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Detailed assembly instructions . . . Dual Tracking ±0-19V Power Supply Pt.2: By NICHOLAS VINEN Last month, we introduced our new Dual Tracking ±19V Power Supply and explained how it works. This month, we show you how to build the PC boards, install them in a case, wire it up and get it going. Both the mains-powered and plugpack-powered versions are covered. A S MENTIONED last month, there are two versions of this supply – one powered via an internal mains transformer and the other via an external AC plugpack. These instructions concentrate on the mains-powered version, which can supply more output current. If you want to build the plugpack version, check the panel at the end of the article. Main board assembly Fig.8 shows the parts layout on the main PC board (code 04206101). Start 80  Silicon Chip by carefully inspecting the underside of the board for any cracks in the copper or short circuits. That done, install the 20 wire links using 0.71mm tinned copper wire. You can substitute 0Ω resistors for some of the shorter links if you prefer, although none have been specified in the parts list. After that, mount the six 1N4148 small-signal diodes (D11-D12 & D13D16). Note that D11 & D12 face in opposite directions. Next solder in all the 0.25W resistors. If you are building the plugpack version don’t forget to install a 150kΩ resistor instead of the 91kΩ resistor to reduce the current limit appropriately. Check each resistor with a DMM before installing it since the colour codes can sometimes be difficult to read. Follow with the 10 1N4004 power diodes (D1-D10). Six of these have a short lead spacing (0.3-inch or 7.62mm), so their leads must be bent right at the diode bodies so that they sit flat on the board. After that, solder the TVS (or the alternative 5W zener diode) into place. Leave some space siliconchip.com.au 10k IC2 LM833 68 IC3 TL074 4004 4004 79L15 D2 10 F D11 D12 4148 4148 47k 47k 10M 10 F (TO VR2) AC POWER IN CON8 (TO FRONT PANEL BOARD) 68 1k 4148 4148 2 4148 1 100nF 4148 15 16 500 9.1k 2.7k 2.7k 3.0k 820 27k 68 VR6 68 4004 D14 D16 D15 D13 VR5 D4 CON1 CON7 68 500 D3 LK1* 78L15 91k* 10k 68 10 F 4004 D1 110k 100nF 22k VR7 500 LED2 11k 4.7k 0.1  5W 4700 F BC559 1k VR4 3.3k 47k 47k 1k CON4 IC4 LM833 0.1  5W 4700 F 4.7k CON5 100nF V– OUT 0V OUT 500 VR3 BC549 4004 100nF 68 1k LED1 4.7k 4.7k 10k 500 IC1 LM833 10 F 100 F 100nF 4.7k TVS 4004 10 F 10 F CON6 (TO VR1) 120 4004 D7 D8 100 F 10 F LM337T CON3 D10 4.7k 0V OUT 120 D9 10 F 4004 4.7k 4004 V+ OUT LM317T D5 4004 4.7k D6 68 7805 CON2 9.1k 820 between it and the board (about 3mm), as it will get very hot if the 5V output is shorted for more than a couple of seconds. Now you can install the four DIP ICs. Make sure they go in the right way around – see Fig.8. Sockets have not been specified but can be used if desired. After the ICs, mount the five trimpots. Regular horizontal or Piherstyle mini trimpots can be used. They all have the same value and can only go in one way. Next on the list are the BC549 & BC559 transistors (Q1 & Q2) and the 78L15 & 79L15 regulators (REG3 & REG4). These are all in TO-92 plastic packages, so check their markings carefully to ensure they each go in the correct position. Be sure to orientate them as shown in Fig.8 and use small pliers to bend their leads if necessary, so that they fit the PC board holes. Once they’re in, install the five 100nF MKT capacitors, then solder the 16-pin IDC socket in place. Line up the notch in the socket as shown on the overlay (ie, towards the top) and check that the socket is sitting flat against the PC board before soldering its pins. LEDs 1 & 2 can now go in (they are orientated in opposite directions), followed by the three screw terminal blocks (CON1-CON3). Be sure to install the latter with their entry holes facing outwards. The four polarised headers (CON4-CON7) can then be installed with their plastic locking tabs orientated as shown. Again, make sure they sit flat against the PC board before soldering their pins. Follow these parts with the 10µF and 100µF electrolytic capacitors, taking care to ensure they are correctly orientated. The two 0.1Ω resistors can then be installed. These resistors only dissipate about a 0.25W each, so they can be mounted flush against the surface of the board. * USE 150k RESISTOR & INSTALL LK1 FOR PLUGPACK-POWERED VERSION ONLY Fig.8: follow this parts layout diagram to build the main PC board. Make sure that all polarised parts are correctly oriented and be careful not to get the regulators mixed up. Mounting the regulators Regulators REG1 (LM317T), REG2 (LM337T) & REG5 (7805T) can now be mounted along the top edge of the board – see Fig.8. Don’t get them mixed up and be sure to push them down all the way into the holes before soldering one pin of each regulator. That done, check that each tab is at a right angle to the PC board before soldering its remaining leads (if it isn’t, re-melt the solder on the first lead and adjust it). siliconchip.com.au The completed main board prototype differs only slightly from the final version shown in Fig.8. It’s connected via a number of cables that terminate in headers and screw terminal blocks. July 2010  81 2 0 1TO 6 0 2CON6 40 TO PANEL METER LE NCON7 AP T N OR F YCON4 LPP U S H C N E B TO TO 1 2 3 4 5 6 S1 V+ VR1 FROM MAIN BOARD