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13.5V 25A power supply for transceivers, Pt.2 In this second article on our 13.5V 25A power supply we give the construction details and the setting up procedure. Most of the work involves assembling the PC boards and mounting the hardware in the chassis, there being relatively little wiring required. By JOHN CLARKE & LEO SIMPSON Because this power supply uses Triac control for regulation, it is quite efficient and therefore dissipates a relatively small amount of power in the chassis. This means that big heatsinks and fans are not necessary and as a result, the overall construction is quite simple. It uses a conventional 3-unit high rack-mounting case with two PC boards mounted on the base panel. There are two transformers, one a large 625VA toroidal type and the other a miniature conventional type. There are also the two iron-cored 64 SILICON CHIP chokes and the four stud type rectifiers which mount on heatsinks on the rear panel. Although two PC boards have been designed for this project, you may end up using only the control board, depending on your constructional approach. The control board is coded SC14105911 and measures 165 x 125mm. As its name suggests, it carries all the control circuitry. The second PC board is coded SC14105912 and measures 180 x 176mm. We used it to mount the two 80,000µF banks of capacitors which were all PC-mounting types. However, if you purchase chassis mounting filter capacitors, this board would not be necessary and the capacitors could all be har:d-wired. We'll talk more on this aspect later but from here on, we'll assume that both boards are to be used. PC board assembly You can begin construction by assembling the control board. First of all, inspect it carefully for any defects such as breaks in the tracks or shorts between adjacent tracks, undrilled component holes and so on. Correct any of these before you install any components. Follow Fig.4 carefully when assembling the control board. You should check your work against this diagram and the circuit diagram (Fig.2) published last month. If you look at the PC board depicted in Fig.4, you will notice that the components in one corner are rather sparsely laid out; ie, they seem to occupy more than their fair share of 12.6V _....... D5 fl 06 ---t:a---- 7805 7905 •G• I 0 • I• 10uF 10uF • Fig.4: here's how to install the parts on the control PC board. Note that the Triac is installed from the copper side of the board (see text) and that IC3 faces in the opposite direction to the other ICs. The SCR is initially left off the PC board & is installed only after the crowbar circuit has been adjusted for correct operation. • G 0 ·O ·O LJ MOC30218 1 space. There is a good reason for this because these components (ie , the MOC3021 optocoupler, the Triac , toroidal inductor L3 and the associated resistors) all operate at mains potential (240VAC). Begin the board assembly by installing the 12 PC stakes. These are for the external wire connections and for the indicator LEDs. Next, install the low profile components such as the ICs, resistors, diodes and links. Be sure to orient the ICs correctly (note: IC3 is oriented differently to the other four ICs). Take care also with the diode orientation and be sure to use the correct diode type at each location. Diodes D5 , D6 and D14 are 1N4002s, while the remaining diodes are all 1N4148s. When installing the three 1W resistors in the mains section of the PC board , push some clear insulating sleeving over the each resistor body and its wire leads. This will prevent accidental contact and the possibility of electric shock. Now you can install the capacitors. All the electrolytics, except for th e 2.2µF bipolar type , must be oriented correctly as shown on the overlay diagram. This done, install the regula- 1 .~ t ... i 9iii¥t:. J·'~ .) 1t t lT '_ ' ----...... "i--.--- --··~-~· --- - 'Ii.ii-·- ~~ -.- ' rfi ._.__-._._-J 1a r~ IIII I ;ij. The parts at the top lefthand corner of the control board all operate at mains potential so be wary of this part of the circuit. Note that plastic sleeving has been fitted to the three 1 W resistors to reduce the possibility of electric shock. tors and transistors , making sure that you install the correct transistor type at each position. Do not install the MCR69-2 SCR at this stage. This will be don e after the crowbar circuit has been adjusted for correct operat ion. Also, the two LEDs should be left off until the PC board is JUN E 1991 65 0 TOL2 ... ◄ ~ TO ANO OES,OF 03 , 04 ~ - \ TO NEGATIVE OUTPUT TERMINAL VIA CURRENT SENSE TO GNO OF CONTROLLER BOARD TO FEEDBACK INPUT OF CONTROLLER BOARD Fig.5: the capacitor bank board carries the 16 10,000µF capacitors plus the four 560 5W resistors. Mount these four resistors slightly proud of the PC board to allow the air to