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Beginner’s Variable Dual-Rail Power Supply If you’re just beginning in electronics, then you’ll probably baulk at building a mainsoperated power supply. This project uses a plugpack which means that you can make your own variable dual-rail power supply without worrying about mains wiring. By DARREN YATES When it comes to experimenting in electronics, power sup­plies are a bit of a “chicken and egg” situation. To experiment with circuits, you need a power supply but unless you have the necessary knowledge already, building a mains-powered supply is beyond most beginners. The alternative is to run all of your circuits from batter­ies or buy a readymade supply. Either option is expensive. So in the interests of making it easier to start experimenting, we’ve come up with this dual-rail power supply which runs from a 16V AC plugpack. It’s capable of providing output voltages ranging from ±1.25V DC to ±15V DC at currents up to 500mA (see Fig.1). The beauty of this design is that it doesn’t require any external mains wiring! All the mains wiring is contained inside the plugpack, leaving you with just the low-voltage AC output which connects straight into the project. In order to keep costs down, the output voltage is varied in 11 switched steps. This eliminates the need for an output voltage meter since the precise value can be directly read off the switch position. The 11 switched voltage ranges are: 1.25V, 1.5V, 3V, 4.5V, 5V, 6V, 7.5V, 9V, 12V, 13.5V & 15V. Both supply rails are protected against short circuits and voltages generated by external loads, while a LED indicator lights if the supply stops regulating. Another worthwhile feature is the provision of a “load” switch. This allows the power to the load to be switched on and off while keeping the supply switched on. The output current capabilities of the supply are relative­ly modest but should be more than adequate for most projects. Fig.1 plots the maximum current that can be delivered at various output voltages. As can be seen, the supply is capable of deliv­ ering 250mA or more for voltages Silicon Chip’s Electronics TestBench  73 Fig.1: this graph plots the maximum output current from the supply for voltage settings between 1.5V & 15V (16VAC 1A plugpack). The supply is capable of delivering 250mA or more over most of the range. from 1.5V up to about 14V, with a maximum of 500mA at 7.5V. Note that these figures assume a 16VAC 1A plugpack supply. By now, some readers will be asking “what is a dual-rail power supply?” It’s quite straightforward really – a dual-rail power supply has both positive and negative output voltage rails, as well as the ground (or zero volt) rail. Most projects and cir­cuits you build will only require the positive output and the ground rail. This is basically the same as if you connected a battery of the same voltage to the circuit you’re building. However, you’ll also come up against circuits which use operational amplifiers (op amps) and these require both posi­ tive and negative supply rails. That’s where the dual-rail power supply comes in. It can power op amp circuits with ease and so is just that much more versatile than a standard single rail supply. An important feature of this design is that the negative supply rail automatically tracks the positive supply rail. This means that the two rails always have the same absolute value. Thus, if you set the positive output to +12V, the negative rail will be at -12V. And here we should clear up a common misconception regard­ ing dual rail supplies. Despite what many people think, it’s quite possible to use the positive and negative rails to obtain a much higher output voltage than is possible by simply connecting between one of these rails and the 0V rail. For example, if you want a 30V single-rail supply, simply set the supply 74 to give ±15V and connect the circuit across these outputs. Another way of looking at this is simply to con­sider that there is 30V between the two outputs. So a dual-rail ±1.25-15V variable power supply can also function as a 2.5-30V single rail supply. How it works The circuit for the Beginner’s Dual Rail Power Supply uses only standard components which you can find in any virtually electronics store. If you’ve got a parts bin handy, you’ll prob­ably have a few parts that are suitable already. Let’s take a look at the circuit – see Fig.2. The plug pack takes care of all of the mains wiring and steps the 240VAC mains voltage down to a suitable 16VAC for our circuit. This is fed via power switch S1 to rectifier diodes D1 & D2 to produce unregulat­ed plus and minus DC rails of about 20V. These DC rails are filtered by two 470µF electrolytic ca­ pacitors and fed to LM317 and LM337 3-terminal