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Mk2 0-30V 0-2A bench supply Part 2 by John Clarke This revised Bench Supply is basic yet feature-packed, with full onboard metering and an adjustable current limit. It’s pretty easy and cheap to build, so is suitable for relative beginners, and handy for various purposes, including powering circuits for testing or development. Let’s get to building it. P art of the reason for the 30V and 2A limits is that they allow us to use a modestly-­ sized transformer that fits neatly alongside the regulator board in a compact 160 × 180 × 70mm benchtop instrument case. The Supply is small enough to stay out of your way but powerful enough for many jobs. You could even stack two or three to have a few different voltages available or connect two in series to form a split supply. Just keep in mind that their current limits will be enforced separately, so if there is a fault, it’s possible that one Supply would go into current limiting while the other(s) wouldn’t. While this is a mains-based project, anyone who is good at following instructions and with reasonable soldering skills should be able to build it safely. Just make sure you perform all the wiring exactly as described using correctly rated wire, and don’t skip any of the required insulation or cable ties. 72 Silicon Chip Before we get to construction, a word about the metering. We tested some low-cost volt/ammeters from eBay but found that they were too inaccurate, which is why we specified the part from Core Electronics. Use caution if you want to substitute another meter, as its readings could be way off. If you missed the first part last month and are wondering why we’re revisiting this design after just one year, it’s because the multi-tapped transformer used in the 2022 version is no longer available. This version uses a readily-available transformer with independent 12V+12V secondaries, each tapped at 9V, wired in series. A small voltage inverter module makes up for the lack of a 30V tap. As with many projects, the first step in construction is soldering the majority of the components to the printed circuit boards. Construction Most of the parts for the Supply Australia's electronics magazine mount on three PCBs. The main 76 × 140mm PCB is coded 04107223 and includes most of the components, while a smaller 56 × 61mm PCB coded 04105222 has the front panel parts such as voltage and current setting potentiometers, indicator LEDs and load switch. A 14-way ribbon cable with insulation displacement connectors (IDCs) joins the two PCBs. The voltage inverter circuitry uses a 37 × 42.5mm PCB coded 04107222 that mounts vertically on the main board using short tinned copper wire links and a support wire at the top. As explained last month, there is the option to use a single 2.5kW multi-turn potentiometer for VR1 or a standard single-turn 5kW potentiometer in conjunction with a 5kW multi-turn trimpot (VR2). If you are using the 2.5kW multi-turn potentiometer, VR2 is not used and must be left off the PCB. During the following process, refer to the PCB overlay diagrams (Figs.3 & 4) to see which parts go where. siliconchip.com.au Begin construction with the main PCB (Fig.3) by fitting the two surface-­ mount components. These are the INA282 shunt monitor (IC2) and the 20mW resistor. For the resistor, we have made provision on the PCB for either two 10mW resistors in series or a single 20mW resistor. Both the resistor and IC are relatively easy to solder. Find the pin 1 orientation marker on the INA282. This can be a dot on the top face, a notch at the pin 1 end of the device, or a chamfer along the pin 1 to 4 edge of the package. Position the IC over the pads and solder a corner pin using a fine-tipped soldering iron. Once soldered, check the alignment against the remaining IC pin leads and PCB pads. Remelt the solder and realign the IC if necessary until each pin aligns with its pad, then solder the remaining pins to the PCB. Any solder bridges can be fixed using solder wick with flux paste to draw up the excess solder. The surface-mounting resistor can be soldered similarly, one end at a time. Straighten the resistor by remelting the solder and nudging it after the first end is soldered should it be skewed. The next components to be installed are the through-hole (axial) resistors. The resistors have colour bands, but it is a good idea to check the values using a multimeter too. Fit the 12 diodes of four types next. They are all polarised and must be orientated as shown in Fig.3 and the screen printing on the PCB. Use the smaller glass-encapsulated 1N4148 diodes for D5, D6 