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Constructional Project Intelligent Dual Hybrid Power Supply PART 2: BY PHIL PROSSER Our new Dual Hybrid Supply has very quiet outputs given its use of switchmode regulators to provide good efficiency and high power output in a small package. The outputs can be used independently or together in series or parallel, all controlled through a single easy-to-use digital interface. We described the circuitry last month, so this article will concentrate on assembling and calibrating the Supply. T here are a few steps in assembling, testing and calibrating this Supply. First, you need to build the four PCB assemblies: two regulator modules, the control board and the front panel board. Then you need to wire them up and put them through some basic checks to make sure they are all functional. Following that, you attach the regulator modules to the main heatsink, prepare the case, mount everything in the case and wire it all up. Once you’ve done that, we’ll take you through the calibration procedure, which is mainly done via menus on the LCD screen, with the aid of a decent multimeter. There’s quite a bit to get on with, so let’s start with populating the regulator PCBs. Building the regulator module(s) Each regulator module is built on a double-sided PCB coded 18107211, measuring 116 x 133mm. Fig.10 is the 64 PCB overlay diagram; it shows which components go where and indicates the correct orientations for polarised components. Refer to it as you build the board assembly. If you are making a dual power supply, only one board needs the LM2575-5 (REG3) and associated components (L3, D12 etc) to be loaded. It is essential to only fit links LK1 & LK2 on the one board with the LM25755 regulator. So install those on one board now – you can use 0W resistors or lengths of tinned copper wire (Bell wire would work too). Construction of the Regulator Module commences with all resistors, except for the 0.01W current sense resistor and 0.05W current sharing resistors. Leaving these off for now will mean that the board lays flatter on the bench, making it easier for you to solder the fiddly components that come later. With those smaller resistors in place, mount all the diodes bar the TO-220 case diode and bridge rectifier, checking carefully that each is in the correct orientation before soldering. Follow these with the 100nF film capacitors, 10μF electrolytic capacitor and remaining MKT capacitors. Before you fit the ceramic capacitors, solder the SMD ICs in place. Then install the small transistors (TO-92 package). Make sure you don’t get the two different types mixed up. We have described how to do this on many occasions. The basic idea is to tack one pin down, check that the placement and orientation are correct, add flux paste to all the pins, solder all the pins, then clean up any bridges which have formed using more flux paste and some solder wick. Pay attention to the orientation of the MAX14930 isolators, IC6 & IC7; they are mounted in opposing directions. We have added markings near pin 1 of each SMD IC to assist. With the SMD chips in place, fit the ceramic bypass capacitors. Practical Electronics | July | 2025 Dual Hybrid Bench Supply, part two There are two 15μF surface-mount tantalum capacitors on the top of the board. These go with the positive end toward the regulator. Double-check their orientation; the positive end should have a stripe. There are also five surface-mount capacitors on the back side of the board; fit them next. Now is a good time to load the components we held back: the 1W, 0.01W and 0.05W resistors. Then mount the headers, connectors and fuse clips and install the fuse. Making the diode heatsink TO-220 diode D3 needs a small heatsink to make it bulletproof. Its dissipation is only high if the Supply’s output is short-circuited, but ideally we want it to handle that continuously. We used a 55mm by 40mm piece of 1.6mm-thick aluminium with a fold in it. We recommend you do the same, as there is no need for a ‘bought one’, and this is the optimum size for the available space. Fig.11 shows how to fold and mount this heatsink. Now fit the larger transistors (Q3 & Q10), three DIP ICs, plus the LM317, LM337 and LM2575-5 regulators. The regulators can be mounted with a couple of millimetres lead length. The +12V regulator (REG1) and negative regulator (REG4) need small flag heatsinks which are attached with an M3 machine screw, crinkle and flat washer, insulating bush and washer, as shown in Fig.12. Before you mount the electrolytic capacitors, attach the heatsink to the TO-220 diode. This will make it easier Here is an example of how to mount the diode to the heatsink. Take note that the heatsink should have a bend in it as shown in Fig.11. Practical Electronics | July | 2025 Fig.10: the Regulator board is somewhat packed but not difficult to assemble. There are just a few SMDs; none with particularly fine-pitch leads. The only components that mount on the underside of the board are five SMD capacitors, all in the upper right-hand section. They are shown in an ‘x-ray’ fashion here. Fig.11: we couldn’t easily find a commercial heatsink to fit in the space around diode D3, so we made one. It’s simple as you just need to cut out a rectangle of aluminium, drill one hole and fold it 90° where shown. Then attach it to the tab, including connecting the heatsink to the device’s cathode for EMI reduction. 