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The prototype was built into a plastic zippy case but could also be built into a 2-piece aluminium box. It provides a fixed + 5V output and 6 switched outputs from ± 3·18V. Build this low cost dual power supply Since completing this project, I wonder how I ever got along without it. It has lots of uses, is simple to build, safe, reliable and almost foolproof. Above all, it is cheap and could save you a fortune in batteries. By JOHN CRICHTON This power supply was originally designed to power op amp circuits which require balanced positive and negative rails, with the option of simultaneously driving TTL circuitry (5V logic) as well. It provides six switchable voltages: ± 3V, ± 6V, ± 9V, ± 12V, ± 15V and ± 18V (the maximum recommended for many op amps) and has an independent + 5V output which can be used with or without the dual rails. Of course, if you feel you will never need the 5 volt supply and wish to power a radio or something similar instead, the 5V regulator could be replaced by a 6, 8 or 12V 78 SILICON CHIP unit on a drop-in basis. This option will be discussed later. Other uses include driving the family Walkman (typically + 3V), trannie ( + 9V), bench amp (say + 12V), 9V battery projects, smoke testing power amplifiers and recharging gel and nicad batteries. Some of these uses will be referred to again later. The circuit is powered by a 16VAC 900mA plugpack transformer and since no separate transformer or mains wiring is required, the project is suitable for the complete beginner as well as those who like to have projects up and working in only a couple of hours. The great advantage of this kind of switched voltage supply over a "wind up the wick" style is that there is no need to tie up a voltmeter to make the voltage selection. With this unit, once you've set the switch you know what the voltage is and that it will remain constant while you work. Voltages can also be changed very quickly merely by using the switch. Two versions are possible and which to build depends on the space available on your workbench. A medium-sized zippy box makes a unit about the size of the average W alkman radio which could be fixed to a shelf or a wall, while a two-piece aluminium case would make a nice free-standing unit on your bench. Both units have identical performance and are equally easy to build. The design is not intended necessarily to be followed in every detail but can be tailored within limits to your precise requirements. The circuit is shown in Fig.1 and is based on the use of three 3-terminal voltage regulators. An LM7805 provides the fixed + 5V output while the LM317 and LM337 provide the balanced rails. If the 5V output is not required, the + 5V regulator can be omitted completely or replaced with another value. Regulators designed to supply 6, 8, 12 or 15V can be used as pin for pin replacements. The plugpack voltage of 16V AC is half-wave rectified by D1 , D4 & D7 and filtered to obtain open circuit voltages of about + 24V on Cl & C5 and - 24V on C3. The balanced ± 24V supplies are fed directly to the LM317/337 regulators while C5 feeds C6 via R7, a 150 5W resistor, to reduce ripple and lower the power dissipation in the 5V regulator. The LM7805 contains active circuitry which maintains close to 5V between its OUT and COM terminals for inputs greater than about 7.5V. C7, a 2.2µ,F tantalum capacitor, is required to maintain stability and improve the transient response of the regulator at high frequencies. DB and D9 are protection diodes, reverse biased in normal use, which protect the regulator against reverse polarity or heavy reverse currents if the output voltage were ever to exceed the input (eg, if the input were shorted). Other components associated with the adjustable regulators play the same roles. The LM317 and its negative counterpart the LM337, are of more recent design than the 78XX series regulators and have better specifications. The pinouts of these regulators are shown on the circuit diagram of Fig.1 (as are the pinouts for the 7805). They develop 1.25V between their OUT and ADJUST terminals and typically only 50µ,A flows between ADJUST and ground. Other housekeeping (chipkeeping?) currents are diverted to VOUT. Consequently, a minimum output current of some 10mA is necessary or the output voltage will rise. In this design, each adjustable regulator is set up to generate a constant current which is passed through a set of 2 700 resistors switched by a 2-pole 6-position switch to "jack up" the output voltage in accurate 3-volt steps. D9 + +v r--<--~>Nr+-'IWr.. &80 ~ VR1 VR2 10011 ...