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· 1.2A {6Ah) 2A Build this 6/12V SLA battery charger This upgraded version of the March 1990 SLA battery charger is both cheaper & easier to build than the original unit. It automatically charges either 6V or 12V SLA batteries at any one of six current settings. By DARREN YATES As with most batteries, it is important that SLA (sealed lead acid) batteries be correctly charged. Incorrect charging procedures can cause considerable damage to a battery's internal structure and can reduce its service life. Unfortunately, most car battery chargers are not suitable for use with SLA batteries and will almost always result in overcharging. This SLA battery charger is a far better alternative. It can be used with both 6V and 12V batteries and automatically adjusts its charging rate to suit the condition of the battery. Like the original version, it's based on the Unitrode UC3906 intelligent battery 22 SILICON CHIP charger IC which monitors the battery and automatically switches to one of three charging modes. Because the battery voltage is continuously monitored and the charging current adjusted accordingly, this charger can be permanently connected to the battery if required. The unit maintains the battery at a constant "float voltage" once it has been fully recharged. A 6-way rotary switch on the front panel of the unit is used to set the maximum charging current to match the battery capacity. All the common battery capacities are catered for, as follows: 1.2, 2.5, 4.5, 6, 10 & 15Ah. The maximum charge currents for these settings are 250mA, 500mA, 900mA, 1.2A, 2A and 3A, respectively. Note that the charge currents provided are at the rate of C/5, where C is the battery capacity in amp-hours. If you have a battery which does not quite match one of the batteries listed, it doesn't matter-just select the nearest setting. For example, if you have a 1Ah battery, select the 1.2Ah rate. There's only one other control on the front panel and that's the 6/12V selector switch. The power on/off switch is located on the rear panel. The front panel of the unit also has three LEDs to indicate which of the three possible charging modes is currently in operation. These charging modes are MAIN (red), TRICKLE (yellow) and FLOAT (green) . We'll explain these three modes a little later on. Another very worthwhile feature of the unit is that it is output short circuit proof. After all, you don't want the unit "blowing up" just because the battery clip leads touch each other. Nor can the unit be damaged by re- SINK 16 verse connected batteries (except for blowing an internal fuse). SOURCE 15 COMPENSATION 14 +VIN Upgraded features Although the March 1990 SLA battery charger was a very successful unit, the amount of internal wiring caused problems for some readers. In particular, readers experienced problems wiring up the 4-pole voltage selector switch. Some constructors also experienced problems with inconsistent operation of the LED indicators. The charger would operate normall y but the TRICKLE or FLOAT indicator LED would refuse to light. The cure is simple: just replace the TL074 op amp with an LM324. Our main goal with the new unit was to make it much easier to build. This has been achieved in two ways: (1) by simplifying the mechanical construction; and (2) by simplifying the switch wiring. The list of new features and improvements is as follows: • the sheet metal case has been re. placed with a cheaper and smaller plastic instrument case; • a cheaper and smaller 60VA power transformer has been substituted for the original 108VA unit; • three of the four 5W resistors have been eliminated from the circuit, thus reducing heat dissipation; • a 3A current meter has been added to indicate the charging current; • a 6A PC-mounting power diode has been used instead of a stud-mounting diode; • CMOS logic switching has been added so that an SPDT toggle switch can be used for voltage selection instead of the previous 4-pole rotary switch. • An extra current range (2A) has been added to cater for 10Ah batteries. 1----41>-013 VOLTAGE SENSE C/L 40--U---I , I C/S OUT 1 0 - - - - - - - - , CIS+ I I Jo---•+' 25mV C/S - 20----1 + I I I 1-----012 CHARGE ENABLE VREF VREF 2.3V AT 25"C -3.9mV/"r. GROUND 60--+---- .,. 