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40V 8A Adjustable Power Supply; Pt.2 Last month we provided the circuit details for this com­pletely revised 40V 8A adjustable power supply. This month, we cover the construction. Most of the parts are mounted on a large PC board and there are only two setting up adjustments. By JOHN CLARKE The new power supply is housed in a large plastic instru­ ment case measuring 355 x 250 x 122mm. Our prototype case is light blue in colour although currently available cases come in grey or black, The case uses an internal steel baseplate to provide adequate strength and requires the addition of aluminium front and rear panels rather than the plastic ones supplied. The case and baseplate are 74  Silicon Chip available from Altronics (see parts list from last month). Most of the circuit components mount onto a PC board meas­uring 80 x 94mm, code 04304981. The remaining components are either mounted on the steel baseplate or onto the front or rear panels. Begin assembly of the power supply by checking the copper pattern on the PC board. It should be free of any shorted, missing or open circuit tracks. Check the pattern against the published artwork of Fig.7 to be sure that the board has no faults. Fig.1 shows the component layout on the PC board. You can start by installing the PC stakes first and then the links which can be made from tinned copper wire or from component pigtails. Note that you need to use 1.25mm diameter tinned copper wire for the links between the drains of Q1 and Q2 and transformer T2. The other links can be made from the standard 0.8mm diameter tinned copper wire. Insert the resistors next. You can use the accompanying colour code in Table 1 to check each resistor value, or easier still, use your digital multimeter to measure them. The 5W resis­tors should be mounted with about a 2mm gap between the resistor body and the PC board. This will allow for free air Fig.1: the component overlay for the PC board. Take care to ensure that all polarised parts are correctly oriented. flow to assist cooling. Diodes D5 & D6 can be installed next, along with zener diodes ZD1 & ZD2 (top left of Fig.1). Take care with their orien­tation. The ICs can be inserted at this stage and be sure that each one is oriented as shown and with the correct type number, before it is soldered in place. Check that there are no solder bridges between the pins. By the way, IC sockets are a worthwhile option here. Next, the capacitors can be installed. The MKT polyester types are marked with a value code as shown in Table 2. When inserting the electro- lytic capacitors, make sure they have the correct polarity, as shown on Fig.1. Transistors Q3 & Q4 are installed by pushing them down so that the lower edge of each device body is about 8mm above the board. The trimpots can go in next. VR3, the 5kΩ trimpot, could be marked 502 rather than 5kΩ. Similarly, the 50kΩ trimpot (VR4) may be marked as 503 and the 500Ω trimpot (VR5) may be marked as May 1998  75 501 rather than 500Ω. This coding is similar in principle to the EIA coding on capacitors. The 3-terminal regulator REG1 is mounted on a small heat­sink. Loosely bolt the device and its heatsink to the PC board and bend the component leads at rightangles so that they can be inserted into the allocated holes. Once you have soldered the leads in place, tighten down the screw and nut. Winding the coils Inductor L1 uses the ETD34 transformer assembly and its winding details are shown in Fig.2. It comprises two 20-turn windings wound side-byside. Use 0.8mm diameter enamelled copper wire and terminate one end on pin 3 and the other end on pin 4. Now carefully wind both wires together for 20 turns. Terminate the wires onto pins 11 & 12. Check with your multimeter that there is continuity between pins 3 & 12 and between pins 4 & 11. One of the ferrite cores can be inserted into the bobbin and secured in place with the steel clip. Now place the 10 x 5 x 0.5mm spacers on the two outside core faces and insert the second core. Fit the clip in place to secure this ferrite core in place. The assembly of L1 is now complete with a 0.5mm gap between its core faces. Since the assembly is symmetrical, L1 can be inserted into the PC board either way around. Transformer T2 is a little more tricky to wind than L1. It is wound on the larger ETD44 bobbin and core Fig.2 (above): these diagrams shows the winding details for T2 and L1. Fig.3: the winding details for toroids L2 and L3. Note that the two windings on both cores are wound in different directions. Table 1: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  2 ❏  3 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  2 ❏  1 ❏  4 ❏  6 ❏  2 ❏  3 ❏  2 ❏  2 76  Silicon Chip Value 1MΩ 220kΩ 100kΩ 47kΩ 33kΩ 27kΩ 22kΩ 18kΩ 12kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 470Ω 100Ω 47Ω 10Ω 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown yellow violet orange brown orange orange orange brown red violet orange brown red red orange brown brown grey orange brown brown red orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown yellow violet brown brown brown black brown brown yellow violet black brown brown black black brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown orange orange black red brown red violet black red brown red red black red brown brown grey black red brown brown red black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown brown black black black brown yellow violet black gold brown brown black black gold brown Fig.4: this diagram shows the physical layout and wiring of the power supply. For clarity, we have used a numbering system in­stead of showing every single wire interconnection. So, for example, points 1 & 2 on the PC board go to points 1 & 2 on LED1 on the front panel, points 7 & 8 on the board go to points 7 & 8 on potentiometer VR1, and so on. May 1998  77 Fig.5: the mounting details for the Mosfets (Q1, Q2) and fast recov­ery diodes (D1-D4). Table 2: Capacitor Codes ❏ Value ❏ 0.1µF ❏ .01µF ❏ .001µF IEC 100n 10n 1n0 EIA 104 103 102 enamelled copper wire, wind on 13 turns as shown in Fig.3. This winding should cover slightly less than one half of the core. The second winding is also 13 turns of the same gauge wire, wound on the second half of the core. Note the direction of the winding. This toroidal inductor is installed in place on the PC board and held in position with two cable ties, as can be seen in the photographs. Check that the coil wires are soldered correctly by checking for continuity with your multimeter. Inductor L3 is wound on a 33mm toroidal core, in a similar fashion to L2. Fig.3 shows the winding details. It uses 1.25mm diameter wire and has 8 turns per winding. Be sure to wind in the directions shown for each winding. L3 does not mount on the PC board but is connected to the power supply output terminals. We will refer to this later. Insulated links Fig.6: this is a load diagram of the power supply to show how much current is available at various voltages. The maximum power output available is 35V at 8A, corresponding to 280W. This is not much less than the 300VA rating of the power transformer. assembly. It has four quadrifilar (ie, 4 wires wound together) primary windings and a bifilar (2 wires) secondary. Again, the details are shown in Fig.2. Start by soldering four ends of 0.8mm enamelled copper wires onto pins 3, 4, 6 & 7. Now carefully wind all four wires together and each side by side for 15 turns. Terminate onto pins 12, 13, 15 & 16. Note that the wire starting at pin 3 must termi­nate on pin 16; pin 4 must connect to pin 15; pin 6 must connect to pin 13; and pin 7 must connect to pin 12. Check the continuity of each with your multi­meter to ensure that these connections are correct. The secondary is wound with two lengths of 0.8mm diameter enamelled 78  Silicon Chip copper wire, starting on pins 1 & 2. Wind both wires at the same time for 20 turns (the direction of winding is unimport­ant) and terminate at pins 17 & 18. Ensure that the winding starting on pin 1 finishes on pin 18 by measuring continuity with your multimeter. Insert the ferrite cores in through the bobbin and secure them in place with the steel clips. There is no spacer required between the cores for this assembly. T2 inserts into the PC board holes with pin 1 oriented as shown. Toroid coils Inductor L2 is wound on a 44mm toroidal core. Using 1.25mm diameter Two lengths of heavy duty hookup wire can now be connected from PC stakes just below pins 1 and 16 of T2 to the allocated pins above inductor L1. Finally, insert and solder in the two Mosfets (Q1 & Q2) and the four power diodes (D1-D4). The leads for these are inserted into the PC board so that there is about 1mm of lead length below the copper side of the board. Mounting the hardware Now that the PC board is complete, work can begin on the case. We will assume that you are building the power supply from a kit of parts which has all the necessary metalwork drilled out. If you are building the power supply up from blank metalwork, then all holes will need to be drilled and deburred or filed to shape before assembly of the components. Using the wiring diagram shown in Fig.4 as a guide, mount all the hardware onto the baseplate. This includes the transform­ er (T1), the This general view of the interior shows most of the wiring details on the PC board. Note that the two windings on the large toroidal core (L2) at right are wound in different directions. mains terminal block, bridge rectifier (BR1), the earth connections and PC board. The bridge rectifier is mounted with a smear of heatsink compound between the lower face and the baseplate before securing it with a 4mm screw and nut. Note: the baseplate cannot be installed in the case until all the hardware is mounted on it. The power transformer is mounted with a large neoprene washer between it and the baseplate and another neoprene washer between the transformer and the circular retaining plate. It is secured in place with a bolt and nut. Tighten the nut so that the transformer cannot slide around. The primary (orange wires) are terminated at the mains terminal block as shown. The secondary windings are paralleled, with the blue and red wires connecting to one AC (~) terminal of BR1 and the yellow and grey wires connecting to the other AC terminal. The solder lugs for the three earth connections on the baseplate are each