65 Constructional Project Heatshrink tubing should be placed over the flying leads to the bridge rectifier as shown. 7.5A rated hookup wire; red for positive, black for negative and yellow for AC. Also use heatshrink tubing to insulate the connections to the bridge rectifier leads. Building the control boards This shows how the boards should look when mounted to the heatsink. Note that in this picture, there is only one LM1084 per board. The final design has two LM1084s and two current-sharing resistors. Fit those as per the overlay diagrams. to get all the bits aligned and tightened. If you forget, you can poke a screwdriver through the hole in the toroidal inductor, but that is much more fiddly. You can now fit the remaining electrolytics; put the larger ones in last as they tend to dominate the board. All bar two of these have the positive (+) lead toward the main heatsink, or to the left with the heatsink at the top. The two 220μF electrolytics do not follow this rule. These are at the input to the MC34167 and have their (longer) positive leads to the left, as demanded by the pinout of this device. Fig.12: we are using pre-made heatsinks for REG1 & REG4; attach them like this. 66 Finally, load the inductors. We put a dab of neutral cure silicone under ours to stop them moving, and recommend that you do the same. At this point, everything except the parts that mount to the main heatsink should be on the board. The bridge rectifier is attached via 150mm flying heads, allowing it to be mounted to the heatsink. Put short lengths of heatshrink tubing over the connection of the flying leads to the bridge rectifier, as shown in our photo above. Route the leads to the rectifier pads on the PCB. We used 15cm lengths of The controller for this project is the same one that was used for the Audio DSP project in the January-March 2020 issues and the Low-distortion DDS Signal Generator from February 2021. The main difference is that here, the PIC32MZ is programmed with the Intelligent Power Supply firmware. The PCB overlay for this controller board is shown in Fig.13. We’ve removed most of the components you don’t need for this project from that diagram, although it won’t hurt if you fit them anyway. As usual, fit the SMD ICs first (watch their orientation and check for bridged pins!), followed by the other SMDs, then the lower-profile through-hole components, finishing off with the taller parts. Besides the ICs, be careful that the cathode stripes of the diodes go in the right locations, plus the SMD LED cathode (which is often marked with a green dot or T-shape). Also make sure that the positive (longer) leads of the electros go to the pads marked with + symbols. If you will be programming your own microcontroller, the HEX file is available for download from our website. It can be programmed in-circuit via CON23, but note that if you plan to plug a PICkit in, it goes to the row of pins closest to the micro, with pin 1 at the end marked with a “1”. Or you can purchase a pre-programmed micro from the Silicon Chip Online Shop, in which case you can skip that step. Fig.13: the powerful PIC32MZbased control board for this project has been used in several previous projects. Some of the components are not needed for this one, so we have left them off this overlay diagram. Solder IC11 first (watch its orientation!), then IC12, followed by the passive SMDs (resistors & capacitors), then the remaining SMDs and finally the through-hole components. Practical Electronics | July | 2025 Intelligent PSU Interface V1 CON1 Current RE2 Metalwork The heatsink used is an Altronics H0545 300mm diecast aluminium type, with the final four fins cut off, as shown in Fig.16. This is to leave room for the power connector and fuse on the rear panel of the recommended case. It might seem an odd thing to do to a perfectly good heatsink, but it is otherwise ideal for the job, just a tad too long! Cutting the heatsink is a 10-minute job using a hand-held hacksaw and a liberal dose of elbow grease. While it might look intimidating, no special tools are required. Clamp the heatsink to a workbench with