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This power supply has balanced positive and negative supply rails and can be controlled by your computer to deliver up to ±25.5V and up to 2.55A. Not only are all the functions of the power supply programmable but you can also use it as a conven­ tional supply with all functions controlled from its front panel. PART 1: BY RICK WALTERS COMPUTER CONTROLLE 56  Silicon Chip W E HAVE PUBLISHED quite a few power supplies in the past but this is the first one to have the option of computer control via the parallel port of a PC-compatible computer. In fact, you can build this project as a conventional power supply with normal front panel controls for voltage, current limit and so on, or with the addition of an extra PC board linked back to your computer’s parallel port, you can have full computer control via an on-screen menu. The computer program allows you two options: (1) full vari­able control of voltage and current from the computer keyboard and (2) monitoring of voltage and current with these values displayed on the VGA monitor. Virtually any PC-compatible computer can be used: 286, 386, 486 or Pentium. The program is not Windows-based, although you could run it from within Windows if desired. Here’s your chance to press that old 286 or 386 into service and make it do someth­ing useful again if it has been relegated to the back room. Specifications 1. Positive & negative supplies, each adjustable from 0V to 25.5V 2. Local individual voltage settings; computer-controlled individual voltage settings; computer-controlled negative track­ing positive 3. Current limiting for both supplies from 10mA to 2.55A 4. Local metering of positive or negative supply voltage 5. Local metering of positive or negative supply current 6. Remote positive voltage setting in 100mV steps from 0V to 25.5V 7. Remote negative voltage setting in 100mV steps from 0V to -25.5V 8. Remote current limit setting from 0 to 2.55A in 10mA steps 9. Remote monitoring of positive and negative output voltages 10. Remote monitoring of positive output current to ±25.5V at up to 2.55A. These odd maximum values come about because we use an 8-bit parallel printer port and an A/D (analog to digital) converter which has a maximum conversion count of 255. To exploit the full conversion range of this device we selected the aforementioned voltage and current values. A front panel switch allows instant changeover from comput­er control to local (front panel) setting capability. LED indica­ tors show whether the supply is in local or computer mode. Why programmable? Why not? There are many processes which require a certain voltage (or current) for a particular time, then a reduced vol­tage after that. Or maybe you want to monitor the current drawn over a long period and you can’t sit watching the power supply all day, can you? You might want to control a plating job for a couple of hours for example, or maybe charge a nicad battery. The charging procedure usually specifies 14-15 hours at 1/10 the rated capaci­ty of the cell. Then, if they are not going straight into serv­ice, they can put onto a trickle charge to keep them topped up. This would be a doddle for this programmed power supply. Fig.1: two completely independent supplies, DC1 and DC2, are regulated by Q2 and Q3 respectively, to produce balanced positive and negative adjustable supply rails. Features The SILICON CHIP Computer Controlled Power Supply can provide up ED DUAL POWER SUPPLY January 1997  57 58  Silicon Chip Fig.2: since the two regulated supplies are essentially independ­ent of each other, a separate ±12V supply is needed to power the op amps. This is provided by IC3 and T1 operating at 27kHz. In the “local” mode the voltages of the positive and nega­tive supplies can be independently set anywhere from zero to a maximum of 25.5V and this voltage is shown on the front panel (RHS) voltmeter, which can be switch­ ed from the positive to the negative supply. Similarly, the current drawn from each supply can be read on the ammet­ er on the lefthand side. This too can be switched from the positive to the negative supply. A single current limit control sets the maximum current which can be drawn from either supply before it changes from constant voltage to a constant current mode. This limit can be read from the front panel current meter whenever the “current limit” switch is pressed. By using a logarithmic potentiometer for this control the current adjustment range obtained is from around 10mA minimum to a maximum of 2.55A. In fact, the front panel ammeter is pretty well useless for readings of less than 100mA and that is why we have provided a scale around the current limit knob, as a guide only. For really accurate current limit settings at low values, you need to resort to the Computer Control mode. Computer control In the “computer” mode the supply is controlled from paral­ lel printer port LPT1 or LPT2 using a GW-Basic program. The output voltage can be set by pressing the “V” or “E” keys for the positive and the “N” key for the negative, then entering a value. The negative rail can be made to track the positive rail merely by hitting the “T” key on the keyboard. The maximum current (current limit) for both supplies can be set by pressing the “I” or “A” keys then entering a value. Incremental changes to the positive voltage (and the nega­tive voltage in tracking mode) can be made by press- ing the + and - keys. The grey keypad keys on the AT keyboard make this a very convenient adjustment. Once the values are set from the computer it can be switched off or another program can be run, as the values are latched on the digital-to-analog interface board. This month we propose to cover the operation of the power supply as a self-contained unit. Next month we will give details of the parallel interface board and key points of the program code used to control the supply. Circuit description Fig.1 shows the block diagram of the power supply, minus the interface circuitry required for computer control. We will describe that circuitry next month. The design approach used in this power supply is quite different from that applied to typical supplies having positive and negative outputs. Normally, for the positive side of the supply, the controlling element, usually a power transistor or Mosfet, is in series with the positive rail. Similarly, a control element is in series with the negative rail. Fig.1 shows two DC power blocks, DC1 and DC2. These are completely floating with respect to each other. Furthermore, the positive rail of DC1 is directly connected via the load switch S2a, to become the positive output rail of the supply. Similarly, the negative rail of DC2 is connected via the other pole of the load switch, S2b, to become the negative rail of the power sup­ply. Between these two rails is the 0V terminal which is also connected to Earth. The negative rail of DC1 is connected via a PNP Darlington power transistor (Q2) and its 0.1Ω emitter resis­tor to the 0V terminal. Hence, Q2 can be regarded as a variable resistor under the control of the voltage and current block comprising IC2a, 2c and 2d. Similarly, the positive rail of DC2 is connected via an N-channel power Mosfet Q3 and its 0.1Ω source resistor to the 0V terminal. Hence, Q3 can be regarded as a variable resistor under the control of the voltage and current block comprising IC1a & 1b. The two voltage and current control blocks are completely independent. The positive and negative output supply rails do not track each other in this circuit, although, as already January 1997  59 Fig.3: the component overlay for the PC board. Note that the rectifier diodes (D9-D16) should have a stress relief loop in both leads. Take care to ensure that all polarised parts are correctly oriented. noted, they can be made to do so under computer control. Fig.1 looks wrong If you are accustomed to reading SILICON CHIP circuits, Fig.1 looks wrong. After all Q2 is a PNP transistor with its emitter connected to 0V –surely that is wrong. Similarly, Mosfet Q3 appears to be connected “upside down” in voltage terms, with its source to the 0V terminal. However, if you look at the arrows which show the direction of currents IL1 and IL2, you will see that they are in the “right” direction for both Q2 and Q3 to function properly. Note also that the negative rail of DC1 is more negative than 0V. Similarly, the positive rail of DC2 is more positive than 0V. This can only happen if DC1 and DC2 are fully floating with respect to each other. Now let us look at the full circuit which is shown in Fig.2. The similarities between it and Fig.1 are that the tran­sistors, IC numbers and DC numbers correspond. Hence, Q2 on Fig.1 corresponds to