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4-Output Universal Voltage Regulator By Jim Rowe & Nicholas Vinen This is our most flexible linear regulator board yet. It has provision for four outputs: adjustable positive and negative outputs and two fixed positive outputs of 5V & 3.3V. It can be fed from an AC plugpack, small transformer or DC supply with balanced outputs. T HIS MODULE was initially design­ ed to power the Balanced Input Attenuator project elsewhere in this issue but it can also be used to power a wide variety of circuits. It can supply balanced rails for op amps and comparators as well as multiple lowvoltage rails to power microcontrollers, digital logic ICs etc. A typical configuration with four outputs might be: +15V, -15V, +5V and +3.3V. These can all come from the same transformer, as long as the current requirements are modest. It can fit into a small jiffy box for lowcurrent applications and this can even be mounted on the back of a (large) plugpack. Alternatively, there are four mounting holes so it can be held in a larger case by tapped spacers. It’s designed to run from an AC plugpack or small AC transformer but a DC supply can also be used provided you don’t need a negative output voltage. Ideally, if a transformer is used, it should have a centre tap although this is not required and indeed most AC plugpacks lack a centre tap connection. The input and main output connect­ ions are made via terminal blocks at either end of the PCB while the 3.3V, 5/6/9/12V and GND terminals are via a Features & Specifications Output voltages: 1.3V to 22V, -1.3V to -22V plus either 12V, 9V, 6V or 5V + 3.3V* Continuous output current: typically 200mA+ per output, depending on voltages Peak output current: up to 1.5A on adjustable outputs, 1A/250mA for fixed outputs Output ripple: typically <1mV RMS on all outputs up to 250mA load Line regulation: <2mV/V (main outputs), <1mV/V (auxiliary outputs) Load regulation: <20mV/A Transient response (1A load step): 500mV drop, 400mV overshoot, 200ms recovery Quiescent current: ~40mA (AC supply), ~25mA (single polarity DC supply) Protection: short circuit, over-current, over-temperature, reverse polarity (with DC supply) * Main positive output must be at least 2V higher than auxiliary output voltage 78  Silicon Chip polarised header. There’s an on-board LED to indicate that power is present. Our last universal regulator, in the March 2011 issue, was somewhat simpler and cheaper to build than this one but it didn’t have as many outputs, nor was its performance quite as good. This new design has quieter outputs which are more suitable for powering sensitive audio gear. In addition, since this one is adjust­able, the two main output voltages can be accurately set without changing any components. Different configurations This design has provision for four regulators as noted above, however if you don’t need four different voltages you can leave some components off to save time and money. Since it’s common to need two or three regulated supply rails, we’re pro­viding a few different options for building the module: •  Version A: this deluxe version in­ cludes all four outputs, two adjustable and two fixed, plus a power LED. It can run off single, dual or centre-tapped transformer secondaries. •  Version B: like Version A, this one has positive and negative adjustable outputs but does not include the two extra fixed positive regulators for circuits where they are not required. siliconchip.com.au •  Version C: similar to Version B but the output voltages are set by fixed resistors and it will only run from a transformer with a single secondary winding. This is the version used to supply the ±15V rails for the Balanced Input Attenuator from a 17VAC plugpack. •  Version D: similar to Version A (deluxe) but without the negative adjustable regulator and associated components. Thus it has three outputs, all positive: one adjustable and two fixed. It can run from an AC or DC supply, including batteries, DC plugpacks and in-line switchmode