and nut also it from the charger immediately. Under normal circumstances, the battery pack should become warm and the "charged" LED should light at the end of the charging period (ie, after about 20 minutes). As soon as the "charged" LED comes on, disconnect the battery pack but leave the charger connected to the 12V source. Now quickly connect your multimeter (set to volts) between pin 1 of ICld and ground and adjust VR2 so that pin 1 switches high. This effectively sets the timer so that it disables the charger shortly after the end of the normal charging cycle. To check the timer action, disconnect the charger from the 12V source, leave it for a minute or so to discharge the circuit's capacitors and then reconnect it, without a nicad battery pack in place. Then check that LED 2 comes on after 20 minutes. When you are using the charger and want to charge several battery packs in succession, remember to disconnect the charger from the 12V source after each pack is charged. This resets the timer and the voltage monitoring circuit. Footnote: the Mega-Fast Nicad Battery Charger can also be used to charge lower voltage packs (eg, 5.6V nicad packs) without any changes to the circuit. lb secures a piece of insulating material to cover the mains terminations on the transformer. (In the kit supplied by Oatley Electronics, this material will be Presspahn or Elephantide ). We suggest that the cut-outs for the PIR lens assembly and the LDR be weather-sealed using a silicone sealant. If possible, try mounting the unit under the eaves of the house, out of the weather. A licensed electrician should be employed to connect the unit to existing house wiring. Note: on boards presently being supplied by Oatley Electronics, it is necessary to modify the pattern asssociated with the relay coil on the PIR movement detector. Instructions on how to do this are being supplied with the board (see Notes and Errata on page 95). lb