Q2 on Fig.2 and so on. Similarly, DC1 on Fig.1 is the same on Fig.2 etc. Having noted the similarities between the two diagrams, let us also comment that references to IN1, IN2, 60  Silicon Chip IN3 & IN4 on Fig.2 have no reference to the circuit operation described this month. They are the inputs for the optional parallel interface board mentioned earlier. Positive supply regulator We start with an 18V secondary which is rectified using four 3A diodes (D9-D12) and filtered with two 4700µF capacitors to produce around 27V DC. This becomes DC1. As noted above, transistor Q2 is the series control element for the positive supply, under the control of op amps IC2a, 2c & 2d. The control is best understood in the following way. Q2’s base is pulled low, turning it hard on, by the resistor connected to the -12V rail. Also connected to Q2’s base are three diodes, D1, D2 & D3 and these effectively shunt current away from the base of Q2 so it is fully controlled rather than being turned fully on. Op amp IC2d provides the voltage control. VR1 sets the output voltage while VR6 sets the feedback to pin 12 so that the output voltage is exactly 5.1 times the voltage on pin 13. IC2d’s output is coupled to Q2 via D1. IC2c & IC2a provide the current control. IC2c amplifies the voltage across the 0.1Ω emitter resistor of Q2. IC2c’s output is fed to mixer op amp IC2a which also gets an input from IC1d, the op amp which sets the current limit in conjunction with VR2. While ever the output voltage of IC2c is less than that set by VR2 and IC1d, the input voltage to pin 3 of IC2a will be negative and its output will sit at -12V. As soon as the output current exceeds the preset limit of VR2, pin 3 of IC2a will go positive causing its output pin 1 to also swing positive. This will pull the base of Q2 positive via D2, reducing the output voltage until the output current matches the limit set by VR2. As you can see, the outputs of IC2a and IC2d are effectively ORed using diodes D1 and D2. Whichever diode’s anode is more positive will reduce the output voltage, so even if the voltage control is demanding 20V output, the current control will reduce it to a voltage which will just supply the preset limit into the load. Soft start When the power supply is first turned on the 470µF capaci­tor associated with diode D3 will be discharged and this will pull the base of Q2 positive, keeping it turned off. The base must be pulled slightly negative, (towards the collector poten­tial) to turn it on. The 91kΩ resistor will slowly charge the capacitor, eventually taking the anode of D3 to -12V. After this D3 will have no further effect. This slow start circuit prevents the output voltage from rapidly increasing to full output when the mains is first switched on, before op amps IC2a & IC2d can gain control. In the meantime the 4.7kΩ resistor will be trying to turn the output transistor on. When the output voltage reaches a level which results in pins 12 & 13 of IC2d being at almost the same potential the op amp will take control and hold the output at this level. The negative supply control system works in a similar manner to that for the positive. In this case we start with another 18V secondary which is rectified using four 3A diodes (D13-D16) and filtered with two 4700µF capacitors to produce around 27VDC. This becomes DC2. The negative rail goes via the LOAD switch S2b to the negative output terminal on the front panel. The supply negative is routed via Q3 and the 0.1Ω resistor to ground. Note that the negative voltage regulator uses an N-channel Mosfet which requires a positive voltage on its drain and a positive gate voltage to turn it on. Therefore all the diodes and supply voltages are reversed. We would have preferred to use a Mosfet for Q2 as well, but P-channel IGFETs are still very expensive and are harder to obtain. In other respects, the voltage and current control and soft start feature work in exactly the same way, via op amps IC1a and IC1b. Because the analog-to-digital converter on the interface board can only operate with positive voltages, the negative output voltage is inverted by IC1c and scaled to 5 volts for full output by the 10kΩ resistor and the 2.2kΩ resistor in parallel with the 18kΩ resistor. The resistors which are connected from the unused inputs of the operational amplifiers to ground are select­ed to reduce the input offsets. 