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High Power Linear Bench Supply Part 3 We have described how our new 8A Linear Bench Supply works and how to assemble its main PCB control module over the last two issues. Now it’s time to finish it off. That involves cutting some holes in the case, mounting the components inside, attaching the front panel controls, wiring it up and doing the final calibration and testing. W e chose to put the Bench Supply in a Jaycar HB5556 instrument case because it’s just big enough to fit everything without making it too large or heavy; it’s reasonably priced and easy to get, easy to work and it has plenty of ventilation for the required cooling air. The following instructions assume you are using that case. If using a different case, make sure that all the parts will fit inside and that nothing will foul anything else; if it’s substantially larger, you should be fine. You also need to ensure that it has adequate ventilation, especially in the top and bottom panels around where the heatsink will be mounted. Ambient air is sucked in through holes at the rear of the case, blown over the heatsink and exits through holes above and below the heatsink fins. Your case will need to have a similar arrangement. 86 Silicon Chip It also needs to be made of steel or aluminium, not only for strength but also so that all of its panels can be Earthed for safety. Any case that meets these requirements can be used, but you will have to vary the instructions regarding where to mount the components inside the case and on the front and rear panels, and adjust the cutout and wiring placements to suit. So without further ado, let’s get to finishing off the Bench Supply. Preparing the case Several holes need to be drilled and cut into the metal instrument case. The front panel hosts the panel meter, control potentiometers, output binding posts, over-current LED and load switch, by Tim Blythman Australia’s electronics magazine while the mains socket and fan cooling holes are on the rear panel. All six panels also have Earth screws to ensure safety. The bottom part of the case also needs to be drilled to mount the transformer, PCB and heatsink. The top and bottom panels are vented; the case is orientated with the vents at the rear, as this is where the fans and heatsink are mounted. It may help you to start by putting the case together, so you understand how all the parts fit, then mark where holes will be drilled in each panel while it is in place. Fig.8 shows the hole locations and sizes for the front and rear panels. We recommended in the article last month that you use the blank PCB and heatsink spacer to mark out the required hole locations in the base, as well as the hole for the transformer mounting bolt. Check now that these are in the right places. The case is made of aluminium, so it siliconchip.com.au Fig.8: use these diagrams to assist you in cutting and drilling the front and rear panels. Note they are 60% of life size, so to copy and use as a template you will need to enlarge them by 166.7%. Downloadable PDFs are at siliconchip.com.au is not hard to work. No holes need to be made in the case sides, but their internal ribs must be trimmed to allow all the components to fit. We recommend test-fitting all the parts before doing any drilling or cutting, to make sure it will all go together properly later. This is especially true if you are making any variations from our design. Rear panel preparation Even though the panels are not flat, they can be held in a vice by placing them between some scraps of timber. This will also help to prevent damage to the enamel finish. We opened up the large holes in the panels using a 3mm drill bit on a drill press, making numerous closely-spaced holes inside the outline. The holes were then joined with a hacksaw, after which the edges were brought to dimension and finished with a file. You may also There isn’t much mounted on the rear panel; just the switched, fused IEC mains input socket and the two cooling fans. The small screw head visible to the right of the mains socket is the main Earth point inside. siliconchip.com.au Australia’s electronics magazine December 2019  87 The front panel of the Bench Supply has two knobs to set voltage and current and a switch to connect or disconnect the load, along with the three output terminals. The red LED above the current knob indicates when thermal limiting is occurring. The LCD screen shows the actual and set voltages, actual current and current limit, plus the heatsink temperature. find a nibbler useful, if you have one that can handle 1mm thick aluminium. For the hacksaw cuts, we removed the blade from the hacksaw, threaded it through the pilot hole, reattach the blade to the hacksaw and then made the panel interior cuts. We suggest that you use a similar technique to make the cutout for the IEC