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FAST CHARGER NICAD BATTER! This Nicad Fast Charger is designed to operate from a 12V car battery. It can charge nicad battery packs from 6-12V at currents up to 6A, or you can wire the circuit to charge battery packs up to 30V at reduced current. and that ultimately leads to cell damage and loss of performance. According to the battery manufacturers, the correct way to recharge a nicad battery pack is to first discharge each cell in the pack to its end point (about 1. 1V) and then recharge it at the 10-hour rate for 14 hours. Clear as mud? OK, let's say that we have a battery pack rated at 1300mAh. This should ideally be recharged at a 130mA rate for 14 hours. So why fast charge nicad batteries? Well, there are many situations where you may want to recharge a nicad pack quickly, particularly if you are involved with radio controlled models. For these applications, the bat- Nicad battery packs are expensive but many people risk damaging them by using primitive fast charging techniques. At its crudest, a fast charger consists simply of a pair ofresistive leads connected to a car battery. The idea behind the resistive leads is to limit the charging current into the battery but that doesn't stop overcharging if you don't constantly monitor the battery and disconnect the leads at the correct time. The same goes for many commercial fast chargers on the market. Often, there is no automatic shut-off feature or the shut-off feature is unreliable. Instead, the charger just carries on pumping current into the cells FUSE +12V PUSH-PULL OUTPUT VOLTAGE CONVERTER STOP FIXED VOLTAGE DROP NICAO BATTERY FEEDBACK CURRENT SENSING RESISTOR 8-BITUPONLY ANALOG TO DIGITAL CONVERTER Fig.1: basic configuration of the Nicad Fast Charger. Power for the circuit comes from a 12V battery & this drives a DC-DC converter which charges the battery pack. The analog-to-digital converter, together with comparator IC8b, functions as a peak detector & shuts down the converter at the end of the charging period. 32 SILICON CHIP tery packs can handle considerably higher charge (and discharge) currents than ordinary nicad cells. Of course, the risks of overcharging and irreversible cell damage are much greater if fast charging is employed. And that's where this project comes in. It detects when the battery is fully charged and automatically switches the unit off to prevent overcharging. Unlike previous designs (eg, our Megafast Nicad Charger in June 1988), this charger uses digital circuitry to detect when the battery is fully charged. This ensures more reliable operation than analog detection techniques - in fact, it's virtually foolproof. In operation, the circuit will recharge a typical nicad racing pack in about 20 minutes. Large battery packs There are several other features which make this charger an attractive unit. First, it can be switched in five 1.2V steps so that it can fast-charge any nicad battery pack from 6-12V at 6A (ie, 6V, 7.2V, 8.4V, 9.6V & 12V). The 6th switch position is labelled "Custom" and that's the position you select if you want to recharge a battery pack to greater than 12V (up to about 30V). It's this ability to recharge large numbers of series-connected cells to voltages greater than 12V that really gives this unit the edge over previous designs. For example, you may be involved with electric flight models and need to recharge a string of 24 cells to 28.8V. To do this however, you have to change the number of turns on the secondary of a transformer during construction from the value normally recommended. In this case, the lower ranges will still work The Nicad Fast Charger is built into a standard plastic instrument case. It can charge nicad battery packs from 6-12V at 6A or you can wire the circuit to charge a custom battery pack (eg, 24 cells in series) at reduced current. but the current capability will be greatly reduced. We'll say more about this in Pt.2 next month. The Nicad Fast Charger is also