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FAST CHARGER NICAD BATTER! Last month, we introduced our new Fast Nicad Charger & described the circuit. In this issue, we give you all the construction details. By JOHN CLARKE & GREG SWAIN Despite the circuit complexity, this project is very easy to build and there are just two simple adjustments. Most of the parts are mounted on a single large PC board coded SC14101911 (167 x 222mm) and this is housed in a plastic instrument case. A large finned heatsink is bolted to the metal rear panel and this provides heatsinking for the Mosfet switching devices (Ql-Q4) and for the rectifiers (D4 & D5). Begin construction by checking the copper pattern of the PC board for any shorts between tracks or open circuit tracks (compare it with the published pattern). A quick check at this stage can save hours of troubleshooting later on. Fig. 7 shows the parts layout on the PC board. Before installing any of the parts, check that the board mounting holes are large enough to accept the self-tapping mounting screws. Similarly, check the mounting holes for diode D1 and for the transformer (Tl). Begin the PC board assembly by installing PC stakes at all the external wiring points, then install the wire links, diodes and resistors. Be sure to use the correct diode at each location and check that they are all installed the right way around. It 's a good idea to install the zener diodes with a loop in one of the leads (see photo) to relieve any stresses due to thermal expansion. The two 0.22Q 5W resistors should be stood off the board by about 2mm so that air can freely circulate beneath them for cooling. This done, install the ICs , small signal transistors and the stud-mount diode (D1). Be sure to install the star washer for D1 on the copper side of the board, as this "bites" into the copper pattern and ensures a good contact. The electrolytic capacitors can now all be installed, followed by the four Mosfets (Ql-Q4) and the two power diodes (D4 & D5). These last six components should all be mounted at full lead length. Inductors This close-up view shows how power diode D1 is mounted. Be sure to install the star washer on the copper side of the board, as this bites into the copper pattern & ensures good electrical contact. Note also the stress loop in one of the leads from zener diode ZDl. The other zener diodes are installed in similar fashion. 62 SILICON CHIP Inductors Ll and L2 are each wound using 9 turns of 1.25mm enamelled copper wire (ECW) on the same Neosid iron powder toroid. These windings are wound on opposite sides of the toroid and in opposite directions to each other (see Fig.7). Scrape Pt.2: The Construction Details FOR ES The Nicad Fast Charger features a very reliable automatic switch-off circuit when operated in fast-charge mode. This prevents overcharging which could ultimately lead to expensive cell damage. the enamel from the ends of the windings and tin them before installing the unit on the PC board. Winding the transformer The transformer is supplied as a bobbin with two E cores, one for the top and the other for the bottom. These cores are held together with a U-shaped clamp which is installed after the transformer is wound. Now take a look at Fig.6. This shows how the primary and secondary windings are terminated on the transformer bobbin. Note that the 4-pin side of the bobbin terminates the primary leads, while the 5-pin side is for the secondary windings. To wind the primary, you will need 1-metre of1.25mm ECW. First, scrape the enamel from one end and solder it to the S1 pin. Now, starting from the bottom, wind on 8.5 turns in a clockwise direction, with the windings laid side by side as you progress up the transformer bobbin. When you have wound on the 8.5 turns, run the lead directly down the side of the bobbin (ie, at right angles to the winding), trim to size and terminate the end on the Fl pin. Wrap a layer of insulating tape tightly around the winding to secure the turns firmly in place. The other half of the primary wind- S1 F1 S2 FZ S3 PRIMARY S4 F3 SECONDARY T1 WINIHNGS Fig.6: the transformer windings must be terminated exactly as shown here, otherwise the inverter circuit won't work. Step-by-step winding details are given in the text. F4 ing starts at S2 and is wound in the same direction directly over the top of the first. Wind on 8.5 turns as before and terminate this winding cin the F2 pin. Another layer of insulating tape is then used to secure this winding. The two secondary windings are wound in exactly the same manner, except that each winding consists of 17.5 turns. Begin the first winding at the S3 terminal, wind on 17.5 turns of 1.25mm ECW and terminate at the F3 pin. Secure this using another layer of insulating tape then, starting at S4, wind on another 17.5 turns and terminate this winding at F4. Install a final layer of insulating tape to secure this last winding, then fit the top and bottom cores to the bobbin and clamp the transformer assembly to the PC board. Tighten the clamp nuts firmly but don't overtighten them, otherwise you'll crack the ferrite cores. Finally, solder the various transformer pins to the PC pattern. FEBRUARY1991 63 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 0 No. 2 5 2 12 1 2 2 5 3 1 1 2 4 2 Value 4-Band Code (5%) 