circulate under them for cooling. The current sense resistor consists of a 150mm length of 1.25mm enamelled copper wire (see Fig.6). which is so ldered to two points on the board. Installing the Triac to be final ly install ed into th e case. Trim pots VR1, VR2 and VR3 plu s th e va ri stor can now be so ld ered into pl ace. Winding the toroid Inductor L3 is wound with 45 turn s of 0.63mm ename ll ed co pper wire on a Neosicl 17/.742 / 22 toroid al former. Cut a 2- met re length of th e wire an d s trai ghten it by anc horing one end in a vycc and pu lli ng th e oth er end with pliers. Pul l th e wire just enough so that it gives a littl e and bec omes d ea d s trai ght. Thi s removes a ll th e littl e kinks and hends an d gives yo u a ni ce strai ght length of wire to work with. The toroid is th en wound in two stages. working from th e cen tre of th e wire, as it wore. Fend one metre of wirr! through tlw cent re of th e toroid and w ind (or thread throu gh) about 66 SII.IC:() ,\ ' Cl/11' 22 turns, as even ly spaced as possible. Thi s d one, w in d the other en d of the wire onto the toroid to make up the full 45 turns . Cut th e ends of the winding to a reasonable length, remove the enamel in su lation by scrap ing it down to bare copper w ith a utility knife, and then sol der the inductor to the PC board. The inductor is th en furth er secured using a tinn ed copper w ire strap The Triac is mounted on the u nderside of the PC board w ith its leads bent upwards into the mounting h oles . Bend the leads at right angles so that the lower face of the Triac body is 6mm below the bottom of the board. This w ill all ow it to be bolted directly to the base of the chassis when the board is mounted on 6mm spacers. That completes the control PC board assembly for the moment. You can now start work on the capacitor board. Capacitor board CAPACITOR CODES 0 0 u ;_j [.J Value IEC Code EIA Code 0.1µF .027µF .022µF .01µF 100n 27n 22n 10n 104 273 223 103 This PC board accommodates the 16 10,000µF capacitors plus four 5W resistors. Begin construction by checking the hole sizes for th e five heavy cables. Th ey should all be drilled to 3mm - see Fig.5 . The assembly of th is PC board is straightforward, since the capacitors can only be installed one way due to the fact that each can has three leads. The third short lead is actually a support lead and is not electrically connected to the capacitor. It must be soldered to the PC board though, to properly anchor the capacitor in place. Due to the large copper area of this PC board , you may have problems soldering with a normal low power (30W) soldering iron. If this is the case, try to obtain a 60W heavy duty iron for the job. Be careful not to overheat the capacitors when using this iron, though. The four 56Q 5W resistors should be installed so that each resistor body sits about 1mm above the board surface. This will allow better ventilation for the .r esistor and avoid any tendency to char the board - these resistors do get very hot. Finally, install three PC stakes to terminate the external connections . This close-up view shows how the wiring to the binding post terminals is run using heavy-duty cables and connectors. Note also the enamelled copper wire loop that forms the current sense resistor. It runs between the capacitor PC board and the negative output terminal (right). Chassis preparation Fig.6 shows where the various components are mounted in the case. Start by temporarily placing all the heavy hardware items in position on the base plate. These include the two transformers, the two iron cored chokes and the two PC boards. Once everything is in position, mark out the various mounting holes, then remove the parts from the chassis. The holes can then be drilled and deburred . Make sure that the mounting area for the Triac is clean and abso lutely free of metal swarf and paint. The paint can be cleaned away using fine grade emery paper. Because the Triac normally only becomes slightly warm during operation, you might think that heatsinking it to the chassis is not really necessary. However, if it is subjected to a quick series of power up/down cycles or if a fault condition occurs, the Triac may become hot and this is when the heatsinking is required. After you have cleaned the paint away-from the Triac mounting area, do the same for the chassis area around the main earth lug (near the mains cord entry point). This must be done to ensure a good earth connection to the chassis. The rack case specified is supplied unassembled. This means that you can mount all the base panel hard- RESISTOR COLOUR CODES D D D D D D D D D D D D D D D D D No. 9 1 2 3 9 2 1 5 5 3 4 Value 470kQ 150kQ 100kQ 47kQ 10kQ 4.7kQ 3.3kQ 2.2kQ 1kQ 680Q 560Q 390Q 330Q 100Q 56Q 47Q 4-Band Code (5%) 5-Band Code (1%) yellow violet yellow gold brown green yellow gold