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A charger for sealed lead acid batteries This new charger is designed especially for 6 and 12 volt sealed lead acid batteries and is based on the Unitrode UC3906 charger IC. It is suitable for charging batteries of up to 15 amp-hour capacity and can deliver up to 3 amps. By DARREN YATES Elsewhere in this issue we have produced a comprehensive article on the characteristics on the Unitrode UC3906 battery charger IC. Here we present the IC in a circuit which will cope with 6 or 12V batteries and has five switchable charge rates. Most sealed lead acid (SLA) battery chargers on the market are quite simple affairs and are relatively cheap, but (and this is an important "but") most do not correctly charge a battery at either the correct current or to the correct 20 SILICON CHIP voltage. Not only that, many do not provide an "end-of-charge" condition and continue to belt current into the battery whether it's fully charged or not. This can do considerable damage to sealed lead acid batteries and greatly shorten their service life. Our charger maintains the battery at a constant float voltage once it has been fully recharged. This means it can remain connected to the battery indefinitely and still keep it in peak condition. This charger will charge either a 6 or 12V SLA battery and has charge rates set to suit capacities of 1.2, 2.6, 4.5, 6 and 15Ah. The maximum charge currents for these settings are 250mA, 520mA, 900mA, 1.2 amps and 3 amps, respectively. If you have a battery which does not quite match one of those batteries listed, that does not matter just select the nearest suitable current. For example, if you have a lAh battery, select the 1.2Ah charge rate. If you have a 3Ah battery, select the 2.6Ah rate. The charge currents provided are at the rate of C/5. C is the battery capacity in amp-hours. So the C/5 rate is C divided by 5. Hence, 15 amp-hours divided by 5 gives a charge current of 3 amps. However, many SLA battery manufacturers quote or recommend a maximum charge rate of C/4 (we took a more conservative approach). So if your battery is not among those quoted, you can divide its amp-hour rating by 4 and select the nearest charge rate provided. For example, if you have a 12 amphour battery, you could charge it on the 15Ah setting provided on our charger (ie, maximum charge rate would be 3 amps). The new charger is housed in a relatively large metal case, measuring 306mm wide, 204mm deep and 96mm high. On the front panel, it has two terminal posts for the charging leads to the battery and two rotary selector switches. The first selects either 6 or 12 volt charging while the second selects the rate of charge. There are four LED indicators. The first, on the righthand side of the panel, is a power LED, to indicate that the circuit is on. On the lefthand side there is a group of three LEDs to indicate the float (red), main (green) and trickle (yellow) charge modes. We will explain these modes a little later in this article. On the rear panel of the charger is the mains switch and two fuseholders, one for the primary of the power transformer and one in the positive output lead. The charger is short circuit proof and cannot be damaged by reverse connected batteries (except for blowing the fuse). How it works Now have a look at the circuit diagram of Fig, 1. As already mentioned, the heart of the circuit is the UC3906 (IC2) and this is teamed with an LF347 quad op amp (ICl) and a BD650 Darlington transistor, Ql. What the circuit does is to continuously monitor the battery voltage and then adjust the charge rate to suit. If you have a 12V battery connected and it is flat, say below 10 volts, the charger will only deliver a small trickle charge. This is because "flat" sealed lead acid batteries should not be charged at a high rate - it can cause damage. While the circuit is in trickle mode, the yellow LED is lit. Once the battery voltage rises above 10 volts, the charger then delivers its maximum charge rate, according to the setting of the charge switch, S2. While the charger is delivering a high cur- PARTS LIST 1 PCB, code SC14103901, 190 x 102mm 1 front panel label, 302 x 90mm 1 metal case, 302 x 200 x 90mm 2 knobs 1 2-pole 5-position rotary switch 1 4-pole 2-position rotary switch 1 red 4mm binding post terminal 1 black 4mm binding post