__ _ _ _ __, 1D00 D1,09 9x1N4002 Rl 7805 FROM 16VAC D3 PLUGPACK + Cl 2200uf C2 + --...,.,_ _ 2.2af· S1a .~.. COM 0 +18V AC2 0----+-----------------------0 DV LM317 -18V 0 oom,~• OUT CJ + S1b 2200uf C4 2.2uf + 06 LM337 VR4 1000 '--~~Nr+-•.,,,,,,,... :~! D4 --~.. IN LOW-COST DUAL POWER SUPPLY Fig.I: a 7805 3-terminal regulator provides the fixed + 5V output while LM317 & LM337 adjustable regulators provide the variable plus & minus rails. Power comes from a 16V AC plugpack. In the prototype, 5 % resistors were selected from a bandolier but these days 1 o/o metal film resistors are widely available if you feel you need that kind of accuracy. The current-setting resistors associated with each regulator are adjusted so that 3V appears across each resistor in the chain (about 1 lmA). Because the number of resistors through which the current passes can be adjusted with switch S1, the JULY 1990 79 cuits! The regulators are mounted on the copper side of the board with their leads bent at rightangles. An ON/OFF switch was deliberately omitted as otherwise the plugpack could be left turned on for long periods with possible dangerous consequences. Instead, a LED indicator on the front panel serves to remind users to turn the supply off at the wall socket. The power supply is not connected to mains earth but is isolated by the plugpack tranformer. Consequently, it may be used without restriction on both earthed and double insulated equipment without any fear of hum loops and the problems they can introduce. Assembly of the PCB The completed PC board is secured to the lid of the case by the regulator mounting screws. The lid, together with an external heatsink, provides the necessary heatsinking for the regulators. voltage at the output can be switched to plus and minus 3, 6, 9, 12, 15 & 18V. If the switch is at the 3V setting, about 1.25V of this will be due to the regulator and the remaining 1.75V to the first resistors in the chain (about 1570). Ideally one would use a combination of fixed resistors here but the tolerance on regulators seems to be about ± 4% so a trimpot is necessary for accuracy. For the same reason, trimpots are required to set the constant currents (about 1120). The fixed resistors associated with the trimpots allow the design settings to be achieved more quickly and with greater accuracy than if trimpots were used alone. Construction As already mentioned, there are two basic versions of the supply with a couple of options which you can select to suit your own application. The same PCB is used in both units. In the zippy box version, the regulators are mounted on the metal lid to remove heat. An external T0-3 heatsink is also recommended but may not be required if only low output currents are drawn and you guarantee no short cir- If~ The PCB design is very simple but it is worthwhile inspecting the board to ensure there are no patchy copper lands or unwanted bridges between tracks. Repair these if necessary and make sure all holes are drilled. Because the PCB is a tight fit in the zippy box, the filter capacitors and trimpots must be small in size. Recent stock from most suppliers is suitable but parts from your junk box are unlikely to fit. PC pins or stakes are used to terminate external wiring connections and these should be mounted first. The resistors and diodes are mounted next, followed by the tantalum capacitors and trimpots. Use the component overlay diagram of Fig.2 as a guide when installing the components. The 5W resistor should be I ~~ ~---rk\ o• .i::,. . . . . . . . The three regulators are mounted on the copper side of the PC board as shown here. Smear the regulator tabs with heatsink compound before bolting them to the lid. 80 SILICON CHIP 1=..: Fig.3: this is the full-size PC pattern. re}. PARTS LIST F / A / g ~~ 1 plastic zippy box, 41 x 68 x 130mm, (DSE Cat. H-2583 or equivalent) 1 PC board, code SC 04107901, 70 x 60mm 1 16V 600mA AC plugpack 1 2-pole 6-position rotary switch 1 knob to suit switch 2 red binding post terminals 1 green binding post terminal 1 black binding post terminal 1 U-shaped heatsink, 60 x 35 x 30mm (Jaycar Cat. HH-8511) 1 2. 1 mm DC power socket 1 2. 