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Li’l PowerHouse Since preparing the first article for the Li’l Power­House we could not let well enough alone. Having seen how good this little switchmode power supply is, we could not avoid the temptation to improve it and so it now has even better regula­tion, and better residual hum and noise. How did we do it? Read on. By PETER SMITH & LEO SIMPSON As with any high-performance circuit, half the magic is in the “core” ICs and most of the rest lies in the PC board layout and associated wiring. And so it is with the Li’l PowerHouse. Having produced the prototype, confirmed that it all worked and prepared the first article for publication, somebody (who shall remain nameless) realised that there were a few tweaks that could be done to the circuit board and output filter network. Could the changes be made? Chief bean counter/publisher: “No! You’ve already changed the PC board 60  Silicon Chip from the one that’s shown in the first article”. Nameless one: “But it’ll be much better!” CBC & P: “No, you’re talking about a new PC board, a com­plete rewire, more photography, egg on collective editorial faces, pushing the deadlines...” Nameless one: “But I’ll work evenings, weekends, come in early in the mornings, make your morning tea...” CBC & P: “You’re on!” And it came to pass. The new PC board looks vaguely similar to the one Part 2 shown on page 61 of the June 2000 issue but in many respects it has been a complete revamp. As often happens, you change one aspect, which leads to another few changes and before you know it the PC board is looking radically different. Not only has the copper track layout for much of the PC board been changed, the output filter network is now quite dif­ferent. The second toroid filter now has two windings on the same core but we have managed to do without one 470µF 63VW filter capacitor and that is a worthwhile saving. Why didn’t we do it this way the first time? Answer: we’re not perfect . . . yet. Amended circuit Fig.5 shows the portion of the circuit which has been changed to incorporate the 2-winding version of toroid L2. These two windings are phased so that flux developed by L2a is can­celled by the flux developed in Despite the relatively minor circuit changes, the final PC board assembly differs quite markedly from the prototype shown last month. Please note: Some constructors have not been able to calibrate the current reading successfully, finding that the current reading is too high and cannot be adjusted low enough with VR2. If you encounter this problem, try reducing the value of the 15kΩ resistor connected to pin 2 of IC2. We suggest a value of 7.5kΩ. L2b. This effectively cancels the DC component of the flux and prevents core saturation. The two inductors on the one core effectively filter any common mode signals. Apart from this change, the circuit is otherwise identical to that published last month but the changed circuit board has also led to significant performance improvements. mounting holes which mate up with three of the integral pillars in the base. Fig.6 shows the complete wiring diagram and includes the component overlay for the PC board. Begin by checking the PC board for any etching defects and undrilled holes. If everything is OK, start by installing the PC stakes at all external wiring points. You will need 21 PC stakes, not 22 as listed last month. Note that the stakes should be a tight fit into their respective holes, before they are soldered. It is no use having PC stakes which fit loosely as they will tend to come away from the PC board when you attempt to solder wires to them. Next, install the resistors and wire links. Table 1 shows all the resistor values and their respective colour codes but you should also use your Building the Li’l PowerHouse The Li’l PowerHouse was built into a folded metal case measuring 200 x 162 x 67mm. Alternatively, it can be built into a standard plastic case measuring 200 x 155 x 65mm, with metal front and rear panels. All the circuitry, apart from the front panel components and the digital panel meter, is mounted on a PC board measuring 126 x 113mm and coded 04106001. If you are using the plastic case, you will find that the PC board has corner Fig.5: this diagram shows the amended output filter circuit. The main change involves the second toroid filter which now has two windings on its core instead of one. These winding effectively filter any common mode signals and together prevent core saturation. JULY 2000  61 Fig.6: install the parts on the PC board and complete the wiring exactly as shown here. Be sure to use 250VAC-rated cable for all mains wiring and sleeve all exposed terminals with heatshrink tubing to avoid the possibility of accidental contact. Note that the earthing details differ from the arrangement shown here if you use are using a plastic case with metal front & rear panels – see text. 62  Silicon Chip multimeter to check each value before it is installed. Resistor R1, the current sensing resistor, is a length of 0.4mm enamelled copper wire, installed as shown. Make sure you tin each end of the wire (scrape off the enamel at each end first) before soldering