socket. Mark its outline on the rear panel and then drill a series of small holes inside the perimeter. Keep the holes well inside the markings. Drill a larger hole (large enough for a hacksaw blade or other small metal saw) inside. Then use a hacksaw blade to cut towards the corners from the large hole in the centre. Take care that the sheet metal does not bend and break on the forward stroke. Once the cuts have reached the corners, the triangular shapes may be flexed along the drill holes, to break them off. Use a file to carefully bring the edges of the cut to their correct dimensions. Keep the mains socket nearby to test fit, as you do not want to take away too much metal. This could cause the receptacle to be not held securely by its tabs. Try fitting the socket at an angle to test the height and width independently. Once the dimensions are correct, gently run a file across any sharp edges of the opening to remove any burrs. Now is also a good time to drill a 3mm hole in the rear panel for the Earth connection. The location is not critical, but placing it near the receptacle will minimise the Earth wire length. Sand the inside of the panel until you have an area of exposed bare metal around 1cm in diameter around the hole. The aim is to make a good metal-tometal connection with the eyelet lug at 88 Silicon Chip the end of the Earth wire. You will also need to drill eight 3mm holes to mount the fans. Test fit the fans to check their locations as there is not much room around the fan guards, and they need a small amount of clearance to allow the filters to be clipped on and off. You may need to space the fan mounts so that they are not hard against each other. Two large holes are required so air can be drawn in by the fans. We traced out a circle using the inside of the fans as a template, but any circular object around 80mm across will be fine (or copy/print Fig.8 to use as a template). Check that the fan guards completely cover your marked hole before cutting it out. Use a similar technique to the IEC receptacle to open out the holes. Drill a series of small holes and then open up the panel with a hacksaw blade and finish by filing down the rough edges. You can now fit the mains socket. Orientate it so that the lead plugs in below the switch, allowing access to the switch from above. Now is also a good time to insert the fuse. While the 6A fuse chosen may seem excessive for a 500W transformer, this is the recommended rating for that transformer. Lower-rated fuses will blow due to inrush current when the unit is switched on. If you want to use a lower value fuse, it will need to be a slow-blow type. Front panel preparation The front panel is treated similarly to the back. Assuming you are using our Five-way Panel Meter, check that your LCD screen’s dimensions match our template and then transfer this to Australia’s electronics magazine the front panel. We have designed an acrylic bezel that suits the LCD on the Five-way Panel Meter, which hides any small inaccuracies in cutting the front panel around the Meter. You can place the bezel over the LCD to see if it matches the dimensions and if so, use it as a template to mark out the front panel. Otherwise, use the LCD dimensions or Fig.8 as your guide. If you have separate panel meters, check their specifications for recommended cutout dimensions, and plan how they should be laid out, leaving room for the binding posts, switch and potentiometers. Cut out the opening for the panel meter(s) using the same technique as for the mains socket. You’ll also need to drill the four 3mm mounting holes. You also need to drill two or three holes for the binding posts; three are required if you want an Earth post, which can come in handy from time to time. Otherwise, the supply outputs are ‘floating’. Check the diameter of the holes required for your posts and drill them with equal spacings. Ours were 9mm. Start these holes by using a punch to locate the centre of the hole and then by drilling with a smaller size to create a pilot hole. Finish with the recommended size drill bit to complete the hole. Similarly, drill a hole below the panel meter for the output on/off (load) switch. Typical panel-mount toggle switches require a 6.5mm hole, but again, it’s best to start with a smaller pilot hole and then enlarge it to the final size before deburring. Below the banana socket holes, add a 3mm hole for the front panel Earth. siliconchip.com.au As for the rear panel, sand the inside to remove the enamel for about 1cm around it. The two potentiometers require two holes each to mount; one for the shaft and a second to hold the locating lug so that the pot won’t rotate. Drill the two holes using the usual technique. Drill a hole for LED1 as well, taking into account the bezel diameter. If you wish to add our front panel artwork, you should do so now. The best way to get