and D9. On the other hand, diodes D1, D4, D7, D8 and D10 are larger 1N4004 devices, while D2 is a larger still 1N5404 diode. The three remaining diodes are zener diodes ZD1, ZD2 and ZD3, which are in medium-size glass packages. ZD1 is 33V (1N4752) while ZD2 and ZD3 are 12V (1N4742) types. Ensure each is installed in the correct position and with the correct orientation. Operational amplifier (op amp) IC1 can now be installed, taking care to orientate it correctly. This can be mounted using a socket or directly on the PCB. Follow with transistors Q2-Q6 and REG2. These all are in TO-92 plastic packages, so be sure the correct device is installed in each location. Q2 is a siliconchip.com.au Fig.3: the components fit on the main PCB as shown here; watch the orientations of the polarised parts. VR2 is only needed if VR1 is 5kW; in that case, it has the adjustment screw towards the top like the other trimpots. Leave Q1, REG1 and the inverter module off until the case has been prepared. Ensure the sockets for CON1 and CON2 are orientated as per the photos, so that the wires entries are not blocked by other components. Fig.4: this board carries the front panel controls and indicator LEDs. Potentiometer VR3 is held to the board using PCB pins, and its terminals are also connected via PCB pins. VR1 is attached using brackets on either side of its body and connected to its three pads (labelled “Anti CW”, “Wiper” and “CW”) via short lengths of wire. Australia's electronics magazine October 2023  73 An internal photo of the completed Supply minus both PCBs, so you can more clearly see where the various other parts mount and how the wiring is run. Note the locations of the three plugs in the lower portion, ready to plug into the main PCB. 2N7000 while Q3-Q5 are BC547s and Q6 is a BC327. REG2 is the LM336-2.5. Mount the trimpots next. These are top-adjust multi-turn types; two are 10kW (VR6 and VR7), one or two are 5kW (VR2 and VR4), while VR5 is 100W. The 10kW trimpots might be labelled 103, the 5kW trimpots as 502 and the 100W trimpot as 101. Be sure to orientate these with the adjustment screws as shown in Fig.3. Note that if using a 2.5kW multi-turn pot for VR1, VR2 is not fitted. Now install rectifier bridge BR1; the diagonally cut corner is the positive side, so make sure that it is orientated as shown. You can install the three and fourway pluggable terminals for CON1 and CON2 now. Ensure these are orientated correctly by inserting the plugs into the sockets first, then rotating them so that CON1’s screw heads face toward CON3 and CON2’s screw heads are toward the edge of the PCB. Then solder the terminals in place, followed by box header CON3, with its notch facing as shown. There are 12 test points located around the PCB. You can fit PC stakes/ pins in each or leave them bare and use your multimeter probe directly 74 Silicon Chip onto the PCB pad instead. It is easier to have a PC stake at TP GND so that you can use an alligator or crocodile clip connection for measurements with respect to 0V. If fitting the PC pins, do that now. Mount the capacitors next. The 100nF, 10nF and 1μF ceramic types can be installed either way, but most of the electrolytic capacitors are polarised and must be inserted with the polarity shown. The positive side usually has a longer lead, while there Fig.5: this shows the components on the voltage inverter module. Ensure the electrolytic capacitors, IC1 and the diodes are all correctly placed and orientated. The electrolytic capacitors lie flat against the PCB. Australia's electronics magazine is a stripe on the negative side of the can. The 10µF capacitor marked NP is non-polarised and can insert either way around. Now fit relay RLY1 and two-way header CON7. Leave Q1, REG1 and the voltage inverter module off for now. Front panel PCB assembly The front panel PCB (Fig.4) has components mounted on both sides. The potentiometers, switch and LEDs are on the top, while CON4-CON6 are mounted on the underside. It is easier to solder in the 14-way box header (CON4) first so that you have full access to solder its pins on the top side of the PCB. It is installed on the underside of the PCB; ensure it is orientated correctly before soldering it in place. Next, install the six PC stakes for VR1 and the three for VR2. Then fit CON5 on the underside of the PCB, with its wire entries towards the nearest PCB edge. Mount switch S2 on the top side of the PCB. This sets the height position for the potentiometers and LEDs; however, LED1 and LED2 are mounted after the front panel holes are drilled and LED bezels are inserted. siliconchip.com.au Fit VR2 next, but