12V supply This inside view shows the prototype with the computer interface board (to be described next month) in place. Note that this board is optional; if you don't need computer control, leave it out and build the supply as described here. astable oscillator running at about 27kHz and it drives transformer T1 via a .001µF capaci­tor. High-speed diodes D7 & D8 act as half-wave rectifiers to produce supply rails of ±12V. IC3 is supplied from the 15V 3-terminal regulator REG1 which provides a measure of regulation for the ±12V supplies. The other 3-terminal regulator in the circuit is REG2, a 78L05 5V device. This provides the reference voltage for the positive and negative supply regulators. REG2 feeds trimpot VR4 and then emitter follower Q1. This then feeds voltage control pot VR1, as well as the current limit pot, VR2. Metering The voltage and metering is fairly straightforward. Meter M2 is scaled from zero to 30V and monitors the output voltage between points TP7 and TP12. It is switched by toggle switch S5 to read the positive or negative output voltage. To monitor current, meter M1 is used to monitor the voltage across the 0.1Ω emitter resistor for Q2 or the voltage across the 0.1Ω source resis­t­-or for Q3, depending on how it is switched by S4. The 1mA meter we used has an internal resistance of 58Ω. This has to be padded out to a total of 300Ω and this is the reason for the series 220Ω and 22Ω resistors. Current limit setting When PB1 is pressed, meter M1 is switched to read the vol­tage at pin 14 of IC1d. This will be -5V for a current limit of 2.55A and because of the series 5.6kΩ resistor and the other This close-up view shows how power devices Q2 and Q3 are mounted on the heatsink (refer also to Fig.4). As the two supplies DC1 and DC2 are floating with respect to ground we need a separate ±12V supply to power the op amps. This is generated by IC3, transformer T1 and the associated components. The 555 timer IC3 is wired as an January 1997  61 Fig.4: mounting details of the transistors on the heatsink. After mounting, use your multimeter to confirm that the metal tabs of the devices are correctly isolated. series resistances, the reading will be close to 2.55. capacitors. T1, REG1, Q2 and Q3 are the last items to be fitted. Construction Transformer winding Having described the power supply circuit we will now describe how you put it together, starting with the PC board. The first step is to check the board for open circuit tracks or shorts. The best way to do this is to hold it up to a bright light and look at the copper pattern from the fibreglass side. An open circuit track will stand out. After repairing any tracks or bridges, begin by fitting the four links and the 28 PC stakes. The resistors, 1N914 diodes and IC sockets are inserted next. Double check the IC socket orientation and diode polarity. Use a multimeter to check the value of each resistor as it is installed. The low profile capacitors and power diodes go in next. The power diodes should have a loop in both leads to allow for ther­mal expansion. Next fit and solder in REG2 and Q1, the three trimpots and the four filter Before you finish the board you will need to wind the high frequency transformer T1. The three windings all use 0.25mm enamel copper wire. The plastic bobbin former for the transformer has the numbers 1 to 8 moulded on the top side. The primary winding starts on pin 4 and the wire is wound on in a clockwise direction and 75 turns later terminated on pin 1. Don’t solder the leads yet. Just wrap them around the former pins using a few turns and leave 15-20mm free. The secondary starts on pin 8 and consists of 145 turns wound clockwise (the same direction as the primary) and terminat­ing on pin 7. Without breaking the wire, put a 30mm loop in it, twist it around pin 6, then wind on another 145 turns in the same clockwise direction and terminating on pin 5. There is no magic in the clockwise direction but it is most important that the primary and secondaries are 64  Silicon Chip wound in the same direction. Slip the ferrite core halves into the former and see how the wires need to be dressed to clear the ferrite. Clean and tin each wire end, wind 2-3 turns around its pin and push the wire down the pin close to the base. Check the ferrites for clearance again and when you are satisfied quickly solder each pin. Don’t