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This handy bench power supply has no expensive meters and offers six fixed dualpolarity supply voltages: ±3V, ±5V, ±6V, ±9V, ±12V and ±15V DC. And for added flexibility, you can use any of the first three outputs and any of the second three at the same time. By JIM ROWE F ULLY VARIABLE DC bench supplies with voltage and current meters are great for checking circuits that operate from odd-ball voltages. They’re also essential for checking the voltage range over which a circuit operates correctly. However, for a lot of work, they can represent overkill. Some of the bells and whistles on a typical supply can even be a drawback, when you’re simply trying out an idea 48  Silicon Chip for a cir­cuit that must work from a bog-standard supply rail. For example, on many low-cost bench supplies, the meters are either too small or too inaccurate to allow you to properly check that the output is set within tolerance. So you generally have to reach for your DMM and check the voltages anyway, before even connecting the supply to your circuit. There can also be a problem when it comes to trying out a circuit that needs multiple supply rails. Most bench supplies have two outputs at most and even these are generally balanced – ie, the positive and negative outputs closely track each other. That’s great when you do want balanced supply rails but not so useful if you want say +12V and -5V. In that case, you generally need a second supply altogether. In fact, if you need three rails – say +5V, -5V and +12V – there’s usually no option but to use a second supply. And if you need a fourth rail, you might well have to use either a third supply or at the very least, two different balanced twin sup­plies. All of which demonstrates the truth of that old saying in electronics: “you can never have too many power supplies!” Multiple fixed outputs For a lot of day-to-day bench work, what would be really handy is a small www.siliconchip.com.au supply with four outputs – especially if these outputs could be easily switch­ ed to select commonly used fixed voltages. Such a supply wouldn’t need any voltmeters, because of the fixed outputs, and for a lot of work it wouldn’t need current metering either. And none of the outputs would need to have a high current/power rating, since most bench work now involves very low power circuitry. This line of thinking culminated in the compact, low-cost four-in-one bench supply described in the January 1998 issue of “Electronics Australia”. It was a very handy little supply and quite popular too but it did turn out to have a few shortcomings. For example, it had a choice of only four output voltages: ±5V and ±12V. Obviously, there are situations where other voltages are required. The other “shortcoming” was that it was not suitable for beginners, because of the transformer and mains wiring inside the case. The idea behind this new design has been to come up with a supply that’s not only more flexible than the 1998 version but easier and safer to build as well. It still offers only four output voltages at once (two bipolar pairs) but these can now each be switched between three pairs of voltages. You can have either ±3V, ±5V or ±6V from one set of outputs and either ±9V, ±12V or ±15V from the other set. Despite this extra flexibility it’s even easier to build than before, because all of the internal wiring is on two PC boards which connect directly together. There’s really no off-board wiring at all. There are no safety worries for beginners either, because the supply gets all its power at very low voltage from an exter­nal 9V/1A AC plugpack. The highest voltages anywhere inside the case are only 9VAC and 27V DC. Like the earlier design though, it won’t deliver really high currents. You can draw up to about 750mA at ±3V, 550mA at ±5V, 450mA at ±6V, 600mA at ±9V, 500mA at ±12V and 350mA at ±15V. This is for each output used singly of course but the figures don’t “droop” too rapidly when multiple outputs are in use – the main limitation is the regulation of the AC plugpack. Fig.1 shows the performance details in graphical form (see also the accompanying specifications panel). www.siliconchip.com.au Fig.1: this graph shows the output current capabilities (blue) for the various fixed voltage outputs. The ripple performance is also plotted (green). As you can see, it still has enough “grunt” for most exper­imental bench work. So although it deliberately lacks a lot of the traditional bells and whistles, it’s still a surprisingly practical unit. The outputs are overload and short-circuit protected and the output terminals are spaced on standard 19mm centres to allow the use of dual banana plugs if desired. The circuit Refer now to Fig.2 for the circuit details. It may seem a bit daunting at first glance but it’s really very straight­ for­ward. First, there