regulators. These provide the adjustable plus and minus supply outputs respec­tive­ly. In the case of the positive rail, the LM317 (REG1) does most of the work. Its output voltage is set by the 120Ω and 2.7kΩ resistors on its ADJ terminal and by the resistive divider string associated with switch S3. These components form the feedback network around the regulator IC. Basically, switch S3 sets the output voltage from REG1 by setting the resistance between the ADJ terminal and the 0V rail. When the ADJ terminal is connected to 0V, the output voltage is +1.25V. This voltage can then by Silicon Chip’s Electronics TestBench PARTS LIST 1 plastic case, 198 x 113 x 62mm 1 PC board, code 04110941, 102 x 57mm 1 front panel label 1 red 4mm binding post 1 black 4mm binding post 1 blue 4mm binding post 1 SPDT toggle switch (S1) 1 DPDT toggle switch (S2) 1 12-position 1-pole rotary switch (S3) 1 knob to suit S3 2 LED bezels 1 16VAC 1A plugpack 1 3.5mm power socket 2 mini U heatsinks 4 rubber feet Semiconductors 1 LM358 dual op amp (IC1) 1 LM317 3-terminal regulator (REG1) 1 LM337 3-terminal regulator (REG2) 6 1N4004 rectifier diodes (D1-D6) 6 1N914 diodes (D7-D12) 2 15V 1W zener diodes (ZD1,ZD2) 2 5mm red LEDs (LED1,LED2) Capacitors 2 470µF 25VW electrolytics 2 100µF 25VW electrolytics 4 1µF 63VW electrolytics 1 0.1µF 63VW MKT polyester Resistors (0.25W, 1%) 1 4.7MΩ 2 330Ω 2 47kΩ 1 270Ω 1 22kΩ 1 220Ω 2 3.3kΩ 1 180Ω 1 2.7kΩ 2 150Ω 3 1kΩ 2 120Ω 1 680Ω 1 56Ω 1 560Ω 1 27Ω 1 470Ω Miscellaneous Machine screws & nuts, washers, hook-up wire. stepped up to a maximum of +15V by using S3 to progressively switch in additional resistors in the string. The 1µF capacitor between the ADJ pin and ground ensures that any residual noise from the mains is kept to a minimum. Finally, the output voltage Silicon Chip’s Electronics TestBench  75 POWER LED1 1k 470 25VW 470 25VW D1 1N4004 330  ZD2 15V ZD1 15V 330  -15V 47k +15V 47k 22k 1 8 +15V -15V IC1a 2 LM358 4 1 1 1 OUT 2.7k LM317 REG1 ADJ 3 IN BEGINNER'S POWER SUPPLY  D2 1N4004 FROM 16VAC PLUG-PACK POWERT S1 D3 1N4004 1 120  100 25VW 15V 13.5V 12V 9V 7.5V 6V 5V 4.5V 3V 1.5V 1.25V S3 D4 1N4004 REG2 ADJ IN LM337 OUT 1k 560  470  680  270  220  150  56  180  150  27  120  D5 1N4004 3.3k 3.3k 0.1 D7 AO I LM317 D8 2x1N914 100 25VW 6 5 4.7M IC1b D6 1N4004 A IO LM337 7 1k A K 4x1N914 D9-D12 LED2 DROPOUT  M1 R2 R1 S2b LOAD S2a 0V V V This is the view inside the prototype. Note the two small heatsinks fitted to the two 3-terminal regulators. Take care to ensure that the regulators are correctly oriented – each device is installed with its metal tab towards the centre of the PC board. from REG1 is filtered by a 100µF electrolytic capacitor and fed to the load via switch S2a. Negative regulation The negative regulator (REG2) works in a similar manner to REG1. It’s made to track the positive rail by using IC1a to provide a mirror of the voltage on the ADJ terminal of REG1. For example, if the ADJ voltage of REG1 is at 10.75V (to produce a 12V output), then IC1a will act to produce -10.75V on the ADJ terminal of REG2. This is achieved by connecting IC1a as a unity gain invert­ ing amplifier. Its inverting input (pin 2) is fed from the ADJ terminal of REG1 via a 47kΩ Fig.2 (left): the circuit uses two adjustable 3-terminal regulators (REG1 & REG2) to provide the positive & negative supply rails. IC1a inverts the control voltage applied to the ADJ terminal of REG1 to drive REG2, while IC1b drives D9-D12 & LED 2 to provide dropout indication. 76 resistor, while the associated 47kΩ feedback resistor sets the gain to -1. The non-inverting input is biased to 0V via a 22kΩ resistor to ensure minimum output offset. The output of IC1a drives the ADJ terminal of REG2 via a 1kΩ resistor. This 1kΩ resistor is inside the feedback loop and is there so IC1a can actually drive the ADJ terminal to the maximum required value of -13.75V (when the output voltage is set to ±15V). This is outside the operating range of the LM358 because its supply rails are ±15V. The result of all this is that the negative output voltage of REG2 tracks the positive output voltage of REG1. The ±15V supply rails for IC1 are produced by zener diodes ZD1 and ZD2, while LED1 provides power indication. Diodes D3, D4, D5 and D6 protect the regulators from any reverse voltag­es which may be generated by capacitive or inductive loads con­ nected across the outputs. Dropout detection When the regulators are working as Silicon Chip’s Electronics TestBench intended, the ripple voltage superimposed on the DC rails will be very low. However, if the current drain is higher than the regulators can supply while still maintaining about 2V between their IN and OUT termi­nals, the ripple voltage will suddenly become quite high. At this point, the output voltage