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Build a 40V 3A variable power supply This month, we complete the 3A-40V Adjustable Power Supply by describing the construction, testing & setting up pro­cedures. Most of the parts mount on a large PC board, so the assembly is straightforward. PART 2: By JOHN CLARKE 66  Silicon Chip A large PC board coded 04202941 (222 x 160mm) carries the bulk of the electronic circuitry, including the power transform­ er. This board is mounted on pillars moulded into the base of the case and secured using self-tapping screws. Most of the remaining parts are mounted on the front panel and are connected to the PC board via insulated leads. Board assembly Fig.9 shows the parts layout on the PC board. Begin by checking the board Fig.9 (facing page): install the parts on the PC board as shown on this combined layout & wiring diagram. The leads marked with an asterisk (*) must be run using 32 x 0.2mm wire in order to carry the heavy currents involved. ▲ The S ILICON C HIP 3A-40V Adjustable Power Supply is housed in a standard plastic instrument case measuring 260 x 190 x 80mm. This is fitted with aluminium front and rear panels, the rear panel providing the necessary heatsinking for the switching regulator (IC1). In addition, these aluminium panels are connected to the mains earth to ensure safety and play an important role in shielding the supply circuitry. Do not, under any circumstances, use plastic panels for this project. for etching defects by comparing it with the published pattern. Usually there will be no problems but it’s always best to make sure before mounting any of the parts. If everything is OK, start the assembly by installing PC pins at all external wiring points, then install the resistors and wire links. Table 1 lists the resistor colour codes but it’s best to also check them on your multimeter as some of the colours can be difficult to decipher. Note that the two 680Ω 5W resis­tors should be mounted about 1mm above the board to allow air circulation, while the 4.7kΩ resistor ACTIVE (BROWN) FUSE EARTH (GREEN/YELLOW) METAL REAR PANEL EARTH TERMINALS CORD GRIP GROMMET GREEN/YELLOW GREEN/YELLOW 1 IC1 D1-D4 100uF 1000uF 4700uF 680  5W D5 NEUTRAL (BLUE) 4700uF 22 1000uF 1.5k 21 L1 PRI 15k 100  VR3 2.2k 680  330pF 15V 0V VR4 1k 10k 100k 47k 91k  15 16 17 1k 2.2k 10k IC5 4053 14 10k 1k 1 D6 1k 47k 220  22k 0.1 0.1 100  IC3 LM339 POWER TRANSFORMER 1k 1M 6.8k 1 4.7k IC2 OP77 15V 0V 100uF IC4 OP77 ZD2 REF1 1 10uF 0.1 10uF 100uF ZD1 1  IC6 7660  L2 R1 0.1 680  5W 18 19 20 100k 10uF 1 13 12 11 10 9 8 7 6 5 4 3 2 0.1 250VAC 0.33 GND  SEE TEXT GREEN/YELLOW S1 10 13 12  17    S4 S2  9 8 7 6 1 5 4 15 16 3  14 GND 11 22 2 I/P 7106 DPM-02 VR2 20 A A K LED1 METAL FRONT PANEL 19 18 BATT S3 21 VR1 K LED2 SOLDER LUG ON POT BUSH February 1994  67 The switching regulator (IC1) is bolted to the rear panel for heatsinking but must be isolated from the panel using an insulating bush & washer. A separate TO-220 style heatsink is fitted to diode D5. The connections to the LCD panel meter are made by soldering leads to the terminals on the back of the PC board. Use a small fine-tipped soldering iron for this job. A few dabs of epoxy resin can be used to hold the panel meter in place. adjacent to zener diode ZD2 must be rated at 0.5W. The link designated R1 must be run using 0.4mm diameter enamelled copper wire (note: this is the current sense resistor). Tin each end of the link (scrape away the enamel at each end first) before mounting it on the PC board. This will ensure a good solder joint at each end of the link. Do not use any other type of wire for this link, otherwise you will have trouble calibrating the supply later on. 68  Silicon Chip Next, install the ICs, zener diodes, diodes, REF1 and the trimpots. Solder only the two outside pins of IC1 at this stage (do not trim the leads) so that it can be later easily adjusted to line up with its mounting hole in the rear panel. Make sure that the ICs and diodes are correctly oriented and be sure to use the correct part number at each location on the board. Zener diode ZD1 should be mounted with a small loop in one end to provide thermal stress relief. Diode D5 is mounted on a small TO-220 style heatsink fitted with two locating lugs. Smear the metal tab of the diode with heatsink compound, then bolt it directly to the heatsink using a machine screw and nut (no mica washer necessary). The resulting assembly can then the fitted to the board and the leads