cardboard protecting its surface and patiently work at it. Finishing off with a file will deliver you a neat result. We taped it up and applied a quick Shutdown 22nF S2 (UNDER) 22nF 22nF 1 22nF RE1 Voltage TP1 0202 MGT GND 10kW 22nF 22nF Exit 10kW 22nF This control module connects to both regulator modules with a multidrop 10-way ribbon cable, which we’ll make up shortly. It also connects to the front panel PCB, shown in Fig.14. There isn’t much to assembling this board. Just fit the two resistors, then the seven caps, followed by the header on the top. That just leaves rotary encoders and buttons, which mount on opposite sides. The encoders are on the top side and the pushbuttons on the underside. Make sure all of those are square and pushed down firmly before soldering their pins. When it comes to the rotary encoder, any of the horizontally-mounting “TT” 20-pulse-per-revolution parts with a switch should work (Mouser part code 858-EN11-VSM1AF20 has been verified as working). These are available with either a D-shaft or spline shaft. Once you have assembled this board, it’s a good time to make up the three ribbon cables, as shown in Fig.15. Cut the 10-way cable to 320mm and 250mm lengths and the 20-way cable to one 160mm length. Crimp on the IDC plugs as shown in the diagram. Note how the cable folds through the strain relief clamps at either end, but not on the sole middle plug. Some IDC plugs might not come with relief clamps. These lengths assume you are using the recommended case and will be sticking to our layout. If you are varying either, you might need longer cables, so check that first. S1 (UNDER) Dual Hybrid Bench Supply, part two Fig.14: this simple frontpanel board carries the two rotary encoders and two pushbuttons used to control the Supply, plus some debouncing components and pull-up resistors. It connects to the control board (shown in Fig.13) via a 10-way ribbon cable with DIL IDC connectors at each end. spray of black paint to the cut section, but you don’t have to do that. There are six mounting holes to drill to 4mm, and ten mounting holes for regulators and brackets. We drilled and tapped these to M3 x 0.5mm. We have laid the PCB out so that the mounting holes are between the fins; if you do not have an M3 tap, you can simply drill these to 3mm and use long screws and nuts to mount the power devices straight through the heatsink between the fins. Note how the mounting holes run along the top and bottom edges of the heatsink. All power device mounting holes are along the middle of the heatsink. In addition to the regulator ICs, the diode bridges are mounted to the heatsink, and there is a bracket in the middle of each regulator PCB. Power supply assembly Now it’s time to fit this all into a neat benchtop case. One of the design goals for this project was to keep interfaces and wiring simple and tidy. This is achieved by the PIC32 communicating with the regulator modules using an SPI interface. If you are into Arduino or Micromite, you could design your own controller. The majority of work now is in preparing the case and heatsink. The case we have specified is an ideal size for the workbench, and provides a professional looking finish to the product. You could use any other case of suitable size, with the only provisos being to ensure the case has adequate mechanical rigidity to secure the transformer and heatsink, and that it can be safely Earthed. Practical Electronics | July | 2025 Fig.15: these are the three IDC cables you will need to make up to connect the boards. The 10-way cable with two plugs connects the control board (Fig.13) to the front panel (Fig.14), the other 10-way cable with three plugs connects the control board to the regulator board(s) (Fig.10) and the 20-way cable connects the control board to the LCD screen module. 67 Constructional Project Mounting the regulator modules to the heatsink requires a little care. Our approach was to fix the mounting bracket, insert all the power devices into their PCB pads and jiggle it around to get them aligned. We then screwed them loosely into their mounting holes and soldered their leads, as follows: 1. Install and screw down the mounting bracket in the middle of the PCB. 2. Bend the leads on the MC34167 to ensure that the device will mount flush to the heatsink. This device is a relatively tight fit. 3. D o t h e s a m e w i t h t h e t w o LM1084IT-3.3 devices, ensuring that you get them to about the right height. 4. Using silicone insulators, insulating bushes, flat washers, shakeproof washers and 16mm M3 screws, loosely mount the power devices to their locations on the heatsink. It is best to do this with the heatsink flat on a desk and the regulator module facing upwards. 5. Tack solder on one pin of each device. 