5-Band Code (1%) 2.2MQ 470kQ 220kQ 100kQ 47kQ 33kQ 22kQ 1% 20kQ 1% 15kQ 10kQ 6.8kQ 4.?kQ 4.?kQ 1% 3.3kQ 1.5kQ 1kQ 820Q 680Q 47Q 10Q 1Q1W 0.22Q 5W red red green gold yellow violet yellow gold red red yellow gold brown black yellow gold yellow violet orange gold orange orange orange gold not appli cable not applicable brown green orange gold brown black orange gold blue grey red gold yellow violet red gold not applicable orange orange red gold brown green red gold brown black red gold grey red brown gold blue grey brown gold yellow violet black gold brown black black gold brown black gold gold not applicable red red black yellow brown yellow violet black orange brown red red black orange brown brown black black orange brown yellow violet black red brown orange orange black red brown red red black red brown red black black red brown brown green black red brown brown black black red brown blue grey black brown brown yel low violet black brown brown yellow violet black brown brown orange orange black brown brown brown green black brown brown brown black black brown brown grey red black black brown blue grey black black brown yellow violet black gold brown brown black black gold brown brown black black silver brown not applicable The PC board now can be fitted into the case. Note that it will be necessary to trim the threaded portion of the stud-mounting diode using a hacksaw so that it doesn't foul the bottom. In addition, some of the internal pillars will have to be shortened using an oversize drill so that they don't foul the board. Installing the Mosfets Once the PC board fits correctly, you can mark out the mounting positions for the Mosfets and power diodes on the heatsink and rear panel. Note that the two outside Mosfets (Ql & Q4) mount on 93mm centres so that their mounting screws fit between the fins of the heatsink. The two inner Mosfets, QZ & Q3, are on 27mm centres so that their mounting screws intercept the flat area between the two sets of heatsink fins. Note: if you use a different heatsink to the one specified, you will have to adjust the positions of the mounting holes accordingly. It's best to mark out and drill the holes in the heatsink first and then use this as a template for marking out 64 SILICON CHIP the Mosfet mounting holes in the rear panel. Position the four holes so that they are 28mm from the bottom of the heatsink and make sure that the heatsink is centred vertically on the rear panel so that it clears the top and bottom panels of the case. The holes for the two power diodes , cordgrip grommets and the fuseholder can also be marked out on the rear panel at this stage. After drilling , carefully deburr all holes using an oversize drill to ensure a smooth mounting surface for the transistors and diodes. The four Mosfet transistors and the two power diodes (D4 & D5) must be insulated from the case using mica washers and insulating bushes. Fig.8 shows the mounting details. Smear heatsink compound on the mating surfaces before screwing each assembly to the rear panel and note that it may be necessary to bend the leads of each device slightly so that it lines up with its mounting hole. As each device is mounted, use your multimeter to confirm that its tab has been correctly isolated from the rear panel. If you do get a short CAPACITOR CODES 0 Value IEC Code EIA Code 0 0 0 0 0.1µF .022µF .01µF .001µF 100n 22n 10n 1n 104 223 103 102 circuit, clear the problem before proceeding further. Final assembly If you don't already have a predrilled front panel, you will have to use the accompanying artwork or a self-adhesive label as a drilling template. If you have a self-adhesive label, it's best to attach this to the front panel before drilling the holes. Drill small pilot holes for the switches initially, then carefully ream them out to the correct size with a tapered reamer. Once this has been done, you can mount the switches and LEDs and complete the wiring as shown in Fig. 7. All wiring to the front panel METAL REAR PANEL ~FUSE '\~1HOLOER TO NICAO BATTERY CORO CLAMP GROMMET - 02 G• O •S • ...___G_•o_; .. I ~ ~ZD3~ 'D3 ~ \L I) - ri·• 0.47uF +,( D ........ ........ 0 Q) 0 0 e3 0 0 •2' 0 0 •,/ FRONT PANEL Fig. 7: check that all polarised components are correctly oriented when mounting the parts on the PC board. Be sure also to use the correct transistor type & zener diode at each location. The pinouts for the Mosfet transistors (Ql-Q4) & the two power diodes (D4 & D5) can be obtained from the main circuit diagram (Fig.5). Note that the metal tabs of these parts must be electrically isolated from the metal rear panel, as shown in Fig.a. The front panel wiring can be run using light-duty hookup wire but use heavy-duty (10A) cable for all rear panel wiring. FEBRUARY1991 65 INSULATING MICA WASHER ~,~jl SCREW r rnllllil(3 -------- CASE 1 T0220 DEVICE Fig.8: mounting details for the Mosfet transistors & fast recovery diodes. Smear all mating surfaces with heatsink compound before bolting each assembly together. The four Mosfet transistors and the two fast recovery diodes (D4 & D5) are all soldered directly to the PC board at full lead length & then bolted to the metal rear panel using TO-220 insulating kits. Check that the mounting surfaces are smooth & free of metal swarf before installing these devices. can be run using light-duty