are four simple rectifier and filter circuits producing raw DC rails from the 9V AC delivered from the plug­pack. Each rectifier then drives an adjustable 3-terminal regula­ tor with a switch to select one of three regulated output voltag­es. It’s mainly the plugpack which sets the unit’s total power rating of around 9W (9V x 1A). The two low-voltage rectifiers are standard half-wave cir­ c uits, each based on a single 1N5404 power diode (D1 & D2) feed­ing a pair of 2200µF filter capacitors. These rectifiers produce about ±13V of unregulated DC under no-load conditions, drooping down to SPECIFICATIONS Outputs: 2 x dual-polarity pairs (VA & VB) plus two common terminals. Output Voltages: 3 x dual polarity low-voltage outputs (VA); 3 x dual-polarity high-voltage outputs (VB), as follows: (1) Low-voltage switch (VA): ±3V <at> 750mA; ±5V <at> 550mA; & ±6V <at> 450mA (2) High-voltage switch (VB): ±9V <at> 600mA; ±12V <at> 500mA; & ±15V <at> 350mA Power supply: 9VAC 1A plugpack Overload and power indication: 4 x 3mm LEDs Load switching: independent toggle switches for each output pair April 2002  49 lower voltages as current is drawn. The 1N5404 diodes have a current rating of 3A continuous and 200A peak, so they should be virtually “unbreakable” here. For the higher voltage outputs, Parts List 1 plastic instrument case, 155 x 160 x 64mm, with metal rear panel (2mm thick aluminium) 2 PC boards, code 04104021 (119 x 124mm) and code 04104022 (134 x 48mm) 6 banana jack screw terminals (2 red, 2 black, 2 green) 2 DPDT miniature toggle switches (S1, S2) 2 2-pole, 3-position rotary switches (S3, S4) 1 DC power socket, 2.6mm (PC-mount) 1 9V 1A AC plugpack 23 PC terminal pins, 1mm diameter round type 4 TO-220 insulating kits 4 M3 x 12mm round head machine screws 4 M3 nuts, flat washers and star lockwashers 2 instrument knobs, 19mm dia. Semiconductors 2 LM317T 3-terminal regulators (REG1, REG2) 2 LM337T 3-terminal regulators (REG3, REG4) 6 1N5404 3A power diodes (D1-D6) 8 1N4004 1A power diodes (D7-D14) 2 3mm red LEDs (LEDs 1 & 3) 2 3mm green LEDs (LEDs 2 & 4) Capacitors 6 2200µF 16VW RB electrolytic 4 1000µF 63VW RB electrolytic 4 100µF 25VW RB electrolytic 4 10µF 16VW RB electrolytic Resistors (0.25W, 1%) 1 9.1kΩ 1 680Ω 1 5.6kΩ 2 560Ω 1 4.7kΩ 1 510Ω 1 3.6kΩ 1 470Ω 3 3.3kΩ 1 430Ω 1 2.4kΩ 1 330Ω 1 2.2kΩ 2 270Ω 2 1.5kΩ 2 240Ω 1 1.2kΩ 1 180Ω 1 1.1kΩ 2 120Ω 1 750Ω 50  Silicon Chip we use half-wave voltage doubling rectifiers, each with a 2200µF series capacitor, a pair of 1N5404 power diodes (D3 & D4 and D5 & D6) and a pair of 1000µF filter capacitors. These rectifiers produce about ±27V of unregulated DC under no-load conditions, which again droops as current is drawn. By the way, the relatively poor regulation of the half-wave rectifiers doesn’t pose a problem. In fact, it helps keep the power dissipation of the regulators down to an acceptable level, by lowering the voltage across the regulators at higher load currents. The maximum power dissipated by any of the regulators is 3.5W, which is reached by the high-voltage regulators when deliv­ering ±9V at 400-450mA. This is acceptable because the regula­ tors are all mounted on a reasonably good heatsink (the rear panel) and have inbuilt thermal overload protection anyway. If they do get too hot, they automatically shut down for a while to cool off. As shown on Fig.2, the positive 3-terminal regulators are LM317T devices while the negative regulators are LM337Ts. Both of these regulator ICs are capable of handling up to 1.5A of current so, like the rectifier diodes, they’re being used quite conserva­ tively here. The regulator circuits all use virtually the same configu­ ration. This is because the LM317 and LM337 regulators work in the same way, acting to maintain a fixed voltage across the resistor connected between their “output” and “adjust’ terminals (240Ω for the positive regulators and 120Ω for the negative regulators). In each case, the regulator keeps the voltage across these resistors fixed at 1.25V. Because virtually all of the current through these resis­tors comes from the output terminal and almost no current flows in or out of the adjust terminal, virtually the same current flows in any resistance we connect between the adjust terminal and ground. So we are able to set the actual output voltage of the regulator by adjusting this lower resistance value, to set up a “bootstrap” voltage drop that’s equal to the desired output voltage less the 1.25V that’s maintained across the upper resis­tor. For example, in the low-voltage positive regulator (REG1), the series 470Ω and 430Ω resistors give a total of 900Ω, which produces +4.75V between the adjust pin and ground. As a result, the regulator’s output is +6.0V (4.75 + 1.25) when these resis­tors are in circuit alone. Similarly, for REG2, the 1.5kΩ and 1.1kΩ resistors alone give an output of +15V, while the 