supplies. Other combinations are possible and, for example, it would be possible to modify Version D so that it has a full bridge rectifier at its input, which might be handy if you want to run it from an AC plugpack (ie, with a single secondary winding). All four versions can be built using the same PCB. Voltage limitations There are many different combin­ ations of voltages that you can get from this board but there are also a few restrictions. These apply mainly to the two auxiliary outputs, which would normally be +5V and +3.3V but there are some other options. The first restriction is that the main auxiliary output (normally 5V), which can deliver 1A, must be at least 2V less than the positive adjustable output. If you want to have a 6V, 9V or 12V output instead of 5V, it’s simply a matter of swapping this fixed regulator for one with a different output voltage. However, the 2V headroom is still required. Also, note that any current drawn from either auxiliary output reduces the maximum available from the main positive adjustable output. Note that if you choose to change the 5V output to a higher voltage, you will lose the 3.3V output as the specified regulator will not withstand a higher input voltage. You can also omit the 3.3V regulator if you don’t need that output. Current capability While the two adjustable outputs are capable of delivering peak currents of up to 1.5A, in practice heat dissipation will limit the continuous current delivery to a fraction of this. Similarly, the higher-voltage fixed output is capable of 1A but it too is normally dissipation-limited. The 3.3V output has no such limitation since it is only siliconchip.com.au rated at 250mA anyway. How much current you’ll get from this board depends mainly on the output voltages and the voltage(s) you’re feeding in. In most cases, we expect constructors will be running it from a transformer (including AC plugpacks) and selecting the right transformer for maximum current and to avoid loss of regulation. Transformer selection Having figured out what output voltages you need and how much current is required by the circuit it’s going to power, use the following procedure to select a transformer or power supply. Let the highest positive voltage that’s required be Vp(max) and the total current required from all positive outputs be Ip(sum). Similarly, let the magnitude of the negative output voltage be Vn and the required negative current be In. For a transformer with a single secondary, the ideal voltage is whichever of these two results is higher: V1 = (Vp(max) + 3.5V + Ip(sum) x 20) x 0.7 V2 = (Vn + 3.5V + In x 20) x 0.7 Whereas for a transformer with two secondaries or a single centre-tapped secondary, use: V1 = (Vp(max) + 3.5V + Ip(sum) x 10) x 0.7 V2 = (Vn + 3.5V + In x 10) x 0.7 For a centre-tapped transformer, double the resulting voltage. It’s unlikely you’ll get a re­sult that’s a round number so choose a transformer with the next highest voltage rating. Often, you will find that you need a transformer with the same AC voltage rating as the highest DC output voltage you have selected, eg, a 15VAC transformer is used for ±15V DC outputs. Now let the transformer secondary voltage be Vac. To calculate the re­ quired transformer VA rating, use the following formula for a transformer without a centre tap: VA = Vac x 1.5 x (Ip(sum) + In) For transformers with a centre tap, use: VA = Vac x 0.75 x (Ip(sum) + In) Note that it’s generally a good idea to choose a transformer with a somewhat higher VA rating if at all possible. This is not just us being conservative; with a circuit like this, because most of the current will be drawn at the Parts List 1 double-sided PCB, code 18105151, 76 x 46mm 1 UB5 jiffy box (optional) OR 4 M3 tapped spacers and machine screws for mounting 1 transformer or plugpack to suit required voltages/currents 4 2-way mini terminal blocks, 5.08mm pitch (CON1,CON2) 1 3-way polarised header (CON3) 2 2kΩ mini horizontal trimpots (VR1,VR2) 3 mini flag (6073B-type) heatsinks (for REG1-REG3) 3 