more efficient than other fast charger designs we have seen. That translates into more charges from the car battery. It also features reverse polarity protection and there is provision for trickle charging as well as fast charging. In addition, the circuit features a low-voltage cutout feature to prev:ent you from flattening your car's battery - the usual source for most radio control modellers. Note, however, that the circuit does not feature automatic shut-off in the trickle charge mode. That feature is reserved for the fast-charge mode only where it is much easier to detect the fully-charged condition. Charge status Three LEDs on the front panel indicate the operating status of the charger. When the Start button is pressed, the Charging LED comes on and remains on until the batteries are fully charged. At this point, the Charging LED goes out and the End Of Charge LED turns on. ·' The third LED is the Error indicator. It lights if the wrong charging voltage is selected for the battery connected. In practice, the unit is very easy to use. You simply connect the battery, select the appropriate voltage and press the Start switch. After that, it's just a matter of waiting until the End Of Charge indicator comes on. Block diagram Now take a look at the block diagram of Fig.1. This shows the basic configuration of the Nicad Fast Charger. Power for the circuit comes from the 12V car battery. This drives a DCDC converter circuit, the output voltage of which is controlled by the current through a current sensing resis- tor in series with the nicad battery pack. In operation, the output voltage of the converter automatically adjusts to maintain a constant voltage across the resistor and thus a constant current through the nicad battery. For example, if the nicad battery is almost completely flat, then the output voltage of the converter automatically adjusts to maintain the correct current. The advantage of this scheme is that the power dissipation of the circuit is low, with losses occuring only in the push-pull converter driver stages and across the current sensing resistor. The ability of the converter to produce an output greater than 12V also makes it possible to charge nicad battery packs to voltages greater than the 12V produced by the battery. The remaining circuitry is used to monitor the charge on the nicad battery and shut the charger down when the battery is fully charged. The full charge condition is detected by the small drop in battery voltage that JANUARY 1991 33 ;- 1.8 1.7 ~ 1.6 ~=.... g 1.5 1.4 1.3 0 ~ / IC4 ---G CLK MSB OUT BINARY COUNTER IC 5 R LSB RESET INPUT 07 06 05 04 03 02 01 00 / I 8-BIT I DIGITAL I OUTPUT I 81 82 83 84 85 86 87 88 REFERENCE DAC IC6 10 15 20 CHARGE TIME (MINUTES) Fig.2: typical 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. This voltage drop is detected by the AID converter, which then shuts the circuit down to prevent overcharging. occurs as the battery heats up under overcharge conditions - see Fig.2. ·It works like this. First, the battery voltage is dropped by a fixed value and then applied to the inverting input of a comparator (IC8b) and also to an up-only 8-bit analog-to-digital (AID) converter. The output of the AID converter then drives the non-inverting input of the comparator. Thus, IC8b compares the analog input and output voltages of the AID converter. Normally, the output of IC8b is low but if the battery voltage falls, the voltage on the inverting input ofIC8b will fall below the voltage on the non-inverting input (since the AID converter can only count up to a peak). The output of IC8b will thus switch high to shut down the pushpull converter circuit and end battery charging. Fig. 3 shows the basic arrangement of the up-only AID converter. It consists of comparator IC8a, a gated clock (IC4), and a binary counter (IC5) with its 8-bit output connected to the 8-bit input of a digital-to-analog converter DAC (IC6). This produces an analog output which corresponds to the count provided by the