apply the heat for too long as the plastic boobin is very soft. Insert the ferrites in the former and wrap a layer or two of sticky tape around them to hold them together. Once the transformer is mounted on the PC board you can put a cable tie around it. With the PC board completed, you can start work on the case. We used a steel baseplate to mount the power transformer and the PC board. It also functions as a heatsink for the 3-terminal regulator REG1. Your first task with the case is to drill the steel baseplate, if you are not working from a kit. You will need holes for the transformer mounting bolt, the mounting screw for REG1 and the two mounting screws for the PC board. Mount the power transformer, the PC board and REG1 to the steel baseplate before installing it in the case. A large single sided finned heatsink is mounted on the rear panel for the two power transistors, Q2 and Q3. The rear panel will need to be drilled to take the heatsink and transistor mounting screws, the cordgrip grommet and fuseholder and the D socket for the interface board. Similarly, the heatsink will need to be drilled for the mounting screws and with holes for the leads of the two power transistors. We drilled individual holes for the three leads of Q2 and a single 10mm hole for the leads of Q3. Both transistors must be mounted with either a mica washer, insulating bush and heatsink compound (see Fig.4) or one of the new thermal washers and an insulating bush. In either case, do not overtighten the mounting screws. Front panel assembly Fit the Dynamark adhesive label to the front panel and then you can drill all the holes for the front panel hardware. The meters will be supplied with their own template as an aid to cutting the circular holes. Fig.5: details of the case wiring. Table 1 shows most of the interconnections between the PC board and front panel. Mount all the switches and meters. You will need to fit a new scale to the ammeter and this is more easily done after the meter is mounted on the front panel. When the time comes, unclip the front cover of the ammeter and remove the two tiny Phillips head screws from the meter scale. Carefully remove the scale and stick the new one January 1997  65 Table 1: Wiring Interconnections Test Point Signal TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 TP9 TP10 TP11 TP12 TP13 TP14 TP15 TP16 E Mains Earth +V ref S1a common I ref S1b common -V ref S1c common Ref supply VR1,2,3 CW +I monitor S4 IN1 Interface PC board +V out S5 & S2a IN2 Interface PC board IN4 Interface PC board -I monitor S4 IN3 Interface PC board -V out S5 & S2b I limit PB1 +5.6V Interface PC board +DC2 S1d LED supply LED1, LED2 Earth VR1 & VR3 CCW, VR1 case All switch actuators, VR1, VR2, VR3 metal onto it. Trim the edges with a utility knife if necessary, then refit the scale and clip the front cover on. The potentiometers will need to have their shafts cut to a suitable length for the knobs. We had to cut 10mm off ours but the shaft length will depend on the supplier. Mount the pots with the terminals facing the local/computer switch, as shown in Fig.5. Fit the two 5mm LEDs in their Destination clips and rotate them so that the two cathodes (shorter leads) are facing each other. Front panel wiring There is a large number of wires between the front panel and the PC board and thus the chance of connection errors is greater. We used a length of 16-way rainbow cable, which made the wiring a little easier. The black lead was used for the E Fig.6: the full size etching pattern for the PC board. 66  Silicon Chip Fig.7: this is the full-size artwork for the meter scale. pin, brown for TP1, red for TP2, orange for TP3 and so on, following the colour code. When we got to TP10 we used the black wire, then the brown for TP11 etc. Follow the wiring interconnection shown in Table 1. The wires to the inter­ face PC board can be left un­ stripped and wrapped with a piece of insulation tape. The switches and controls must be earthed, as the front panel is plastic. With a piece of emery paper, remove the plating from the case of each potentiometer where you want to solder the earth wire, then tin it well, before you actually solder the wire. Using large solder lugs, loop an earth wire from switch to switch and connect to one potentiometer case. Connect the nega­tive control potentiometer case to the earth bolt on the chassis