M3 x 10mm machine screws and nuts (for mounting heatsinks) 2 grommets to suit input/output cables (optional) Semiconductors 1 LM317T adjustable positive regulator (REG1) 1 LM337T adjustable negative regulator (REG2) 1 7805T 5V 1A regulator* (REG3) 1 MCP1700-3.3/TO LDO 3.3V regulator (REG4) 8 1N4004 diodes (D1-D8) 1 3mm LED (LED1) Capacitors 2 2200µF 25V electrolytic 3 100µF 25V electrolytic 2 10µF 25V electrolytic 2 1µF multi-layer ceramic 4 100nF multi-layer ceramic Resistors (0.25W, 1%) 1 3kΩ 0.5W 2 1kΩ 1 1.5kΩ 2 100Ω 2 1.1kΩ 2 10Ω 0.5W Notes: (1) Some parts may be omitted, depending on which version is being built. (2) For wider voltage adjustment range, reduce 1kΩ resistor value. 500Ω trimpots can be used instead for a narrower adjustment range. (3) *A different 78xx series regulator may be substituted in some cases (see text). In this case, REG4 is not fitted and the 3.3V output is not functional. voltage peaks, these calculations will underestimate the I2R losses in the transformer and so it will get hotter than you might expect. Thus a transMay 2015  79 Table 1 Power Supply Conguration Options Power Supply Adjustable Output(s) Auxiliary Output(s) Dropper resistor(s) 9VAC plugpack or transformer, 4.5VA ±9V 200mA each 5+3.3V* 200mA total 0Ω (wire links) 18VAC centre-tapped transformer, 4.5VA ±9V 200mA each 5+3.3V* 200mA total 0Ω (wire links) 12VAC plugpack or transformer, 6VA ±9V 200mA each 5+3.3V* 200mA total 10Ω 0.5W 12VAC plugpack or transformer, 6VA ±12V 200mA each 5+3.3V** 200mA total 0Ω (wire links) 24VAC centre-tapped transformer, 6VA ±12V 200mA each 5+3.3V** 200mA total 0Ω (wire links) 15VAC plugpack or transformer, 7.5VA ±12V 200mA each 5+3.3V** 200mA total 10Ω 0.5W 15VAC plugpack or transformer, 7.5VA ±15V 200mA each 5+3.3V# 200mA total 0Ω (wire links) 30VAC centre-tapped transformer, 7.5VA ±15V 100mA each 5+3.3V# 300mA total 0# (wire links) 17VAC plugpack or transformer, 8VA ±15V 200mA each 5+3.3V# 200mA total 10Ω 0.5W 36VAC centre-tapped transformer ±15V 200mA each 5+3.3V# 200mA total 10Ω 0.5W 36VAC centre-tapped transformer ±17V 200mA each 5+3.3V## 200mA total 0Ω (wire links) 48VAC centre-tapped transformer*** ±24V 200mA each 5+3.3V## 150mA total 0Ω (wire links) 12V DC plugpack or lead-acid battery +9V, 200mA 5+3.3V* 200mA total 0Ω (wire link) 15V DC plugpack or switchmode supply +12V, 400mA 5+3.3V** 250mA total 0Ω (wire link) 18V DC plugpack or switchmode supply +15V, 400mA 5+3.3V# 250mA total 0Ω (wire link) 24V DC plugpack or lead-acid battery*** +12V, 100mA 5+3.3V** 80mA total 0Ω (wire link) Note: current ratings selected for maximum 2W dissipation per heatsinked TO-220 package; higher currents possible with sufficient airflow. For example, add 50% to all current values for 3W dissipation per package. * alternative auxiliary output: 6V DC   ** alternative auxiliary outputs: 6V or 9V DC   *** use 1000µF 35V capacitors # alternative auxiliary outputs: 6V, 9V or 12V DC   ## alternative auxiliary outputs: 6V, 9V, 12V, 15V or 18V DC former with a somewhat higher rating (say 50%) is desirable. If that all seems too hard, have a look at Table 1. We’ve done these calculations (plus more explained below) for a number of common configurations. Assuming your needs match up with those, you can simply read the supply options from the table. Regulator dissipation Having chosen a transformer, it’s now a good idea to check that the regulator dissipation will be reasonable. If it’s too high, the regulators could overheat and shut down; this is unlikely to cause any damage but it will prevent your circuit from working properly! Let the adjustable positive output voltage be Vp1 and the auxiliary positive voltages be Vp2 (normally 5V) and Vp3 (normally 3.3V). Similarly, the maximum current drawn from each output is Ip1, Ip2 and Ip3. Dissipation can then be approximated as: DISreg1 = (Vac x 1.4 – Vp1) x Ip(sum) DISreg2 = (Vac x 1.4 – Vn) x In DISreg3 = (Vp2 – Vp1) x (Ip2 + Ip3) DISreg4 = (Vp3 – Vp2) x Ip3 The results are in Watts. As stated 80  Silicon Chip earlier, you don’t really