this is to download the PDF from the SILICON CHIP website, print it out and laminate it. Note that the front panel is wider than an A4 piece of paper is long, so it will look better printed on A3 so that no joins are needed. Alternatively, you can print on selfadhesive sheets (see siliconchip.com. au/Help/FrontPanels). Mounting the front panel components Solder a 20cm length of black 10Arated wire to the black binding post, and a 20cm length of red 10A-rated wire to one terminal of the output switch. A second 5cm length of red wire is then soldered between the other switch terminal and the red binding post. Insulate the solder joints with heatshrink tubing. Strip back the last 5mm of both free wires for connecting to CON1 on the main PCB. If adding an Earth binding post, attach a short length of 10A green/yellow wire stripped from mains flex or a mains cord, and crimp or solder an eyelet (ring) lug to the other end. It will attach to the front panel Earth screw later. The binding posts and output switch can now be secured using the supplied nuts and washers. Orientate the switch so that it makes the connection from the red binding post to CON1 + on the PCB when it’s down (the standard position for ‘on’ in Australia and New Zealand). Thread the potentiometer shafts through the panel from the back and locate the lugs into the smaller holes to stop the potentiometers from rotating. Secure at the front with mounting nuts and fit the knobs. We used spline shaft potentiometers, which allow the knobs to be attached at almost any angle. If you have D-shaft potentiometers, you may need to rotate the front part of the knob later so that the pointer sweeps over an appropriate range (these can usually be prised off with a knife). Now mount the rest of the front panel hardware. Fit the LCD bezel by threading a 12mm M3 machine screw through each corner, then feed the screws through the holes in the front panel. Secure with M3 nuts at the back of the panel. If your LCD has mounting holes which are too small to fit an M3 screw, these can be carefully enlarged with a 3mm drill bit, ideally in a drill press. Avoid inhaling the fibreglass dust which results. Five-way Panel Meter LCD can then be threaded over the back of the machine screws and held in place by four more nuts. Attach the IDC cable to the header, ensuring the marked pin 1 on the cable lines up with that on the PCB. Finish by pushing the LED with bezel through the hole you drilled for it earlier. Transformer and main PCB If you haven’t already marked out and drilled the required holes in the bottom of the case, use the populated PCB, heatsink spacer and transformer to determine where the holes need to go. All of these need to be drilled to 3mm and deburred, except for the transformer mounting bolt hole which will need to be larger. Measure the diameter of the supplied bolt; around 8mm should do. Before drilling those holes, it’s a good idea to slot the front and rear The main requirement for the SPST “LOAD” switch (mounted under the display) is that it must be capable of handling the whole output current – up to 8A DC. Practically, this means you’ll need a 10A DC switch – don’t be tempted to use one only rated for 10A AC – it’s not enough! siliconchip.com.au Australia’s electronics magazine panels into the case to make sure that the internal components will not foul anything mounted on either panel. Test fit the transformer and PCB according to the markings, to ensure that everything fits as expected, then drill the holes. You may need to remove the side panels as they are likely to conflict with the PCB and transformer mounting positions. You can test fit these later to confirm how they need to be trimmed. We needed to trim away some of the internal parts of both side panels on our prototypes, as the side panels protrude slightly into the case near their fastening holes and screws. Check that there are no collisions between the PCB, transformer and front and rear panel hardware. Keep in mind that the fans and their spacers will sit between the heatsink and the rear panel. You might also like to check that the transformer’s leads reach the mains plug receptacle and the bridge rectifier tabs on the PCB. If everything appears correct, then drill the holes in the base. The smaller holes for the PCB and heatsink that sit in the vented region of the base can be tricky to drill, but if they end up slightly out of the marked positions, that should not be a big problem. In the worst case, you will just have to enlarge these holes slightly. Also drill a 3mm hole for the mains Earth in the base. Place it near the mains receptacle, but clear of the vented region. As with the other Earth holes, sand the area around it to expose the underlying metal. The transformer is quite heavy so take care not to drop it while working with it. Feed the bolt through the bottom of