270Ω and 180Ω resistors in the REG3 circuit give an output of -6V, and so on. To set the two lower output voltages for each regulator, we simply switch in additional shunt resistors across these lower resistors, to reduce their value and hence the voltage drop. For example, in the REG1 circuit, we switch in a 3.3kΩ resistor to lower the regulator’s output voltage to +5V, or the parallel 2.2kΩ and 680Ω resistors to bring it down to +3V. Exact­ly the same arrangement is used for the other three regulators. Note that the two low-voltage regulator outputs (REG1 & REG3) are set using switches S3a & S3b, while the high-voltage regulator outputs (REG2 & REG4) are set using S4a & S4b – ie, each pair of outputs is tied together. As a result, S3 and S4 are respectively marked “VA SELECT” and “VB SELECT” on the front panel, to ensure easy operation. In addition, load switches S1a & S1b allow you to switch the two low voltage outputs together, while S2a & S2b perform the same role for the two high-voltage outputs. These switches are miniature toggle types and are positioned quite close to each other on the front panel. So with a little dexterity, it’s quite easy to switch all four outputs on or off within a few millisec­onds of each other. As shown in Fig.2, each regulator has a 100µF filter capacitor across its output and a 10µF capacitor from its adjust pin to ground to provide additional filtering. There are also reverse-biased diodes connected between each regulator’s output and input (D7, D9, D11 & D13) and between the output and adjust terminals (D8, D10, D12 & D14). Fig.2 (facing page): the circuit uses four simple rectifier and filter circuits to produce raw DC rails from the 9V AC delivered from the plug­pack. Each rectifier then drives an adjustable 3-terminal regula­tor to derive the fixed output voltages. www.siliconchip.com.au www.siliconchip.com.au April 2002  51 status indicator, based on LEDs 1-4 and their series resistors. This means that should one of the regulators begin to shut down in response to an overload, that output’s LED will become dim – so you’ll at least be warned of an overload situation. That’s the cue to hit the appropriate switch and investigate the cause of the overload! This simple system works quite well in practice, despite its low cost. Construction Fig.3: install the parts on the main PC board as shown in this diagram. Note, however, that REG1-REG4 are not installed directly on the board – instead, you have to install PC stakes at each of their lead positions and the regulators are then later soldered to these stakes (after mounting them on the rear panel). The “upper” diodes are included to protect the regulators against damage if the outputs are accidentally connected to a voltage higher than that across their input filter capacitors. This can happen, for example, if you turn off the power to the supply’s plugpack and then suddenly turn on one of the two load switches, thus connecting its regulator outputs to charged bypass capacitors in an external circuit. The “lower” diodes similarly protect the regulators from damage due to any charge remaining in the 10µF filter capacitors when the AC input power is removed. To save costs and keep the circuitry as simple as possible, there’s no current monitoring or limiting, apart from that pro­vided inside the regulator chips themselves. However, each of the four outputs has a simple LED The complete supply is housed in a standard plastic instru­ ment case measuring 160 x 155 x 65mm. Inside the case, everything is mounted on two compact PC boards which solder together at right angles: a main board which is mounted horizontally in the lower half of the case and a switch/ output terminal board which mounts vertically behind the front panel. The main board is coded 04104021 (119 x 124mm) and carries all the components used in the rectifiers. The four 3-terminal regulators also mount along its rear edge, so they can be bolted to the rear panel which acts as the heatsink (the usual plastic rear panel is replaced by a 2mm-thick aluminium plate). Also on this board are the basic components used in each regulator cir­cuit and the power supply AC input connector (CON1). The vertical PC board is coded 04104022) (134 x 48mm) and supports mainly the front-panel components: ie, voltage selector switches S3 & S4, load switches S1 & S2, the six output terminals and the four indicator LEDs. Also on this board are the LED series resistors and the voltage selection resistors switched into the regulator circuits by S3 & S4. The connections between the two boards are made via 11 PC terminal pins, which solder to circular pads near the bottom of the vertical board Fig.4: this is the parts layout for the vertical PC board. Refer to the text for the details on mounting rotary switches S3 & S4. Eleven PC stakes have to be soldered to the otherwise vacant pads along the bottom of the board. These are installed from the copper side and connect to matching pads on the main PC board (see Fig.5). 