need to worry about the dissipation of REG4 as it will normally be less than 0.5W. REG1REG3 can handle about 2W each before you risk them shutting down; more in free air (say 3W) and even more if you have forced air (eg, a fan blowing over the heatsinks). If using a DC supply, replace the Vac x 1.4 term with the maximum DC input voltage the regulator will experience. For example, if it’s being powered from a lead-acid battery which could be charged during use, to be safe, substitute 15V. It’s a good idea to calculate the sum of all four figures, especially if you’re planning to put the board in a jiffy box. This will give you an idea of how much heat will be coming off the board. More than a few Watts total and the jiffy box will get mighty warm! Note that if you have had to choose a transformer with a higher than ideal voltage rating (due to availability, etc) and the dissipation values for REG1 and REG2 look a little on the high side, the board does have provision to fit a couple of 0.5W dropping resistors before the regulators. These will allow you to reduce the dissipation of each regulator by around one third to one half watt each; not a major reduction but possibly enough to prevent them from overheating and shutting down. If you do want to do this, calculate the required resistor values as follow: Rp = (Vac x 1.4 – 3.5 – Vp1) ÷ ( Ip(sum) x 3 ) Rn = (Vac x 1.4 – 3.5 – Vn1) ÷ ( In x 3 ) Round to the next lowest preferred resistor value. For the Bal­anced Input Attenuator power supply, we had to use a 17VAC plugpack to get the Earth connection (ideally we would have used 15VAC). The output voltages are ±15V and the current requirement is around 180mA each. If you do the calculations, you’ll come up with 10Ω, which is what we used. The dissipation in REG1 & REG2 then reduces to: DISreg1 = (Vac x 1.4 – Vp1 – Rp x Ip(sum)) x Ip(sum) DISreg2 = (Vac x 1.4 – Vn – Rn x In) x In In our case, this leads to a reduction in dissipation of about 0.33W each. Note that this does not change the total dissipation; it merely moves some of it away from REG1 and REG2 and into the added resistors. This means you can’t really reduce the dissipation per siliconchip.com.au REG4 MCP1700-3.3V Fig.1: the circuit for Version A. It’s based on a mains transformer with a 30V centre-tapped secondary (or two 15V secondaries) and has two adjustable outputs (REG1 & REG2) and fixed +3.3V & +5V outputs (REG3 & REG4). The adjustable outputs can be independently set from +13.2V to +17V and -13.2V to -17V. GND A 230V 15V 0V +5V 15V ~ A N 2200 µF 100nF 25V K K VR1 2k 25V K IN D1–D8: 1N4004 A SC +Vo A K 100 µF 0V –Vo D8 E A D7 100Ω 3.0k 0.5W A OUT K A regulator by more than we did or you risk burning out the resistors. Running from a DC supply If using a regulated DC supply or battery, the considerations are much simpler. Around 3V headroom is required, so for example with a 12V DC supply the highest available output voltage will be 9V. For a battery, calculate using the lowest expected terminal voltage. The current drawn from the DC supply is simply the sum of the current drawn from each regulator output, plus the quiescent current of around 25mA. As mentioned earlier, you can’t use the negative output if the regulator board is running off DC. Circuit description The full circuit is shown in Fig.1 and this is version A. Here we’re assuming that the power supply is a mains transformer with a 30V centre-tapped secondary, or two 15V secondaries connected in series. These secondaries connect to a bridge rectifier formed by diodes D1-D4 on the board via terminal block CON1, to charge up 7805 MC P1700 IN K UNIVERSAL REGULATOR MK2 siliconchip.com.au CON2 REG2 LM337T LED 20 1 5 λ K D6 100 µF K ADJ A LED1 K VR2 2k 100nF D3 * LINK OUT OR CHANGE THESE RESISTORS TO ALLOW A WIDER RANGE OF OUTPUT VOLTAGES 1k* 1k* 10 µF 2200 µF 100nF D5 A 100nF 10 µF A A 100Ω D4 E 100 µF K ADJ D2 1 µF OUT IN ~ CT +3.3V REG1 LM317T K CON1 0V CON3 OUT IN D1 A 1 µF REG3 7805 GND T1 OUT IN OUT GND IN GND GND OUT LM337T LM317T OUT ADJ OUT IN IN ADJ IN OUT VERSION A: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE DUAL OUTPUTS PLUS TWO FIXED POSITIVE REGULATORS two 2200µF electrolytic capacitors to roughly ±20V. REG1 then regulates the +20V to somewhere between +13.2V and +17V, depending on the setting of VR1. Similarly, REG2 regulates the -20V rail to between -13.2V and -17V depending on how VR2 is set. The lower limits of these voltages are determined by the ratio of the 1kΩ and 100Ω divider resistors while the upper limits are determine by the need to have at least 2V of headroom for the regulators, when taking into account the ~1V ripple expected on the input capacitors with moderate (~100mA) loads on the regulators. Thus, if you need a lower output voltage you can reduce the 1kΩ values or link these resistors out entirely. Similarly, you could change the 2kΩ trimpots to lower values (eg, 500Ω) to give a narrower adjustment range. This would make accurately setting the output voltage easier but would require the initial range (determine by those fixed resistors) to be set fairly accurately. When choosing fixed resistor values, factor half of the resistance of VR1/VR2 into the equation, so that these pots will be roughly centred at the required output voltage. The formula to select these resistors is: Vout ÷ 0.0125 - 100Ω. Subtract half the trimpot resistance then pick the closest resistor value. The 10µF capacitors from each ADJ terminal to ground greatly improve the ripple rejection for REG1 and REG2. That’s because they reduce the impedance between the ADJ terminal and GND, which would otherwise be limited by the value of the resistors used in the divider. There are also 100µF capacitors at each regulator output to improve transient response. Diodes D6 & D8 prevent the regulator outputs from being pulled negative at switch-on/switch-off by a load connected directly between +Vo and -Vo. This is an especially common problem when a transformer with a single secondary is being used, as depending on which part of the mains cycle power is applied, either the positive or negative rail will come up first and any capacitors across the output (typically within the load) will cause the other output to be pulled in the wrong direction. LED1 is connected across both out­ May 2015  81 D1 A T1 A 230V 15V REG1 LM317T K CT 15V ~ K ADJ ~ 0V OUT IN CON1 D2 A N 100nF K 2200 µF 25V VR1 2k 10 µF 1k* E 100nF 2200 µF 25V IN * LINK OUT OR CHANGE THESE RESISTORS TO ALLOW A WIDER RANGE OF OUTPUT VOLTAGES 100 µF 0V –Vo E A 3.0k 0.5W A REG2 LM337T D1–D8: 1N4004 A SC +Vo D8 D7 100Ω OUT UNIVERSAL REGULATOR MK2 K K A LM337T LM317T LED 20 1 5 CON2 A K K ADJ A λ K D6 VR2 2k 100nF D3 K LED1 K 100 µF 1k* 10 µF A D5 A 100nF D4 K A 100Ω OUT ADJ OUT IN IN ADJ IN OUT VERSION B: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE DUAL OUTPUTS Fig.2: Version B is similar to Version A but omits the two fixed voltage regulators. This is the version to build if you only require split supply rails that can be set anywhere from +13.2V to +17V and -13.2V to -17V. Trimpot VR1 adjusts the positive rail, while VR2 adjusts the negative rail. puts and will light as long as there is more than a few volts between them. REG3 provides the +5V rail and this runs from the output of REG1. There is quite a large voltage drop from the input filter capacitor (in this case, around 20V) and the 5V output so this arrangement splits the dissipation between REG1 and REG3, both of which would normally be fitted with a heatsink. It also means the 5V rail will be very quiet and virtually free of 50/100Hz ripple. Input bypassing is provided by REG1’s output capacitor while a 100µF electrolytic capacitor provides output filtering. REG4 derives the 3.3V rail from the 5V output and has 1µF ceramic capacitors at both input and output. REG4 can only handle an input voltage of up to 6V, thus REG3 is required if it is to be used. It can provide up to 250mA output and will only dissipate (5V - 3.3V) x 0.25A = 425mW at full load, well within the capabilities of the small TO-92 package (625mW). Both the 3.3V and 5V rails are available at CON3 while the two main outputs and mains earth are at CON2. Note that you could change REG3 to a higher-voltage type of regulator if required but then you would have to leave REG4 out as it will not handle the higher input voltage. Note also that a mains earth connection is made between CON1 and CON2 but is not joined to the rest of 82  Silicon Chip the circuit. This would normally be connected to ground at the load end. In the Balanced Attenuator project, this allows for an Earth Lift switch to disconnect the two should the circuit be earthed elsewhere. Other versions Fig.2 shows version B of the circuit in which REG3 and REG4 are not fitted and the associated components have also been deleted. This is how you would build the board if you only need the two main (±) outputs, ie, without 5V or 3.3V rails. Fig.3 shows version C which is the same as version B but with two changes: (1) Trimpots VR1 and VR2 have been omitted. This reduces the cost slightly and gives fixed output voltages within about ±5% of the selected values (due to regulator and resistor tolerances). However, note that you may not be able to select resistors of exactly the value required to set your desired output voltage, thus the difference could be more than 5%. (2) A 17VAC plugpack has been used and this does not have a centre-tapped secondary. As such, diodes D2 and D4 have been omitted since they are not used and D1 & D3 operate as two half-wave rectifiers. The disadvantage is that the filter capacitors are only recharged alternately at 50Hz rather than simultaneously at 100Hz how- ever there is little choice as few AC plugpacks have centre-tap connections available. As explained earlier, this is the version used to power the Balanced Input Attenuator presented elsewhere in this issue. The circuit in Fig.4 is similar to Fig.1 but all the components associated with the negative output have been removed. This is shown powered from a transformer with a centre-tapped secondary but a DC supply could also be used, with its negative output connected to the CT terminal of CON1 and its positive output to either of the remaining terminals (ignoring the earth connection, which could be left unconnected). Note that the current-limiting resist­ or value for LED1 has been reduced as it is now running from a lower voltage without the presence of the negative rail. Construction Once you have decided which version to build, calculate the required resistor values to set the output voltage ranges. If you are fitting the optional voltage-dropping resistors you will need to calculate their value too, otherwise you will be fitting wire links in their place. Refer to the overlay diagram appropriate to the configuration you are building, which will be one of Figs.5-8 (or a variation thereof). siliconchip.com.au D1 A 17V/1.25A AC PLUGPACK N ~ 17V 230V E OUT IN 0.5W CON1 A REG1 LM317T 10Ω K CT 2200 µF 100nF A 100Ω D5 LED1 A 100nF 25V ~ K ADJ 1.1k 10 µF λ K K CON2 D6 100 µF +15V A K 1.1k 10 µF 2200 µF 100nF 100nF 25V D3 K IN 0.5W UNIVERSAL REGULATOR MK2 SC 3.0k 0.5W D7 A REG2 LM337T D1,D3,D5-D8: 1N4004 20 1 5 E A 100Ω OUT OUT ADJ K A K LM337T LM317T LED A 0V –15V D8 K ADJ 10Ω A 100 µF IN IN OUT IN ADJ OUT VERSION C: UNTAPPED TRANSFORMER SECONDARY, DUAL ±15V OUTPUTS Fig.3: Version C uses a 17VAC plugpack (ie, no centre-tap), with D1 & D3 operating as half-wave rectifiers. In addition, trimpots VR1 & VR2 have been omitted and the output rails set to ±15V by the 100Ω and 1.1kΩ resistors. This is the version that’s used to power the Balanced Input Attenuator described elsewhere in this issue. REG4 MCP1700-3.3V OUT IN GND * LINK OUT OR CHANGE THIS RESISTOR TO ALLOW A WIDER RANGE OF OUTPUT VOLTAGES D1 230V +3.3V 0V +5V OUT IN A T1 15V CON3 REG3 7805 GND A 1 µF 15V ~ OUT IN K ADJ ~ CT D2 A 100 µF REG1 LM317T K CON1 0V 1 µF 2200 µF 100nF 25V K A 100Ω D5 LED1 A 100nF VR1 2k 10 µF 1k* K D6 100 µF λ K CON2 1.5k A N +Vo 0V –Vo E E LED D1,D2,D5,D6: 1N4004 A SC 20 1 5 K IN K A UNIVERSAL REGULATOR MK2 