first cut its shaft so that the length from the top of the threaded mounting boss to the end of the shaft is 15mm. VR2 is supported by PC stakes soldered to the potentiometer body. You need to scrape off the passivation coating in the area where the PC stakes will be soldered so that the solder will adhere. Solder it so that the top of the threaded section matches that of switch S2. Once it is in place, make the electrical connections to the potentiometer using PC stakes. Mounting VR1 The mounting method for VR1 depends on whether you are using a single-turn or multi-turn pot. The circular cut-out allows the multiturn potentiometer to pass through the hole. Solder right-angle brackets to the back of the PCB and use a cable tie to position the pot as shown above. Connect short wires from the pot terminals to the wiper, anti-clockwise and clockwise terminals on the PCB. Similarly, if using a single-­ turn pot, it is held in position by right-angle brackets soldered to the pot body and the PCB. The brackets need to be soldered to the PCB such that they reach the pot body and there is some overhang from the cut-out. Again, you will have to scrape off the passivation coating from the pot body where you will solder the brackets. For a single-turn pot, solder its terminals directly to the PC stake connection points. The main PCB (with voltage inverter attached) and both sides of the front panel PCB are shown at actual size. Compared to the original Bench Supply design, the main PCB uses a larger PCB which fits around the transformer. The front panel PCB is unchanged. Voltage inverter The -8V inverter module is assembled as shown in Fig.5 (also see the separate article starting on page 90 last month). We use a 220W 1W resistor for R1 and a 12V 1W zener diode (ZD1) to regulate the supply for the inverter to 12V. Make sure the electrolytic capacitors, IC1 and the diodes are all correctly placed and orientated. The finished PCB is installed vertically on the main PCB using short lengths of 0.7mm tinned copper wire or component lead off-cuts. Connect the Vin, GND and Vout on the inverter module to the main PCB at its matching Vin, GND and Vout pads. siliconchip.com.au Australia's electronics magazine October 2023  75 Making the ribbon cable Fig.6 shows how the IDC line sockets are attached to the ribbon cable. Ensure the 14-way wire and sockets are orientated correctly, with the notches positioned as shown, before compressing the connectors. You can do this by placing a small piece of soft timber (such as radiata pine) over each side of the connector and compressing it with a G clamp or bench vice. Alternatively, you can use a specialised IDC crimping tool. Metalwork Close-ups of the components and wiring behind the front panel. Note the mains switch insulation. Now it’s time to drill and shape holes in the baseplate of the enclosure and the heatsink, as shown in Fig.7. Rectangular and similarly-shaped cut-outs can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing the job to a smooth straight finish. The power switch hole must be sized so that it stays clipped in when inserted into the cut-out, so take care when shaping it. The banana sockets have ovalshaped holes (“F”) that can be made by first drilling round holes and then using a round file to elongate them. There are four holes for mounting the regulator, power transistor and thermal switch on the rear panel; these are the holes marked “A” not near the mains input socket. After drilling them, clean them up around the edges on both sides with a deburring tool or a larger drill bit, so there are no sharp edges around the hole perimeters. This will avoid puncturing the insulation pads for the regulator and transistor and allow the heatsink to sit flat against the rear panel for maximum heat transfer. Fig.6: fit the IDC line sockets to the cable as shown here. This way, pin 1 is correct on both sockets but having them on opposite sides makes routing the cable easier once everything is in the case. Note that some sockets don’t come with the third locking bar over the top, in which case the ribbon cable isn’t looped. 