soldered. Note that the locating lugs on the heatsink go through two matching holes in the PC board. Bend these lugs slightly to secure the heatsink in place. The capacitors can now all be installed on the PC board but watch the polarity of the electrolytic types. Take care when installing the three 100µF electrolytic capacitors; two of these are rated at 63VW while the third is rated at just 16VW. The latter is installed adjacent to ZD1. Winding the transformers Inductors L1 and L2 can now be wound and installed on the PC board. L1 is made by winding 50 turns of 0.8mm enamelled copper wire on its plastic bobbin former. Begin by pre-tinning one end of the wire and soldering this to terminal 10. This done, wind on the first layer (with each turn adjacent to the other) and cover it with a single layer of insulation tape. The remaining layers are then wound in exactly the same manner until 50 turns have been made, with each layer covered by a single layer of insulating tape. When the 50 turns are on, solder the wire end to terminal 4 and wind a couple of layers of tape over the completed windings. Before assembling the transformer, the centre leg on one of the ferrite core halves must be filed down so that there is a 1mm gap between the centre cores. You will need a flat file for this job – keep the file square to the ferrite core surface to main­tain an even gap across the entire face. A short length of 1.0mm-diameter wire is used as a feeler gauge to check the gap at regular intervals. When the gap is correct, the cores can be inserted into the bobbin and the metal retaining clips snapped in place. L2 is wound on a toroid former using two 1-metre lengths of 1.5mm enamelled copper wire – see Fig.10. There are two separate 14-turn wind­ ings, L2a and L2b, and these must be wound in the directions shown to ensure correct phasing. Wind the turns on firmly and strip and tin the Fig.10: inductor L2 is made by winding two separate 14-turn coils on a toroid former. Wind the coils exactly as shown here, to ensure correct phasing. wire ends to ensure good solder joints to the PC board. L1 and L2 can now both be installed as shown in Fig.9. Note that a plastic cable tie is used secure L2. Finally, transformer T1 can be secured to the board using 4mm screws, washers and nuts. Preparing the case Some of the integral pillars on the base of the case must be removed in order to accommodate the PC board. Fig.11: the mounting details for IC1. Smear all mating surfaces with thermal grease before bolting the assembly together. To do this, first fit the board to the base and use a felt-tipped pen to mark its five mounting pillars (ie, the five directly beneath the board mounting holes). This done, remove the PC board and remove all the unused pillars using an oversize drill. The five remaining mounting pillars should also be cut down by about 1mm, so that the transformer will fit within the case when the lid is on. In addition, the case lid has a small raised bar running across its centre and this should be removed using side cutters or a sharp chisel. If you are building the power supply from a kit, the front and rear panels will be supplied pre-punched, while the front panel will also come with screen printed labelling. Altern­ative­ly, if you are starting from scratch, drill a mounting hole for two earth lugs in the top lefthand corner of the panel, then mount the two earth lugs using a countersunk screw plus nuts and washers (note: use a couter­sunk dress February 1994  69 Use plastic cable ties to lace the wiring together & make sure that none of the mains leads can come adrift & short against the case or other parts. The fuse & power switch (S1) are both covered with heatshrink tubing, to prevent accidental contact with the 240V AC mains. screw if the front panel is supplied screen printed). The front panel label can now be fitted and used as a drilling template for the various holes. It’s always best to drill small pilot holes first and then carefully enlarge them to size using a tapered reamer. The square cutouts for the LCD panel meter and for switches S1 and S2 can be made by first drilling a series of small holes around the inside perimeter of the marked areas, then knocking out the centre pieces and filing each cutout to shape. The DVM-02 module is initially held in the front panel by making it a force fit, so be careful not to make its cutout too big. A small dab of epoxy resin along each side of the module (applied from the back of the front