52  Silicon Chip www.siliconchip.com.au Here’s what the completed assembly looks like before it’s installed in the case. We sandwiched two 1mm-thick aluminium panels together to make up the rearpanel heatsink but you can use a single 2mm-thick panel. and to rectangular pads along the front edge of the main board. As well as making the connections, these pins also hold the two boards together at 90°. Putting it together Assembling the supply is easy, particularly if you tackle it in the following order. First, inspect both PC boards and make sure they’ve been trimmed to the correct sizes and that there are no solder bridges between tracks. This done, begin the main board assembly (Fig.3) by fitting three PC terminal pins to each regulator position along the rear edge – ie, 12 pins in all. Next, fit the 2.6mm power connector CON1 to the board, followed by the eight wire links. Note that most of the links can be made using bare tinned copper wire (eg, component lead off­ cuts) but the two longest www.siliconchip.com.au links should be made using insulated hookup wire. With the links in place, you can then fit the resistors, 1N4004 diodes and finally the larger 1N5404 power diodes. Make sure the diodes are all fitted with the correct polarity, as shown in the overlay diagram, and be sure to fit the correct diode in each location. Once the diodes are fitted you can fit the electrolytic capacitors, again taking care with their polarity. Your main board should then be complete and you can put it aside while you work on the second board. Begin the assembly of this board (Fig.4) by checking that the holes have been drilled to the correct sizes to accept the larger items, such as the rear of the output terminals and the rotary switch connection lugs. That done, fit all of the resistors, again using the overlay diagram as a guide. The only other items to fit to the board at this stage are the two rotary switches but first you have to trim their control shafts to about 10mm from the threaded mounting ferrule. That done, rotate each switch shaft fully anticlockwise, remove its locking nut and star washer, and move the indexing collar three positions anticlockwise. Finally, replace the star washers and mounting nuts to lock the collars down. Each switch should now operate over three positions (in­stead of six). You might also want to file a “flat” on each switch shaft (if one isn’t already present), to help prevent the knobs from working loose later. The flat should be diametrically opposite the switch locating spigot, when the rotor is in its centre position After the shafts have been trimmed and given flats, both switches can be fitted to the board, with their locating spigots directly above the shafts (see Fig.4). You may need to straighten their lugs a little, to allow them to mate with all of the board holes correctly. April 2002  53 also a good idea to file the holes for the output terminals with “flats” on each side as suggested by the artwork, to prevent them rotating and working loose later. You may also want to provide small “blind” holes above the main mounting holes for switches S3 and S4, to accept the switch locating spigots. Check also that the holes for the 3mm LEDs will in fact accept the LED bodies without too much force. The ideal hole size is where the LED will just fit snugly, without being loose. The adhesive label can now be attached to the front panel and the holes cut out using a sharp utility knife. This done, mount the toggle switches and output terminals. The switches should be fitted with the nuts adjusted so that the switch bodies are reasonably close to the panel, with the threaded ferrules protruding 1.5mm or so beyond the front nuts (this is to facili­ tate board mounting later on). The green terminals are fitted in the two centre “Common” positions, with the black terminals for the negative outputs and the red terminals for the positive outputs. If your toggle switches are fitted with standard “solder lug” terminals instead of PC terminals pins, now is the time to fit short lengths (say 20mm) of tinned copper wire to the Fig.5: this cross-section diagram shows how the 3-terminal regulators are attached to the rear panel (using TO-220 insulating kits) and their leads bent so that they can be soldered to the matching PC stakes on the PC board. The diagram also shows how the two PC boards are connected together. That done, solder all the lugs to the board’s copper pads, with the switch body in contact with the front of the board. The next step is to fit the four LEDs in their correct positions, as shown in Fig.4. Just tack-solder one lead of each LED at this stage and DON’T cut any of their leads short – they’re just being positioned for final