9 OVER-CHARGE INDICATE POWER 7 INDICATE OVEn.GHARGE 8 TERMINAL UC3906 Fig.1: internal circuit of the UC3906 intelligent battery charger IC. It monitors the battery voltage and automatically switches to one of three charging modes: trickle, main charge or float . _J _______________ _ INPUT SUPPLY VOLTAGE r----EE - - - - : : - - - - - . . . : - VDC C CHARGE VOLTAGE -=--~~VF_ A - ---------------CHARGE CURRENT I --------- ------IT STATE LEVEL OUTPUT oc INDICATE OUTPUT The UC3906 IC The heart of this project is the UC3906 intelligent SLA battery charger IC from the Unitrode Corporation. As we mentioned back in March 1990, it is a tricky IC to work with and is easily damaged. However, with a little care, it works very well. Fig.1 shows the basic internal structure of the IC. It contains five op amps which monitor the battery and current settings and control the driver circuitry to determine the charging rate. What makes this IC unique is its - - - - - - 011 TRICKLE BIAS VREF +VIN 5 0 - - - - DC TERMINATE INPUT (C/S OUT) ·::~ I I Off ~~ STATE 1 , I I I I ---t--- . -t---_1 ~ I - _ _ _ _.___I I STATE 2 _ ____ j ,, STATE 3 . . _ , _ _ I- • I STATE 1 Fig.2: these graphs show the voltage & curre)lt waveforms for the various charge states. If the battery is flat, it is trickle charged at current IT until voltage VT is reached. The circuit then switches over to main charge (point B) & finally to float charge when the overcharge voltage (Vod is reached (point C). specially-designed internal voltage reference. This sits at 2.3V at 25°C but has a negative temperature coefficient of -3.9mV/°C. This means that as the temperature rises, its voltage reduces by 3. 9m V/°C and this closely matches AUGUST 1992 23 PARTS LIST Semiconductors 1 PC board, code SC14109921, 225 x 124mm 1 front panel label, 245 x 73mm 1 0-3A meter scale 1 plastic instrument case, 262 x 190 x 82mm 1 M-2165 60VA transformer (Altronics Cat. M-2165) 1 SPST mains panel switch 1 M205 bayonet fuse holder 2 M205 PC mount fuse clips 1 5A M205 fuse 1 500mA M205 fuse 1 cordgrip grommet 1 TO-220 mounting kit (mica washer & plastic bush) 1 2-pole 6-position rotary switch 1 SPST toggle switch 1 MU-45 50µA FSD panel meter 1 red 4mm binding post 1 black 4mm binding post 2 solder lugs 1 2-way mains terminal block 1 21 mm x 6.4mm shaft Collett knob 3 5mm LED bezels 1 3-core mains power cord with moulded 3-pin plug 1 0.5m length of 10-core rainbow cable 1 300mm length of heavy-duty figure-8 cable 1 5kQ 5mm trimpot (VR1) 3 15 x 4mm machine screws & nuts (for power transformer & earth lugs) 2 15 x 3mm machine screws & nut (for bridge rectifier & mains terminal block) 1 10 x 3mm machine screw & nut (for 01) 5 No.6 self-tapping screws Hookup wire, solder, heatsink compound, tinned copper wire for links, heatshrink tubing. the temperature coefficient of an SLA battery. This thermal tracking is important because it ensures that the battery is always charged to the correct voltage, regardless of temperature. In particular, it avoids overcharging and possible damage to the battery in cold weather. The UC3906 also contains the logic which is used to switch the charger from one state to another, as well as operating the LED indicators. Fig.2 shows the voltage and current wave- forms for the three possible charging states: MAIN, TRICKLE and FLOAT. Take a look at the graphs for the charge voltage and the charge current. If the battery voltage is below VT (in this circuit, 10.5V for the 12V range), the UC3906 switches to the trickle state (IT) and charges up the battery at about 30mA. This is done to prevent damage to an overly-flat battery. When the battery voltage reaches 10.5V (point B in Fig.2), the "charge enable" comparator inside the UC3906 24 SILICO N CHI P 1 UC3906N intelligent SLA battery charger IC (IC1) 1 LM324N quad op amp (IC2) 1 CMOS 4066 quad analog switch (IC3) 1 TIP126 Darlington power transistor (01) 1 BC547 NPN transistor (02) 1 15W 1W zener diode (ZD 1) 1 3.3V 400mW zener diode (ZD2) 1 PW04 400V 6A bridge rectifier (BR1) 1 PX6007 or R250H 6A rectifier diode (01) 1 5mm red LED (LED1) 1 5mm yellow LED (LED2) 1 5mm green LED (LED3) Capacitors 1 4700µF 25VW electrolytic 2 0.1 µF 63VW MKT polyester 1 .022µF 63VW MKT polyester Resistors (0.25W, 1%) 1 560kQ 1 360kQ 1 220kQ 1 180k.Q 1 110kQ 