panel) is then used to secure the LCD module in position. On the rear panel, you will need to drill holes to accept the mains fuse 70  Silicon Chip (F1), the cord grip grommet and three solder lugs. The wiring diagram (Fig.9) shows the locations of these holes. In addition, you will also have to drill a mounting hole for IC1. The location of this mounting hole can be determined by fitting the PC board inside the case and sliding the rear panel into position. Mark out and drill the hole, then carefully deburr it using an oversize drill so that the surface is perfectly smooth. Finally, refit the rear panel and adjust IC1 as necessary before soldering its three remaining pins to the PC board. Fig.11 shows how IC1 is isolated from the rear panel using a mica washer and insulating bush. Smear all surfaces with heat­sink compound before bolting the assembly together (note: heat­sink compound is unnecessary if you use one of the new silicone impregnated washers). Finally, check that the metal tab of IC1 is indeed isolated from the rear panel using a multimeter switched to a low ohms range. The PC board assembly can now be attached to the base of the case using five self-tapping screws and the various hardware items mounted on the front and rear panels – see Fig.9. Before mounting the potentiometers, cut the shafts to a length to suit the knobs and note that a large solder lug is fitted to the shaft of VR1. A similar large solder lug is also fitted to the GND output terminal. Important: if the aluminium panels are anodised, you will need to scrape away the anodising from around the earth lug holes to ensure good electrical contact. Final wiring Fig.9 shows the final wiring details. Begin this work by stripping back the outer insulation of the mains cord by 170mm, so that the leads can reach the mains switch (S1) on the front panel. This done, push the mains cord through its entry hole and clamp it securely to the rear panel using the cordgrip grommet. The Neutral (blue) mains lead goes directly to switch S1, while the Active (brown) lead goes to S1 via the fuse. Slide some heatshrink tubing over the leads before soldering the connec­tions. After the connections have been made, the tubing is shrunk over the switch and fuse to prevent accidental contact with the mains. The green/yellow striped lead from the mains cord connects directly to the rear panel earth using a crimp lug terminal. Additional green/yellow earth wires are then run from the rear panel earth to the front panel, from the front panel to the power transformer frame, and finally from the solder lug on VR1 to an earth terminal at top right on the rear panel. Note that the two earth leads running between the front and rear panels are critical in obtaining low residual hash in the supply output. Do not leave these leads out. Light-duty rainbow cable is used for wiring the LEDs, while most of the remaining leads are run using light-duty hook-up wire. The exceptions are those leads marked with an asterisk (*). These must be run using 32 x 0.2mm wire in order to carry the heavy currents involved (ie, to the transformer secondary termi­ nals, to the output terminals and to switch S2). Note that the heavy-duty leads running from near L2 on the PC board to switch S2 are twisted to prevent noise pick-up from the switchmode circuitry. Use plastic cable ties to The centre leg on one of the ferrite core halves used for L1 must be filed down so that there is a 1mm gap between the centre cores when the inductor is assembled. The photo below shows how the ferrite core is pushed into the plastic bobbin. lace the wires together, to give a neat appearance. In addition, use several plastic cable ties to lace the mains wires together. This is an important safety measure as it prevents any wire that may come adrift from making accidental contact with any part of the metalwork or vulnerable low-voltage circuitry. Be warned that the wiring to switch S4 may present a few problems if the switch specified in the parts list is not used. This is because some momentary pushbutton switches have their common (C) terminals in the middle and their normally open (NO) and normally closed (NC) contacts on the RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 1 2 1 1 3 1 1 2 1 5 1 1 2 Value 1MΩ 100kΩ 91kΩ 47kΩ 22kΩ 15kΩ 10kΩ 6.8kΩ 4.7kΩ 2.2kΩ 1.5kΩ 1kΩ 680Ω 220Ω 100Ω 4-Band Code (1%) brown black green brown brown black yellow brown white brown orange brown yellow violet orange brown red red orange brown