terminal 1 240VAC mains switch 1 9mm rubber grommet 1 8mm cable clamp 2 3AG safety fuseholders (Jaycar Cat. SZ-2036) 1 3AG 5A fast-blow fuse 1 3AG 1 A fast-blow fuse 4 1 0mm 3mm-tapped metal spacers 3 solder lugs 1 2-way insulated terminal strip 1 18V 6A transformer (Jaycar Cat. MM-2000 or equivalent) 1 insulating kit to suit T0-220 transistors 1 insulating kit to suit stud mounting diode Resistors ( 1/ 4 W) 1 1MO 1% 1 2 560k0 1 % 4 3 220k0 1 % 1 3 180k0 1 % 1 1 47k0 1% 1 1 18k0 1 % 1 3 10k0 5% 1 2 3.9k0 5% 2 1 2.7k0 1% 2 Semiconductors 1 LF347, TL074 quad op amp (IC1) 1 UC3906 charger IC (IC2) Miscellaneous Screws, nuts, washers, heavyduty hook-up wire, heatsink compound, solder. rent, the green LED will be lit. Once the battery is fully charged, which usually takes four hours or so depending on how discharged it was, the charger will change over to the "float" mode. This maintains the battery at a constant terminal voltage, dependent on temperature. And as you would expect, this is when the red (float) LED is lit. All the charge functions are controlled by the UC3906 while the three LEDs are driven by op amps in the quad package. Power for the circuit comes from a transformer with an 18 volt secondary which feeds a bridge rectifier and 4700µF capacitor to give about 24 volts DC. This supply is fed to pins 3 and 5 of the UC3906 (IC2) and to the emitter of the Darl- 1 BYX98-300 or equivalent 1 OA stud mount power diode (D1) 1 1N5404 3A power diode (D2) 1 BD650 Darlington transistor (01) 1 PA40 rectifier bridge 1 3.3V 400mW or 1 W zener diode (ZD1) 2 5mm red LEDs (LED 1 , LED 4) 1 5mm yellow LED (LED 2) 1 5mm green LED (LED 3) 4 5mm LED bezels Capacitors 1 4 700µF 35VW pigtail electrolytic 3 0.1 J,tF metallised polyester (greencap) 1 .0012µF metallised polyester 2.2k0 5% 1 kO 5% 1 kO 1 % 6800 1 % 4700 5% 4 700 1 % 3900 1% 4. 70 5% 5W 3.30 5% 5W ington transistor, Ql, via a paralleled group of four 5-watt wirewound resistors which have a resultant resistance of 0.970. This composite resistance is monitored by IC2 via switch S2. This part of the circuit looks a little confusing but is quite straightforward really. What happens is that switch S2 is used to "tap off" part of the voltage developed across the composite resistance and feed it to pin 4 of IC2 . IC2 then turns on Ql just hard enough to ensure that the voltage between its pins 4 and 5 does not exceed 250mV. This is how the circuit maintains the selected charge rate. By using S2 to "tap off" the voltage across the composite resistance, there is no need to MARCH 1990 21 The SLA Battery Charger uses a large power transformer so that it can deliver the maximum 3-amp charging current. Take care with the mains wiring and sleeve all exposed terminals with plastic tubing to prevent electric shoclc. switch the charging current. Hence a light duty rotary switch can be used instead of one with heavy duty contacts. Because the UC3906 can only supply a maximum current of 25mA, Ql is specified as a BD650 Darlington transistor which has a minimum gain of 750 at a collector current of 3 amps. This means that the maximum current that the UC3906 will ever have to deliver is a tiny 4 milliamps. Typically, the current from the UC3906 will be much smaller, less than one milliamp. The collector of Ql is connected to the positive output terminal of the charger via a 10-amp stud mounting diode, Dl. This prevents any damage to the circuit which could be caused when a battery is connected to the output while no 22 SILICON CHIP power is applied to the circuit. Diode D2 and the 5A fuse provide protection against batteries connected the wrong way around. Voltage switching S3, a 4-pole 2-position rotary switch, is the voltage selector. All four poles of the switch work by switching shunt resistors in and out of circuit, depending on whether the 6V or 12V mode is selected. Note that all the resistors associated with S3 are 1 % types. This is necessary to ensure that the circuit provides correct voltages across the battery at all times. The two 0. lµF capacitors at pins 8 and 14 on IC2 and the .0012µF capacitor between base and collector of Ql ensure that the circuit is stable and not able to oscillate at supersonic frequencies. LED indication Three op amps in ICl drive three light emitting diodes, as mentioned previously. All three are connected as comparators. ICla