4100kQ 1 91kQ 1 82kQ 1 47kQ 2 22kQ 118kQ 6 10kQ 1 8.2kQ 1 6.8kQ 1 4.?kQ 1 3.9kQ 51kQ 1 680Q 2 390Q 1 330Q 1 0.22Q 5W Miscellaneous switches the charger into the "main charge" state. As can be seen from the charge current graph, it jumps up to the maximum current level. In practice, this level will depend on the setting of the "charge current" switch set (ie, from 250mA to 3A). What happens now is that the battery voltage steadily rises towards a maximum of 14.6V. However, the charge current begins to decrease when it reaches 95% of this voltage. This is shown as point C in Fig.2. By the time the battery voltage reaches 14.6V or the "overcharge" voltage, the charge current has tapered off to about 120mA (point D) and the charger switches into the "float" state. What happens now is that it switches off momentarily and allows the battery voltage to drop to 13.8V. When a voltage of 13.8V (point E) is reached, the charger then supplies about l00mA of current to maintain this voltage indefinitely, or until a load is placed across the battery. If a load is connected, the battery voltage drops. Once it reaches about 13.ZV (or 10% below 14.6V), the charger kicks back into "overcharge" mode and supplies its main charge current to the load. A similar sequence of events occurs when a 6V battery is charged. Voltage selection is achieved simply by changing the bias settings to the op amps inside the UC3906. Load current If the load current is less than the selected charge current range, the charger will supply the required current but both the MAIN and FLOAT LEDs will light. This indicates that the battery is not being significantly depleted and that the charger is able to handle the load. Conversely, if the load current is higher than the current range, the charger will switch into the main charge mode and light the MAIN LED only. This indicates that the charger is now supp1ying its maximum current to the load and that the battery is also snaring some of the demand. For example, if the charge current switch is set to 1.2A and the load draws 2.5A, the charger will light the MAIN LED only. If the load is less than 1.2A, both the MAIN and FLOAT LEDs will light up. Once the load is removed, the charger will automatically resume '11 like the feeling of our new tligital troubleshooting scope. ~~ ~ 0. ~ i Q..r;J~i:,;:: ·- I - ! ~ ~EJ··· -a ....... ,!J . . . ! -. ' Now there's a 100 MHz digital scope that handles just like analog. instantly to the slightest control change. Digital oscilloscopes have certain advantages that are hard to overlook. But for troubleshooting, many engineers still prefer analog scopes. Simply because they like the way they handle. But when it comes to troubleshooting, the HP 54600's digital performance leaves analog and hybrid scopes far behind. At millisecond sweep speeds, the display doesn't even flicker. Low-rep-rate signals are easy to see without a hood. The HP 54600 changes that. It looks like a 100 MHz analog scope. All primary functions are controlled directly with dedicated knobs. And itfeels like one. The display responds It has all the advantages that only a true digital scope can provide. Like storage, high accuracY, pretrigger viewing, hard copy output, and programming. And since it's one . of HP's basic instruments the HP 54600 gives you all this performance at a very affordable price. So if you like the feel of analog control, you'll like the way our new digital scope handles troubleshooting. To find out more call the Customer Information Centre on 008 033 821 or Melbourne 272 2555. rJ,'n9 ~/!II HEWLETT PACKARD A Better Way. Just rete·ased: the HP 54602A scope with bandwidth up to 250MHz JIVTHTMl25/A ! ...-----------------------------<11----------~-v+ 100k 100k Z01~ 15V 1W VR1 5k 4700 + 25VWr BR1 PW04 F2 5A llATTERY 01I 680(1 180k + 16 V+ 15 10k 11 15V 4A 12 -:- IC1 UC3906N .0221 'i 18k _ ___,.___ V+ F1 500mA 10k 13 240VAC A N E 14 10 V+ ~ 560k 1k B IC3c EOc VIEWED FROM BELOW BCE LED3 FLOAT GREEN -:- K 12V 6V 47k .,. -:- A 10k ~K .,. 