6. Where there is a misalignment, reflow the solder on the offending pin to adjust it. 7. Secure the MC34167, then the LM1084IT-3.3 devices. You can access the mounting screw for the MC34167 through the gap between the 4700uF capacitors. 8. Once everything is aligned and there is no stress on the PCB, gently tighten all the mounting screws. Watch out that tightening the screw does not twist the device around, and make sure you don’t overtighten them. 9. Now solder all the pins. 10. Mount the bridge rectifier on the heatsink now. It should already be wired to the PCB. 11. While you have the boards in this location, attach two 15mm long M3 threaded standoffs to the regulator module using 6mm M3 screws, flat and shake-proof washers. This will ensure it sits neatly on the desk. When mounting the MC34167, it is easiest to stand the heatsink on end, slip the regulators into their holes and get the insulator in the right spot. Then using long nose pliers, line up the screws with the insulating bush and washers in the hole so you can do it up. Repeat this process for the LM1084 regulators, then the other regulator module (assuming you’re building two). Now put your multimeter on a high ohms range (eg, 20MW) and check the resistance between the heatsink and the tab of the TO-220 devices. There should be an open circuit in each case. If not, remove the device and check what has gone wrong; check in particular for burrs on the screw hole in the heatsink. This process is repeated for the second module. Initial testing Connect the 20-way cable between the control board and LCD (being careful to line up pin 1 at both ends). Also attach the 10-way cable with two plugs between the control board and front panel board (the same comment applies) and the other 10-way cable between the control board and the one or two regulator boards. You can make some initial checks at low power and without mounting anything to the heatsink. Just don’t draw high currents! Install jumpers on JP1 & JP2. If you are using only one module, select channel one only. For dual rails, select channel one on one board and two on the other. If you are not using our microcontroller-based control board, you do not need to install these and should not have loaded the DAC, ADC or opto-isolators. During this testing, if it has not been mounted to the heatsink yet, make sure that the bridge rectifier Fig.16: this heatsink drilling pattern suits two regulator modules. The holes marked “A” are for mounting the heatsink to the case, while the two sets of holes marked “B” are for attaching the PCB-mounted semiconductors and bridge rectifiers on each module to the heatsink, plus a bracket to prevent the heatsink/PCB assembly from flexing too much. If you can’t tap the holes for M3, they are positioned between the fins, so you can drill through and use long machine screws, washers and nuts. 68 Practical Electronics | July | 2025 Dual Hybrid Bench Supply, part two can’t short anything out. Either place it somewhere safe or wrap it in insulating tape. Initial testing can be done by injecting ±15V DC into the board. Still, if you don’t have a suitable dual supply (maybe that’s why you’re building this one?), you can instead solder a 10W 5W resistor across a blown M205 fuse and use this in place of the onboard fuse while applying about 24V AC to the input terminals. If you have the dual DC supply, connect +15V to the rectifier side of the fuse, the external power supply ground to a ground point, such as the “-” on the bridge rectifier, and -15V to the large via just next to the 3300μF capacitor. You can solder a piece of wire into this via and clip a lead to it. Switch on and allow it to settle. The current draw should be less than 200mA. Check for the following voltages: • +12V (typically closer to 11.5V) on pin 2 of REG1 (LM317). This is also on the cathode of D2, just below the regulator. This should be within a volt of the expected value. • +5V (5.1V actual) on pin 2 of REG2 (LM317). This is also the cathode of D10, just below the regulator. This should be within 0.5V of the expected value. • -4.5V (-4.5V actual) on pin 3 of We left a 4mm gap between the big capacitors to allow a screwdriver to get to the tab of the TO-220 devices. It is tight, but enough for a standard Philips screwdriver. It’s easiest to start by holding the screw with longnose pliers. REG4 (LM337). This is also on the anode of D17 next to the regulator. This should be within 0.5V of the expected value. • +5V on LK1, generated by REG3 (LM2575). This should be within 0.5V of the expected value. If any of these are outside of the expected ranges, check the following: • Is the supply current high? Feel for components getting warm. • Look for solder bridges. • Check that the electrolytic capacitors are in the right way around. • Check that the regulators, diodes and ICs have been installed with the correct orientations. Assuming that’s all OK, verify that the pre-regulator is working. With no controller connected, the output should be set to 0V automatically. That means the pre-regulator should be producing around 5V. You can probe this on the output side of the 220μH inductor (the pin away from the MC34167). The exact voltage is not critical, but it should be between about 5V and 6.6V. If this is not as expected, check the following: • If you have an oscilloscope, set it to measure 5V/division and probe pin 2 of the MC34167. You should see some serious switching waveforms. It might not be switching at 72kHz, as the regulator will be unloaded and possibly running in discontinuous mode. • Check for solder bridges in the switchmode area. If the output is in the range of 0-0.5V, check for shorts around the schottky catch diode (D3). • Check the voltage on pin 1 of the MC34167. It should be close to 5V. Remainder of case Once you have finished preparing the heatsink, move on to the rear panel. Fig.17: the case’s front and rear metal panels need to be drilled and cut as shown here. The large rectangular opening at the rear allows the regulator PCBs to be admitted into the case after being attached to the main heatsink. The main heatsink then bolts to the rear of the case via the six holes marked “A” around the cutout. See the text for advice on how to cut the large holes. Practical Electronics | July | 2025 69 Constructional Project Remove it and drill and cut the holes, as shown at the bottom of Fig.17. To make the rectangular hole and the D-shaped hole for the mains socket, we drilled large holes in each corner of the cutouts and used a handsaw with a metal blade to cut along the outlines. Other approaches would be to use a jigsaw with a metal blade or a rotary tool (eg, a Dremel) with a metal cutting disc. In all cases, be somewhat careful as the material in the recommended case is aluminium, and you will easily bend it once cut. Once this is mounted to the heatsink, it will regain its strength. We have used a large hole to allow the complete heatsink assembly, with regulator modules, to slip in from the back. Now present the heatsink and reg- ulator modules to the rear panel. The assembly should slip through the large cutout, and the mounting holes in the heatsink should line up with those in the rear panel. If there is a minor misalignment, simply drill the offending holes to 4mm or so. Fix these using 16mm M3 machine screws, flat and star washers and nuts. Finally mount the IEC panel male socket and fuse holder. The IEC socket is fixed using 16mm M3 screws, flat and star washers and M3 nuts. The base needs a few holes drilled, shown in Fig.18. We have provided locations for the regulator module mounting holes. Still, given the variability in how you have mounted the PCB to the heatsink, you will be better off putting some masking tape at the identified locations, installing the rear panel with regulator modules mounted and marking the exact locations before drilling those holes. While you are there, mark and drill the remainder of holes in the baseplate: the Earth post near the IEC connector, the toroidal transformer mount, the two holes for the terminal block and four holes for the control PCB. Cut Presspahn or similar insulating material and place this under the terminal block. Mount and secure the terminal block with 16mm M3 machine screws and nuts. Remove the paint around the Earth post and mount a 16mm M3 screw with a shake-proof washer and M3 nut. Reserve another shakeproof washer, M3 nut and solder lug to attach the mains Earth wire. Mount the control PCB using four Fig.18: this shows where you need to drill holes in the bottom of the case to mount the regulator modules, transformer, control board and terminal blocks for terminating both the low-voltage and high-voltage windings on the main transformer. It’s best to check where exactly your regulator modules sit when mounted in the case to ensure their holes are drilled accurately. 