VR2 LED3 LIMIT+ 1 2 LED5 TO CON8 DROPOUT 0V FROM MAIN BOARD 0V +5V S2 5k LINEAR VOLTS ADJ 1k LINEAR CUR LIMIT 15 V+ 16 V– V– LED4 LIMIT– OUTPUTS ON/OFF 0102 © +5V EARTH Fig.9: follow this layout diagram and the photograph below when building the front panel PC board. Note that the binding post terminals are soldered to the board after they have been mounted on the front panel. RETE M LE NAP D CL LCD PANEL Q 0571) R OTPAMETER DA YA(ALTRONICS LPSID 0102 © 30160240 Once the regulators are in place, all that’s left are the two large electrolytic capacitors. Once again, take care with their orientation. Front panel board assembly The smaller front panel board is coded 04206102 and the parts layout is shown in Fig.9. Begin by installing the four wire links. You can either use 0.71mm tinned copper wire or 0Ω resistors if preferred. That done, use flat pliers to bend the pins of the two 16mm potentiometers 82  Silicon Chip Fig.10: this assembly is only required if you are using the Altronics LCD panel meter. The LED panel meter already comes on its own PC board with a header. at right angles, so that they project out in the same direction as the shafts. Bend them as close to the potentiometer bodies as possible, then remove the nut and washer from each pot. Next, take the 5kΩ potentiometer (VR1) and insert the shaft through the indicated hole on the PC board, with the body on the copper side. When it is correctly orientated, the metal tab will fit through the small adjacent hole. The potentiometer’s pins should just touch the three corresponding pads but it’s OK if they don’t quite reach – solder can bridge the gap. Line the pins up with the pads and place the washer and nut over the shaft. Do the nut up finger-tight, then centre the pins in the pads and solder all three. Once the pins have been soldered, tighten the nut down firmly. It is also a good idea to flood the pad around the small metal tab with solder. It probably won’t adhere to the metal of the pot, but it will help prevent strain on the soldered pins when the shaft is rotated. Repeat these steps for the 1kΩ potentiometer (VR2) which goes alongside. The next step is to install the 6-way, 2-pole rotary switch (S1). There are two different ways it can be orientated and only one is correct. The mounting procedure is as follows: (1) Rotate the switch shaft all the way in one direction then back two positions (ie, back two clicks). (2) Fit the switch to the board so that the flat part of its shaft faces towards the bottom of the board (ie, towards toggle switch S2). (3) Solder one of the outer pins and check that the switch body is sitting flat against the board. Check it from all angles since it has four plastic “feet” and they must all be touching the surface. If this checks out, solder the diagonally opposite pin, then check the switch again before soldering the remaining pins. The rotary switch shaft needs to be shortened and now is a good time to do it. It only takes a few strokes with a hacksaw to cut through the soft plastic. Cut it so that its end lines up with the ends of the potentiometer shafts (ie, about 28mm above the board surface), then use a fine file to remove any swarf. This will leave a stub a little under 10mm long, which is just enough to attach the knob. It’s better to err on the side of cutting it slightly too long and file it down if necessary. Next, install switch S2. It must be orientated so that the switch actuator operates vertically. Press it down hard into the holes until it is sits flat on the PC board, then secure all nine tabs by flooding each hole with solder. LEDs3-5 can now be installed. The middle one (LED5) is a red highbrightness type, while the other two are standard amber or orange LEDs. They must be installed with their bodies 12mm above the PC board. The easiest way to do this is to use a cardboard spacer. Cut a piece of cardsiliconchip.com.au Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o o o No.   1   1   1   1   1   1   4   1   1   1   2   2   8   1   1   2   4   2   1   2   7 Value 10MΩ 150kΩ 110kΩ 100kΩ 91kΩ 82kΩ 47kΩ 27kΩ 22kΩ 11kΩ 10kΩ 9.1kΩ 4.7kΩ 3.3kΩ 3kΩ 2.7kΩ 1kΩ 820Ω 470Ω 120Ω 68Ω board 12mm wide and place it between the legs of the first LED to be installed. Insert this LED into the PC board with the flat side orientated as shown and push it down against the cardboard spacer. It’s then just a matter of ensuring that the LED is perpendicular to the board before soldering its leads. This procedure is then repeated for the other two LEDs. LCD adaptor board assembly This board (code 04206103) is only necessary if you are using the LCD panel meter. Aside from the meter, the only other component mounted on it is an 8-way right-angle polarised header. Fig.10 shows the assembly details. Begin by mounting the polarised header. Orientate it as shown and make sure it is sitting flat against the board before soldering it in place. The LCD can now be fitted. It goes in with the label that carries the model number (720000) towards the top. The adjustment trimmer on the rear of the LCD meter should also be closer to the top. The alternative LED panel meter comes with a header already mounted on the rear (see Fig.11), so an adaptor board isn’t necessary. Attaching the ribbon cables Unfortunately, headers can not be siliconchip.com.au 4-Band Code (1%) brown black blue brown