where the mains earth is connected. Slip individual lengths of heat­ shrink over each mains switch lead and shrink them, then slide a large piece over the complete switch. You can’t be too careful with 240 volts! MICROWAVE PARTS & REPAIRS WARNING!: All microwave repairs must be done by a qualified microwave technician. All text within is to be used as a guideline only. We recommend reading “MICROWAVE OVEN OPERATION AND SERVICING MANUAL” (code: MAN-MICRO, cost $19.95) for full safety instructions. Shailer Park Electronics will NOT take liability in any form for safety, health or work done. MICROWAVE OVEN LAMPS Hard to Find Range of Microwave Resistant Lamps Code Volts Watts Baseφϕ $ CL818 240V 25W 13mm $8.50 CL819 125V 25W 13mm $9.50 CL821 240V 20W 15mm $8.50 CL822 125V 20W 15mm $9.50 Base φ MICROWAVE SHORT PROTECTOR Blowing mains fuse? This short protector may be blown. It’s located across the high voltage cap which holds approximately 2300V. This short protector can be tested by first unplugging mains lead and then discharging the high voltage cap with a 1kΩ resistor. The short protector can then be safely measured out of circuit. REPLACE SHORT PROTECTOR IF FOUND DEAD SHORT. Code: 2X062H $14.95 MICROWAVE HIGH VOLTAGE CAPACITORS MICROWAVE HIGH VOLTAGE CAPACITORS Code Value Voltage Cost Is your microwave oven blowing the main fuse? The high voltage capacitor may be faulty. These high voltage, low tolerance capacitors are used in microwave ovens to complete a resonance circuit with the magne­tron which is inductive. A faulty capacitor may upset the lead-lag factor of the resonance circuit and cause the transformer to labour (hum) or blow short protector and/or main fuse. The high voltage capacitor, which holds approximately 2300V, can be tested by unplugging the mains lead and then discharging the capacitor with a 1kΩ resistor, after which it can be safely measured out of circuit. REPLACE CAPACITOR IF FOUND FAULTY OR DEAD SHORT MWC65 MWC70 MWC83 MWC85 MWC86 MWC90 MWC95 MWC100 MWC105 MWC110 MWC113 MWC114-6 MWC120 0.65µF 0.70µF 0.83µF 0.85µF 0.86µF 0.90µF 0.95µF 1.00µF 1.05µF 1.10µF 1.13µF 1.14µF 1.20µF 2300V 2300V 2300V 2100V 2100V 2100V 2100V 2100V 2100V 2100V 2100V 2100V 2100V $35.50 $36.50 $39.50 $36.50 $39.50 $39.50 $39.50 $50.50 $42.50 $44.95 $45.50 $44.95 $44.95 MICROWAVE OVEN ROOF LINING Does your microwave throw sparks inside cavity? The roof lining may need replacing. This lining is made of a special material to diffuse the microwave beam for even distribution. You will find the lining if you open the door and look up inside the cavity; it is a flat sheet held in by screws or clips. With age, the microwave beam will burn through this lining causing sparks inside. We supply 13cm x 17cm sheet, simply cut and shape to size. MICROWAVE OVEN ROOF LINING Code Type Size 13cm Price MRL20 Microwave 13cm x 17cm $15.50 MRL50 Microwave 13cm x 17cm $17.95 17cm MICROWAVE FUSES Our range of original microwave fuses are time delayed, ceramic tube, with brass nickel plated contact cups and have a high breaking capacity of 500A/500V. Never use conventional fuses as they may explode and shatter throwing pieces of glass inside the food cavity, which may be a health risk. If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.winradio.com/ MICROWAVE FUSES Code Rating Length Price AF010P 6.3A 5mm x 20mm $2.50 AF011P 8A 5mm x 20mm $2.50 AF012P 10A 5mm x 20mm $2.50 AF019L 6.3A 6.35mm x 32mm $2.50 AF020L 8A 6.35mm x 32mm $2.50 AF021L 10A 6.35mm x 32mm $2.50 MICROWAVE TURNTABLE BELTS Code Dimensions (A x B x C) Length Cost MWB95 95 x 7.0 x 0.6 300 $11.65 MWB100 100 x 7.5 x 0.6 320 $11.75 MWB105 105 x 4.0 x 1.0 330 $11.80 MWB110 110 x 7.0 x 0.6 340 $11.70 MWB165 116 x 4.0 x 1.0 520 $15.65 MWB210 210 x 2.5 square 650 $14.95 MWB260 260 x 3.0 square 800 $14.90 MWB280 280 x 3.0 square 880 $13.30 MWB175 175 x 2.5 round 550 $19.95 MICROWAVE TURNTABLE MOTORS Postage & Packing $3.50 SHAFT A 2.5 rpm Code: MWM91 Cost $34.95 SHAFT B 5 rpm Code: MWM16 Cost $36.95 ORDER HOTLINE: (07) 3209 8648. FREE CALL: 1800 63 8722. FAX: (07) 3806 0119 SHAFT C 2.5 rpm Code: MWM159 Cost $39.95 SHAILER PARK ELECTRONICS KP Centre, Cnr Roselea & Lyndale St, Shailer Park, Qld 4128. January 1997  67 PARTS LIST 1 PC board, code 04101971, 160 x 83mm 1 instrument case, 355 x 250 x 122mm, Altronics H-0490 or equiv­alent 1 baseplate, Altronics H0492 or equivalent 1 front