70 Practical Electronics | July | 2025 Dual Hybrid Bench Supply, part two 15mm Nylon standoffs and 6mm M3 screws with fibre washers under the heads. This assures good separation of the mains Earth from the control PCB. Front panel The front panel needs a few holes and a large cutout for the LCD. Read through this section and check the measurements of your parts before cutting. This front panel will be right there on your workbench for a long time, so make it neat! The details are in Fig.17. Drill and deburr the holes as shown. For the LCD cutout, provided you are using the acrylic panel with paint to hide the cutout, you can err on the large side for the hole, as the paint will hide the hole you cut. Mount the switches and connectors, then attach the control PCB using the rotary encoders to secure it to the front panel. That just leaves the LCD screen. Mounting the display in a manner that insulates the LCD bezel from the case is fiddly. We found that some panels supplied have the LCD panel ground pin connected to its metal bezel. The display has 2.5mm mounting holes, so unless you have plenty of 16mm M2.5 machine screws and nuts on hand, drill these to 3mm. Clean up any burrs that result. Now take the 3mm acrylic sheet and cut it as shown in Fig.19 (or purchase a laser-cut version from the Silicon Chip Online Shop). The mounting holes on our display were 88mm by 65mm apart. We carefully drilled mounting holes through the acrylic as shown, and came up with an arrangement that mounts the acrylic to the front panel and holds the LCD to the acrylic rather than the LCD bezel touching the case. You could be lucky, and your LCD bezel may be isolated from the case – but please do not assume this to be the case, as we want isolation between the power supply and mains Earth. This arrangement gives you good clearance. To make things neat, after cutting and adjusting the cover to fit the LCD and case, we masked off the inside of the cover as shown, then spray painted it black. Once the masking was taken off, we had a neat black shadow line that hides the cut in the case and gives a professional appearance. The assembly of the acrylic cover, the case and LCD is as shown. The acrylic cover mounts to the metal case Secure the acrylic bezel to the case with the M3 screws, then slip the LCD screen onto their shafts and tighten it up against the acrylic with insulators under the nuts. Fig.19: as it’s tough to make a clean and rectangular cutout in the metal panel for the LCD, we designed this plastic bezel to cover up the screen surround. You can order a laser-cut clear bezel with these dimensions from our website (at the same time you order the PCBs etc), but you will have to paint the outer area yourself. Practical Electronics | July | 2025 71 Constructional Project not desired. These mains Earth points need to be wired back to the front panel Earth lug using green/yellow striped mains-rated wire. The final connections to the front panel are the outputs from each of the two regulators, made with red and black (or blue) 7.5A-rated cable. Make sure these are secure. Ours were 24cm and 34cm in length after twisting together and terminating. While you are here, it is worth terminating the mains side of the toroidal transformer to the terminal block. Mains wiring using M3 (or M2.5) machine screws. The hole in the case is slightly larger than the bezel on the LCD, so the LCD can then be secured using four more machine screws and fibre washers. At this point, all parts of the case should be cut and drilled, and the PCBs mounted and ready to wire up. Refer now to the wiring diagram, Fig.20. There are three control cables, which are routed as shown. Move on to the front panel and lowvoltage wiring. We have included the wiring of the bridge rectifiers for completeness, although this should have been addressed while building the regulator module(s). When connecting the wiring to the front panel, solder two 100nF 50V capacitors across the pairs of output terminals. These reduce the noise from the switch-mode section of the PSU on the output. Also install 10nF capacitors from each of the negative outputs of the two channels to mains Earth. Without this, the capacitive coupling through the mains transformer will induce substantial floating voltages on the channel outputs. Now it’s time to mount everything else in the case. Start by locating the mains transformer and wiring the outputs to the terminal strip as shown. We have used colour coding to match the Altronics M5525C 25 + 25V transformer. Check yours before proceeding as an error in this part of the circuit would not be good. 72 We used 13cm and 25cm pairs of 7.5A-rated cables (red) from the terminal strip to the AC inputs of the regulator modules. Twist these to keep things neat, and tuck them away between the boards. We have made provision on the front panel for mains Earth access at each of the outputs. In some circumstances it can be handy to connect one of the outputs to Earth, but other times it is Use mains-rated cables for all wiring. Be careful to check this and if you have someone you trust, get them to look over it too. With the fuseholder and IEC socket installed, fit the Earth screw at the rear of the case. Make sure you scrape the paint off the case in the bolt area and use star washers on top and bottom. Do it up securely. First, connect the Earth wire from the IEC socket to the Earth lug using a solder lug and heatshrink tubing, to keep things