it to the PC board. By the way, don’t use anything other than 0.4mm enamelled copper wire for this job, otherwise you may have trouble calibrating the supply later on. Note also that the 680Ω 5W resistor should be mounted with its body about 2mm above the PC board because it will get quite hot when the supply is set to deliver close to 40V DC. Next, install the ICs, the diodes and zener diode, REF1 and the trimpots. Solder only the two outside pins of IC1 at this stage (do not trim the leads) so it can be easily adjusted to line up with the mounting hole in the rear panel. Note that while we have specified the 1A LM2575HVT-Adj device, you may be supplied with the higher-rated LM2576HVT-Adj which will work just as well (and also has the advantage of being a little more rugged). Make sure that the ICs and diodes are correctly oriented and be sure to use the correct device at each location on the board. IC sockets are optional (we used two as part of our proto­type testing). Zener diode ZD1 should be mounted with a small loop at one end to provide thermal stress relief. The capacitors can be installed next and make sure that all the electrolytics are installed the right way around otherwise there could be pyrotechnics when you first turn it on. Winding the inductors Both the inductors were specified as prewound types in the parts list in last month’s issue but we include the details here for those who want to make them. To make L1 you will need to tightly wind 60 turns of 21 B&S (0.71mm) enamelled copper wire onto a Neosid 33mm diameter pow­ered iron toroid (Neosid 17-742-22; Altronics L-5120). The turns are made side by side – do not overlap. A total of 210cm of wire is required. Scrape the enamel off the ends of the winding, tin them with solder and then solder the inductor into the PC board. Fig.7: the winding details for inductor L2. Two windings are required, each consisting of 13 turns of 22 B&S (0.63mm) enamelled copper wire. Once it is soldered in place, the toroid can be secured to the PC board with two ‘blobs’ of hot-melt glue or non-corrosive silicone sealant. For L2, two windings are required. Each winding is 13 turns of 22 B&S (0.63mm) enamelled copper wire wound tightly onto a 14.8mm Neosid powered iron toroid (Neosid 17-73222; Altronics L-5110). Again, the turns are made side by side – do not overlap. Two 40cm lengths of wire are required and they must be wound exactly as shown in Fig.7 to ensure correct phasing. Case preparation If you have purchased a kit with a pre-punched case and silk-screened front panel, you don’t need to worry about these next few steps. But if you are starting with a plastic instrument case with blank metal front and rear panels, you have some work ahead of you. For a start, some of the integral pillars on the base of the case must be removed in order to accommodate the PC board. To do this, first sit the board in the base, against the back panel, and use a felt-tipped pen to mark three mounting pillars (ie, the three directly be­neath the board mounting holes). This done, remove the PC board and remove all the unused pillars using an oversize drill. Note that you will also need to remove any pillars which will be underneath the transformer when it is mounted. Next, the various holes and cutouts in the front and rear panels need to be made. A hole needs to be drilled in each panel to mount an earth lug with a screw, nut and star washer (note: use a countersunk dress screw if the front panel is supplied screen printed). Choose a convenient point on the far lefthand side of the front panel for a common earth point and drill for a 3mm counter­sunk bolt. Drill a second 3mm hole in the rear panel in a roughly complementary position and run a length of wire from there to the common earth point on the front panel. Secure all four earth lugs to this point. The front panel label can now be fitted and used as a drilling template for the various holes. It’s always best to drill small pilot holes first and then carefully enlarge them to size using a tapered reamer. The square cutouts for the LCD panel meter and for switch S1 can be made by first drilling a series of small holes around the inside perimeter of the marked areas, then knocking out the centre pieces and filing each cutout to shape. Both the switch and panel meter clip into place with their integral retaining lugs. On the rear panel, you will need to drill holes to accept the mains fuse (F1), the cordgrip grommet and three solder lugs. Refer to the photos for the locations of these holes. In addi­tion, Fig.8: the mounting details for IC1. Its metal tab must be electrically isolated from the metal case using an insulating pad and bush. JULY 2000  63 Table 1: Resistor Colour Codes  No.   3   6   1   1   1   1   2   1   1   2 Value 1MΩ 100kΩ 27kΩ 15kΩ 4.7kΩ 1.5kΩ 1kΩ 470Ω 300Ω 100Ω you will also have to drill a mounting hole for IC1. The location of this mounting hole can be determined by fitting the PC board inside the case and sliding the rear panel into position. Mark out and drill the hole, then carefully deburr it using an oversize drill so that the surface is perfectly smooth. Finally, refit the rear panel and adjust