6/12V SLA BATTERY CHARGER 22k -;- Fig.3: although based on ICl, the circuit also uses IC2a-c to drive the indicator LEDs and IC3 to switch resistors in & out of circuit for voltage range selection. IC2d forms part of the selector circuit for the three lower current ranges. charging the battery, depending on its condition. It's this flexibility that makes this charger unique - it can be left on the battery indefinitely and will look after it under all conditions. Circuit details Now take a look at Fig.3 which shows the full circuit details. Two other !Cs are used in addition to the UC3906 (ICl). These are an LM324N quad op amp (IC2) and a 4066 quad analog switch (IC3) which performs most of the functions of the 26 SILICO N CHTP voltage selection switch in the previous circuit. Power for the circuit comes from the mains and is applied to transformer Tl via switch S1 and a 500mA fuse. The 15VAC secondary of the transformer then feeds bridge rectifier BRl and a 4700µF filter capacitor to give about 21V DC. This DC supply rail is applied to pins 3 and 5 of !Cl and to the emitter of Darlington transistor Ql via a 0.220 5W resistor. The 0.22Q 5W resistor forms part of the current limiting circuitry. Meter Ml monitors the current through this resistor, with VRl used to calibrate the meter for a full scale reading of 3A. Transistor Ql (TIP126) acts as the main pass element of the circuit. It is controlled by the drive current from pin 16 ofICl. Q1 's collector then feeds diode Dl which protects the UC3906 from damage if a battery is connected to the output while no power is applied to the circuit. Diode D2 and the 5A fuse in series with the positive output terminal protect the circuit if a battery is connected the wrong way around. If a battery is wrongly connected, D2 con- Most of the parts, including the large power transformer, are mounted on a single PC board. Take care with the mains wiring & sleeve all exposed terminals with plastic tubing to prevent accidental electric shock. ducts heavily and blows the' 5A inline fuse. Current selection The maximum charge current is set by the 0.22n 5W current sensing resistor and the 250m V reference source at pin 4 of IC1 (see Fig.1). The voltage developed across the 0.22Q sense resistor is compared with the voltage at pin 4 and the current through it is then adjusted accordingly by IC1. There are two modes by which this current sensing and control take place. The first mode applies to the 1.2A, 2A and 3A settings and in these cases, switch S2 taps off the voltage developed across the 0.22Q resistor, via a voltage divider consisting of the 8.2kQ, 4.7kQ and 10kQ resistors. With the 1.2A setting for example, S2 taps off the full voltage developed across the 0.22n resistor; ie, 250mV/0.22ll For the 2A range, switch S2 selects the voltage at the junction of the 10kQ and 4. 7kQ resistors. Since the voltage between pins 4 and 5 is limited to 250mV, the voltage developed across the 0.22n resistor will be higher, at around 443mV; ie, (250mV/(8.2kQ + 4.7kQ)) x (8.2kQ + 4.7kQ + 10kQ). Similarly, when the 3A range is selected, the voltage developed across the 0.22n resistor is around 698mV. Since the maximum current provided by the charger passes through the sense resistor, this resistor cannot be too large otherwise__ there will be too much voltage loss and too much power dissipated. On the other hand, the resistor cannot be too small, otherwise the voltage developed across it for the low current ranges will not be enough. That was the dilemma we faced in the original design and so the sense resistor was much larger. In this new design, we have tricked the circuit into "seeing" a much larger sense resistor than is really there, when the ranges below 1.2A are selected. This is accomplished by op amp IC2d and its associated resistor string. This op amp acts like a current sink and it reduces the voltage which must be developed across the 0.22Q resistor for a given charging current. For example, when the 500mA current range is selected, only 110mV will be developed across the 0.220 sense resistor. However, pin 4 of IC1 is fed with 250mV because IC2d and the tapped resistor string provide the remaining 140mV. A similar process occurs for the 250mA and 900mA ranges. So by using IC2d, we've been able to do away with three of the 5W resistors from the original design. Voltage selection The charging voltage is selected by switching parallel resistors in or out AUGUST 1992 27 ----- -------- EARTH TO ~NEL 0 0 ~;wr,a&1nw::ca .an. ~; O! ~ •il ' ~ VR1 l::I POWER TRANSFORMER BATTERY Fig.4: use heavy-duty cable when wiring up the output terminals & note that D1 the 0.22n 5W resistor are mounted proud of the board to allow the air to circulate beneath them for cooling. Qt is bolted to the rear panel (see Fig.6). & of circuit to obtain the correct bias levels for pins 12 and 13 of ICl. Switching of the resistors is accomplished by 4066 analog switches, in IC3. 