is driven from pin 9, the "over-charge indicate" output of IC2. When pin 9 goes low, the output of ICla goes high to turn on LED 3 and indicate that the charger is delivering full charge. IClc is driven from pin 10, the "state level control" of IC2. When pin 10 goes high, the output of IClc goes low to turn on LED 4, the float charge indicator. Fig.1 (right): the circuit is based on the UC3906 charger IC. This monitors the battery voltage and switches between three charge states: trickle, charge & float. The output of ICl (pin 16) drives Darlington transistor Qt to control the charging current. c.) N 0 co ....::r:co ("'J ::c > ~ :::>IC:'.:W ___ - r _ CASE nh, V+ • i ~K BCE ; 0.1 I • • - - 4700 35VWI ...r:=<!A40y J:• ~ N-~-------~ ~-----~ 240VAC A ~ S1 POWER 680!:J 10k I -r I I SAH ¾-fri ..~. 1 390!l t. 1k _4.5AH 4.7!l 5W I 3.3!:J SW I .,. v+ .,. MAIN A CHARGE LE03 GREEN K t- 6 IC2 UC3906 I Ll8 Ll2 D3 I I 2.7k tt'h" I.~·:. 1 ·~ .t4 0.1+ 116 ~ r 13 10 12 ll 15 ---, "00" K_ 01 BYX98300 O~IM UNIVERSAL 6/12V BATTERY CHARGER V+ -!:- POWER~ LE01 REP . . . n.l!·. o 470!:J r;. r . t / '- 6V SJd __ 12V ~A 1'/o 47k ~ 220k 1¾ 1% S3c 012" ~ , 1M ~ 560k 1 1/o I S3b 12V 6V 1~0k~80k 11, 1'/, S3a v+ v+ ' -:, 1k 7 • - I 1Dk1 -t ~ - + LE04 REo - CHARGE v~~ TRICKLE CHARGE BATTERY r+ 1N5404 02 1 220k 1 1/a '8 1.. k1 Vo \ 2•0 f 18k 1% 1'Ak f 18D - RESISTORS D D D D D D D D D D D D D D D D No 1 2 3 3 1 1 3 2 1 1 4 1 1 1 1 1 Value 1MO 560k0 220k0 180k0 47k0 18k0 10k0 3 .9k0 2 .7k0 2.2k0 1 kO 1 kO 6800 47 0 0 4 700 3900 4-Band Code (5%) not applicable not applicable not applicable not applicable not applicable not applicable brown black orange gold orange white red gold not applicable red red red gold brown black red gold not applicable not applicable yellow violet brown gold not applicable not applicable 5-Band Code (1%) brown black black yellow brown green blue black orange brown red red black orange brown brown grey black orange brown yellow violet black red brown brown grey black red brown brown black black red brown orange white black brown brown red violet black brown brown red red black brown brown brown black black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown yellow violet black black brown orange white black black brown Note: where 5% tolerance resistors are called for in the parts list, 1 % types may be used . IC1 b drives " trickle charge" indicator LED 2. When IC2 is in the trickle mode, current is supplied from pin 11 and via the 4700 resistor to the battery. While this is happening, the voltage at pin 11 of IC2, and hence pin 6 of IClb, will be above the reference voltage at pin 5. This causes IClb's output to go low and light LED 2. When the battery voltage rises sufficiently, IC2 switches off its output at pin 11 and turns on the charge output at pin 16, to drive Q l. When this happens, the voltage at pin 11 drops from around + 24V to the battery voltage. This causes pin 6 of IC1 b to drop below its reference input at pin 5 and LED 2 goes out. Some readers may think that pins 9 and 10 could be used to drive LEDs directly, eliminating the need for ICla and IClc. However, these "open collector" outputs cannot sink very much current, typically 5mA or less, and so the op amps are necessary. By the way, the unused op amp in the IC1 package is not shown. On the printed circuit board, its inputs (pins 9 and 10) are tied to the OV line while its output, pin 8, is left with no connection. Putting it together Light duty hook-up wire can be used to wire the front-panel switches since they switch low currents only. Bind the leads together as shown. We used sockets for the ICs but you can solder them directly into circuit. 24 SILICON CHIP All the circuit components except the LEDs, the stud mount diode, switches and power transistor are mounted on a printed circuit board measuring 190 x 102mm (code SC14103901). Both the stud mount diode and the transistor are bolted to the rear wall of the chassis for heatsinking. You will need to drill holes in the chassis to mount the four LEDs, two rotary switches, power switch, two fuses, the transformer, the Darlington transistor, bridge rectifier, stud diode, the printed circuit board and any other hardware. We suggest you drill all these holes before any assembly work takes place. Use the Scotchcal front panel, if available, as a template for drilling