IC1 as necessary before soldering its three remaining pins to the PC board. Fig.8 shows how IC1 is isolated from the rear panel using a mica washer and insulating bush. Smear all surfaces with heatsink compound before bolting the assembly together (note: heatsink compound is unnecessary if you use a silicone impregnated wash­ er). Finally, check that the metal tab of IC1 is indeed isolated from the rear panel using a multimeter switched to a low ohms range. Note that the legs of IC1 should not be under any stress when it is finally bolted into position. The PC board assembly can now be attached to the base of the case and the hardware items mounted on the front and rear panels. Before mounting the potentiometers, cut the shafts to a length to suit the knobs. Note that we have installed a 10-turn pot for the Voltage Adjust control (VR1) in our prototype but this is an optional feature. It is worthwhile having though. Final wiring Fig.6 also shows the wiring details. Begin this work by stripping back the outer insulation of the mains cord by 170mm, so that the leads can reach the mains switch (S1) on the front panel. This done, push the mains cord through its entry hole and clamp it 64  Silicon Chip 4-Band Code (1%) brown black green brown brown black yellow brown red violet orange brown brown green orange brown yellow violet red brown brown green red brown brown black red brown yellow violet brown brown orange black brown brown brown black brown brown securely to the rear panel using the cordgrip grommet. The Neutral (blue) mains lead goes directly to switch S1, while the Active (brown) lead goes to S1 via the fuse. Slide some heatshrink tubing over the leads before soldering the connec­ tions. After the connections have been made, the tubing is shrunk over the switch contacts and fuse to prevent accidental contact with the mains. Our prototype was built into a folded metal case and the earth­ing details are as shown in Fig.6 and the photos. We chose a point just in front of the power transformer. Use a 3mm bolt and star washers to secure the three earth lugs as per the wiring diagram. Make sure that you scrape away the paint from underneath these lugs and from under the transformer mounting bolts, to ensure a good earth connection. If you are using a plastic case with metal front and rear panels, the earthing details are different. In this case, the green/yellow striped lead from the mains cord connects directly to an earth lug terminal which must be securely bolted to the front panel. Additional green/yellow earth wires are then run from the front panel earth to the rear panel and from the front panel to the power transformer frame. Important: if the aluminium panels are anodised, you will need to scrape away the anodising from around the earth lug holes to ensure good electrical contact. All switches and pots are wired with light duty (1.5A or less) hook-up wire. Load switch S2 and transformer T1 should be wired with medium duty (5A or more) hook-up wire. The 0.33µF capacitor at the output is wired 5-Band Code (1%) brown black black yellow brown brown black black orange brown red violet black red brown brown green black red brown yellow violet black brown brown brown green black brown brown brown black black brown brown yellow violet black black brown orange black black black brown brown black black black brown directly across the backs of the output terminals (see Fig.6). The connections to the digital panel meter are made using a 13-way SIL connector and ribbon cable. You can cut down a larger socket strip or glue two smaller ones together. Note that the two wires running from pot VR1 to the rear of the board near IC1 are twisted and routed along the bottom of the case, hard up against the side (see photos). The idea here is to reduce noise pickup from inductor L1 as much as possible while minimising wire length. In addition, use several plastic cable ties to lace the mains wires together. This is an important safety measure as it prevents any wire that may come adrift from making accidental contact with any part of the metalwork or vulnerable low-voltage circuitry. Be warned that the wiring to pushbutton switch S4 (Set Current) may present a few problems if the switch specified in the parts list is not used. This is because some momentary pushbutton switches have their common (C) terminals in the middle and their normally open (NO) and normally closed (NC) contacts on the outside, whereas the switch we used has its common terminals at one end. If your switch has its common terminals in the middle, the wiring shown in Fig.6 will no longer be relevant and you will have to work out the connections from the circuit diagram pub­lished last month (Fig.4). The common, NO and NC terminals will usually be marked somewhere on the body of the switch. Note that the wire connecting the pole of S4b to the PC board (and to Keep all wiring neat and tidy and use plastic cable ties to bind the mains wiring. Note that a separate earth lead must be run to the transformer frame if you are using a plastic case, rather than the metal case shown here. the 100kΩ resistor at pin 2 of IC4) should be omitted at this stage. It will be