mounting later. Take care to ensure that the LEDs are correctly oriented – the anode lead is the longer of the two (see Fig.2). Before you can proceed any further with this board, you have to prepare the front panel (that’s because they combine to form an integrated assembly). So the next step is to drill and/or ream the holes in the front panel, using a copy of the artwork as a template. It’s Table 1: Resistor Colour Codes  No.   1   1   1   1   3   1   1   2   1   1   1   1   2   1   1   1   1   2   2   1   2 54  Silicon Chip Value 9.1kΩ 5.6kΩ 4.7kΩ 3.6kΩ 3.3kΩ 2.4kΩ 2.2kΩ 1.5kΩ 1.2kΩ 1.1kΩ 750Ω 680Ω 560Ω 510Ω 470Ω 430Ω 330Ω 270Ω 240Ω 180Ω 120Ω 4-Band Code (1%) white brown red brown green blue red brown yellow violet red brown orange blue red brown orange orange red brown red yellow red brown red red red brown brown green red brown brown red red brown brown brown red brown violet green brown brown blue grey brown brown green blue brown brown green brown brown brown yellow violet brown brown yellow orange brown brown orange orange brown brown red violet brown brown red yellow brown brown brown grey brown brown brown red brown brown 5-Band Code (1%) white brown black brown brown green blue black brown brown yellow violet black brown brown orange blue black brown brown orange orange black brown brown red yellow black brown brown red red black brown brown brown green black brown brown brown red black brown brown brown brown black brown brown violet green black black brown blue grey black black brown green blue black black brown green brown black black brown yellow violet black black brown yellow orange black black brown orange orange black black brown red violet black black brown red yellow black black brown brown grey black black brown brown red black black brown www.siliconchip.com.au This close-up view of the rear panel shows how the four 3-terminal regulators are mounted. Note that the regulators must all be electrically isolated from the rear panel using TO-220 insulating kits (see Fig.5). They are connected into circuit by soldering their leads to matching PC stakes on the main PC board. top four lugs of each, pointing directly backwards along the lug axis but with a small loop around the side of each lug before solder­ing – to make sure it can’t drop off when you later solder it to the PC board pad. You should now be ready to mate the front panel and the vertical PC board together. This involves pushing the rotary switch shafts and their threaded ferrules through the front panel holes (you have to remove the locking nuts first) and at the same time pushing the rear spigots of the output terminals and the leads on the rear of the toggle switches through the corresponding holes in the board. It’s a bit fiddly but not too difficult if you take it carefully. Once the two are mated together, you may need to adjust the positions of the mounting nuts and washers for the toggle switch­es so that the switch positions fore-and-aft will allow both panel and board to be truly parallel to each other, with a space of very close to 16.5mm between them everywhere. Tighten the toggle switch nuts at this point, followed by the rotary switch nuts – but carefully, so you don’t strip the plastic threads or slip and scratch the front panel. www.siliconchip.com.au You should now be able to solder the ends of the output terminal spigots to their large pads on the PC board. The toggle switch leads can then also be soldered to their respective pads. That done, you can untack each LED from its initial posi­tion and carefully push it forward until its body fits snugly in the corresponding front panel hole. Its leads can then be sol­dered properly to the board pads and any excess finally trimmed off. The next step in preparing this board/panel assembly is to lay it face down on the bench and fit the 11 PC terminal pins which will connect it to the main board. These are all fitted from the copper side, so their main length protrudes backwards from the board. Solder each one carefully to its pad. The two boards can now be mated The rear panel is pretty uninspiring – just the four screws that secure the regulators plus a hole for the power socket. April 2002  55 Fig.6: these full-size artworks can be used a templates for drilling the front and rear panels. Note that the holes for the for the banana jack terminals have straight sides, so profile these carefully. together, by soldering these same 11 terminal pins to the rectangular pads along the front of the horizontal board. This is best done with the main board upside down (ie, copper side up) and with the other board/panel assembly also upside down but held at right angles using a small strip of 18 x 32mm wood or similar as a guide. It’s a good idea to just tack solder the pins at each end first and then make sure everything is aligned properly in terms of both the 90° angle