brown green yellow brown brown brown yellow brown brown black yellow brown white brown orange brown grey red orange brown yellow violet orange brown red violet orange brown red red orange brown brown brown orange brown brown black orange brown white brown red brown yellow violet red brown orange orange red brown orange black red brown red violet red brown brown black red brown grey red brown brown yellow violet brown brown brown red brown brown blue grey black brown fitted to the front-panel board because it is too close to the front panel itself. As a result, all the wires to the front panel are soldered into place, the exception being the earth wire which is attached via a crimped eyelet connector. All the permanently soldered wires either have connectors which mate with headers on the main board and display panel, or they go to screw terminal blocks. This makes it quite easy to remove the main PC board if necessary. First, it is necessary to cut ribbon cables of appropriate lengths. Here’s the procedure: (1) Take a piece of 16-wire ribbon cable and cut it into three lengths: 200mm (A), 120mm (B) and 120mm (C). (2) Separate lengths A and B into smaller wire bundles as shown in Table 3. You can do this by making a small nick between strands (with fine scissors) and then gently pulling the wires apart until you are left with two smaller ribbons. Cut these ribbons to the lengths shown in the table. (3) Take the Voltage Adjust, Current Adjust and LED Power cables and separate the strands at each end by 10mm. Strip 5mm of insulation from each (an automatic stripping tool will make this much easier, as it will strip all the wires in the ribbon at once). Tin 5-Band Code (1%) brown black black green brown brown green 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 violet black red brown red red black red brown brown brown black red brown brown black black red brown white brown black brown brown yellow violet black brown brown orange orange black brown brown orange black black brown brown red violet black brown brown brown black black brown brown grey red black black brown yellow violet black black brown brown red black black brown blue grey black gold brown the exposed conductors. (4) Solder these three ribbon cables to the front panel board where indicated on the overlay (ie, at the “TO CON6”, “TO CON7” and “TO CON4” positions) . The cables enter from the noncopper side. The order of the colours doesn’t matter, but if you make them match the wiring diagram (Fig.11), it will be easier to follow. (5) Install a polarised header connector at the other end of each of these cables. It’s just a matter of crimping and then soldering the header pins to the various leads before pushing them into the plastic header blocks (a soldering stand with alligator clips is handy for holding the pins as they are soldered). Be sure to double-check that the connector orientation will be correct once the pins are inserted. The small metal tags on the pins line up with the rectangular slots in the sides of the blocks. A small flat-bladed screwdriver (1mm wide) can be used to push the pins into the header blocks until they click into place. The same screwdriver can be pressed into the slots to remove the pins if they have been inserted in the wrong locations but doing this is tricky so it’s best get them right the first time. (6) Once those connectors are finished, July 2010  83 84  Silicon Chip siliconchip.com.au HEATSHRINK SLEEVING 15V 5 6 3 2 1 4 (PANEL METER) 0V M6674L TRANSFORMER 30V MAIN EARTH POINT +5V FRONT PANEL FRONT PANEL PC BOARD 5V OUT VOLT ADJ TVS V+ OUT 0V OUT 1 2 (TO VR2) LED PWR 4004 15 4004 16 REG5 4004 4004 POWER IN 4004 REG1 4004 4004 4004 4004 4148 MAIN BOARD 4148 REG2 4148 0V OUT V– OUT 0V PAD ON FRONT-PANEL BOARD – SEE FIG.9 Fig.11: here’s how to install the parts in the case and complete the wiring for the mains-powered version. Be sure to use fully-insulated spade connectors for all connections to the IEC socket and fit cable ties to the low-voltage and signal leads so that they cannot possibly contact the IEC connector if they come adrift. POWER LED 470 Ω RESISTOR IEC CONNECTOR WITH FUSE & DP SWITCH ALL CONNECTIONS TO IEC CONNECTOR VIA 4.8mm FULLY INSULATED SPADE CONNECTORS 4148 (THIS SECTION OF REAR PANEL SHOWN TILTED BACK) 4004 4148 INSULATE EXPOSED STRIP & TERMINAL WITH HEATSHRINK & SILICONE SEALANT 4148 separate and strip the 6-wire cable that goes to the LCD panel. Solder it to the front-panel PC board in the indicated location. (7) At the other end, pull the wires apart until each separate strand is 20mm long. Fit each wire with a header pin, then push them into the 8-way polarised connector block as shown in Fig.11 (make sure they click into place). Note that the order the strands are inserted into the block is not the same as their order within the ribbon. (8) There will be two free positions at the righthand end of the header for the meter power leads. Separate and strip the ribbon (red & black wires) but only tin one end. Crimp and solder that end into two header pins, then insert them into the remaining positions for the 8-way header. (9) Strip 10mm of insulation from the other end of this cable. Do the same at one end of the Power Indicator ribbon and also for the single 5V Power strand. (10) Take the ground strand of the Meter Power ribbon and twist it