connected later. Testing Before applying power, carefully check your work for any wiring errors. This done, wind VR1 fully anticlockwise and set VR2, VR3, VR4 and VR5 to their centre positions. Switch on the supply and check that the voltage across ZD1 is about 5.1V. If so, check the reading on the digital display. It should show about .000 for both positions of S3, the meter switch. Now adjust VR5 for a reading of exactly .000 on the panel meter. If everything is OK at this stage, you can check the supply voltages to each IC. Connect your multimeter negative lead to the cathode of ZD1 and check the voltage at pin 7 of IC2 and IC4, pin 8 of IC3 and pin 8 of IC5. These should all be at +5.1V. Pin 4 of IC2 should be at about -5V. If at any stage the voltages are incorrect, switch off immediately and correct the problem before proceeding. Voltage calibration Now turn off the supply and connect the missing wire from the pole of switch S4b to the PC board. This done, set the meter switch to the Volts position, turn the power supply on again and check that the output voltage is adjustable from 1.23V up to about 42V (depending on the mains voltage at your location). Connect a digital multimeter to the output terminals and with no load connected, set the Load switch (2) on. Now set the output voltage to read 39V on the digital multimeter (DMM) using the Voltage Adjust control VR1. Connect your DMM between pin 6 of IC4 (marked TP1 on Fig.6) and 0V (marked GND) and set the DMM to read millivolts. Adjust VR4 for a reading of 390mV on the DMM. Check the reading on the digital panel meter. If it is not 39.0, JULY 2000  65 Photo A: taken at an output of 12V and 160mA, this scope photo shows the high frequency ripple of about 2mV peak to peak. Note that the ripple and noise is now about 10 times better than we specified last month. Photo C: taken with the same output settings as photo A, the scope timebase has now been switched down to 5ms/ div to show the 100Hz ripple. This time the noise on the waveform is the residual 52kHz ripple shown in photo B. Photo B: taken at an output of 12V and just over 1A, this scope photo shows that the high-frequency ripple is not much higher than for the lower current condition and is 3mV peak to peak. The noise on the waveform is actually jitter caused by superimposed 100Hz ripple. Photo D: again taken with an output setting of 12V and 1A, these scope waveforms demonstrate the switchmode and filter operation. The top trace is taken from the input to L1 while the lower is the waveform at the input to L2. After passing through L2 the waveform becomes the same as shown in photo B. adjust the trimpot at the rear of the DPM for the correct reading. Current calibration Rotate the Current Limit pot (VR2) fully clockwise and set the meter switch (S3) to the Current position. Now connect a 4.7Ω 5W resistor across the output terminals, set the Load switch (S2) on and connect your DMM across the 4.7Ω resistor. Power up and adjust the output voltage to read 4.7V on the DMM using the Voltage Adjust control (VR1). Adjust trimpot VR3 for a reading of 1.000A on the DPM. Note that the 66  Silicon Chip 4.7Ω resistor will get very hot during this procedure – don’t burn yourself or singe your dining room table! Note: the DPM will read somewhere around .010 with no load connected. This is normal and is the result of the input offset of op amp IC2. This offset also results in a small difference between the current set reading and the actual current limit value although in most circumstances this small error should be of not consequence. Switch off the Load switch and press the Current set switch and check that the display reading can be varied from .010 up to at least 1.100A by adjusting the Current Limit control. Note that the overload LED may light when the control is fully anti­ clock­ wise. This is normal and the LED will extinguish when the current limit reaches about 10mA (0.01 on the display). Check the current limiting The current limiting feature should now be checked for correct operation. To do this, leave the 4.7Ω resistor in circuit and switch on the Load switch. Rotate the Current Limit control anticlockwise until the overload LED lights. This should initial­ly occur at These digital scope plots show the same waveforms as in photo D. Note the remnant switching spikes in the lower trace. These are completely removed by the second stage of filtering in L2. Fig.10: this full-size artwork can be used as a drilling template for the front panel. The cutouts can be made by drilling a series of small holes and knocking out the centre piece. Fig.9: this is the full-size etching pattern for the PC board. can observe the output ripple and noise. Right; it’s all finished and set to use. 1A but you should now be able to set lower current limits by further reducing the control setting. The power supply will “squeal” during current Footnote: kits for this project will be available shortly from Altronics in Perth. You can order on their toll-free number SC 1800 999007. limiting but this is quite normal. Finally, you can check the power supply on various loads and if you have access to an oscilloscope, you JULY 2000  67