the case, then place one of the rubber gaskets over its shaft on the inside. Lower the transformer into place, rotating it so that the wires are close to where they need to connect. The second rubber gasket goes on top of the transformer, followed by the dished metal plate with its convex side facing down. Slide the small washer in place, thread the nut onto the bolt and tighten it up to a reasonable degree, so the transformer is held securely in place. Do not overtighten it or you could damage the transformer windings. Remove the two 9mm tapped spacers from the PCB that are nearest to the heatsink. Alternatively, if you haven’t already fitted them, fit the two spacers December 2019  89 furthest from the heatsink but leave the other two off. Getting the PCB into position in the case can be tricky due to the weight of the transformer. We found that it was possible to balance the case on its edge by using the weight of the transformer to hold it upright. Start by feeding one M3 x 10mm Nylon machine screw through the base of the case and into the heatsink, making sure to thread it through the acrylic spacer. Then fit the other three Nylon machine screws to hold the heatsink in place. This should also hold the PCB in place, for now. Metal screws cannot be used on the heatsink as this would connect the live heatsink to Earth. Use two machine screws to secure the front of the PCB to the bottom of the case. Now is a good time to attach the feet to the case. We used taller feet than those included with the enclosure, as those were so short that the transformer mounting bolt head was still touching the bench with them in place. Taller feet also provide more space for cooling air to escape via the underside vents. Rear panel and fan mounting The fans can now be fitted. They are mounted to the rear panel on spacers. Ideally, they should be as close as possible to the heatsink, but not touching. Take one fan and thread four 32mm machine screws through the corner holes. Fasten them to the fan using the 15mm-long M3 tapped spacers. These will sit against the rear panel, so if there is room to bring the fans closer to the heatsink, nuts or washers can be placed under the spacers. Just make sure that the fans don’t touch the heatsink fins. Now separate the fan filters/guards into two pieces and place the fans on the inside of the rear panel and the guards on the outside. Attach the fans using 9mm long M3 machine screws through the guards and rear panel, and into the tapped spacers attached to the fans. Clip the fan filters back into place on the guard frames. With the PCB and transformer in place, you can mark and cut the required cutouts in the side panels, to clear the internal components. You can see how much material we had to remove in our photos. There is a fair degree of overlap between side, top and bottom panels, so slight inaccuracies in cutting the side panels will be hidden. The two 80mm fans we used were specifically chosen for their high flow rate. They’re Digikey P122256 24V models, available from digikey.com If you substitute other fans they may not have the essential cooling properties of these ones. Firmly hold the side panel in a vice using timber off-cuts to protect the finish. Make the marked cuts with a hacksaw. If the panel vibrates as you are sawing, try clamping it closer to where the cut is being made. Check that the panels now clear the transformer, PCB and heatsink. Once everything fits together correctly, dress any sharp edges of the side panels with a file. The side and top panels will also need to be Earthed. This can be done via the remaining sections of the mounting tabs. These are already slotted, so you don’t need to drill any holes. Just remove the enamel from a small area on one of these tabs, where the Earth eyelet will be attached later (see photo at left). Use an area near the back of the side panels, as the Earths will all connect back to the rear panel. For the top panel, choose a location opposite the Earthing location on the bottom panel, which is otherwise clear of components. Drill a 3mm hole and sand the inside of the panel as for the others. Making the final connections It’s not immediately obvious here but each of the mains spade connectors on the IEC (input) socket (upper left of pic) are covered with a clear shroud. Also note each of the removable case panels has its own Earth wire attached, connecting back to the main Earth point on the rear panel (alongside the IEC socket). 