binary counter. Thus, when the binary counter has a count of 00000000, the analog output will be at 0V. Conversely, when the counter has a count of 11111111, the analog output will be at its maximum - say 5V. For the remaining 254 counts in between these two extremes, the analog output is incremented by 34 BATTERY SAMPLE I ~ GATED CLOCK COMPARATOR SILICON CHIP Fig.3: block diagram of the up-only AID converter. When the output ofIC8a is high, IC4 clocks binary counter IC5. This counter then drives digital-to-analog converter IC6 and this produces an analog output which corresponds to the count in the binary counter. As soon as the sampled battery voltage drops below the DAC output, IC8a's output goes low & stops the clock, thus freezing the DAC output at its peak. about 20mV for each count. Because of the very small increment between each count, it is necessary to include a reference voltage for the DAC so that the analog output is repeatable and accurate. The analog output from the DAC is fed to the inverting input of comparator IC8a where it is compared with a sample of the battery voltage on the non-inverting input. This comparator controls a clock circuit. When the DAC output is lower than the battery voltage, the comparator output is high and so the clock signal is applied to the binary counter. This in turn increments the binary counter and so the analog output voltage from the DAC rises. When this voltage just exceeds the sampled battery voltage, IC8a's output switches low and stops the clock. Thus, the DAC analog output voltage is held (or frozen) at its peak. Fig.4 shows the waveforms involved in this operation. Note that the steps shown for the DAC output are not to scale, since in reality, they only increment in 20mV steps. When first powered up, the DAC output is at 0V because the binary counter (IC5) is initially reset. Thus, the comparator output is initially high, the clock is enabled and the DAC output steadily increases until it just exceeds the sampled battery voltage. The comparator output then goes low, the clGJck stops and the DAG output remains steady. If the battery voltage later rises again due to charging, the comparator again switches high and restarts the clock to increment the DAC output voltage. However, when the battery voltage subsequently falls (ie, when it is being overcharged), the DAC output remains as it is since the comparator output stays low. Thus, the DAC is a peak hold circuit which registers the peak battery voltage and holds it until the counter is reset. Circuit details Fig.5 shows the final circuit of the Nicad Fast Charger. Despite the apparent complexity, it's really quite straightforward. In fact, you should be able to identify most of the circuit COMPARATOR OUTPUT CLOCK OUTPUT ANALOG VOLTAGE (DOTTED) _____ ~---_,,.,. ✓--, ' ..... __ _ Fi'g.4: these waveforms show how the DAC output increases until it just exceeds the sampled battery voltage during charging. Notice how the comparator output again switches high & enables the clock when the battery voltage later rises due to charging. The DAC output then rises until it again exceeds this new level. functions by comparing it with Fig.1 and Fig.3. The push-pull converter circuit is right at the top of Fig.5 and includes ICl (TL494), Mosfet transistors Q1Q4, transformer Tl, and rectifier diodes D4 and D5. ICl is really at the heart of the converter circuit. This is a dedicated switchmode IC from Texas Instruments and it includes all the necessary circuitry for generating complementary square wave pulses at its pin 9 & 10 outputs. These outputs are pulse width modulated by internal error amplifiers to regulate the output voltage of the converter. The inputs to the internal error amplifiers are at pins 1 & 2 (+Cl & -Cl) for one amplifier, and at pins 16 & 15 (+CZ & -C2) for the other. In addition, the IC contains an internal sawtooth oscillator, a 5V reference (pin 14) and a "dead time" control comparator. The latter is included to prevent the push-pull outputs at pins 9 & 10 from rising and falling at exactly the same time. The dead time input is at pin 4 and is tied to the 5V reference at pin 14 via a 4. 