the front panel. The GROMMET I (n) POWER TRANSFORMER ~- LED3 MAIN BATTEAY CHARGE g ~ 10 _ ( : ~ 0 ;) "~j ,3 14 Fig.2: Q1 and D1 must be isolated from the chassis (see Figs.3 & 4) while the four 5W resistors and D2 should he mounted proud of the PCB to aid heat dissipation. Use heavy-duty cable to wire the output terminals. printed board can be used as a template for its mounting holes. Before beginning assembly of the board, check it carefully for breaks or shorts in the copper pattern. It's much easier to find and correct any defects at this stage. Although we did not use them for our prototype, we suggest you use PC pins for all the wire connections to the board. They should be installed first. Use the wiring diagram of Fig.2 as a guide during assembly of the printed board and subsequent wiring of the chassis. With the PC pins installed, you can start soldering in all the small components such as the resistors, diodes, links and small capacitors. Elsewhere in this article is a table showing the colour codes for 5 % MARCH 1990 25 The BD650 Darlington transistor is mounted on the rear panel which provides heatsinking. Use heavy-duty cable to wire the emitter and collector leads. and 1 % resistors. While the table will help you select the right resistor, particularly when 1 % types are involved, we do suggest that you check each value with a digital multimeter before it is soldered into place. Note that where the parts list calls for resistors with a 5 % The BYX98300 diode is also mounted on (but isolated from) the rear panel to ensure adequate heatsinking. Fig.3 shows the mounting details for this device. tolerance, you may naturally use 1 % types instead. When soldering in the power resistors, make sure they sit a few millimetres above the board, as they get quite warm when charging at 3 amps. Check the polarity of the 4700µ.F filter capacitor, diode DZ and zener diode ZDl when they are being installed. Insert the two ICs last. Check that they are correctly oriented [both in the same direction) before soldering their pins. When the board is complete, check your work carefully and then set it aside so that work can proceed on the chassis. Wiring the chassis ~ - SOLDER LUG (§)- e~ MICA WASHER INSULATING BUSH C) (9.)(<at>cQ) MICA WASHER PLAIN WASHER -LOCK WASHER ®-NUT Fig.3: mounting details for the BYX98300 diode. After mounting, use your multimeter to check that the body of the diode is correctly isolated from chassis. 26 SILICON CHIP Both the transformer and the bridge rectifer must be bolted to the base of the case. This done, you can run all the wiring to the primary and secondary of the transformer. The mains cord should pass through a grommeted hole in the rear of the chassis and be secured with a cable clamp [or you could use a cord grip grommet). The Active [brown) and Neutral [blue) leads of the mains cord should be stripped and tinned and secured in the insulated terminal block. The earth lead should be terminated at the solder lug adjacent to the transformer. Both the mains power switch and the two fuses should have shrink-on sleeving fitted over their contacts, INSULATING BUSH \ T~ SCREW T0220 DEVICE MICA WASHER ! I NUT { - § HEATSINK (REAR OF CASE) / Fig.4: mounting details for the BD650 Darlington transistor. Use your multimeter to check that its metal tab has been correctly isolated from chassis. after the wires have been soldered. This will help prevent any accidental shorts or electric shocks. We used miniature bezels to mount the four LEDs. To use these, you clip the front section of -the bezel into the panel, then insert the LED and then fit the locking clip onto the back of the bezel. You can then use a length of 6-way ribbon cable to wire the three charge indicator LEDs. When installing the stud mount diode, an insulating kit must be used to isolate the device from the chassis (see Fig.3). This consists of two mica washers, plastic bl!.sh, flat washer, lock washer and nut. A solder lug needs to be fitted over the threaded stud to make the cathode connection. Make sure that the mounting hole is deburred Problems? ... and you don't have our NEW 1990/91 148 pf1ge electronic parts and accessories catalogue ... Its our latest TRADE catalogue for the consumer ARISTA ... Your one-stop problem solver . 