1 mm DC power plug +v LEDl(;YB K FROM 16VAC PLUGPACK DV -v Fig.2: check that all parts are correctly oriented during assembly and use PC stakes at external wiring points. The three regulators are mounted on the copper side of the board (see photo). Install the 5W resistor so that it sits off the board by 5mm to aid cooling. mounted so that it stands about 5mm off the board to improve cooling. Carefully check the polarity of the electrolytic capacitors against the overlay diagram before soldering them to the board. Leave the regulators until the box is ready and you get a chance to fit parts together for a dry run. MICA WASHER BUSH NUT \ SCREW i \ :~~~ :s-1 ~ DEVICE 1 METAL REAR PANEL ' FINNED HEATSINK Fig.4: mounting details for the LM317 & LM337 3-terminal regulators. The metal tabs of these devices must be electrically isolated from the metal lid & heatsink. You can now solder the 10 2700 range resistors between the 12 terminals on the rotary switch, as in the photograph and the overlay diagram of Fig.2. Front panel hardware The front panel, and specifically the spacing of the binding posts, was designed with finger room in mind. However, most multimeters use a standard 19mm spacing and matching dual plugs are available. If you wish to use 19mm spacing, use the appropriate front panel layout as a starting point. In the plastic version, clearances dictate that the + , - and common terminals need to lie along a line near the centre (the line of the switch is recommended). Mark up, drill and ream the holes in the front of the zippy box for the output terminals, LED and switch. Remember to drill a hole for the 2.5mm power socket in the side of the box. Fit the binding posts and the range switch with its resistors in position and drop the PCB in on top so that the filter capacitors fit Semiconductors 1 7805 5V 3-terminal regulator (REG1) 1 LM31 7 adjustable 3-terminal regulator (REG2) 1 LM337 adjustable 3-terminal regulator (REG3) 9 1 N4002 silicon diodes (D1-D9) 1 red LED (LED 1) Capacitors 3 2200µF 25VW PC electrolytics 3 2.2µF 35VW tantalum Resistors (0.25W, 5%) 1 1.2k0 1W 12 2700 2 1200 2 820 2 680 1 150 5W Miscellaneous Insulated hookup wire , screws, nuts, lockwashers, Dymotape or white Letraset for labelling, heatsink compound, solder. snugly between the switch and the terminals. The PCB must be recessed about 10mm into the box so that the lid will fit when the regulators are in place. You will need to cut away some of the plastic ribs in the box with a wood chisel or a hot knife to do this. Leaving just enough clearance to reach the lid screws at the side of the T0-3 heatsink, mark the line of the regulator fixing holes and drill them in the aluminium lid. They will ]ULY1990 81 DISKS! LIFEIIME WARRANTY 0. 0150210 01 50220 0150310 0150320 1-9 BOXES 1 O+BOXES $7.40 $6.95 $19.50 $18.95 $22.50 $21 .50 $49.95 $47.95 DESCRIPTION NO BRAND 5.25 DISKS DSDD NO BRAND 5.25 DISKS DSHD NO BRAND 3.5 DISKS DSDO NO BRANO 3.5 DISKS DSHD C201100 DISK STORAGE CASE (1 Ox5. 25 disks) C201200 DISK STORAGE BOX (100x5.25 disks) C201300 DISK STORAGE BOX (80x3.5 disks) H230100 DISK HEAD CLEANER H230200 DISK HEAD CLEANER $3.50 $15.95 $17.95 $7 .95 $7.95 5.25 DISKS 3.5 DISKS FD45010 5.25 360K DISK DRIVE HALF HEIGHT FD45020 5.25 1.2Mb DRIVE HALF HEIGHT FD45030 3.5 SWITCHABLE 760K OR 1.44Mb $159.50 $179.50 $219.50 FC45000 4 WAY DISK CONTROLLER 360K/760K/1 .2Mb/1.44Mb $139.00 MT23010 $3.00 DIGIT AL MULTIMETER Measures DC 0-1 OOOV, AC 0-500V Current 0-10 amps, Resistance 0-2MEG Includes: Test Leads, Manual , Spare Fuse and Battery MT23020 DIGITAL MULTIMETER AUTORANGING $169.50 -::~t\ Measures DC 0-1 OOOV, AC 0-700V Current 0-1 0 amps, Resistance 0-2MEG Continuity Buzzer. Includes: Test Leads, Manual, and Carry Case. ~.~ ~ · STANDARD MAIL RATES WITHIN AUSTRALIA $1.00-$9.99 ........ . .......... . . . . ......... $3.00 $10.00-$29.99 .................... .. ....... . $3.50 $30.00-$99.99 .......... . ................... $5.50 $100.00-$200.00 . ........................... $6.50 ~ 1 ::11R:Rc:~RR:A~~RP~:!