together with the ground strand of the Power Indicator ribbon. Tin them both, then do the same for all three 5V power strands. (11) Crimp and solder both tinned junctions into header pins. Push them into the 2-way connector block, taking care to get them the right way around. (12) Separate, strip and tin the other ends of the Power Indicator ribbon and 5V Power strand. (13) Take cable C (120mm) and crimp it to the 16-way IDC connector using an IDC crimping tool or, in a pinch, a vice. Fig.12 shows the details. If you are using rainbow cable, then use the blue wire to indicate pin 1. (14) Separate the strands at the other end of the cable by 12mm and strip 5mm of insulation from the ends. An automatic wire stripper will do about six at a time. Once you are finished, twist and tin the exposed strands of each wire. (15) Solder these wires to the DIL pads on the copper side of the front-panel board (ie, at the “TO CON8” position) – see Fig.12. Pin 1 of CON8 on the main board goes to pin 1 of the DIL pads on the front-panel board (top-right as viewed from the copper side), pin 2 goes to pin 2 and so on. The easiest way to do this is to first insert eight wires (every second one in the ribbon) into their holes down one siliconchip.com.au Table 3: Wiring Cables From To Purpose Strands Length Cable Set Main Board Front Panel Voltage Adjust 3 200mm A Main Board Front Panel Current Adjust 3 160mm A Main Board Front Panel 5V Power 1 105mm B Main Board Front Panel LED Power 2 140mm A Main Board Front Panel Misc. Signals 16 120mm C Main Board Power LED Power Indicator 2 200mm A Main Board Panel Meter Meter Power 2 160mm A Front Panel Panel Meter Meter Signals 6 120mm B REAR OF FRONT PANEL PC BOARD 16-WAY IDC SOCKET (120mm LENGTH OF 16-WAY IDC RIBBON CABLE) Fig.12: use this diagram to make up the 16-way ribbon cable that connects between CON8 on the main board and the 2 x 8-way pad arrangement on the front-panel board. side and solder one at either end. The other six wires on that side can then be soldered, after which the eight wires on the other side can be inserted and soldered individually (you will need tweezers or needle-nose pliers to insert them into their holes). Finishing the front panel If you are building the supply from a kit, the plastic front panel will probably be supplied pre-drilled and cut. It is also possible that the baseplate and/or rear panel will already be cut to size and drilled. If this is the case, skip the sections explaining how to prepare these panels. The drilling templates for the front and rear panels can be downloaded from the SILICON CHIP website. Leave the panel meter cut-out for last. The larger rectangular cut-out is for the LED panel meter, while the smaller cut-out is for the LCD meter. Either way, to make the cut-out, drill a 3mm hole a few millimetres inside each corner, then a row of holes between each of these corner holes. Do not let any hole go outside the outline. That done, use a cutting tool to remove the plastic between each hole, then knock out the centre piece. LED panel meter mounting If you are using the LED panel meter, begin by removing the two screws at the rear to detach the bezel. Be careful not to scratch the red Perspex window, which you can remove now (otherwise it will fall out). Now carefully file the edges of the panel meter cut-out until it is a clean rectangle which the bezel posts can fit through. Avoid making the hole too large and keep it horizontal – the tighter the fit, the better. The front-panel artwork can now be downloaded from the S ILICON CHIP website. Print it out, laminate it and attach it to the front panel using double-sided tape, then cut the holes out using a sharp hobby knife. Next, insert the bezel through the panel from the front and carefully place the red perspex inside it. Attach the display to the rear side using the two screws you removed earlier. Because the front panel is relatively thin, the LED panel meter will be free to move backwards and forwards. To July 2010  85 Earthing The Transformer Safely EARTH WIRES TO BASEPLATE & FRONT-PANEL BOARD REAR PANEL STAR LOCKWASHERS CRIMP LUGS M4 x 15mm SCREW, NUTS AND STAR LOCKWASHERS MAINS EARTH WIRE Fig.13: this diagram shows how the earth leads are secured to the rear panel. The second nut locks the first, so that the assembly cannot possibly come loose. EARTH LEAD INSULATED CRIMP EYLET LOCKING NUT STAR LOCKWASHERS M4 x 15mm SCREW & NUT BASEPLATE NB: CLEAN LACQUER AWAY FROM MOUNTING HOLE Fig.14: a similar scheme to that shown above is used to secure the earth leads to the baseplate. solve this, place the panel face-down so that the meter is pressed firmly against it, then flow a generous amount of hot melt glue around the edges, filling the gap between the plastic panel and the meter. Don’t disturb the panel until it has cooled to room temperature. This isn’t a permanent attachment – you can remove the glue if you pull hard enough with a pair of pliers – but if you use enough glue it won’t come loose of its own accord. LCD panel meter mounting Alternatively, to mount the LCD panel meter, first remove the two plastic clips on either side of the display. This is done by pulling each clip backwards until it rotates, un-hooks and detaches. Now carefully file the edges of the panel meter cut-out until it is a clean rectangle which the rear portion of