panel label, 345 x 118mm 1 160VA toroidal mains transformer with two 18V secondaries (T2) 1 1mA 30V scale, panel meter, 58 x 52mm (M2), 1 1mA 58Ω, panel meter, 58 x 52mm (M1) 1 0-3A meter scale 1 4PDT miniature toggle switches (S1) 4 DPDT flat shaft miniature toggle switches (S2-S5) 1 3-core mains lead with moulded 3-pin plug 1 2AG panel fuseholder 1 1A 2AG slow-blow fuse 3 16mm aluminium knobs 1 red binding post 1 black binding post 1 green binding post Semiconductors 2 LM324 op amps (IC1, IC2) 1 555 timer (IC3) 1 BC338 NPN transistor (Q1) 1 BDV64B PNP Darlington transistor (Q2) 1 MTP75N06 N-channel Mosfet (Q3) 1 7815 15V regulator (REG1) 1 78L05 5V regulator (REG2) 6 1N914, 1N4148 signal diodes (D1-D6) 2 1N4936 fast rectifier diodes (D7, D8) 8 1N5404 3A diodes (D9-D16) 2 5mm red LEDs and mounting clips (LED1, LED2) Capacitors 4 4700µF 50VW PC electrolytic 2 470µF 25VW PC electrolytic 5 100µF 25VW PC electrolytic 2 47µF 50VW PC electrolytic 2 10µF 50VW PC electrolytic 5 0.1µF MKT polyester Similarly with the fuseholder, sleeve each connection then sleeve the complete holder. Testing Before you turn on the power use 68  Silicon Chip 1 .01µF MKT polyester 1 .0022µF MKT polyester 2 .001µF MKT polyester Resistors (0.25W, 1%) 2 91kΩ 1 3.9kΩ 1 51kΩ 1 2.2kΩ 1 22kΩ 2 1.5kΩ 4 18kΩ 2 1.2kΩ 2 10kΩ 1W 5% 3 510Ω 11 10kΩ 1 470Ω 1 8.2kΩ 1 220Ω 1 5.6kΩ 1 82Ω 1 5.1kΩ 1 47Ω 5 4.7kΩ 1 22Ω 1 4.3kΩ 2 0.1Ω 2W 5% Potentiometers 2 10kΩ 24mm linear potentiometers (VR1, VR3) 1 10kΩ 24mm log potentiometer (VR2) 1 2kΩ 25-turn top adjust trimpot (VR4), Altronics R-2378 or equiv­alent 1 100Ω 25-turn top adjust trimpot (VR5), Altronics R-2370 or equivalent 1 1kΩ 25-turn top adjust trimpot (VR6), Altronics R-2376 or equivalent Miscellaneous 1 cordgrip grommet to suit mains cable 2 TO-220 mounting hardware 1 TO-3P mounting hardware 300mm 20-way rainbow cable 500mm 20-way rainbow cable 500mm 16-way rainbow cable tinned copper wire 28 PC board stakes 5 6.5mm lugs 2 solder lugs 2 100mm cable ties 50mm 3mm heatshrink 100mm 16mm heatshrink 7 3mm x 10mm machine screws 2 3mm x 15mm machine screws 1 3mm x 20mm machine screw 12 3mm hex nuts 11 3mm flat washers 10 3mm spring washers your multimeter to test for continuity from TP1 through to TP16 on the PC board to the destination of the other end of the wire (see Table 1). Turn the front panel switch off, plug the lead into a mains outlet and turn it on. Switch the front panel mains switch on and watch for smoke or meters against the stop and listen for buzzing noises. If it passes the smoke test (no smoke), things are look­ ing good. DC voltages You should measure about 27V DC on each of the 10kΩ resis­tors near the filter capacitors. The voltage from D7’s cathode to ground should be around +12V to +12.5V, while D8’s anode should be around -12.5V to -13V. With the voltmeter switch set to + volts, the meter should follow the rotation of the “Volts Positive” knob. A similar situation should occur with the meter switched to “- volts” and with the “Volts Negative” knob being rotated. Current limit Turn the “SET mA” control anticlockwise, then quickly press and release the current limit pushbutton. If the meter didn’t move hold the button down and rotate the control clock­wise. The meter should move up the scale to around 25. If everything is fine up to this point, you are on the home straight. All you have to do now is the final calibration. With a digital multimeter connected to the negative output terminals, rotate the “Volts Negative” control fully clockwise. Adjust VR4 until the output voltage is -25.5V. Next, turn the “Volts Positive” control fully clockwise and after connecting the multimeter to the positive output terminals, adjust VR6 until the output is +25.5V. Note: the negative output voltage must be set before the positive adjustment is carried out. Output current Connect a 2.2Ω 10 watt resistor in series with a multimeter that is capable of reading 2A DC and connect them across the positive terminals of the power supply. Set the positive voltage so that 2A is flowing through the resistor. Adjust VR5 so that the voltage on TP8 (pin 8 of IC2) is 3.92V. The front panel cur­rent meter should indicate about 2.0. This completes the calibration of the power supply. Now you can put it into service and become familiar with it before fit­ting the interface PC board to be described in the next issue. SC