~~::Ec~~ ~::E:F :::~:~~CREDIT CHARGE ORDERS MINIMUM PURCHASE $10.00 ~ ~ /)/\ AUST-TEK L/b F• EC• RONICS PO BOX 1422 CAMPBELLTOWN, NSW, 2560 You've Gotta Getta Goldstar from: A.C.T. John Pope Electrical 80 6576 N.S.W. D.G.E. Systems (049) 69 1625 e W.F.Dixon (049) 69 5177 e Macelec (042) 29 1455 • Newtek (042) 27 1620 • Novacastrian Electronic Supply (049) 62 1358 e Obiat Pty Ltd 698 4776 e Digital 709 6511 e Geoff Wood 4271676 N.TERRITORY J Blackwood (089) 84 4255, 52 1788 e Thew & McCann (089) 84 4999 QUEENSLAND St Lucia Electronics 252 7466 e B.A.S. Audiotronics 844 7566 • The Electronics Shop (075) 32 3632 • Solex (Townsville)(077) 72 4466 • Xanthos Electrical (079) 27 8952 S.AUSTRALIA Force Electronics (08) 212 5505 VICTORIA G.B. Telespares 328 4301 • The Electronic Components Shop 670 6474 e_Factory Controls (052) 78 8222 • Mektronics 587 3888 • Truscott Electronics 723 3094 W AUSTRALIA Atkins Carlyle 4811233 e Leda Electronics 361 7821 PAPUA NEW GUINEA T E (P.N.G.) Pt Moresby 25 6322 Lae 42 6246 Now for the smoke test! Set all trimpots to their mid-positions and the switch to minimum, then connect a voltmeter between the adjustable outputs. Switch on and monitor the temperature of each regulator tag with your fingers. If any of the regulators gets hot, switch off at once and recheck everything. If the voltages are high but stable and nothing else seems to be wrong check that the switch is wired in the correct sense. It is all too easy to connect the wrong ends of the resistor chains to ground. If all is well, check that the outputs of the variable regulators are similar, positive and negative, and breathe a sigh of relief. Now switch the voltages up a click or two and check that the outputs change in sympathy. This close-up view shows the mounting details of the 3-terminal regulators. Use your multimeter to check that the tabs of the adjustable regulators have been correctly isolated from the heatsink. Note the cardboard insulator between the regulators & the PC board. be about 25mm from one end. Now use the lid as a template to modify the central hole in the T0-3 heatsink to fit the regulator positions. Remove any burrs from the holes and ensure that the lid and heatsink will fit tightly together to conduct heat away from the regulators. Spread a little thermal paste between them. You can now mount the regulators on the board and then attach them to the lid using insulating kits and thermal paste (LM317 & LM337 only). The metal tab of the 7805 regulator can be bolted directly to the metal lid. Tighten the fixing bolts carefully to ensure that the regulators cannot rotate and check that there are no shorts between adjacent pins. Now use a multimeter (switched to a low "Ohms" range) to ensure that the adjustable regulator tabs are correctly insulated from the lid and from each other. Note that the lid is connected to ground on the PCB via the tab on the 7805. Now that the PCB is attached to the lid, cut a piece of cork or card as an insulator/spacer to fit between the PCB and the lid and glue it in position near the filter capacitors. This done, cut a piece of thick card about 25 x 60mm and slip it between the board and the regulators to prevent shorts between the PCB and the fixing nuts. Tighten the switch and the terminals on the inside of the plastic box and wire diodes between each terminal and ground. These will protect the regulators against reverse connected charged capacitors in the external circuitry. Setting up The PCB assembly should be' checked out and adjusted before mounting it in the box. To do this, connect up the LED and the input sockets to the board with suitable lengths of hookup wire and connect the 2.5mm connector to the plugpack. On the switch, make sure that the "common" sides of the two resistor chains are connected together and to the two poles of the switch and to earth on the PCB. The other ends of the chains go to the trimpots associated with each adjustable regulator. Use short lengths of suitable colour-coded hookup wire to connect the PC stakes to the output terminals (wrap one turn round each stake before soldering to obtain a durable joint). Use thicker wire (or two lengths of hookup wire) to connect the earth on the PCB to the common terminal. Check all the wiring. Adjustments The next step is to adjust the constant currents. Connect your voltmeter across the first 2700 resistor on the switch at the "hot" end of the chain associated with the negative regulator. Adjust trimpot VR4 carefully to obtain 3.0 volts. Now repeat this step for the corresponding positive regulator resistor and its triinpot (VRl}. Now that the currents are set, you can adjust the other trimpots (VR2 & VR3} associated with each regulator to obtain the plus and minus 3V outputs at the minimum switch setting. That's all there is to it. When you are ready to fit everything together, put a dab of superglue on the side of the LED before fitting it into its hole