secured with a 3mm machine screw, nut and star washer. The PC board is mounted on the baseplate with 6mm spacers and secured with machine screws and nuts. Do not forget the solder lug which is mounted adjacent to the three 1000µF electro­lytic capacitors. The baseplate can now be secured with eight self-tapping screws which tap into integral pillars in the base of the case. Rear panel assembly You can begin the rear panel assembly by attaching the fuseholder (F1) and securing the mains cord into the cord-grip grommet. The Earth wire (green/yellow striped wire) is attached to the solder lug as shown in Fig.4. Make sure that the Earth wire is attached properly to the solder lug and that it is not a dry joint. Alternatively, crimp lugs can be used in place of the solder lugs. Slide a length of heatshrink sleeving over the Active (brown wire) from the mains cord and solder the wire to the centre leg of the fuseholder. Solder another brown wire to the second fuse terminal and slide the heatshrink sleeving over the fuseholder body. This second brown wire and the Neutral (blue) wire attach to the insulated terminal block, as shown. Mosfets Q1 & Q2 and the four diodes (D1-D4) are attached to the rear panel with machine screws. Fig.5 shows the mounting details. Note that the large finned heatsink is secured to the back of the rear panel with the same screws. Apply a smear of heatsink compound between the heatsink and rear panel before mounting. If mica washers are used, these will require a smear of heatsink compound May 1998  79 Fig.7: check your PC board against this full-size etching pattern before installing any of the parts. on both sides of the washer before assembly. If silicone washers are used instead, heatsink compound is unnecessary. Check that the metal tabs of the Mosfets and diodes are isolated from the metal panel by measuring the resistance with a multimeter. The thermal cutout switch, TH1, 80  Silicon Chip is secured with two 3mm screws and nuts. Front panel assembly The front panel should be supplied with a screen printed label and with the cutouts for the meters and other components already provided. If the panel is not supplied with all holes drilled, the meter packaging provides a cardboard template for the necessary cutouts. New scales will need to be installed on the meters to show voltage and current. Firstly, remove the clear plastic escutcheon by undoing the screws on each side of the meter. The meter scale is removed by undoing the small screws on either side. Then carefully slide the scale away from the meter, taking care not to damage the pointer. Finally, install the new scale and replace the plastic cover. Before mounting the meters on the front panel, install the countersunk earth screws which are below the (-) terminal on the voltmeter and below the (+) terminal on the ammeter. Then attach the meters with the supplied spring washers and nuts. Mount the potentiometers (VR1 & VR2), switches S1-S4 and the output terminals on the front panel. Attach the earth solder lugs to the screws and secure these with a star washer and nut. Wiring it up Now the power supply can be wired up. When wiring the mains switch (S1), be sure to use 250VAC-rated wire and slide heatsh­ rink sleeving over the switch body to insulate the terminals. Shrink both the switch and fuseholder sleeving with a hot air gun to secure it in place. Use cable ties to neatly secure the wires together at the fuseholder, terminal block and switch. This is a safety measure, so that if one wire comes adrift, the other wire or wires will keep it in place and prevent it from shorting to the case. Complete the earth wiring from the rear panel to baseplate, baseplate to front panel and GND terminal to front panel using green/yellow mains-rated wire. Use heavy duty hookup wire where indicated to prevent excessive voltage drops and to prevent them fusing. The remaining wiring can be done using medium-duty hookup wire. For clarity on the diagram we have used a numbering system instead of showing every single wire interconnection. So, for example, points 1 & 2 on the PC board go to points 1 & 2 on LED1 on the front panel, points 7 & 8 on the board go to points 7 & 8 on potentio­ meter VR1, and so on. Use a variety of colours so that it will be easier to check the wiring once completed. Install insulating sleeving over the leads to the LEDs. Do not forget the wire from the earth lug on the right hand lower corner of the PC board to the PC stake on the board. The 0.1µF 250VAC capacitor solders across the (+) and (-) output terminals and inductor L3 mounts above this capacitor and is wired as shown. The two power Mosfets and four fast recovery diodes are mounted on the rear panel. Their mounting screws also retain the finned heatsink on the back of the rear panel. The wiring of the front panel is quite tight in parts so you will need to follow the diagram of Fig.4 quite closely. When the wiring is complete, check your work very carefully to ensure that all components are in their correct place on the PC board and that the wiring is correct. You can now bundle the wiring with cable ties where appropriate. Once you are sure everything is correct, insert the fuse into the