7805 MC P1700 OUT GND IN GND GND LM317T OUT OUT ADJ OUT IN VERSION D: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE POSITIVE OUTPUT PLUS TWO FIXED POSITIVE REGULATORS Fig.4: Version D has fixed +3.3V & +5V outputs based on REG4 & REG3, plus a single +13.2V to +17V adjustable output based on REG1. It’s similar to Version A but does away with the parts associated with the adjustable negative output rail. Note that linking out or changing the 1kΩ resistor allows a wider range of output voltages to be set (all versions). Start by fitting the resistors, keeping in mind any variations in value. If your version requires any wire links, form these from the resistor lead off-cuts and solder them in place. Follow with siliconchip.com.au the 1N4004 diodes, being careful to match up the orientation of each with the appropriate overlay diagram before soldering. There are between four and eight diodes depending on the version. Fit the ceramic capacitors next, followed by trimpots VR1 and VR2. If you don’t need to be able to adjust the outputs and have selected appropriate resistors to give the required voltages, May 2015  83 D7 LM317T 18105151 18105151 D8 4004 5V A D5 LED1 V 5 1- –Vo V0 0V V51+ 4004 E 4004 CON3 0V 10 µF 100 µF EARTH +Vo D6 1 µF CON2 1 µF REG4 VR1 VR2 7805 1k 100Ω 100nF + D1 25V 220 0µ 2200 µF 4004 1k 100Ω 25V 2200 22 0 0 µF D2 4004 D3 CON1 4004 ~ TUP NI CA V 7 1 ~ 100nF REG1 + ~ C 2015 + CT 3.0k REG3 { { + 15V-0 -15V AC IN ~ LM337T + 100nF MAINS EARTH 100 µF 10 µF100 µF + 4004 + 100nF D4 4004 REG2 DC OUTPUTS 3.3V K PWR VERSION A: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE DUAL OUTPUTS PLUS TWO FIXED POSITIVE REGULATORS Fig.5: this Version A board layout corresponds to the circuit diagram of Fig.1. All parts are installed on the PCB, with the adjustable outputs available at CON2 and the fixed +3.3V & +5V outputs at CON3. D7 D8 V 5 1- –Vo V0 0V V51+ EARTH +Vo D6 CON2 4004 10 µF 100 µF E 4004 1k VR1 VR2 100Ω 100Ω 100nF LM317T 18105151 18105151 1k 25V 2200 22 0 0 µF + D1 REG1 25V 100nF 4004 220 0µ 2200 µF D3 4004 D2 4004 CON1 ~ TUP NI CA V 7 1 ~ ~ C 2015 + { ~ CT + 15V-0 -15V AC IN LM337T 3.0k DC OUTPUTS + MAINS EARTH 10 µF100 µF { 100nF + 4004 + 100nF D4 4004 REG2 4004 A D5 LED1 K PWR VERSION B: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE DUAL OUTPUTS Fig.6: follow this PCB layout to build Version B if you only require adjustable split rail outputs (ie, 13.2V to +17V and -13.2V to -17V). Note the heatsinks fitted to the regulators. link them out as shown in Fig.7. Next, dovetail the pairs of 2-way terminal blocks to form two 4-way blocks and place them on the PCB with the wire entry holes facing the nearest edge of the board. Ensure they are pushed down flat before soldering the pins. REG4 can then go in, if you are fitting it. If so, crank its leads out (eg, using small pliers) to suit the holes in the PCB. CON3 can be fitted next, assuming you are using either of the auxiliary outputs. The smaller electrolytic capaci- tors go in now. Be careful with their orientation; in each case, the longer positive lead goes towards the bottom of the board, as shown in Figs.5-8. You will probably need to crank the leads out to fit the PCB pads and depending on the size of the 100µF capacitors, you may find you need to bend them sideways a little in order to avoid interfering with adjacent components (see photos). Now secure each TO-220 regulator you are using firmly to a small flag heatsink using an M3 x 6mm machine screw, shakeproof washer and nut. Table 1: Resistor Colour Codes   o o o o o o No.   1   1   2   2   2 84  Silicon Chip Value 3kΩ 1.5kΩ 1kΩ 100Ω 10Ω 4-Band Code (1%) orange black red brown brown green red brown brown black red brown brown black brown brown brown black black brown Make sure that each regulator is fitted straight on the heatsink, then drop it into place on the PCB. Check that its leads are inserted evenly and then solder and trim them. Repeat for any other regulators being installed. The larger electros can now go in, then all that’s left