28 SILICON CHIP For a 12V battery, the unit is set to change state at the following battery voltages: trickle voltage= 10.5V; overcharge voltage= 14.6V; and float voltage= 13.8V. For a 6V battery, the relevant voltages are: trickle voltage = 5 .1 V; overcharge voltage = 7.2V; and float voltage = 6.5V. To select the 6V range, IC3a and IC3c are closed and IC3b and IC3d are opened. When the 12V range is selected, the analog switches are re- RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No. 1 4 1 2 6 1 5 2 Value 560kO 360kO 220kO 180kO 110kO 100kO 91kO 82kO 47kO 22kO 18kO 10kO 8.2kO 6.8kO 4.7kO 3.9kO 1kO 6800 3900 · 3300 0.22O5W versed. Transistor QZ provides an outof-phase signal so that the switches can be made to work alternately from a single pole switch (S3). The circuit works like this. When S3 is closed to select the 6V range, pins 6 & 13 are pulled high and IC3a & IC3c turn on. IC3a connects a 1 l0kO resistor in parallel with the' 180kO resistor on pin 12 of ICl, while IC3c connects a 360kO resistor in parallel with the 560kO resistor between pins 10 & 13. At the same time, transistor QZ turns on and pulls pins 5 & 12 of IC3 low, thus turning IC3b & IC3d off. Conversely, when S3 is opened, pins 6 & 13 ofIC3 are at 0V and so switches IC3a & IC3c are turned off. QZ also turns off which means that switches IC3b & IC3d are now turned on. IC3b connects a 91kO resistor in parallel with the 18kO resistor between pins 12 & 13 ofICl, while IC3d connects a 220kQ resistor in parallel with the 47kO resistor on pin 13. Op amps IC2a, IC2b & IC2c are used to drive the indicator LEDs. IC2a is driven by pin 1, the current sense output. When pin 1 goes low, the output of ICZa (pin 14) goes high and turns on LED 1 to indicate that the 4-Band Code (1%) 5-Band Code (1%) green blue yellow brown orange blue yellow brown red red yellow brown brown grey yellow brown brown brown yellow brown brown black yellow brown white brown orange brown grey red orange brown yellow violet orange brown red red orange brown brown grey orange brown brown black orange brown grey red red brown blue grey red brown yellow violet red brown orange white red brown brown black red brown blue grey brown brown orange white brown brown orange orange brown brown not applicable green blue black orange brown orange blue black orange brown red red black orange brown brown grey black orange brown brown brown black orange brown brown black black orange brown white brown black red brown grey red black red brown yellow violet black red brown red red black red brown brown grey black red brown brown black black red brown grey red black brown brown blue grey black brown brown yellow violet black brown brown or... ;ige white black brown brown brown black black brown brown blue grey black black brown orange white black black brown orange orange black black brown not applicable charger is delivering full charge. ICZb drives TRICKLE charge indicator LED 2. When ICl is in trickle charge mode, current is supplied from pin 11 to the battery via a 6800 resistor. While this is happening, the voltage on pin 11, and hence on pin 5 of IC2b, is above the reference voltage on pin 6 and thus pin 7 switches high and lights LED 2 to indicate that the charger is in TRICKLE mode. ICZc is driven from pin 10 (the state level control) ofICl. Pin 10 goes high at the end of the main charging period and turns on FLOAT indicator LED 3 via ICZc. Trickle current The trickle current is set by the 6800 resistor between the output and pin 11 to approximately 30mA, although this figure is not critical. In any event, pin 11 cannot supply any more than 40mA maximum. Practical example Let's consider what happens when a 12V battery that has discharged to 8V is connected to the charger. Initially, the circuit senses that the battery voltage is below 10.5V and this switches pin 11 to the supply voltage so that the battery trickle charges