fuseholder. Also check that there is continuity between the Earth pin on the mains plug and the aluminium front and rear panels and the baseplate. There should be a zero ohm reading on your multimeter when these connections are tested. Testing Attach the lid to the case and apply power to the circuit. If there are no May 1998  81 explosions, switch off the power and remove the lid of the case. Incidentally, when you are first powering up a big power supply or amplifier, it is a good idea to wear a pair of goggles. It is a very rare occurrence for an electrolytic ca­pacitor to fail at switch-on but when they do fail it can be pretty spectacular. Attach the negative lead of your multimeter to the negative (-) output terminal on the PC board located near trimpot VR4. Set your multimeter for 0-20VDC and switch on the supply. Check for +12V at the output of REG1 (the righthand pin), on pins 8, 11 & 12 of IC1, pin 1 of IC2 & IC3, pin 7 of IC4 and pin 4 of IC5. There should be +5V at pin 14 of IC1. If at any stage the readings are incorrect, switch off the power and find the fault before proceeding further. Measure the voltage on the output terminals and check that it can be adjusted from close to 0V up to about +45V. You may need to change ranges on your multimeter as you do this, if it is not an auto-ranging model. If the output voltage does not change when you vary the voltage control, check that the load switch is on and that switch S4 is set for the adjustable position. Calibration Fig.8: here are the full-size artworks for the two meter scales. They can be cut out and used direct if required. These analog oscilloscope waveform photos show the ripple and noise on the output of the power supply when it is delivering 8A. You can compare this with the equivalent digital oscilloscope waveforms published last month. Photo 82  Silicon Chip You can calibrate the voltmeter by comparing its readings against those from your digital multimeter. Typically, the accu­racy of an analog meter movement such as in this pow- 1 (left) shows the ripple at a high scope timebase speed (10µs/div), while photo 2 (right) shows the ripple at a low timebase speed (>2ms/div). Overall ripple is about 50mV RMS. The large finned heatsink is bolted to the rear panel to prevent the output devices from overheating and self-destructing. er supply is about ±3% of full scale deflection (F.S.D.) which is nowhere near as good as the typical digital voltmeter. For best results, you need an output voltage setting which is close to FSD and in this case, that means around +45V or so. Calibrate the voltmeter by adjusting VR4 until the reading on the voltmeter matches that on your digital multimeter. Note that when you make comparisons at lower voltages, there could be an error of 1V or more which is still within the specifications of an analog meter but pretty poor when compared to your digital multimeter. Now switch S4 to the 13.8V position and adjust VR3 for a reading of +13.8V on your digital multimeter. If you are lucky, the reading on the analog meter will be pretty close to 13.8V. If not, don’t worry about it. The ammeter is calibrated by setting your multimeter to its 5A range and connecting it in series with a 0.22Ω 5W resistor across the output terminals. Adjust the output voltage so that a reading of 4A is obtained on the multimeter. Now adjust trimpot VR5 for a reading of 4A. Note that the current adjust control should be rotated fully clockwise during the current calibration to avoid current limit­ing. Other methods Note that this is only one method of calibrating the ammet­ er. Other methods include connecting a known resistance, as measured by your multimeter, across the output terminals and measuring the voltage. The current flow is the voltage divided by the resistance. Now adjust VR5 for this calculated reading. You will need to ensure that the resistance can take the power and that the resistance does not change with current. Even high power resistors will change their resistance as they heat up so they do need to be kept cool, if accuracy is to be obtained. You can make a suitable load resistor from an electric jug element with a tapping taken part way along the wire coil. This coil can be immersed in water to provide adequate cooling. You can check the current limit facility by winding down the current adjust control when a load is connected. When in current limit mode, the overload LED should light. Also, check the operation of the current adjust feature when the set current switch is pressed. You should be able to wind down the current adjust knob until the overcurrent LED just lights. Press the current set switch with the load off to see if it has the same reading. Also pressing the current set switch with the load on should show close to 0A since this is the reserve current read­ing. Finally, we have produced a load diagram of the power supply to show how much current is available at various voltages – see Fig.6. The maximum power output available is 35V at 8A, corre­sponding to 280W which is not much less than the 300VA rating of the power transformer. For settings above 35V, the available current is less but it is still quite SC respectable at 6.5A at 41V. May 1998  83