brown black yellow gold yellow violet orange gold brown black orange gold yellow violet red gold orange orange red gold red red red gold brown black red gold blue grey brown gold green blue brown gold orange white brown gold orange orange brown gold brown black brown gold green blue black gold yellow violet black gold yellow violet black orange brown brown green black orange brown brown black black orange brown yellow violet black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown blue grey black black brown green blue black black brown orange white black black brown orange orange black black brown brown black black black brown green blue black gold brown yellow violet black gold brown JUNE 1991 67 <at> \ _ BxHEATSINK SEQIB!!fG / SCREWSAll!)_<at>.!§_ <at> L2 BROWN MAINS ERMINAL BLOCK CONTROLLER PCB LE01 <at> LE02 •K •A •A •K - - - - 7.SA MAINS RATED WIRE === 3.2mm2 INSULATED COPPER WIRE FRONT PANEL + Fig.6: here's how everything fits together inside the rack-mounting case. This circuit carries quite high currents, so be sure to use the correct gauge of wire where this is indicated. Sleeve all mains connections with heatshrink tubing to avoid the possibility of electric shock. ware before the side panels are fitted. The front panel can be now be drilled to accept the power switch , the two LEDs, the binding post terminals and the fuse holder. You can use th e front panel artworks to guide you when marking the positions of thes e components. Note that th e holes for 68 S1uccm C111P the LEDs must be drilled so that they line up with their mounting positions on the control PC board. Make these holes big enough to accept the clip-in bezels. To make the rectangular cutout for the power switch, drill one large hole in the centre and a series of closely spaced smaller holes around the inside perimeter. You can then file out the hole so that the switch fits neatly in position, with the plastic side lugs holding it securely. Once all the holes have been drilled and deburred, the self-adhesive Dynamark labels can be attached to the front panel. Check that they are correctly aligned with the holes and that they are square with the panel before sticking them down (once the e (l) e~SECURING SCREWS TO REAR PANEL HEATSINK - INSIDE CASE Fig.7: the four studmounting diodes must be isolated from the heatsinks using mica washers & insulating bushes as shown here. Smear both sides of the mica insulating washers with heatsink compound before bolting the diodes in position, then use your multimeter to check that the diodes are correctly isolated from the heatsink. Be sure to use the correct diode type at each location. GROMMETS FOR LEADS THROUGH REAR PANEL adhesive grabs, that's it). The holes in the two panels can then be cut out with a sharp utility knife. You can now install the front panel components and attach the front panel to the case. Rear panel The rear panel must be drilled to accommodate the fuse holder, the mains cordgrip grommet and four 6mm ID rubber grommets. In addition, you have to drill eight mounting holes for the heatsinks plus clearance holes for the four stud-mounting diodes (D1-D4). The first step is to drill the heatsinks - see Fig. 7. There are four mounting holes on the outermost fins plus two diode mounting holes for each heatsink as shown. Make the diode mounting holes large enough to accept the insulating bushes and carefully deburr the holes to avoid puncturing the mica insulating washers. Once the heatsinks have been drilled, you can use them to mark out the rear panel mounting holes and diode clearance holes. Use a small pilot drill to drill the diode clearance holes to begin with, then carefully ream them out to 25mm using a tapered reamer. heatsink carries two 70HFRZ0 (reverse polarity) diodes. If you mix them up, you will blow fuses and maybe even blow the diodes. Note that stud diodes D1 and DZ on the main wiring diagram (Fig.6) are "R" types (reverse polarity}, while D3 and D4 are normal polarity types. Normal polarity stud diodes have the cathode (K) connected to the threaded stud and the anode to the centre lug. Reverse polarity stud diodes have the anode connected to the stud and the cathode to the centre lug (see Fig.7). The diodes specified (ie, 70HFZ0) are rated at 70 amps and have a Z0V inverse rating. However, you can also use higher inverse voltage versions such as the 70HF40 (40V) or 70HF60 (60V). To wire the diodes, you will need the diode mounting hardware, four Utilux lugs and heavy duty cable. The cable we used is actually figure -8 speaker cable w ith a conductor cross section of the 3.2 square millimetres (DSE Cat W-2015 or Jaycar Cat WB1732). You will need one metre of this figure-8 cable which can then be split into two lengths. Mounting the stud diodes Once all the drilling has been completed, the four stud diodes can be mounted on