at about 30mA. At the same time, IC2b switches its output high and lights the TRICKLE indicator LED (LED 2). Once the 10.5V threshold is reached, the enable comparator in ICl is turned off and the internal blocking diode keeps the voltage at pin 11 of ICl at the battery voltage. This pulls the non-inverting input ofICZb below its inverting input and so LED 2 now turns off. Series pass transistor Ql now turns on and feeds the selected charge current to the battery. This causes the current sense amplifier to turn on, which pulls pin 1 low. This low is coupled into pin 8 of ICl and also into the inverting input of IC2a. The output of IC2a now goes high arrd turns LED 1 on. The battery continues charging until the voltage nears 14.6V. ICl then begins to throttle back the charge current until it drops to about 120mA, this being set by ICl's internal 25mV source and the 0.220 resistor. When the Voc overcharge voltage (14.6V) is reached, the current sense AUGUST 1992 29 comparator output goes low, thus switching off its transistor and pulling the overcharge terminal (pin 8) high. This also pulls the non-inverting input ofIC2a high and turns LED 1 off again. If you look at Fig.2, you will see that the state level output goes high at this point as well. In the circuit, this results in pin 10 ofICl going from 0V to about 2.ZV, as set by its associated resistors. This output is fed directly into pin 10 ofIC2c which switches its output (pin 8) high and thus turns on .:;:=-~~i1 0 LED 3. ~ ICl also now turns off pass transistor Ql, allowing the battery voltage to drop naturally from 14.6V to 13.BV. Once this level is reached, Ql is allowed to pass about 90mA of current to keep the battery at this voltage indefinitely. The .022µF capacitor between pin 12 and ground removes any tendency for the circuit to oscillate slowly between charge states. If an external load drawing greater than 90mA (approx.) is applied to the battery at this stage, the battery voltage drops until it reaches 95% of 13.BV, or 13.1 V. The voltage amplifier and current limit comparator now set the driver circuitry to deliver the maximum selected current to the load. When the load is subsequently removed, the charger automatically selects the correct charge state according to the battery condition. w w :::r: Construction A Most of the components, including the power transformer, fit on a PC board which is coded SC14109921 (225 x 124mm). Before mounting any of the parts, carefully check the board for shorts or breaks in the tracks. If you find any, use a dash of solder or a sharp artwork knife as appropriate to fix the problem. Fig.4 shows the PC board assembly. Install the wire links first, followed by the resistors, diodes and zener diodes. Note that the resistors should all be 1 % types. Check each resistor with your multimeter before installing it on the board and don't confuse the two zener diodes. Diode Dl and the 5W resistor should be mounted about 5mm above the board to allow the air to circulate for cooling. The capacitors, trimpot VRl and the fuse clips can now be installed. u <[ 30 SILICON CHIP a P O SC14109921 Cl! w _J a.. Cl! w :E 0 Cl! □ u... (I) z: <[ Cl! t- Q SILICON CHIP 1992 Fig.5: this is the full-size etching pattern for the PC board. Be sure to orient the fuse clips correctly but don't snap the fuse in just yet. A 16-pin IC socket can be used for the UC3906 IC (optional), while the other two ICs can be soldered directly We Only Skimped OnThe Price. Introducing The Fluke Series 10. Fluke quality: Made in the USA by Fluke , with the same rugged reliability that's made us the world leader in digital multi meters. 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For a free product brochure , contact your local Fluke distributor today. Autoranging with manual option: Your choice, depending on your situation. Sleep Mode: Shuts itself off if you forgei, extending long battery life even further. ~ New! Slide switch~ few pushbuttons co:~o1 all functions: Designed for true one-hand operation. Capacitance: Autoranging from .001 µF to 9999 µF. No need to carry a dedicated capacitance meter. Fluke 10 4000 count digital display 1.5% basic de volts accuracy 2. 