will fall quite rapidly if even more current is called for and the ripple will go even higher. What this means of course is that the power supply is unable to provide sufficient current to the load and is dropping out of regulation. This undesirable condition is indicated by the dropout indicator circuit and this is based on IC1b and diodes D9-D12. IC1b is connected as an inverting amplifier with a high gain, as determined by the ratio of the 4.7MΩ feedback resistor to the impedance of the 0.1µF input capacitor and the 3.3kΩ resistors which monitor the positive and negative supply rails. The two back-to-back diodes, D7 & D8, limit the maximum input signal to ±0.7V. When ever either regulator drops out of regulation (eg, if an output is shorted to ground), the ripple output increases greatly. Because it operates with such high gain, IC1b squares up this signal to produce a square-wave LED1 K S1 V+ 180  150  27  A 0V V-  150  56 1 S3 1 11 56 0 0W 22 27 0 680  47 LED2 K S2 0 2 3 4 A D3 ZD1 1 1k 1uF REG2 1k PLUGPACK SOCKET output at pin 7. This output drives a bridge rectifier consisting of D9-D12 via a 1kΩ current limiting resistor. The bridge rectifier in turn drives LED 2 and this begins to glow when the ripple at one of the regulator outputs exceeds about 4mV peak-to-peak. By the time the ripple reaches 19mV p-p, the LED is fully alight. An optional metering circuit is also shown on Fig.2, although we haven’t included it in the prototype (the appropriate connection points are on the PC board). All you have to do is calculate what resistance should be added in series with the meter to give a full-scale reading at 30V. For example, if you have a 0-1mA meter movement, then by Ohm’s Law R = V/I = 30/.001 = 30kΩ. Making R1 4 3 IC1 LM358 470uF 330 120  3.3k 0.1 D10 D7 D6 D12 100uF D11 2 1 1uF 100uF R1 3.3k D2 470uF D9 47k 330 1uF ZD1 D8 22k D1 1k 4.7M 2.7k 1uF 120  47k REG1 D5 R2 METER D4 Fig.3: use medium-duty (24 x 0.2mm) hookup wire for all wiring connections & take care to ensure that switch S3 is wired exactly as shown. Resistors R1 & R2 can be left out of circuit if you don't intend installing an output meter. = 27kΩ and R2 = 2.7kΩ will be near enough, especially when the internal impedance of the meter is taken into consideration. Construction All of the components for the Beginner’s Power Supply are installed on PC board coded 04110941 and measuring 102 x 57mm. Before commencing construction, check the board carefully against Fig.4 for any shorts or breaks in the tracks. If you find any, use a dash of solder or a small artwork knife where appropriate to fix the problem. Fig.3 shows the parts layout on the PC board. Start by installing PC stakes at the external wiring points, followed by the wire links, resistors, diodes, capacitors and ICs. Make sure that all polarised parts are correctly oriented and check the resistor values on your multimeter before mounting them on the board. Table 1 shows the resistor colour codes. Note that diodes D1-D6 are all 1N4004 types, while the remaining diodes are the smaller 1N914 types. Pin 1 of the IC is adjacent to a small notch or dot in one end of the plastic body. The metal tabs of the two 3-terminal regulators must be oriented exactly as shown on Fig.3; ie, the metal tab of each device goes towards the centre of the board. Do not confuse these two regulators – REG1 is an LM317 type while REG2 is an LM337. Once mounted, they can be fitted with small Silicon Chip’s Electronics TestBench  77 finned heatsinks to aid cooling. After the board assembly has been completed, you can in­stall the resistors around switch S3. As supplied, this switch will be a 12-position type. It is easily converted to an 11-position type by lifting the locking ring at the front of the switch bush and rotating it to position 11. This done, solder the resistors to the switch terminals exactly as shown on Fig.3, starting at terminal 1 and continuing in an anticlockwise direction to termi­ nal 11 (note: in most cases, the terminal numbers are marked on the back of the switch). If you have a switch that doesn’t have the terminals marked, here’s an easy way to find terminal 1. All you have to do is rotate the switch fully anticlockwise, then use your multi­ meter to find which terminal is now connected to the wiper. This will be terminal 1 and you can begin by soldering the 27Ω resis­ tor to it. The remaining resistors can then be installed exactly as shown. Check the resistor values carefully as they are mounted. If you make a mistake, then one or more of the voltage ranges will be wrong. It’s also a good idea to trim the resistor leads back as you go so that you don’t end up with a tangled mess. Don’t forget the wire link between the switch wiper (near Fig.4: this is the full-size etching