90 Silicon Chip Australia’s electronics magazine The leads to the fans, LED, panel meters and thermistor can be plugged into their respective board connections. The leads for the banana sockets screw into terminal block CON1. Ensure that they are connected with siliconchip.com.au the correct polarity, ie, red wire to + terminal. Mains wiring The transformer needs to have spade crimp lugs fitted to mate up with the IEC plug receptacle and bridge rectifier. The transformer we used has two 115V AC primary windings, which are intended to be connected in parallel for 110-120V AC mains and in series for 220-240V AC mains. The secondary windings are 40V AC each, and in this application, they need to be wired in parallel. Also, the integral DPST switch in the IEC input socket is not joined internally to mains Active or to the fuse. It instead has separate spade lugs to make connections. So we will need two short leads, one brown and one blue, to make these connections. Make sure there is no chance that a mains cord can be plugged in while you are working on the mains side of the circuit. Cut a 100mm length of brown wire and another 100mm length of blue wire, stripped from 10A-rated mains flex or a spare 10A mains cord. Strip both ends of both wires and securely crimp spade lugs onto them. Insulate the exposed metal using heatshrink tubing. Once you’ve made up those two wires, plug them into the rear of the IEC socket, with one going from the fused Active terminal to one pole of the switch and the other going from the incoming Neutral lug to the other pole of the switch. Do not connect them both to the same switch pole! Now is also a good time to insulate the exposed metal strip on the back of the IEC socket using neutral cure silicone sealant, to make working on the inside of the Supply a bit safer. To wire the transformer primaries in series, solder the grey wire to the purple wire and cover the joint using two layers of heatshrink tubing. Remember to slip the tubing over the wires before soldering them. If you are using a different transformer than the one we specified, check the manufacturer’s instructions for wiring it up to a 230V AC supply. Next, fit spade connectors to the transformer’s brown and blue (primary) wires and insulate them with heatshrink tubing. Push these onto the two remaining switch terminals on siliconchip.com.au A B A close-up of the rear of the Supply showing (A) the main earthing point and (B) the Presspahn insulation forming a barrier between the high and low voltage sections. Don’t leave these out: they’re for your safety! the mains socket, so that the wires going to the two switch poles match (ie, brown/brown and blue/blue). It’s essential that you now use multiple cable ties to tie all the mains wiring around the IEC input socket together, so that if any of the wires come loose, they won’t flap around the case and potentially make contact with the heatsink, PCB or any other non-mains conductors. You will also need to fit a Presspahn insulating barrier alongside the heatsink and PCB, so that if a mains wire does somehow come loose, it cannot come in contact with those parts. Cut the sheet of Presspahn to 105 x 208mm and score it 20mm in from one long edge, making a 208 x 20mm foldable section. Now fold that part by 90°, place it in the case alongside the heatsink and drill two holes in the base, through the bottom of the case, close to each end. Attach it to the case using 6mm M3 machine screws and nuts. The photo opposite shows what it will look like when you’ve finished. This piece will come close to touching the lid when it’s attached forming an insulation barrier between the heatsink/PCB and the mains wiring. You will need to use side cutter to make two cuts along the top edge and fold it down, for the transformer secondary wires to pass through. Again, see the photo for an idea of how this was done on our prototype. Earth wires The next step is to make and fit the panel Earths. Five green/yellow wires are required with eyelet connectors crimped to each end. These will go from the rear panel Earth screw to the Australia’s electronics magazine other panels. A sixth wire is needed, with a spade lug at one end (to suit the mains socket) and an eyelet at the other, to go to the rear panel star Earth point. None of the crimp connections need to be insulated. Cut the Earth leads to length, giving enough slack so that you can pull the panels apart later, and so that they can avoid any components which might be in the way. The lead for the top panel should have more slack than the others, as it will need to allow the top panel to be detached and moved out of the way while still being connected to Earth. Once the wires have been made up, plug the spade terminal onto the Earth terminal of the mains socket. Thread a 12mm M3 machine screw through the rear panel hole, then place a star washer over the screw shaft, followed by the six Earth wire eyelets. Secure with an M3 hex nut and tighten well. Then add another nut on top, doing it up moderately tight, to act as a locknut. Now terminate the other end of the five remaining Earth leads to the five other panels similarly. The screw heads should be on the outside the case, with the eyelet connected to each panel through the star