76 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.7: the shapes and sizes of some of the cut-outs are critical, so file them to shape carefully and periodically test to see if the parts fit in the holes. For example, the panel meter will fall out if its hole is too large, as will the rocker switch. For the binding posts (marked “F”), drill round holes, then elongate them to ovals using a round file. It would give even better heat transfer to the heatsink if you cut out a rectangular hole for the transistor, so the transistor and its insulating pad can be mounted directly against the heatsink instead of the rear panel of the case. However, we found that mounting onto the rear panel provided sufficient heat transfer to the heatsink, satisfactory for most uses of the Bench Supply. Still, if you require a high current at low voltages for an extended period, having this cut-out will reduce the transistor temperature. Once the drilling and cutting are finished, temporarily install the mains IEC input connector and then place the heatsink against the back panel with its side about 1mm away from the IEC connector and the top edge in line with siliconchip.com.au the top edge of the rear panel. Mark out the positions for the transistor, regulator and thermal switch holes on the heatsink through those already in the back panel. Make sure all the holes will be within the central mounting area of the heatsink and not through the fins, or the screws won’t fit. Once you’ve checked that, drill them in the heatsink, then deburr them for a smooth finish on the heatsink. regulator can later be attached to the rear panel via the pre-drilled holes using machine screws (temporarily secure the transistor and regulator to the rear panel with M3 screws and nuts). Adjust the leads so that the device tabs sit flat against the rear of the case. Then, ensuring the PCB is straight and not skewed in the case and the standoffs are directly on the base, solder the leads to the PCB on the top side. Case assembly Next, mark out the locations for the Attach the four 6.3mm-long M3-­ standoff mounting holes in the base tapped spacers to the corners of the of the case. Also mark out the mountmain PCB using 5mm M3 machine ing holes for the transformer. This sits screws. Next, insert the power transis- between the left edge of the PCB in-­ tor and the regulator leads into their between the PCB cutout and the left allocated holes in the PCB. edge of the case, leaving equal clearSlide the PCB so the transistor and ance on both sides. Australia's electronics magazine October 2023  77 Fig.8: the internal case layout and wiring. Take care that your unit is wired up exactly as shown here, especially the mains wiring, and don’t skimp on the cable ties, insulation or Earthing. 78 Silicon Chip After that, remove the transistor and regulator mounting screws. Solder the transistor and regulator leads on the underside of the PCB. Now drill the holes for the PCB and transformer (see Fig.8 for the component layout in the case). Also, drill the Earth lug holes in the base and scrape away the paint from around the holes so the Earth connections will be against the metal, not the paint. Attaching the heatsink The heatsink is a little taller than the enclosure. There are two ways of stopping the heatsink from touching the workbench, as the enclosure mounting feet are not tall enough to prevent this from happening. One option is to add extra spacers between the feet and the case, such as two M3 Nylon washers under each foot to raise the enclosure a little. This prevents the heatsink from touching the bench. Use the longer self-tapping screws supplied with the enclosure to secure the mounting feet. The second method is to cut the bottom of the heatsink off, so it is 67mm tall. That can be done with a hacksaw or a metal cutting saw. After you’ve sorted that out, apply a smear of heatsink compound to the rear of the heatsink. Press it onto the rear panel in its correct position and install the thermal cut-out using 15mm-long M3 machine screw and nuts. Leave the screws loose for the moment, so there is movement to adjust the mounting. Insert the 20mm screws for the transistor and regulator through the heatsink, then feed them through the rear panel. Place the TO-3P silicone washer for Q1 and TO-220 washer A close-up of the thermal switch wired up and insulated. for the regulator onto the screw ends. Now you can re-mount the PCB, with the mounting screws for the regulator and transistor passing through the device holes. Push the insulation bush into the regulator mounting hole before attaching it with a hex nut. For the transistor, add a steel washer against the device before attaching the nut. Secure the PCB to the base with M3 × 5mm screws and then tighten up the screws for the thermal cut-out, transistor and regulator, ensuring the heatsink stays square against the rear panel. The main PCB is attached to the base using four M3 × 5mm screws with Nylon washers. The washers allow the screws to tighten into the standoffs without touching the screws that enter from the top. Front panel label The panel label (see Fig.9) can