pattern for the PC board the centre) and terminal 11. The Beginner’s Power Supply is designed to fit into a plas­tic zippy case measuring 198 x 113 x 62mm. The front panel is actually one of the long sides of the case, while the PC board is mounted on the bottom of the case. The whole unit is then turned upside down so that the lid becomes the base. The first step is to attach the front panel label (bottom nearest the lid), then use this as a drilling template for the front panel items. The PC board can also be used as a template to mark out its four mounting holes, while an additional hole will be required in the rear panel to accept a 3.5mm power socket. Note that it’s best to initially drill all holes to 3mm. These can then be enlarged where necessary using a tapered ream­er. Final assembly Once the holes have been completed, mount the various items in place. Fig.3 shows where each component should be placed. Note that the range switch (S3) must be oriented so that RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  2 ❏  1 ❏  2 ❏  1 ❏  3 ❏  1 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  2 ❏  2 ❏  1 ❏  1 78 Value 4.7MΩ 47kΩ 22kΩ 3.3kΩ 2.7kΩ 1kΩ 680Ω 560Ω 470Ω 330Ω 270Ω 220Ω 180Ω 150Ω 120Ω 56Ω 27Ω 4-Band Code (1%) yellow violet green brown yellow violet orange brown red red orange brown orange orange red brown red violet red brown brown black red brown blue grey brown brown green blue brown brown yellow violet brown brown orange orange brown brown red violet brown brown red red brown brown brown grey brown brown brown green brown brown brown red brown brown green blue black brown red violet black brown Silicon Chip’s Electronics TestBench 5-Band Code (1%) yellow violet black yellow brown yellow violet black red brown red red black red brown orange orange black brown brown red violet black brown brown brown black black brown brown blue grey black black brown green blue black black brown yellow violet black black brown orange orange black black brown red violet black black brown red red black black brown brown grey black black brown brown green black black brown brown red black black brown green blue black gold brown red violet black gold brown 5 DUAL TRACKING POWER SUPPLY 6 7.5 4.5 LOAD 9 3 12 1.5 13.5 1.25 + DROPOUT 0V - 15 POWER Fig.5: this full-size artwork can be used as a drilling template for the front panel. the pointer on the knob aligns with the 1.25V marking on the front panel when the switch is rotated fully anticlockwise. Binding posts are used for the three output terminals. We suggest that you use red for positive, black for 0V and blue for the negative. The PC board is secured in the case using machine screws and nuts, with additional nuts under each corner of the board acting as spacers. The wiring can now be completed as shown in Fig.3. It’s a good idea to use different coloured wire for each section, as this will make it easier to check your wiring later on. Take care with the orientation of the LEDs – the anode lead is always the longer of the two and the cathode will be adjacent to the flat edge on the LED bevel. Testing Now for the smoke test. Connect a 16VAC 1A plugpack supply, switch on and use your multi­meter to check the voltage between the “+” and “0V” terminals for each switch posi­ tion. In each case, the measured voltage should correspond to the switch position. The negative rail can then be checked in similar fashion; ie, by connecting the multimeter between the “-” and “0V” terminals. If everything checks out, the power supply is ready for use. If you strike problems, check the supply rails to the 3-terminal regulators and to IC1. You should find +20V on the IN terminal of REG1, -20V on the IN terminal of REG2, +15V on pin 8 of IC1, and -15V on pin 4 of IC1. If any of these voltages are incorrect, switch off and check D1, D2, ZD1 and ZD2 as appro­priate. If the measured output voltages don’t correspond to the switch settings, check the resistor string around S3. You may have some of the resistors in the wrong positions. Additional heatsinking As the unit stands, the output current capability is limit­ed by the modest amount of heatsinking. That’s because the two 3-terminal regulators have inbuilt thermal overload protection which means that they automatically throttle back when they start to get too hot. As an option, you can slightly increase the output current capability by increasing the heatsinking. This additional heat­ sinking can be obtained by substituting an aluminium lid for the plastic lid of the case. The two regulators are then bolted to the lid using TO-220 isolating kits (ie, a mica washer and insulating bush) to provide electrical isola­tion and their leads connected to the PC board via SC flying leads. Silicon Chip’s Electronics TestBench  79