washer, with the screw held in place by a nut done up tightly. The front Earth binding post (if fitted) should have its eyelet placed on top of the front panel Earthing eyelet. The final connections to be made are from the transformer secondaries to the bridge rectifier (BR1) on the heatsink. To parallel the secondaries, solder or crimp the orange and black wires into a spade together and insulate it with heatshrink tubing. Do the same with the yellow and red wires, December 2019  91 The underside of the Power Supply case, showing the locations of the holes required for the transformer (the big black bolt), the heatsink (Nylon screws on/near ventilation holes) and the PCB mounting pillars (right side of pic) The single screw on the left side is for the case Earth. All holes are 3mm with the exception of the transformer mounting (we used a 8mm bolt). into a second spade lug. Again, if you are using a different transformer, you should check this configuration as it may be different. Plug the two spades on the AC lugs on the bridge rectifier. Check that everything else has now been connected Final assembly The back, front and sides of this case can be tricky to assemble. You might find it easier to join the front, back and sides together as a unit and then slot this onto the bottom panel. Screw two of the panel screws into the sides, securing them (and thus the front and rear panels) to the bottom. Check that these screws do not foul the transformer or PCB as you do this. They are much longer than necessary, so can be trimmed, if it comes to that. You can test fit the case lid as well. It should slot onto the remainder of the case, with the last two screws used to secure it. But leave it loose for now, as we will need access to the PCB for the final tests and calibration. Now is a good time to tidy up the wiring. Use cable ties to secure the wires into neat bundles (you should have already tied the mains wiring together). The slotted ribs on the side panels are great places to attaching the cable ties, holding the wire bundles out of the way. This is also a good chance 92 Silicon Chip to run your eye over everything and make sure you can’t spot any wiring or construction problems. Final testing Ensure nothing is connected to the supply outputs and that the front panel knobs are wound down to their minimums. Connect mains power and switch the unit on via the rear panel switch, keeping yourself well clear of all the internals. It’s best to leave the wall socket switch off, ensure the IEC input socket switch is on, then stay clear of the unit while switching it on at the wall. The front panel meters should light up and should all have readings close to zero; if they do not, power off and check for problems. The temperature reading on the Five-way Panel Meter should be around ambient. If the temperature is above 20°C, then the fans may start up. Connect a multimeter on its volts range to the output terminals, with the output switch on (down). The reading should be 0V. If not, shut down and check for faults. If all is well, turn up the current limit pot to slightly above zero, maybe to around one-tenth of its range. At the zero position, the output is completely inhibited. Slowly advance the voltage pot; you should see the voltages on the Australia’s electronics magazine meter rise. If this is the case, then we can calibrate the voltage display. Dial up the voltage until you get 50V DC across the output terminals. If it does not reach 50V at its maximum, adjust VR1 to allow this. Now adjust VR5 and VR6 until their respective meters (set voltage and actual voltage) are both showing 50V. This will probably be at around a third of their range from the minimum position. So far, all the work is being done by REG3. We will now test that the Supply will hand off to the current boosting transistors at higher currents. Dial the voltage pot down to the minimum and connect a 1kΩ resistor (1/2W is fine) across the output binding posts. Now dial the voltage up to 20V; this will be just below the power limit of a 1/2W 1kΩ resistor. Check the voltage across the 68Ω resistor near REG3. It should give a reading of around 0.6V, the base-emitter switch-on voltage of transistor Q3. If the reading is above 1V, then REG3 is passing all the current, and the transistors are not taking the load. Power off the unit, give it a minute for the capacitors to discharge and check for problems around the heatsink-mounted transistors. Assuming all is well, dial the voltage and current down and remove the 1kΩ resistor. We can now calibrate the current meters. You can connect an ammeter (or multimeter at 10A setting) directly across the outputs, although this will involve running the PSU at maximum dissipation. It is a good idea to connect a high-power series load resistor if you have one. We want the Supply to be delivering 8A to provide the