9% basic ac volts accu racy 1.5% basic ohms accu racy Fast contin uity beeper Diode Te st Sleep Mode Two -year wa rranty . ~ .Fast, accurate tests· and measurements: AC and DC voltage measurements to 600 volts, oh ms to 40 MQ; audible continuity test; and diode test. 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F L UKE AND PH I L I PS T H E T &M A L L I ANCE For further information contact: Philips Scientific & Industrial. Tel: (02) 888 0416 FLUKE ® Light-duty hook-up wire can be used to connect the front panel switches, LEDs and the meter but be careful not to transpose any of the connections. Note that the PC board was modified slightly after the photographs were taken. to the board. Note that the ICs all face in the same direction. The two transistors can also be installed at this stage. Mount Ql at full lead length so that it can later be bolted to the rear panel for heatsinking (see Fig.6) . Case assembly The next step is to mount all the front panel hardware. If the front panel has not been supplied pre-drilled, it will require holes for the switches, LED bezels, output terminals and the meter. It's best to use the front panel label as a drilling template for these holes. Carefully attach the label to the panel, then drill pilot holes at the points indicated and ream them to size. The meter is supplied with a drilling template for the large cutout required. This cutout can be made by drilling a series of small holes around the inside perimeter of the marked circle and then knocking out the hole and filing it to a smooth finish. The meter requires a new scale to be attached and this should be supplied with the kit. To install the new scale, first unclip the front plastic cover and remove the two meter scale screws. This done , remove the old scale by sliding it under the meter 32 SILICON CHIP pointer, then attach the new scale and refit the cover. The metal rear panel can now be drilled to accept the bridge rectifier mounting screw, fuse Fl, the earth lug mounting screw, switch S1 and the mains cordgrip grommet. The exact location of these components is not critical and can be gleaned from Fig.4. A mounting hole is also necessary for transistor Ql and this should be marked out by temporarily install- INSULATING MICA WASHER . ,~JI SCREW r llllllll(3 -----CASE ' T0220 DEVICE Fig.6: mounting details for the TIP126 Darlington transistor (Q1}. Check that the rear panel mounting area is smooth & smear all mating surfaces with heatsink compound before bolting the assembly together. ing the PC board and the rear panel in the case. A square cutout is required for switch S1 and, as for the meter, this can be made by drilling a series of small holes and then knocking out the centre piece. File the cutout to a smooth finish but be careful not to make it too big. Final wiring Once all the holes have been drilled, mount the hardware items on the front and rear panels and bolt the transformer and mains terminal block to the PC board. The 12-position rotary switch is converted to a 6-position switch by removR1g the nut from the threaded boss and changing the position of the locking ring (located at the front of the switch). Fig.4 shows the chassis wiring details. You can use light duty hook-up wire for most of the connections but note that heavy duty cable must be used between the PC board and the output terminals. The wiring to the rotary switch and_LEDs can be run using rainbow cable (note: LED 2 is oriented in the opposite direction to the other two LEDs). The mains cord enters through the rear panel and is clamped using the cordgrip grommet. Terminate the Active (brown) and Neutral (blue) leads to the mains terminal block as shown, and solder the Earth (green/ yellow) to one of the earth solder lugs on the rear panel. The transformer metalwork is earthed by a lead that runs from the rear panel to a large solder lug that's secured by one of the transformer mounting nuts. Use mains-rated cable for the connections to the power switch (S1) and to the fusefolder (Fl). These connections should be sheathed in heatshrink tubing to guard against accidental contact with the mains. Don't connect the transformer secondary to the bridge rectifier just yet - we 'll come to that shortly. At this stage, the PC board should be secured to the matching standoffs on the bottom of the case and transistor Ql bolted to the rear panel. Fig.6 shows