7µF capacitor. When the dead time input is at the reference voltage (5V), the output transistors are off and as this voltage drops to ground, the dead time decreases to a minimum. In this circuit, the dead time control is used to provide a soft start. When power is first applied, the 4.7µF capacitor pulls the dead time input (pin 4) to +5V and thus prevents the output transistors inside ICl from switching on. The 4. 7µF capacitor then charges via the 4 7kQ resistor on pin 4 and as it does so, the duty cycle of the output transistors gradually increases until full control is gained by the error amplifiers. Error amplifiers Now let's look at the role of the two error amplifiers. The first error amplifier, with inputs at pins 1 & 2, is primarily used to shut down the converter when the nicad battery is fully charged. Its noninverting input (pin 1) is connected to the +5V reference while the inverting input (pin 2) is connected to a control line which, in turn, is controlled by comparator IC8lr. When this control line is above +5V, the converter functions normally. However, when the line drops below PARTS LIST 1 Plastic instrument case, 259 x 190 x 82mm 1 PC board, code SC14101911, 167 x 222mm 1 Dynamark front panel label, 249 x 75mm 1 metal rear panel, 249 x 76 x 1.5mm 1 finned heatsink, 110 x 74 x 33mm 1 Neosid 17/742/10 iron powder ring core 1 Siemens EC-41 N27 ferrite transformer kit 1 panel mount 3AG fuse holder 1 10A 3AG fuse 2 cordgrip grommets 1 SPOT miniature momentary pushbutton switch (S1) 1 2-pole 6-way rotary switch (S2) 1 DPDT miniature toggle switch (S3) 4 5mm red LEDs (LED1-LED4) 3 5mm LED bezels 5 T0-220 insulating mounting kits 6 10mm-long screws and nuts to suit insulating bushes 26 PC stakes 2 battery clamps to suit 12V battery 1 polarised socket (to suit nicad battery pack) 1 1-metre length twin of automotive wire (1 0A or greater) 1 3.5-metre length of 1.25mm enamelled copper wire 1 450mm-length of 0.8mm tinned copper wire 1 1.5-metre length of light-duty hookup wire 4 self-tapping screws 4 rubber feet 2 10kQ miniature horizontal trimpots Semiconductors 1 TL494 switchmode IC (IC1) 1 4050 hex buffer (IC2) 1 LM833 dual op amp (IC3) 1 555 timer (IC4) 1 4020 binary counter (IC5) 1 DAC0800 DIA converter (IC6) +5V, the output of the error amplifier switches high and reduces the pulse width to zero, thus effectively shutting the converter down to end battery charging. A secondary function of this control line is to shut down the converter 1 TL072 dual op amp (IC?) 1 LM393 dual comparator (IC8) 4 BUZ71, MTP3055 Mosfets (Q1-Q4) 2 BC337 NPN transistors (Q5,Q6) 1 BYX98 10A stud mount diode (D1) 21N40021A diodes (D2,D3) 2 BYW29, MBR1645 16A Schottky diodes (D4,D5) 1 15V 5W zener diode (ZD1) 2 30V 1W zener diodes (ZD2,ZD3) 1 3.3V 400mW zener diode (ZD4) 2 5.6V 400mW zener diodes (ZD5,ZD8) 1 9V 400mW zener diode (ZD6) 1 zener diode (ZD7 - see text) 1 LM336-2.5V reference diode (REF1) 1 MCR100-6 SCR (SCR1) Capacitors 1 2200µF 50VW PC electrolytic 1 2200µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 5 10µF 16VW PC electrolytic 5 4. 7µF 16VW PC electroly1ic 1 2.2µF 16VW PC electroly1ic 2 1µF 16VW PC electroly1ic 1 0.47µF 16VW PC electrolytic 2 0.1 µF monolithic 1 0.1 µF metallised polyester 3 .022µF metallised polyester 1 .01 µF metallised polyester 1 .001 µF metallised polyester Resistors (0.25W, 5%) 1 2.2MQ 2 4.7kQ 1 470kQ 2 4.7kQ 1% 2 220kQ 5 3.3kQ 5 100kQ 1 1.5kQ 2 47kQ 31kQ 1 33kQ 1 820Q 1 22kQ 1% 1 680Q 0.5W 1 20kQ 1% 2 47Q 1 15kQ 410Q 12 10kQ 1 1Q 1W 1 6.8kQ 2 0.22Q 5W WW Miscellaneous Solder, heatsink compound. if the voltage on the 12V car battery drops below a preset level. This is achieved by connecting the control line to the +12V supply via a voltage divider consisting of a lkQ resistor and an 820Q resistor. Thus, if the battery voltage drops below 11 V, the JANUARY 