best calibration. Dial up the voltage slowly; if you only have an ammeter connected across the outputs, you should not see a voltage reading much higher than 1V (depending on lead and load resistance). If it goes much higher, that suggests that there is a problem with the current limiting. The voltage will be higher if you have a series resistor connected. As you advance the current limit pot, assuming the set voltage does not match the actual voltage, that means that current limiting is occurring. The fans should start running if they are not already. Continue winding it up until the meter shows 8A. If it does not reach siliconchip.com.au 8A, then adjust VR2 to fine-tune the maximum current limit. Now adjust VR7 and VR8 until the Five-way Panel Meter (or your individual panel meters) show 8A for both the set current and actual current. These pots will need to be wound to around 2/3 of their range from the minimum. At this stage, the Supply will be dissipating close to 400W, so the temperature will be steadily rising and the fans will be working harder as it does. You can use a contactless (IR) thermometer to check the heatsink temperature, which should be close to what’s shown on the Panel Meter. If you leave the current set to 8A, you can test the thermal limiting. When the temperature reading gets to around 80°C, the limiting LED should come on, and the current will drop. You may also hear the fans run a bit harder too. This is not a ‘boost’ mode, just the effect of the sagging DC voltage disappearing as the load is reduced. If the temperature keeps rising past 80°C with no change in the output current, then shut the Supply down and check for faults in that part of the circuit. If it does enter limiting, then the Supply is working as designed. Dial the current and voltage down to their minimums and let the fans run for a moment so that the heatsink cools down, then turn it off and disconnect your test load. Finishing up Now that everything is functional, it’s just a matter of a few finishing touches. Secure any loose wiring with the cable ties. The wires on our transformer were not too long, so they did not need to be fixed to anything. If yours are significantly longer, you can use self-adhesive plastic cable clamps to tidy them up. The fan and thermistor wires can be bundled together and fixed against the right-hand side panel with adhesive wire clips. Similarly, the output wires to the binding posts should be attached to the base of the case with adhesive wire clips. The other wires to the front panel can be bundled together with cable ties. Since they do not travel far, they should not need to be secured to anything else. siliconchip.com.au The Earth wires should be clipped in place if there are any that might move around excessively. Take care with the lead for the top of the case if it has a lot of slack. You may like to fit a cable clip to the inside of the top of the case to secure it. Secure the top panel in place with the two supplied screws. The High Power Linear Bench Supply is now complete. Variations While we aimed for 50V output voltage in our design, necessitating the 57V rail, you can use a lower voltage transformer too. As long as the 24V regulator can still deliver 24V, the Supply will still work. To use a lower voltage transformer, you may need to reduce the value of the 220Ω 5W resistor, to ensure the input of REG1 always stays above 26V. You can also adjust the upper output voltage limit downwards using VR1. VR1 may even need to be increased in value (eg, to 20kΩ or 50kΩ) if a very low output voltage is desired. The current capacity of the output transistors is much higher than the 2A each we have chosen, but thermal considerations limit their operation. You could tweak the PSU to provide a higher output current if the input voltage (and thus total dissipation at zero output voltage and maximum current) is reduced. The PCB tracks, CON6 and the wiring can handle up to 10A, so this is about the practical limit without making major changes. Note that you may need to reduce the value of the 27kΩ resistor in series with trimpot VR2 to set the current limit to 10A. Fan considerations We chose a particularly high-powered pair of fans to ensure that the output transistors will be cooled as much as possible. The 33Ω series resistor is suitable for these fans, but may not drop enough volts if different fans are used, particularly those with a lower current draw. Its value should be chosen to provide a 9V drop (from 57V to 48V) at the typical current draw of the chosen fans. A 5W resistor should be suitable for up to around 500mA under these conditions. SC Australia’s electronics magazine Fig.9: this front panel artwork is reproduced here at 75% life size, so will need to be copied at 133% to fit the panel. A full size version can be downloaded from siliconchip.com.au December 2019  93