how Ql is insulated from the rear panel using a mica washer and insulating bush. Check that the mounting area is smooth and smear all mating surfaces with heatsink compound before bolting the assembly together. Finally, use your multimeter to confirm that there is no connection between the metal panel and the transistor tab. The transformer secondary voltage should now be checked to ensure that it is correct before it is connected to the bridge rectifier. Wire in the C-F connection on the transformer as shown in Fig.4, then install a 500mA fuse in the mains fusefolder and switch on. You should get a reading of 15-17VAC across the transformer secondary (ie, between points B & D). If not, switch off and check the transformer wiring. As a final check, measure the voltage across the capacitor when the FLOAT LED is alight. You should get a reading of 13.8V (approx.) when the 12V range is selected and 6.9V (approx.) when the 6V range is selected. Note that these figures may vary slightly due to component tolerances. Switching on Meter calibration Assuming that everything is OK, connect the transformer secondary to the bridge, set VRl to mid-range and re-apply power. The FLOAT LED should immediately come on and you should be able to measure about 24V DC across the 4700µF capacitor (ie, between the + and - terminals of the bridge rectifier). If either of these things do not occur, switch off immediately and check for wiring errors. If all is well, set the charger to 12V and 250mA, and connect a 220 resistor across the output terminals. This should cause the TRICKLE LED to light. The TRICKLE LED should go out and the FLOAT LED should come back on again when the resistor is disconnected. Finally, VRl should be adjusted to accurately calibrate the meter. To do this, connect a battery to the charger (preferably lOAh or 15Ah) and connect your multimeter in series with one of the leads to monitor the current. Set the current range, apply power and adjust VRl so that the reading O.Jl the charger's meter matches that on the multimeter when the MAIN LED is lit. If the battery is already fully charged (ie, the FLOAT LED comes on), connect a load to discharge the battery until the MAIN LED comes on. The charger will then supply the maximum selected current to the battery, thus allowing you to accurately adjust VRl. SC 0 .L CLASS-2.5 You can now afford a satellite TV system MU·45 For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the following K-band system from only: Fig. 7: this artwork is used to replace the existing meter scale. The old scale is removed by unclipping the front plastic cover & undoing two screws. $995.00 Here's what you get: *A 1.6-metre prime focus dish antenna, complete with all the mounting hardware. Now set the charger to 6V and connect a 100 resistor across the output terminals. As before, the TRICKLE LED should light and then go out again when the resistor is disconnected. You can now simulate a battery by connecting a 4700µF (or larger) electrolytic capacitor across the output terminals, together with a parallel 2. 2k0 bleed resistor. When the charger is turned on (250mA current range selected), the TRICKLE LED should light for a few seconds, after which the unit should rapidly cycle first to the MAIN LED and finally to the FLOAT *or better). One super low-noise LNB (1.4dB *magnetic One Ku-band feedhorn and a signal polariser. * 30 metres of low-loss coaxial cable with a single pair control line. * lnfrared remote control satellite receiver with selectable IF & audio bandwidth, polarity & digital readout. Your receiver is pre-programmed to the popular AUSSAT transponders via the internal EEPROM memory. This unit is also suitable for Cband applications. LED. Call, fax or write to: AV-COMM PTY LTD PO BOX 386, NORTHBRIDGE NSW 2063. Phone (02) 949 7417 Fax (02) 949 7095 All items are available separately. Ask about our C-band LNBs, NTSCto-PAL converters, video time date generators & Pay TV hardware. I I II -----------· Name. ...................................... ........ I I II I Address ...... .. .... .. .... ..... ........... ..... .... I I ........................... P/code ................ II Phone ............ ...... .. .......................... I I I II I I ACN 002174 478 01/92 ~-----------~ AUGUST 1992 33