the panel can fit through. Avoid making the hole too large and be sure to 86  Silicon Chip If you have lacquered the baseplate or if it is made from PC board material, you will need to separately earth the transformer frame. This means that the earth point on the baseplate will have two eyelet lugs attached to it rather than just one – see Fig.11. First, cut an 80mm length of green/yellow mains rated wire and crimp an eyelet lug on both ends. That done, scrape the passivation layer off the transformer’s frame surrounding the mounting hole nearest the earth lug. A file with a broad, flat end will do this job quickly. The earth lugs on the baseplate should be secured in the same manner as the rear-panel earth lugs – see Fig.14. This involves using a star washer on either side of the earth lugs and two nuts so that the assembly can’t come loose. keep it horizontal – the tighter the fit, the better. Next, attach the front panel artwork as described in the previous section, then insert the panel meter through the panel from the front. That done, re-attach each side clip by latching one side on and rotating the other side into place – the opposite of the procedure used to remove it. It should click into place. You can then press it forward until it’s holding the display to the panel. If the fit isn’t quite perfect and the display can still move, use some hot melt glue along the sides at the rear to secure it in place. Completing the front panel The next step is to mount the five binding posts on the front panel. Fig.9 and the photos indicate which colour goes where. Loosely attach them in place, then undo the nuts on the rotary and load switches (S1 & S2) on the front-panel board and remove the washers. The piece of metal under the washer on the rotary switch (used for indexing) should be left in place. The front-panel board can now be fitted to the front panel by inserting the switches and LEDs through their corresponding holes. At the same time, the binding post terminals should fit through their corresponding holes on the board. Push the board towards the front panel until the rotary switch and load switch are flush against it. The binding post terminals should all protrude through the rear of the PC board by an equal amount. Once it is in place, refit the nuts and washers to the switches and tighten the nuts down firmly to secure the board in place. Check that everything is nice and straight and make sure that the front panel fits properly into the slot in the case. The LEDs should project evenly through the front panel. If they don’t you will have to remove it and adjust their height. Now, temporarily unscrew the end caps of each binding post so that you can see the wire insertion holes. Rotate each binding post so that its hole is easily accessible. They should be horizontal for the V+, 0V and V- posts and vertical for the +5V and Earth posts. Once each post has been orientated correctly, solder it to its PC board pad and use pliers to fully tighten the nuts. Install the three knobs now. The larger knob is for the rotary switch and is secured using a grub screw. The other two knobs push on to the pot shafts but first make sure that the pointers are facing the right direction relative to the D-shaped hole at the rear. If the pointers don’t face directly away from the flat portion, use a sharp knife or very thin screwdriver to prise the face off, then insert it with the correct orientation. All that remains to complete the front-panel assembly is to install the green power LED. Push it through its hole and use a generous blob of hot melt glue at the back to prevent it from falling out. When the glue has cooled, trim its leads to about 10mm, leaving the anode lead slightly longer, then trim the 470Ω resistor leads and solder one end to the anode – see Fig.11. Making the base plate The baseplate is made from an aluminium sheet or copper clad PC board material. Cut the sheet to 248 x 76mm using a guillotine or hacksaw, then drill holes as per the drilling diagram (available on the SILICON CHIP website). It’s a good idea to spray the baseplate with clear lacquer so that it won’t oxidise (even if it’s made from siliconchip.com.au This is the view inside the completed mains-powered version. Note the use of numerous cable ties to secure the low-voltage and signal wiring, so that it cannot come loose and contact the IEC connector. aluminium). If you do this, be sure to scrape away the lacquer surrounding the earth lug hole, so that the earth lug makes good electrical contact with the baseplate. You will also need to separately earth the transformer if you lacquer the baseplate – see panel. Now check that the baseplate’s mounting holes line up with the plastic posts in the case. Since the cases can vary slightly in the post spacing, you may need to slightly enlarge some holes. Once it is correct, remove the 9mm tapped Nylon spacers from the main PC board and attach them to the top side of the baseplate instead, using the drilling template as a guide to their locations. The baseplate can now be secured siliconchip.com.au inside the case using six No.4 selftapping screws. Once it’s in, slide the rear panel into the case and rest the main PC board on top of its spacers. Check that, with the regulators right up against the rear panel, the PC board lines up with its spacers. If not, you will have to enlarge the baseplate mounting holes in the appropriate direction. Note that it may also be necessary to slightly bend the regulator legs so that they sit parallel with the rear panel. Rear panel assembly The aluminium rear panel holds the IEC power input connector and doubles as a heatsink for the main regulators. If it’s not supplied with the case, you will have to cut a sheet of aluminium to 224 x 155mm and drill it as shown in the drilling template. Note that although the regulator mounting holes are marked on the drilling template, your regulators may be offset slightly, depending on how they’ve been soldered to the main board. For this reason, the best approach is to temporarily mount the main PC board in the case and slot the panel into place. The regulator tab holes can then be marked directly on the panel, rather than relying on the template. The IEC socket outline can be marked using a scribing tool or sharp knife. Once that’s done, drill a 6-7mm hole inside the outline and use a nibJuly 2010  87 Our mains-powered prototype used the Altronics 3.5-digit LCD panel meter. Alternatively, the specified Jaycar LED panel meter can be used if you prefer a really bright readout. bling tool to complete the cut-out. The socket must ultimately be a tight fit so do this carefully, leaving a small margin inside the marked outline. A small file is then used to finish it off. Again, it is a good idea to apply clear lacquer to the finished panel but if you do so, you must scrape it off around the earth lug mounting hole on both sides. Once the panel is ready, press the IEC socket through and it should snap into place. After that, slide the rear panel into the case and check that everything lines up. Assuming it’s OK, remove the base plate from the case and attach the transformer to it using four M4 x 15mm machine screws, spring washers and nuts. Make sure the primary wires are towards the rear of the baseplate. It’s now necessary to make up the following cables using green/yellow mains rated wire: (1) Eyelet lug to 4.8mm fully-insulated spade lug, 90mm (IEC socket to rear panel earth point); (2) Eyelet lug to eyelet lug, 170mm (rear panel earth point to base plate earth point); (3) Eyelet lug to eyelet lug, 200mm (rear panel earth point to front panel binding post). Only a ratcheting crimping tool can make safe and secure connections, so be sure to use one. Don’t use a nonratchet type as supplied with cheap automotive crimping sets – the earth connections are vital to ensure safety and a non-ratchet crimper can not be relied on. 88  Silicon Chip Note that you should also insulate the exposed metal strip and terminal on the IEC socket using heatshrink and silicone sealant – see Fig.11. These three earth cables are now attached to the rear panel. To do this, first insert an M4 x 15mm machine screw through the earth point on the rear panel and place an M4 star washer over the thread. Follow this with the eyelet lugs from all three cables, then place another star washer on top and install the nut. Tighten this nut down very firmly, then fit a second nut in place. This will securely lock the first nut into position so that the assembly can not possibly come loose – see Fig.13. It’s now just a matter of running the earth leads to their destinations as shown on the wiring diagram (Fig.11). The cable with the spade lug goes to the Earth lug of the IEC socket and must be pushed all the way on. The second cable goes to the earth point on the baseplate and is secured as shown in Fig.14. The third cable is fastened to the front-panel board, adjacent to the Earth post. The latter is connected using an M4 x 10mm machine screw, earth lug, star washer and nut. The nut goes between the PC board and the panel itself, so use pliers to hold it in place while you tighten the screw. As mentioned earlier, if there is lacquer on either panel around the earth points, it must be scraped off (on both sides) before the earth screws are installed. MICA WASHER M3 NUT M3 x 10MM SCREW INSULATING BUSH REG1, REG2 & REG5 REAR PANEL Fig.15: REG1, REG2 & REG5 must be insulated from the rear panel using a mica washer and insulating bush, as shown here. The baseplate (without the main PC board) can now be reinstalled in the case, ready for the next step. Attaching the regulators Regulators REG1, REG2 & REG5 can now be attached the rear panel. They rely on the panel for heatsinking but must each be electrically isolated from it using TO-220 insulation kits (ie, mica washers and insulating bushes). Fig.15 shows the details. Begin by inserting M3 x 10mm machine screws through the three regulator mounting holes in the rear panel. That done, smear the three TO-220 mica washers with thermal transfer compound on both sides, then slide them over the screw shafts. The main board can now be slipped into position siliconchip.com.au Hot-melt glue is used to secure the panel meter and the power LED is position (plugpack-powered version with LED panel meter shown). with the three screws passing through the regulator tabs (you will need to hold the screws heads in place while you do this). Next, fit a plastic insulating bush to each regulator, then fit an M3 nut over the end of each screw and do it up finger tight. The main board can then be fastened to the baseplate using four M3 x 6mm machine screws, after which