is the power indicator LED. We arranged for ours to poke out through the lid of the jiffy box. To do this, solder it with the bottom of the lens 26mm from the top of the PCB. This is close to full lead length (about 5mm short). Ensure the longer anode lead goes into the hole to the left of the board, ie, with the orientation shown in Figs.5-8. Testing & setting up There isn’t much to check. Connect your power supply temporarily to CON1 and power it on. Verify that LED1 lights, then measure the output   Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 1µF   1µF   1u0   105 100nF   0.1µF 100n   104 5-Band Code (1%) orange black black brown brown brown green black brown brown brown black black brown brown brown black black black brown brown black black gold brown siliconchip.com.au D7 D8 V 5 1- 0V V51+ V0 E 4004 4004 LM317T 18105151 18105151 –15V +15V D6 10 µF 100 µF CON2 100Ω 1.1k 1.1k 100nF 10Ω 100Ω 2200 22 0 0 µF 25V 25V + D1 220 0µ 2200 µF 4004 D3 CON1 TUP NI CA V 7 1 ~ 100nF 4004 REG1 + ~ C 2015 EARTH DC OUTPUTS + ~ 3.0k LM337T + 17V AC IN 10 µF100 µF + 100nF MAINS EARTH FROM PLUGPACK 4004 REG2 { 10Ω 100nF + This photo shows the Version A board fitted into a UB5 plastic case. The power LED pokes through a hole in the lid. 4004 K A D5 LED1 PWR VERSION C: UNTAPPED TRANSFORMER SECONDARY, DUAL 15V OUTPUTS Fig.7: the Version C PCB layout has fixed ±15V DC outputs and runs from a 17VAC plugpack (see parts list for Balanced Attenuator). This is the version to build to power the Balanced Input Attenuator. Below is the assembled PCB. siliconchip.com.au LM317T 18105151 18105151 E 10 µF 100 µF 4004 5V A D5 LED1 V 5 1V0 CON2 0V V51+ CON3 0V 1.1k 100Ω 100nF D1 1 µF EARTH +Vo DC OUTPUT D6 7805 4004 REG4 D2 4004 CON1 ~ TUP NI CA V 7 1 REG1 100nF 4004 1 µF VR1 C 2015 ~ ~ 2200 µF 25V + If you want to mount the board in a UB5 jiffy box as we did (and as we recommend for the Balanced Input Attenuator power supply), you will need to make some minor modifications. You can’t slide the board into the pre-cut notches since the components are too tall, so you need to cut new notches 4mm tall at the bottom of each of the eight ribs using side-cutters and then pliers to remove the remainder. The board will then snap into the bottom of the case, with some cajoling. Next, drill a 3mm hole in the upperleft corner of the lid for the power LED. This goes 10mm from the long side and { CT + Putting it in a box 15V-0 -15V AC IN ~ REG3 + voltages and ensure they are correct. If VR1 and/or VR2 are fitted, simply adjust them to get the required voltage(s). If you can’t, you may need to change the associated fixed resistors. 1.5k MAINS EARTH { The completed unit can be attached to the back of a plugpack supply as shown here. It’s shown taped into position here but could also be secured using silicone adhesive. 3.3V K PWR VERSION D: CENTRE TAPPED TRANSFORMER SECONDARY, ADJUSTABLE POSITIVE OUTPUT PLUS TWO FIXED POSITIVE REGULATORS Fig.8: here’s how to install the parts to build Version D. It has fixed +3.3V & +5V outputs at CON3, plus an adjustable +13.2V to +17V output at CON2. 23mm from the short side of the lid. Check the position with respect to the PCB before drilling it. Two holes are required in the lefthand and righthand ends of the box for the input and output cables. Because the terminal blocks mount so close to the ends of the box, these will need to be made fairly high up and then the individual wires looped down to reach the board. You may wish to fit grommets in these holes, with the right diameter for the cable you’re using. For the Balanced Input Attenuator, the input cable from the plugpack has three wires and these are connected as shown in Fig.7. The output goes to a 4-wire shielded cable fitted with a 5-pin DIN plug. The wiring details for this cable are shown in the Balanced Input Attenuator article (page 70). Once the unit has been tested and the lid screwed onto the box, you can then use double-sided tape to attach it to the rear of the plugpack itself – see SC adjacent photo. May 2015  85