be made using overhead projector film, printed as a mirror image so the ink/ toner will be between the enclosure and film when affixed. Use projector film that is suitable for your printer (either inkjet or laser) and affix it using clear neutral-cure silicone sealant. Roof and gutter silicone is suitable. Squeegee out the lumps and air bubbles before it cures. Once cured, cut out the holes through the film with a hobby or craft knife. For other options and more detail on making labels, see the page on our Fig.9: this front panel label can be downloaded as a PDF from the Silicon Chip website and printed out to form a label for the case. There is an alternative label without voltage markings to suit a multi-turn potentiometer. siliconchip.com.au Australia's electronics magazine October 2023  79 79 website: www.siliconchip.au/Help/ FrontPanels Insert the two LED bezels for the LEDs into the front panel and place the LEDs into the holes from the top side of the PCB, taking care to orientate them with the longer lead to the anode (“A”) side. Push the LEDs down onto the PCB but do not solder the leads yet. Break off the locating spigot on potentiometer VR3 (and single-turn potentiometer VR1, if used) and mount them onto the front panel with the washer on the pot side and nut on the outside. Then mount the on/off switch with one nut on first, to set the depth that the panel sits into the threaded section, then place the second nut on the outside to hold it in place. Move the LEDs off the PCB, insert them into the bezels and solder the LEDs in place. The front panel PCB is held in position by the switches and potentiometers; there is no need for extra support. If you wish, you can add 15mm-long standoffs at a couple of the corners. Now attach the pot knobs. For VR2, ensure the pointer is correctly positioned so it points to the end stops on the front panel label at both rotation extremes. Remaining parts Mount the IEC connector to the rear panel using M3 × 15mm screws and Summary of major test points TP1 is the negative voltage applied to REG1 via VR1 and VR2. It is measured with respect to GND (or V- at CON2) and can range from -1.2V to -1.3V. VR6 is adjusted to provide a 0V output at V+ on CON2 when VR1 is fully anti-clockwise. TP2 is the -2.49V reference. It is measured with respect to GND (or V- at CON2) and adjusted via VR7. TP3 is the current limit setting, measured between TP3 and TP10 at CON6, that ranges from 0V to 2V when correctly adjusted. The upper and lower thresholds are adjusted by VR4 and VR5, respectively. CON6 allows the current limit setting of VR2 to be measured using a multimeter or other floating voltmeter. TP4 is the negative supply and should read -8V to -9V relative to GND. TP5 is the output of current monitor IC2, giving 1V per amp of load current, measured with respect to TP2 (-2.490V). TP6 is the negative voltage applied to IC1a. TP1, the output of IC1a, should be within a few millivolts of TP6. See above for the significance of TP1. TP7 should be near 0V, rising toward 0.6V when power is switched off, measured with respect to GND. This is the AC detection voltage for the relay switching. 0V = AC detected, 0.6V = no AC detected. TP8 should rise from 0V to 13.6V with respect to GND over several seconds when power is first applied and drop quickly to near 0V when power is switched off. The time the voltage takes to rise from 0V to 13.6V is the switch-on delay. TP9 should be at about 12V with respect to GND, generated by zener diode ZD2. TP10 is the current setting offset to compensate current readings at TP5 (see TP3 above). TP21V is the positive supply and should measure around 21V with respect to GND. 80 Silicon Chip Australia's electronics magazine nuts, and the transformer to the base using four M4 × 10mm screws, star washers and nuts. The panel meter can be installed next. This is intended to slide and clip into the panel cut-out, but the top and bottom clips will not compress because they impinge on the seven-segment displays. The solution is to lever out the side clips to allow the internal PCB and displays to come out of the surround, then insert the surround through the front panel. The top and bottom clips can now be compressed so the meter can sit in the front panel. Once it’s in place, reinstall the meter internals. Mains wiring All mains wiring must be done using mains-rated cable. Be sure that brown wire is used for Active and blue wire is used for Neutral. The green/yellow-­ striped wire is for the Earth wiring only (see the wiring diagram, Fig.8). Connect the mains leads to the IEC connector and use a cable tie to secure the