tidy. Make sure this is long enough that it will not be strained. Next, run all the Earth wires from the main Earth screw in the base to the panels shown in Fig.20, including The Regulator modules mounted to the heatsink slide straight into the case. We used nutserts on the rear panel, allowing us to screw the heatsink straight into them, making assembly a breeze. If you build a reasonable amount of sheet metalwork, do yourself a favour and buy a nutsert tool! Practical Electronics | July | 2025 Dual Hybrid Bench Supply, part two F RO N T PANEL PC B RE2 1 FRONT PANEL (INSIDE VIEW) LCD MODULE RE1 POWER SWITCH EARTH LUGS 100nF 100nF LCD ADAPTOR PCB 1 10n F 1 0nF INTELLIGENT POWER SUPPLY WIRING DIAGRAM (50% OF FULL SIZE) 25 V + 25V 300VA POWER TRANSFORMER ALPHA LCD CON12 CON6 DSP SPI1 1 1 2 1 20 19 GRAPHICAL LCD CON7 1 CON8 1 CONTROLLER BOARD CON5 CON10 PORTB 1 SPI2/I2S 1 JP5 CON23 ICSP 1 PRESSPAHN 1 1 + + + + + + + + + + + + + + + + + + + – + + + + + EARTH LUG + + + – + + + + + ~ ~ + + + CHANNEL 2 REGULATOR BOARD ~ ~ + + + + + + + ~ + BRIDGE RECTIFIER + + + ~ + + + + CHANNEL 1 REGULATOR BOARD BASE OF CASE G ND CON9 PORTE 1 25V + 25V 300VA TOROIDAL TRANSFORMER +7VDC CON11 1 + BRIDGE RECTIFIER EARTH LUG IEC PLUG PANEL MTG 17 Fig.20: this wiring diagram should make clear all the connections needed to complete the Supply. Ensure the Earth lugs are making good contact with the bottom of the case and the rear panel; if necessary, clean off any paint or coating around their mounting holes and use shakeproof washers to ensure they ‘bite’ properly. All mains wiring must be properly insulated, including at the rear of the front panel power switch and the rear panel mains input socket. 12 REAR PANEL (INSIDE VIEW) FUSEHOLDER Practical Electronics | July | 2025 73 Constructional Project Presspahn (or a similar insulator) is required under the mains terminal block for safety (shown along the right edge of the case). This photo shows the wiring in place. Make sure that all of the metal chassis panels are connected to mains Earth when assembled either via the securing screws or Earth wiring. the heatsink (eg, using one of the existing bracket mounting screws to attach it). Then using brown wire, connect the Active line from the IEC socket to the fuseholder, and from there to the power switch. We used a 6.3mm crimp connector here; you could solder it directly, provided you insulate the connection properly. Again, keep things secure, and use cable ties to ensure that, should any wire break or joint fail, the ends will be controlled and not create a hazard. Using light blue wire, run the Neutral connection from the IEC connector to the power switch. Ensure that you connect the IEC input to the bottom (switched) pins on the power switch. This way, when the power is off, the unused switch terminals will be connected to the transformer, not the mains. For safety, put heatshrink tubing on the unused power switch pins anyway. At this point, everything should be wired up and ready to go. contains garbage data, it will choose its own defaults to get things running. Do not rely on this as they might not be suitable for you! You can now power the unit back up, and should be able to fully control and monitor voltages and currents from up to two regulator modules. The initial setup procedure is: 1. Click the exit button to the lower left of the voltage set dial. This brings up the setup menu. 2. The voltage dial will allow you to select between three sub-menus: Track, Power and Cal. Enter the Track menu. 3. If you have a single output, N/A will be shown. Otherwise, select dual tracking or independent rails. 4. Enter the Power menu. For the number of rails, select single or dual-channel mode. 5. Set the absolute maximum current limit; this should be 5A in most cases. This can be set lower to limit current below that which the transformer VA rating allows; for example, if you are letting students loose with the Supply. 6. Dial up the maximum output voltage until the stated “required transformer voltage” matches your transformer. 7. Dial in the correct transformer VA rating. The recommended transformer is 300VA. 8. Enter the Cal menu and check the following as an initial starting point for both channels: 8.1 Output offset measured at zero volts set = 0mV 8.2 Set Correction Coefficient = 1.000 8.3 Read Correction Coefficient Scale = 1.000 8.4 Set Current offset = 0mA 8.5 Current Correction Coefficient Scale = 1.000 Calibration and use First, make sure you have the CH1 and CH2 jumpers on! When you power the unit up initially, if the EEPROM 74 Practical Electronics | July | 2025 Dual Hybrid Bench Supply, part two Now check that the Supply generally works. You can set the voltages for channels 1 & 2 using the left-hand rotary encoder. To swap