the three regulator screws can be tightened (hold the nuts with pliers so that they can’t rotate). Finishing the wiring All that is left is to finish the chassis wiring as shown in Fig.11. This mainly involves the wiring to the IEC socket and the transformer secondary connections. First, make up the short cable sections which join the incoming Active and Neutral terminals to the switch terminals on the IEC socket. These must all be mains-rated wires, correctly colour coded and fitted with 4.8mm fully-insulated spade lugs at each end. Heatshrink can be used to improve the insulation if necessary. As before, you must use a ratcheting crimping tool to attached these fullyinsulated spade connectors. Once complete, push them on hard so that they can’t come loose. Next, trim the transformer’s primary leads so that they are long enough to reach the two top switch terminals, adding a little extra so that they can flex slightly. Strip the ends, scrape away the enamel, attach 4.8mm fullyinsulated spade connectors and fit them to the IEC connector as shown. The remaining leads that connect to the front panel are soldered to pads from the copper side. To do this, place the front-panel face-down and fill each remaining hole with solder. Twist the exposed conductors of the wires tightly together and tin them. It’s then siliconchip.com.au just a matter of re-melting the solder while pushing the tinned wire through the hole in the centre of each pad. Once the wiring is completed, slot the front panel into the case and plug the various attached headers onto the main board – see Fig.11. Don’t forget the wires that run to screw terminal blocks CON1-CON3 on the main board. Use heavy-duty hookup wire for the transformer connections to CON1 and for the connections to CON2 & CON3. The wiring to the power LED is run using ribbon cable, as described previously. Cable ties As shown in Fig.11, numerous cable ties are used to secure the wiring in place. These are necessary to ensure that if a lead does come adrift, it cannot possibly reach the back of the IEC socket and thus come into contact with the mains voltage. In particular, keep the lead to the 30V terminal on the transformer short and secure it as shown in Fig.11. The leads to the front panel power LED must also be bound to other cabling so that the LED’s leads cannot possibly contact the IEC socket if it comes adrift (the wiring diagram is not to scale). Similarly, the leads to CON2 & CON3 must be secured right at the terminal blocks. You are now ready to test your new power supply. Testing the supply If you have built the mains-powered version, install the 500mA fuse in the IEC connector now. The fuseholder can also store a spare fuse so if you have one, it’s a good idea to fit this as well. The step-by-step test procedure is as follows: (1) Plug a mains cord into the IEC connector and use your multimeter to check the earth connections (ie, check for continuity between the earth pin of the plug, the rear panel and the baseplate). (2) Set the voltage and current knobs mid-way and set the rotary switch fully anti-clockwise. (3) Switch both the load and power switches off (ie, up), then plug the unit into a 240V AC wall socket. (4) Switch on and check that the power LED lights. The other three front panel LEDs may light briefly but should then stay off. The display should show a value in the range of 7.00-13.00V. If any of these conditions are not met after a couple of seconds, switch off immediately. If the power LED doesn’t light, check the fuse and the power LED wiring. If they look OK, it could be a problem with the main board. Conversely, if the power LED lights but the display is not working, check the display wiring. If the display works but an incorrect value is shown, it could be a wiring or main board problem. If one (or both) of the limit LEDs is stuck on, there could be a problem with the current limit potentiometer wiring or the main board. If the dropout LED lights, the main board probably has a fault. (5) If all is well, turn the voltage knob up and down and check that the voltage reading changes in response. (6) Turn the rotary switch one click to the right and check that the negative rail can also be correctly adjusted over the same range. (7) Turn the load switch on and off and check that it has no effect on the output voltage. Trimming the supply Five adjustments are necessary for maximum accuracy of the readouts and current limit settings. These are best made when the supply is warm, July 2010  89 Using the supply When using the unit, switch the display selector to either V+ or V- to adjust the output voltage. This can be done with the load switch on or off. To adjust the current limit, switch the display selector to LIMIT and turn the second knob until the desired limit current is indicated on the display. During use, the current drawn from either output can be viewed by switching the display selector between the I+ and I- positions. If the current limit is exceeded, the corresponding 90  Silicon Chip 16V AC INPUT V– OUT 3-PIN MIC SOCKET V+ OUT TVS VOLT ADJ ASK ABOUT SUPPLY WIRING TO CON1 5V OUT 10160140 ylppuS h cneB gnik carT HEATSHRINK SLEEVE TO CON1 470 Ω RESISTOR so fit the lid and leave the supply switched on with no load for about 30 minutes (it can be trimmed cold but then readings will be less accurate once it