wires together and insulate using the rubber boot after it is cut so that the main section is 30mm long. This is so there is room for the transformer. Pass the wires through the boot before fitting it. The Earth wire from the IEC connector must go straight to the Earth mounting point on the case. This is attached using a crimp eyelet secured to the base with a 10mm M4 screw, star washer and two M4 nuts. If you haven’t already done so, scrape the paint away from around the hole to ensure the Earth connects to the metal of the case and not just the paint. The wires connect to the mains switch using female spade crimp connectors. Be sure to cable tie the wires together to prevent any broken wires from coming adrift. Additionally, cover the rear of the switch and the spade connections with 25mm diameter heatshrink tubing. Connect the transformer secondaries to CON1 using 7.5A-rated wire. Next, connect the IDC cable between the two boards and wire up the meter. The supply ground (thin black wire) for the meter is not connected and can be either cut short or connected to the NC terminal at the centre of CON5. That centre terminal is used as a wire keeper; it makes no electrical connection. Attach the banana sockets to the siliconchip.com.au front panel, wire them up to CON2 (black for negative, red for positive) and connect the Earth terminal to the chassis. Testing and calibration Before applying power, check your wiring carefully and ensure all mains connections are correct. If you are using a socket for IC1, insert it now with the proper orientation. Take care that none of its leads fold under its body during insertion. Wind VR1 fully anti-clockwise and VR3 a little clockwise from fully anti-clockwise. This sets the Supply to its minimum output voltage at a low current. Wind VR6 fully clockwise by turning it until a faint click is heard, or if you don’t hear a click, wind for 20 turns in the clockwise direction. This prevents the regulator output voltage from going negative initially before being set up correctly. Switch power on, and the voltmeter should show around 1.2-1.3V. Check that you can increase the output voltage by rotating VR1 clockwise. Do not go above 35V as the output capacitor is only rated to handle 35V. If the Supply does not appear to be working at this stage, recheck your construction. In particular, check that there is about -8V (or similar) at TP4 and about 21V at TP21V. Check that TP1 is around 0V. Re-check the component placement and soldering. Once the voltages appear correct, it is time to make adjustments. Firstly, the precision reference needs to be set. Measure the voltage between TP GND (or the negative output terminal on the front panel) and TP2, and adjust VR7 for a reading of -2.490V. Once adjusted, the regulator can be set to produce a minimum of 0V. This is done by initially winding VR1 fully anti-clockwise and measuring between the Supply’s output terminals. Adjust VR6 anti-clockwise until the reading just reaches 0V. Next, we set the maximum 30V output range. This is only if you are using a single-turn potentiometer for VR1. For the multi-turn potentiometer, ignore this step since VR2 is not fitted. For the multi-turn pot, the maximum voltage will be close to 30V when VR1 is wound fully clockwise, possibly a little more. Carefully adjust VR1 clockwise and stop where the voltage is 30V or when the pot is fully clockwise, whichever comes first. If the pot has reached full clockwise rotation and the voltage is less than 30V, adjust VR2 clockwise until you get a 30V output. If 30V is reached before full rotation, adjust VR2 anti-clockwise and VR1 clockwise a little each time until 30V is reached with VR1 fully clockwise. The current limit range is adjusted by rotating VR3 fully clockwise and measuring between TP2 and TP3. Adjust VR4 to obtain 2V. That sets the maximum current to 2A. The minimum current setting alters the lower end of VR3 to cancel out the offset voltage of IC2. To set this, rotate VR3 fully anti-clockwise, then measure between TP5 and TP10 and adjust VR5 for 0V. It shouldn’t be necessary to readjust VR4 again for the maximum current limit as the voltage adjustment made with VR5 will only change the maximum current setting by about 20mV, which is insignificant compared to the original setting at 2A. But you could SC tweak it again if you want to. The Supply should look like this once you have finished fitting all the parts and wiring them up. After checking it works, all that remains is to attach the lid using two of the supplied screws on either side. siliconchip.com.au Australia's electronics magazine October 2023  81