hookup wire but be sure to use heavy-duty (10A) wire for all wiring to the fuseholder, 12V battery and nicad battery. The leads to the 12V battery can be fitted with large automotive battery clips while the output leads should be fitted with a polarised socket to match the plug on the battery pack. Testing Initially, set VR1 and VR2 about 90% anticlockwise, set the Fast/ Trickle switch to FAST, and set the The PC board is secured to integral plastic standoffs moulded into the bottom of the case. Note that it will be necessary to cut off part of the threaded portion of the stud-mounting diode so that the board will fit. 66 SILICON CHIP Battery Select switch to 6V. Now connect a 12V power supply and check for the correct supply voltages on all the ICs. Check also that the pin 1 output of IC3a is at +5V and that the pin 7 output of IC3b is at +9.55V. If you strike problems, switch off immediately and clear the fault before proceeding further. If everything is OK, press the Start switch and check that the Charging LED lights. This LED should then extinguish after a few seconds and the Error and End Of Charge LEDs should come on. Now disconnect the power supply and connect the unit to a 12V car battery. Press the Start switch again and immediately check the output voltage of the charger (ie, check across the output leads to the nicad battery). You should get a measurement of about 24V or higher, since the output terminals are open circuit. Now short the output terminals, press the Start switch once more and check the voltage across the paralleled 0.22n 5W resistors. Adjust VR1 for a reading of 0.66V. This sets the charging current to 6A. Similarly, switch to Trickle charge and adjust VR2 to set the standard charge rate for your nicad cells. For 1300mAh cells, this will be 130mA (ie, C/10). This means that the voltage across the 0.22Q resistors must be set to 14mV (ie, V = IR where I = 130mA and R = 0.11n). At this stage, your Nicad Fast Charger is ready for work but a few final checks are in order. Connect the charger to a discharged battery pack, select fast charge and the appropriate voltage range, and press the Start button to begin charging. Now check that the voltage across the 0.22Q resistors Fig.9: this is the full size artwork for the PC board. It is a good idea to carefully check this pattern against your etched board before mounting any of the parts, as it is easier to spot any errors at this stage. In particular, check for breaks in the copper pattern & for shorts between adjacent tracks. FEBRUARY1991 67 (.!) ~ • 0 z (.!) a: <C J: (.) I- Cl) ::::> (.) > C\I • > • me ,- I- (.) w w U) >• o::t' co >. C\I ...J Cl) • > a: w ~ • CD > I'- > a: w_ U) • a: w (!} a: • <( J: <C J: (.) LL 0 (.) Cl t- z w <( LL I- en • C <( (.) a: ~ CJ) z I w I- CJ) ,....., <C .,, ' LL 1-:u:· 1•••• • , ' llttu•" :::::: ······· ··1····,::· 1111~1 II 68 ~fi~I SILICON CHIP • ~a a: CD I-a:- (.) a: Ww w (.!) ...J zw <((/) w J: (.) (.!) a: 0 ...J :::ii::: (.) a: I- - The output leads should be fitted with a polarised plug to match the battery pack. This will prevent you from connecting the leads the wrong way around. is still 0.66V. Also, check the voltage across the nicad battery pack - it should gradually rise towards its final end point voltage of about 1.8V per cell. While the battery pack is charging, check that the DAC output follows the battery voltage by monitoring pin 1 of IC7. This should rise as the nicad battery voltage rises. Similarly, check that pin 5 of IC8 is 80mV less than pin 1 of IC7. Finally, continue monitoring the nicad battery voltage until it reaches a plateau and begins to fall. The charger should automatically switch off as soon as the battery voltage falls 80mV below its peak value. Custom cell packs The custom selection on the Battery Select switch is provided for special applications; eg, you may want to charge 24 cells in series. As stated earlier, this involves winding more turns on the secondaries of the transformer. In addition, you will have to choose a value for ZD7 (see Fig.5) to suit your particular application. When winding the transformer, you have to provide at least enough turns to give 1.8V per cell. This means that for 24 cells, enough turns must be wound on to give 44V. In this design, each turn on the secondary gives about 1.38V. Thus, a 44V output requires 32 turns, to which we add an extra three turns to provide a reasonable margin. This brings the total to 35 turns on each half of the secondary (ie, an output of about 48V). Note that you will now have to use 0.8mm ECW instead of the 1.25mm wire so that the windings will fit on the bobbin. ZD7 is selected so that the voltage fed to pin 3 of IC8a is less than 8V when the cells are fully charged. For 24 cells , we expect 44V and from this we can subtract the 1.8V drop across LED 1. Thus, 44 - 1.8 - 8 = 34.2V, which means that ZD7 can be a 35V 1W zener diode. Finally, note that the charging current cannot now be adjusted for 6A, as the maximum power that the charger can deliver is limited to 100W. In this case, you will have to settle for a current of about 2A (ie, 100W/48V = 2.08A). SC Fig.10 (left): this artwork can be used as a drilling template for the front panel. You can also use it to make up your own artwork if you don't wish to purchase a commercial panel.