has warmed up). When you are ready, remove the lid and get a plastic adjustment tool or a flat-bladed screwdriver with an insulated handle. The adjustments can then be made as follows: (1) Set the rotary switch to I+ and adjust VR3 to get a reading as close to zero as possible. Turning VR3 clockwise should increase the reading and you will need to make a series of very small adjustments to find the minimum. If it won’t go near zero no matter you do, there could be a problem with IC1 or its adjacent components. (2) Turn the switch clockwise to I- and adjust VR4 to trim IC2 in the same manner. (3) Connect a DMM between the V+ (red) and 0V (black) binding posts and check that the load switch is on. Turn the rotary switch anti-clockwise to V- and adjust VR5 until the reading on the display is as close to the reading on your multimeter as possible. (4) Turn the rotary switch clockwise one step to V-, then move the positive multimeter lead to the V- (blue) binding post and adjust VR6 until the display reading matches that on the multimeter. (5) Turn the rotary switch clockwise to V±, connect the DMM between the V+ (red) and V- (blue) binding posts and adjust VR7 until the reading on the panel meter matches that on the multimeter. If VR7 has insufficient range to properly trim the V± reading, either increase its 68Ω shunt resistor or remove this shunt resistor entirely. That completes the setting-up procedure. You can now install the lid and attach the two machine screws which hold it in place. POWER SWITCH 5 6 3 2 1 4 (PANEL METER) TO FRONT PANEL PC BOARD FRONT PANEL POWER LED Fig.16: this version eliminates the mains wiring and power transformer by using an external 16V AC plugpack supply. It’s wired as shown here. Building The Plugpack-Powered Version I F YOU ARE BUILDING the plugpack powered version, the PC boards can be installed into the smaller case as specified in the parts list last month. Alternatively, you can use the same case that’s used for the mains-powered version. As before, the drilling templates (ie, for the baseplate and front panel) can be downloaded in PDF format from the SILICON CHIP website. The baseplate is 190 x 60mm and the rear panel is 170 x 127mm. Note also that Altronics sell an anodised panel (H0468) to suit this case. We have not provided a rear-panel Limit LED will light. Switching the display to V+ or V- will show how far the voltage has dropped to enforce the current limit. For higher voltage outputs, select the ±V readout and adjust the voltage control as required. The load should be connected between the V+ (red) and V- (blue) terminals. The current limit will operate as normal, although only one current limit LED may light due to slight differences in the op amps or the current sense trimming. If more current is drawn from the siliconchip.com.au This internal view shows the completed plugpack-powered version of the supply. It can be built into a smaller case than the mains-powered version. The magnified inset shows the ribbon cable connection to the front-panel board. drilling template as you simply mark and drill holes for the three regulator tabs, plus a 16mm hole at the other end to take a 3-pin microphone socket. This socket accepts a matching connector from the external plugpack supply. For the larger case, you can use the same drilling templates as for the mains-powered version with just a few changes: (1) Do not drill the transformer mounting holes or the earth lug hole in the baseplate. (2) Instead of making the IEC socket cut-out on the rear panel, drill a 16mm hole for the microphone socket. (3) Drill the power LED hole in the front panel 15mm higher than indicated. (4) Drill a 6.5mm hole 30mm below the power LED for the power switch In either case, install the microphone socket and follow Fig.16 to wire up the power supply. Once this wiring is complete, all that remains is to solder the 3-pin microphone plug to the plugpack lead. To do this, first shorten the bare wires from the plugpack so that they project 15mm from the sheath. Strip 5mm from each end and tin them. Next, remove the screw holding the microphone plug together, along with the two screws that hold the metal clamp at the rear of the plug. The lead can then be fed through the rear of the plug and the wires soldered to the appropriate tabs on the connector. Make sure that the earth (green/yellow) wire goes to the pin nearest the U-shaped depression running along the plug body. The other two wires can go to either tab, since it is an AC plugpack. Finally, reassemble the plug and the job is done. output than is available at the set voltage (see the load graphs published last month), the red dropout LED will light. This indicates that the output voltage is fluctuating. To resume proper regulation, reduce the voltage and/or current until the LED goes out. The dropout LED can also light if the load impedance varies rapidly. Finally, under some conditions, if the supply is switched off then on again soon after, the negative output current sense display may not operate correctly. To solve this, connect a 1N4148 or 1N4004 diode between the output of IC2a and ground, with the anode to ground. This diode can either be soldered to the underside of the main PC board or to the electrically connected pads on the rear of the front panel board. SC siliconchip.com.au July 2010  91