brown green orange brown brown black orange brown blue grey red brown yellow violet red brown red red red brown brown green red brown brown black red brown blue grey brown brown red red brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown white brown black red brown yellow violet black red brown red red black red brown brown green black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown green black brown brown brown black black brown brown blue grey black black brown red red black black brown brown black black black brown February 1994  71 Fig.12: check your etched PC board against this full-size pattern before installing any of the parts. The board is coded 04202941 & measures 222 x 160mm. 72  Silicon Chip . (+) . (-) . GND . SET DROPOUT . . OVERLOAD POWER CURRENT LIMIT METER .V . A. . . 3A-40V ADJUSTABLE POWER SUPPLY Before applying power, carefully check your work for any wiring errors. This done, wind VR1 fully anticlockwise and set VR2, VR3 and VR4 to centre position. Switch on the supply and immediately check that the voltage across ZD1 is about 9V. If so, check the reading on the digital display. It should show about 1.23 volts if S3 (the Meter switch) is in the “V” position, or about 0.00 amps if it is in the A position (note: the least significant digit will be incorrect until VR4 is adjusted later on). If everything is OK at this stage, you can check the supply voltages to each IC. Connect your multimeter negative lead to the cathode of ZD1 and check the voltage at pin 7 of IC2 and IC4, pin 3 of IC3, pin 16 of IC5 and pin 8 of IC6. These should all be at +9V. Pin 4 of IC2 should be at about -9V. If at any stage the voltages are incorrect, switch off immediately and correct the problem before proceeding. The output voltage from the power supply should be adjust­ able from 1.23V up to about 43V, with the dropout LED lighting at about 42V (depending on mains voltage). Check that the voltage reading on the panel meter changes from 2-digit resolution after the decimal point to 1-digit resolution at 15-18V. When the panel meter is set to read amps, the display may initially read several digits above or below 0.00. This can be corrected by adjusting VR4. This done, set the Current Limit control (VR2) fully anticlockwise and press the Set switch (S4). Check that the display still reads 0.00 – if not, adjust VR4 accordingly (the adjustment will only be slight). Now press the current set switch and check that the display reading can be varied from 0.00 up to at least 4.00A by adjusting the Current Limit control. Note that the overload LED may light when the control is fully anticlockwise. This is normal and the LED will extinguish when the current limit reaches 10mA (0.01 on the display). When measuring voltage, the panel meter should be accurate to 1% without calibration. However, if you have an accurate voltmeter, the trimpot on the back of the DVM-02 can be adjusted to give even greater accuracy if required. For current readings, the panel meter is calibrated by first connecting a 4.7Ω 5W resistor across the output and setting the supply to deliver 4.70V. The Current Limit control should now be rotated at least half-way, to prevent the current limit fea­ture from operating. This done, switch S3 to the “A” position and adjust VR3 until the meter shows 1.00 amps. Warning – the resistor will become quite hot during this procedure. The current limiting feature should now be checked for correct operation. To do this, leave the 4.7Ω resistor in circuit and rotate the Current Limit control anticlockwise until the overload LED lights. This should initially occur at 1A but you should now be able to set lower current limits by further reducing the control setting. The power supply will squeal during current limiting but this is normal. Finally, you can check the power supply on various loads and if you have access to an oscilloscope, you can observe the SC output ripple. LOAD Testing Fig.13: this full-size artwork can be used as a drilling template for the front panel. If you buy a kit, the panel will be supplied pre-punched & screen printed. . VOLTAGE ADJUST . outside, whereas the switch we used has its common terminals at one end. If your switch has its common terminals in the middle, the wiring shown in Fig.9 will no longer be relevant and you will have to work out the connections from the circuit diagram (Fig.5). The common, NO and NC terminals will usually be marked somewhere on the body of the switch. February 1994  73