and mount the power socket firmly. Pack the wiring carefully into the box and fit the lid. Check that you get the expected voltages from each terminal before connecting anything to your new power supply. As it stands, the unit is a sealed system which contains heat generating components. If you expect to drive the 5V supply hard for hours at a time, fit a 2-4mm thick washer under each fixing screw or' the lid to allow air to circulate over the 5W resistor and both surfaces of the lid. This small modification will improve heat dissipation and continued on page 99 ]ULY1990 83 Dual power supply: ctd from page 83 hence reliability. A larger heatsink may also be necessary. Performance This unit can be thought of as two power supplies in one. As a dual voltage variable supply it is capable of delivering up to 150mA at ± 18V and in excess of 250mA at ± 15V or less at very low ripple, and with excellent line and load regulation. The Serviceman's Log: ctd from page 45 ranges from 4k0 to 400k0, ± 1.2% for the 4MO range and ± 3.0% for the 20MO range. The accuracy for frequency measurements is ± 2 % on the three ranges from 4kHz to 400kHz. The unit also has a continuity and diode test range with audible alarm and a data-hold feature which stores the last measurement. Priced at $169.50, the Metex M-818 is available from all Jaycar Electronics stores. 48V switchmode supplies from Amtex ed in restoring the colour, and was beginning to regret ever having started the job. Then I thought, "Damn it! I'm never likely to need the chroma board out of this junked set. Why not swap it for the one in this set?" And it was no sooner said than done. That fixed it and the old Thorn went home with a new lease of life. Its picture tube is likely to last another 15 years but whether the circuit boards will last that long is another matter. Then again, there are likely to be more of these old Thorn sets coming in with dud tubes, so perhaps I can keep this one going beyond my retiring age! How it works This new range of switchmode supplies provide an output of 48 volts DC with output currents ranging from 1A to 10A. Each unit has short circuit and output overload protection as well as switchable 110/230 VAC inputs. Also included is soft start power up circuitry and input surge protection. For further information, contact Amtex Electronics Pty Ltd, 13 Avon Road, North Ryde, NSW 2113. Phone (02) 805 0844. With time to think, I evolved the following theory about the workings of the degaussing circuit. At the moment of switch-on, the PTC thermistor, X199, is cold and its resistance is low. This allows almost the full voltage to be applied to the degaussing coils and Z199 which, at the high voltage, offers minimum resistance. In its fixed voltage guise, it can provide in excess of 550mA at 5V with short circuit protection provided by R7 and internal foldback current limiting. As a combination unit, with both supplies in use, the 5V specifications still stand but, as you might suspect, the 18V output may not be usable depending on how much current is drawn from the fixed supply. The lower voltage supplies are unlikely to be affected if current demands a re reasonable. ~ As Xl 99 heats up and its resistance rises, the voltage applied to Z199 and the coils will fall and the reduced voltage across Zl 99 causes its resistance to rise. But this would allow X199 to cool and increase the current through the coils. So the function of R199 is to maintain sufficient current through Xl 99 to keep it warm and its resistance relatively high. At the same time, most of the current it does pass will flow through R199, rather than the coils. Subsequently, a colleague came up with two text books which confirmed this theory: "Colour Television;' by R. N. Patchett, and "PAL Colour Television for Servicemen" by W. C. Cook. That leaves one point still unexplained; the 80V I measured across the degaussing circuit, connected to an open circuit transformer winding. How come? It was another of those (expletive) digital multimeter misreadings. Apparently there was enough capacitance, or leakage, across the break in the transformer winding to develop 80V at the picoamps needed to drive the DMM. Later, using a 100,0000/V analog meter, I could not detect any voltage. ~ Is Your Product Getting The Exposure It Deserves? Consumers need to see your product if you want them to buy it Contact Paul To Reserve This Space - (02) 979 5644 ]UL Y 1990 99