1991 35 junction of the voltage divider drops below 5V and the error amplifier turns off ICl. The second error amplifier, with inputs at pins 15 & 16, is used to regulate the output voltage of the converter. In operation, the voltage across the current sensing resistor (2 x 0.22Q 5W resistors in parallel) is fed to the non-inverting input at pin 16 and compared with a sample of the +5V reference on the inverting input at pin 15. Thus, if the current through the 0.22Q resistors (and thus through the nicad battery) rises above a preset value, the output of the error amplifier also rises and this reduces the width of the pulses from ICl to bring the current back down again. Conversely, if the current falls below the desired value, the error amplifier output also falls and the pulse width increases. The gain of this error amplifier is set by the 4 70kQ feedback resistor between pins 3 and 15. Trimpots VRl & VR2 are used to set the voltage on the inverting input (pin 15) , with S3a used to select either trimpot wiper to provide fast or trickle charging. The complementary PWM outputs from ICl appear at pins 9 & 10 (El & E2) and are switched at a rate of about 20kHz. This frequency is determined by the 33kQ resistor and .00lµF ea~ pacitor on pins 5 & 6, which set the frequency of the internal oscillator to about 40kHz. Actually, El & E2 are the uncommitted emitter outputs of complementary switching transistors inside ICl. These emitters drive paralleled buffer stages IC2a, IC2b & IC2c on one phase of the output waveform and IC2d, IC2e & IC2f on the other. Their outputs then drive Mosfet transistors Ql , Q2 , Q3 & Q4 which in turn switch the 1;:; ".i"' SILICON CHIP . > ~;;: ;!: f-i•· ,-.;;: "' > I c::,C> "'"' N ... "'C'? ~ ;;: .,;1; "'+ ~ + ci~ I• ~ 1(fj C, C, o.i ,------1 --- - - -~+~ --.;1 .. "' ~~ ICw ""'-' '-' :c I• I• ~ I• "'"' H•· ~ =... t; C, ~ ~ C, C, N '-' IC -:;j + c = /;:; !iil .... "' . "'- 8 - 36 ~ C, C ;:; "' N \!! I -= ... ~ c N '-' =-'-' I· =!~ > Fig.5 (right): the final circuit uses ICl which is a dedicated sw itchmode device to drive Mosfet transistors Q1Q4 in push-pull mode. These then drive step-up transformer Tl, the output of which is rectified to produce a DC voltage to charge the battery. IC8a, IC4, IC5, IC6 & IC7 form the AID converter shown in Fig.3. As soon as the sampled battery voltage falls by 80m V from its peak, the output of IC8b switches high & turns on SCR1 to shut down the converter circuit (ICl, Q1-Q4 & Tl). ~ > u. w IC > ;!"' ;:; - '-' I + C, !;; \:? + f-1•· .... ., ~ >"f ~ ,-.;;: ~t; ..... ... ~ ~~ :cZ ~~~i~ ::, E '-' I· ~ 1;:; ► w::, IC>- '+ I="~!,; o-1•· i\; C, "'"' ~t;jN.:.: ICc, U ...cccn>- 'I t,.;I ..... co co ..... -<: ::0 §: >' :z: Jt 16VW+ 1 10k 22k -1%, .,. 10 Jt 16VWJ ~i~C8f • UNDER <+5V CHARGE OFF >+5V NORMAL CHARGE .022 1% 20k +9.3V .>' +5V 220k 10k +9.5V 8200.,. # +9.55V REF +12V .,. t t ~, 1 \)I ; t 1 .i.: 16VW.:r: 08 1N4148 .,. 10k 1.5k S3b 10k r--------+12V I 0.1! .,. +5V REF +12V VOLTAGE DROP DETECT ., .,. 0 LM336 KGA - +A VIEWED FROM BELOW 0 C103 ~K OAC OUT 0 ,; EOC 8 4.7 .J.: 16VWJ .022! 100k 100k +5V 4.7k 1% CLOCK IC4 555 IC5 4020 4.7k D12 011 VLC 1 r .,. .01I COMP $ V· m 5 ~ 1 1a 19 110 111 1~ 81 82 83 84 85 86 87 88 - VREF+ IC6 glO OAC0800 2IIO lLRfF• 15 1- file I 13 10 .i,,: 16VW! +12V -, vwI 10 ~ 15 f .- +5V 101 +1iv 16VW"I"!. COUNTER 011010 09 08 07 06 05 04 5 14 12 13 6 4 5 7 10•cK ~6 NiCad FAST CHARGER 4.7k +5V +9.5V 4.7k 1% 010 1N4148 10 _.:16VWJ ,----+---------------------•12V ERROR CLOCK STOP 6.8k 10k 06 BC337 ~ .~ AGK 0 MCR100 15k 4.7. 16VW+ BATTERY_V_OLTAGE I 11,:11,,,;:;, \ \ 1>,ri,?11/iI ' ... . _, .,, '1::;;, '""~ TY N E DRIVES!! 5 'I•" 360K DRIVE • 500K unl ormatted • IBM ' compa1,b1e xr Cat.Cl 1901 $175 51/4" 1.2 M/BYTE COPAL DRIVE . "'(,,, 20 MEG HARD DISK WITHOUT CONTROLLER .............. $399 20 MEG HARD DISK WITH CONTROLLER ............. ,499 tUl&I #ri: t·i ;I •l•l l#J: 28ms ACCESS WITH IDE/FDD CONTROLLER ............ $495 80 MEG HARD DISK 28 ms ACCESS WITH IDE/FDD CONTROLLER .......... $895 • 1.6 M/Byte unformatted • IBM• AT• conpa!ible OKI LASER PRINTER Introducing the new gen~ration !n page printers, the OKILASER 400. The affordable LED page printer designed for the small business. Rel!able and compact, the OKILASER 400 flts neatly Into the smallest of offices. 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X11105 ...................................... $249 Power supplle to suit X11102 ......................................$225 IC8b, even if the nicad battery pack is completely short circuit. The output of the fixed voltage drop selected by S2 represents the battery voltage and is applied to pin 3 of comparator stage IC8a. Its pin 1 output controls the reset input (pin 4) of 555 timer IC4 which is wired as an astable oscillator with a frequency of about 100Hz. This frequency is set by the two associated lO0kQ resistors and the .022µF capacitor on pins 6 & 2. The finned heatsink is fastened to the rear panel via the mounting screws used to secure the four Mosfet transistors. Its job is to substantially increase heat dissipation & thus keep the Mosfet transistors cool. primary of transformer Tl. Thus, when pin 10 of IC1 goes high, Ql & Q2 turn on and switch the Sl terminal of the transformer primary to ground. When pin 10 goes low again, these transistors switch off and Q3 & Q4 switch on and drive the other half of the primary. The lOQ resistors in the gates of the Mosfets are there to ensure that the paralleled Mosfet pairs share the current equally, while DZ, ZD2, D3 & ZD3 protect the Mosfets by suppressing spikes generated by the transformer. In summary then, the power Mosfets in each phase of the circuit alternately switch the Sl & F2 terminals of a centre-tapped transformer primary to ground, so that the transformer is driven in push-pull mode. The resultant AC waveform is then stepped up by the secondary of the transformer and rectified using Schottky diodes D4 & D5 . Inductors 11 & 12 and the associated 2200µF capacitor filter the diode outputs and the resultant DC output is then applied to the nicad battery. This battery (which may consist of many individual cells) is connected across the converter output in series with the parallel 0.22Q current sensing resistors. Thus, the voltage developed across the 0.22Q resistors depends on the current through the nicad battery. This voltage is filtered using a 10kQ resis40 SILICON CHIP tor and 0. lµF capacitor and fed back to one of the error amplifiers to regulate the converter output, as described previously. In addition to the nicad battery pack, the converter output also drives the Charge LED (LED 1) via S2, a fixed voltage drop , a 680Q resistor and transistor Q5. During charging, SCR1 is off and Q5 is turned on via the lkQ resistor connected between Sl and the anode of SCRl. Thus, when Q5 is on, LED 1 lights to indicate that charging is in progress. Fixed voltage drop Switch S2 selects the fixed voltage drop and this in turn sets the charging voltage to suit the nicad battery pack. In position 1, the voltage drop is provided solely by the lO0kQ resistor on the anode of D6. This is the 6V range for charging a 6V battery pack or five 1.2V cells in series. The remaining switch positions select LED4-ZD7 for the 7.2V, 8.4V, 9.6V, 12V and custom ranges respectively. These selected voltage drops are there simply to reduce the voltage applied to the inverting input of comparator IC8b, since the DAC tracking circuitry can only operate from a range of 5 to 9.5V. Because of this limited operating range, D6 and its associated components are included to ensure that there is always at least 5.6V on pin 6 of Normally, the output of IC8a is high and so IC4 is enabled and it clocks binary counter IC5 via diode DlO. DlO and the associated lOkQ pullup resistor ensure that this clock signal swings only between +5V and+ 12V to match the supply rails to IC5. In order to operate correctly, the DAC requires a reference voltage to provide a fixed current via the 1 % 4.7kQ resistor to its VREF+ (pin 14) input. The 9.55V reference used is derived from the +5V reference of IC1 via op amp IC3b which has a gain of 1.91. In addition, the +5V reference from IC1 is buffered using unity gain non-inverting stage IC3a to provide a +5V supply rail for IC5 & IC6. IC7a (TL072) converts the output current from pin 4 of IC6 to a voltage output and applies this to the inverting (pin 2) input of IC8a. The 1 % 4.7kQ feedback resistor across IC7a ensures that its output voltage is within the 5V to 9.55V range. As soon as the DAC output exceeds the sampled battery voltage, IC8a switches its pin 1 output low and stops the clock (IC4). This freezes the binary counter and thus also freezes the DAC at its peak count as described previously. However, if the battery voltage subsequently rises again (ie, as charging proceeds), IC8a will again switch its output high and the DAC output (pin 1, IC7a) will again increase to slightly greater than the battery voltage before freezing at this new level. End-of-charge detection In addition to driving ICBa, the output of IC7a is also reduced by 80mV and applied to the pin 5 input of IC8b. This 80mV reduction in level is derived by using IC7a to drive a voltage divider network (3.3kQ & 100kQ) which is connected in parallel with a 2.5V reference diode (REF1). The resulting 80mV-reduced signal is A single large PC board makes it easy to build this project, despite the circuit complexity. There are just two trimpot adjustments to make when the assembly is completed: the fast & trickle charge rates. then derived from the tap of the voltage divider and applied directly to IC8b where it is compared with the sampled battery voltage. As stated earlier, when the nicad battery is fully charged, further charging results in a slight reduction in output voltage (see Figs.2 & 4). So, as soon as the nicad battery voltage falls by 80mV from its peak value, pin 7 of IC8b goes high and turns on SCRl via D9 and a 3.3kQ resistor. This then pulls the control line to pin 2 of ICl to about 3.4V (1V across the SCR + 1.8V across LED 2 + 0.6V across D7) and so the converter shuts down and battery charging ceases. Because it is now forward biased, LED 2 lights to indicate the end of charge condition. At the same time, the SCR pulls the base of Q5 low and this turns off to extinguish charge indicator LED 1. Once triggered, the SCR can only be switched off by reducing the curr!;lnt through it to zero. This task is performed by Reset (Start) switch Sl which also resets binary counter IC5, by pulling its reset pin to +lZV. S3b disables the AID converter circuitry by holding the reset line to IC5 high, when the trickle charge mode is selected. This means that the batteries are left on charge when trickle charge is selected. There is no automatic shutdown for this mode. This is not a problem though because trickle charging is not likely to cause cell damage. Error detector Comparator IC7b provides an error warning if SZ is set to a range that is too low to suit the battery on charge. When this happens, IC7a's output will eventually rise above 9.3V and this triggers IC7b which switches its output high. This triggers SCRl via ZDB & DB to shut down the charger and at the same time lights the error LED (LED 3). Note that ZD8 is necessary because IC7b's output does not go fully .to ground since it is an bp amp rather than a comparator. Without the zener diode, the SCR would be triggered by IC7b as soon as power was applied to the circuit. The output ofIC7b also drives transistor Q6 via ZD8 and a 15kQ resistor. When IC7b's output switches high, Q6 turns on and pulls pin 4 of IC4 low to prevent further clocking of IC5. What about the reverse situation, where the range selected is too high for the battery being charged? In this situation, the battery will never fully charge and LED 1 (Charge) will usually remain off or be very dim. That's all we have room for this month. Next month, we will give the full construction details. SC JANUARY 1991 41