the heatsinks. There is a trap here for young players, so pay attention . One h eatsink has two 70HFZ0 diodes on it while the other The four stud-mounting diodes pass through 25mm-diameter clearance holes drilled in the rear panel. The connections to these diodes are run using heavy duty (3.2mm 2 ) cable and are made using a heavy duty iron before the diodes are attached to the heatsinks. JUNE 1991 69 Smear heatsink compound on the back surfaces of the heatsinks before bolting them to the case. The heatsink on the left carries the 70HFR20 diodes while the one on the right carries the 70HF20s (make sure that you don't get them transposed). Tighten the diode nuts firmly to hold them in position. The Triac is installed from the copper side of the control board & bolted directly to the chassis. No mica insulating washer is necessary here since the Triac specified is an isolated tab type. 70 SIUCON CH71' It is necessary to solder the leads to the diode lugs before mounting them on the heatsinks. If you don't, the heatsinks will draw too much heat away from your so ldering iron for reliable soldering. Note that the bared ends of wire should have their plastic tracer inserts cut before the wire is twisted and inserted into the lugs. To make for a professional job, we used heatshrink tubing over each lug. The diodes can now be mounted on the h eats inks as shown in Fig. 7. Before bolting each diode down , smear both sides of the mica insulating washers with heatsink compound to improve heat transfer. Each diode is supplied with a star washer but this should be discarded since it does not allow enough thread length for the diode stud. Do not overtighten the nuts. Once the diodes are mounted, use your multimeter to check that the diode bodies are indeed correctly isolated from the heatsink. Note that the heatsink is anodised and that this provides some degree of insulation. For a good multimeter probe contact, use the exposed aluminium in the mounting holes. If everything checks out OK , the heatsinks can be bolted to the rear panel. Once again, it's a good idea to use heatsink compound on all mating surfaces to improve thermal transfer. Feed the diode leads through the grommets in the rear panel, then screw the rear panel to the case. Wiring All that remains now is the chassis wiring. Strip back the insulation on the mains cord so that the Active (brown) and Neutral (blue) wires are about 35cm long, to reach the power switch with slack to spare. The Earth (green/yellow) lead is cut back to about 10cm and soldered to the earth lug near the cord entry point. Secure the earth solder lug to the chassis with a machine screw and nut plus a star washer. The brown Active lead connects directly to the fus eholder on the rear panel. Pass this lead through some heatshrink tubing before soldering the wire to the fuse holder. This done, solder the remainder of the Active Below: all the major hardware items are easily accommodated inside the rack-mounting case. Use cable ties where necessary to keep the wiring tidy & be sure to install the mains wiring in a professional manner. 13.SV 25A POWER SUPPLY POWER OUTPUT REGULATED CROWBAR + '----13.SV _ ___.I 30A FUSE Fig.8: these artworks can be used as guides when drilling the front panel. JUNE 1991 71 .I a Fig.9: check your control board against this full-size pattern & repair any defects before mounting the components. wire to the second terminal of th e fus e holder and pass it through the tubing. Now the tubing can be pushed onto th e fuseholder to cover th e terminals and shrunk down using a hot air gun. Before soldering th e leads to th e pow er switch (S1), slide some large diameter insulating tubing over the wires so that it can be shrunk right ov er the switch body (see photo). Th e transformer secondary wires are terminated in Clipsal 563k16 connectors. As suppli ed, these connectors have a blind end on the plastic insulation and this will have to be cut off to allow wire entry from both ends. Strip the enamel away from the transform er secondary wire ends before securing them in th e connectors. Th e wires from th e cathodes of diod es 03 and 04 and the anodes of diodes 01 and 02 al so terminate in the: C:lipsal connectors. As noted previousl y, be very careful to ensure that this wiring is correct. The 2111£2 current sensr: rnsistor from 72 SIUCO N C/111' the capacitor PC board to the negative output terminal consists of a 150mmlength of 1.25mm enamelled copper wire. This wire is simply formed into a single large loop and soldered to the PC board and to the output terminal lug. Note that the eyelet lugs supplied as part of the binding post terminals are inadequate for the current rating of the power supply. Use heavy duty eyelet lugs for all connections to these terminals. Transformer T2 (2851) has a centretapped secondary, although