between them, push the dial and it will click. Channel 1 or 2 will be highlighted on the screen. You can set current limits for channels 1 and 2 using the right-hand dial. Similarly to above, pushing the dial will swap between the channel 1 and 2 limits. By clicking either dial, you will save all settings. Now perform calibration. Check that the calibration offsets are zeroed as described above, or else this procedure will be confusing: 1) Output offset This sets the zero for measured voltage, taking out any offset. Set the Channel 1 voltage to 0.00V on the main menu, and measure the output voltage from the regulator module. Ours was -4mV; it should not be a large value. Go into the CAL menu. The first screen says “Ch#1 Output Offset Measured” (see Screen 1). Adjust the voltage dial in the opposite direction to your reading until the output reading is close to 0V. An output voltage within a few tens of mV of zero is acceptable. If you have to dial in a significant value, check your build as this should not be required. 2) Voltage correction coefficient This step sets the scale correction for output voltage, correcting for gain errors in the DAC and feedback network. Go back to the main menu and set the output to a high output voltage that you can measure accurately. For many meters, 19.99V is a good value. This will depend on the transformer you have selected. Now go to the second calibration screen (Screen 2) and adjust the Voltage dial until you read 19.99V (or your chosen value) on your voltmeter. Our coefficient was 0.966, as shown. A value between about 0.85 and 1.15 would be reasonable, although it’s likely to be in the range of 0.95 to 1.05. Do not worry about what the “Meas” voltage says on the main menu just yet! 3) Voltage reading correction coefficient scale This step sets an ADC measurement correction to ensure voltage measurements displayed in the main menu are accurate. With the voltage still set to 19.99V, click on the Voltage READ Practical Electronics | July | 2025 correction coefficient scale menu. You will see the output voltage at the bottom of the screen as measured by the Regulator module for that channel – see Screen 3. Adjust the Voltage dial until you get a reading of 19.99V (or your chosen value) on the bottom of the calibration screen. Our calibration factor was 1.037; values between 0.85 and 1.15 are reasonable. Screen 1 Screen 2 4) Current reading offset This step makes sure that you get current readings of 0mA when no current is flowing. With the voltage still set to any value, but no load connected to the power supply, click onto the Current Read Offset menu. At the bottom of the screen, you will see the current as measured by the Regulator module for that channel (Screen 4). Now adjust the Voltage dial upwards until you read a current on the bottom of the calibration screen, then dial it back to get zero. 5) Current scaling coefficient This step sets the calibration scaling for current measurements, so displayed currents are accurate. You need a dummy load for this test. Any highpower resistor will do; you can use quite a low voltage from the power supply, so two 1W 5W resistors in parallel will do for a short test. This keeps the dissipation to 4W per resistor. Depending on your transformer setup, choose a current that is close to your maximum; say, 4A for a 5A unit. Check that the current limit is set above this value. If you cannot set it high enough, go back and check your transformer configuration. Now put an ammeter in series with the resistor and dial the voltage to achieve your target current. Next, adjust the Voltage dial until you see the correct current reported on the bottom of the calibration screen – see Screen 5. Then click on the EXIT button. After that, press the voltage dial to swap channels and SAVE the calibration data. Calibration is complete for channel 1, so repeat the whole procedure for channel 2. You should find that measured values are within 1% or so of the actual values. We have not attempted to make a laboratory-­grade voltage source here, but the ADC we have chosen does have better than 0.1% resolution. Long-term Screen 3 Screen 4 Screen 5 precision will depend on the stability of the +5V internal voltage rail. Current measurement will be similar in terms of precision and stability. You will notice that if the current output is within 5% of the limit current, we highlight the “I” symbol on the user interface. Similarly, if the output voltage is too low or high, we highlight the “V” symbol. There are headers for LEDs that you can wire to the front panel for over-current indication too, if that takes your fancy. This completes the assembly and setup of the Intelligent Power Supply. We think this will be a valuable addiPE tion to most workbenches. 75