and the side-to-side positioning. Once all is well, you can then solder all the pins to their pads to complete the assembly. At this point, you can fit the control knobs to the rotary switch shafts, ready to adjust the output voltages. The module is now essentially finished (apart from the regulators which are fitted during the final assembly) and 56  Silicon Chip can be put aside while you prepare the rear panel. Rear panel work In order to provide reasonable heatsinking for the four regulators, the rear panel should ideally be made from 2mm-thick aluminium sheet. I didn’t have this available so I used two 1mmthick pieces in “parallel”. There are only five holes to drill/ ream in the rear panel – 4 x 3mm-diameter holes for the regulator mounting screws and 1 x 8mm-diameter hole to clear the power input socket. Their posi­tions are shown in Fig.6, so there shouldn’t be any problems with them. Just make sure you don’t leave any burrs around the 3mm holes in particular. A countersink bit or a large drill bit can be used to remove any metal swarf and make the edges smooth. With the rear panel drilled, the next step is to crank the three leads of each regulator IC forward, so that they end up immediately behind the terminal pins on the rear of the main PC board after final assembly. This is done by gripping each regulator’s leads with a pair of needlenose pliers about 4mm from the body (just after the leads narrow) and then bending all three upwards at 45°. The pliers are then used to grip them a further 5mm along, after which they are bent back down again by 45° (see Fig.5). The four regulators can now be fitted to the rear panel but first make sure that all the mounting holes are smooth and free of metal swarf. Fig.5 shows the mounting details. Note that each regulator must be electrically isolated from the rear panel using insulating bushes and mica washers. Smear all mating surfaces with silicone grease www.siliconchip.com.au 04104021 C 2002 04104022 C 2002 Fig.7: these are the full-size etching patterns for the two PC boards. Check your etched boards carefully for any defects before installing the parts. before bolting the regulators down. Alternatively, you can use silicone-impregnated thermal washers instead of the mica washers, in which case you don’t need the thermal grease. Make sure that you mount each regulator in the correct location – the two LM317Ts mount on the lefthand side of Fig.3, while the LM337Ts are on the right-hand side. When you have fitted them all, it’s a good idea to check with a DMM or ohmmeter to ensure that there’s no connection between any of the regulator leads and the panel. If you do find a short between any of the leads and the rear panel, remove the regulator and locate the source of the problem before refitting it. Final assembly The next step is to fit the board and front panel module into the lower half of the case. You do that by sliding the ends of the front panel carefully down into the front case slot. This should allow the main board to sit flat on the www.siliconchip.com.au case support spigots, with the mounting holes located over the centre hole in each spigot. If the alignment isn’t quite right, you may need to remove the board assembly again so that you can enlarge one or two of the board holes in the required direction. That done, refit the board assembly and install four 6mm x M3 self-tappers to hold it in position. The rear panel (and its 3-terminal regulators) can now be installed in the rear case slot. This should position each set of cranked regulator leads behind their corresponding PC terminal pins (in fact, they should be just touching). Check that all the leads are correctly aligned before soldering them to their respective PC pins. Checkout time If you’ve followed these instructions carefully, your supply should work correctly as soon as you plug the lead from the 9V AC plugpack into CON1. Each of the two pairs of LEDs should glow as soon as you switch on each pair of supply outputs using the two toggle switches. You can then check each of the output voltage pairs using your DMM. Check that you get the correct readings for each position of the two rotary switches – all voltages should be within about ±1% of their nominal values, under no load conditions. About the only possibilities for error are fitting the electrolytic capacitors or diodes incorrectly to the main PC board; mounting the regulators in the wrong positions on the rear panel; mixing up some of the resistors on the vertical PC board, or fitting one or more of the LEDs the wrong way around. So if your supply doesn’t work properly, check these possibilities first after quickly switching off. And that’s it – you’ve just finished making yourself a very handy little four-in-one bench supply. All that should remain is fitting the top of the SC case and putting it to use! April 2002  57