the centre tap (blue) is not used. The two yellow wires from the transformer connect to the controller PC board. Cut back the ce ntre tap lead so that it cannot accid entally contact any part of the chassis or PC board. The remaining wiring can now be completed as shown in Fig.6. Use cable ties and clips where necessary to keep the wiring neat and tidy. Testing Now that th e wiring is complete, the power supply is ready for testing. Do not apply power yet! First, rotate trim pots VR1 and VR3 fully anticlockwise and set VR2 almost fully clockwise. Before switching on th e supply, we recommend that you secure the lid to the cas e. This will protect you against any catastrophic fault which may cause molten material to be ejected from the power supply. Does this sound silly? No it isn't because there is a large transformer in the supply which can deliver very high currents in the event of a wiring fault or malfunctioning component. For this reason, go back and check all your wiring very carefully against the wiring diagrams. When you are satisfied that everything is correct, install the two fuses, switch off power switch S1 and plug the mains lead into the wall. Connect a multimeter to the output terminals of the supply, set to read up to 20VDC. Now apply power. The DC output voltage should increase slowly up to The controller PC board can now be secured in position and the Triac bolted to the chassis. Use heatsink compound between the Triac mounting tag and chassis. No mica washer is required here since the Triac specified is one with a fully insulated tab. Overload current setting Fig.10: this is the pattern for the capacitor bank PC board, reproduced here at half size. The full size board measures 180 x 176mm. somewhere around 14V. If there is no voltage or the fuse blows, switch off immediately and unplug the supply from the mains. Be sure to sort out the problem before reconnecting the mains power. If you are getting voltage from the supply, then it is probably operating correctly. To check this, adjust VR1 to see if the voltage at the output can be Sleeve the body of the mains switch with heatshrink tubing after the leads have been attached to prevent accidental contact with the mains. varied. While adjusting VR1, check that the "Output Regulated" LED lights for output voltages between 11.8 and 14.4V. The current overload can be adjusted in one of two ways. The first method simply adjusts the voltage at the wiper of VR2 while the sr,cond method requires a 25A load. A 25A load can be made up using a number of 12V automotive light bulbs, conn ected in parallel to give 350W. When the power supply is loaded by these, trimpot VR2 is adjusted so that the current limit is just above the 350W load. At the onset of limiting, the voltage will fall suddenly. When this point is found, rotate VR2 slightly anticlockwise. To adjust without a load, connect your multimeter between the wiper ofVR2 and ground and adjust VR2 for a reading of 60m V. This will set the current limit to about 30A. Your new power supply is now ready for use. SC UHF HIGH GAIN ANTENNAS Crowbar trip point Disconnect power and temporarily connect a 47kQ resistor across the 47kQ resistor associated with trimpot VR1. This w ill allow the output voltage to be adjusted above 15V. Now rnr:onnr,r.t powr,r and adjust VR1 for an output voltage of 15V. This done, adjust VR3 by rotati ng it slowly clockwise until the crowbar LED just lights. The crowbar LED can be -reset by switching off power and leaving it off for a minute or so. Now, leaving the VR3 trimpot setting as is, reduce the output voltage using VR1 and then slowly increase it to check that the crowbar trip point occurs at 15V. If not, readjust VR3 and check it again. Once the crowbar level has been correctly set, the SCR can be installed on the PC board. Now check that when the output voltage reaches 15V, the crowbar operates by pulling the output voltage to about 1V. The temporary 47kQ resistor can now be removed and the output voltage reset to give 13.5V with no load. antenna existing systems. triangula powder coated receiving elements ensures excellent UHF reception compared to other UHF antennas of similar size and price. Two models are available: the TVA 14 for Band Four and the TVA 15 /or Band Five UHF reception. Both models are supplied with back reflectors to prevent ghosting as well as a waterproof entry box designed to accept 75 ohm coaxial cable without the need for addittional baluns. They also have predrilled holes for securing the tilt adjustable metal mounting bracket in either a horizontal or vertical position. Imported and distributed by: ARISTA. ELECTRONICS Available through the following retailers: Bernys. Bridgepoint. Military Road. Mosman. 2088. NSW. (02) 969 1966. Ritronics. 56 Renver Rd. Clayton. Victoria. 3168. (03) 543 2166. JUNE 1991 73