'I"'" 0 CJ) C") 0 'I"'" -:t' 'I"'" (.) en Fig.5: here is an actual size artwork for the PC board. before mounting the diode, otherwise the mica washer may be damaged. The Darlington power transistor also needs an insulating kit and this consists of a screw, rectangular metal washer, mica washer, plastic bush, flat washer, lockwasher and nut. Fig.4 shows the mounting details for this device. When mounting both the stud diode and the Darlington transistor, use a little heatsink compound on both sides of the mica washer to im- ...Stylus .. . ... Plugs, Jacks and Sockets ... ...Batteries .. . ...Cable .. . ... Tools and Technical Aids .. . ... Plug and Power Packs .. . ... Car/Auto Accessories .. . ... Boxed Hi Fi Speakers .. . ... Raw Replacement Speakers ... ... Speaker Accessories ... ... Telephones and Intercom .. . ... Public Address Accessories .. . ...Security and Alarm Accessories .. . .. .TV/Video/Antenna Accesories .. . ... Videocam Accessories ... ...Audio Accessories ... ... Headphones ... ...Computer Accessories .. . . .. Microphone Accessories .. . ... Mixers, Amplifiers, Equalizers ... Just about anything you want. .. Get your catalogue complete with "Recommended Retail Prices" free from your local ARISTA dealer or send $2.50 P & H and your return address to: ARIST~ ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW, 2141 MARCH 1990 27 The rear panel carries the on/off switch, two fuses, and mounting hardware for D1 and the Darlington transistor (Ql). prove heat conduction to the chassis. Before making any connections to the stud diode and the Darlington transistor, check that they are isolated from chassis. You can do this by switching your multimeter to a high "Ohms" range and then checking the resistance between the device (diode stud or transistor collector) and chassis. You should get an infinite ohms reading, confirming that the device is isolated from the chassis. Before mounting the front panel hardware, the Scotchcal panel The four 5W resistors and diode D2 are mounted a few millimetres proud of the PC board as they get quite warm when charging at 3A. 28 SILICON CHIP should be fitted, if you have obtained one (from the printed circuit manufacturers listed at the back of this magazine). Before fitting the panel, make sure that all the holes are thoroughly deburred. For the rotary switches, wire the voltage selection switch first. The best way to do this is to measure out the distance from each connection point on the board to the corresponding position on the switch so that all the leads can be laced or tied together neatly. The leads for the current selection switch are much easier to wire up as all but one of them come from adjacent positions on the board. The wiring should then be looped around behind the voltage switch so as to keep them separated. Note how we have used cable ties to keep the wiring tidy. Once the wiring has been completed, check it against the wiring diagram of Fig.2. With this done, you can apply power and check the voltages. The voltage across the 4700µF capacitor should be about 24 volts DC. Set the charger to 12V and connect a 3300 resistor across the output terminals. This should cause the trickle LED to light, indicating that the voltage at the terminals is less than 10.5V. Disconnecting the resistor should cause the trickle LED to go out and the float LED to light. Now set the charger to 6V and connect a 1200 or smaller resistor across the output terminals. Again, the trickle LED should light, indicating that the voltage at the output terminals is less than 5.1 V. Disconnecting the resistor should cause the trickle LED to go out and the float LED to light. You can now simulate a battery across the output terminals by connecting a large electrolytic capacitor across the output terminals, together with a parallel 2.2kn (or thereabouts) bleed resistor. We suggest a capacitor of 4700µF or higher. Now when the charger is turned on, the trickle LED should light for a few seconds and then, all of a sudden, there should be a very fast sequence through the LEDs from the trickle to float. As a final check, measure the voltage across the capacitor when the float LED is alight. When 6V is selected, the voltage should be close to + 6.9V. When 12V is selected, the voltage should be 13.BV. All that remains now is to secure the lid and the charger is ready to recharge all those flat SLA batteries lying around the place. ~