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s time goes on more and more people are deciding to have a dabble in the fascinating world of laser light. Whether they want to set up a high speed data link, experiment with optics or put on laser light shows, there is a great deal of interest. And who are we to argue with this trend? In fact, this particular laser power A supply and laser tube combination has been used as the basis for a simple light show. More about this later. Bigger power supply The design featured in our November 1990 issue is suitable only for low powered laser tubes, rated at less than 1.5 milliwatts. To get more laser light, you n eed a bigger supply and the bat- Build a bigger laser tery powered approach is no longer really practical. The unit presented here could be run from a car battery but we expect that most people will want to run it from a DC power supply which needs to be able to deliver about two to three amps. Without beating around the bush, let's now have a look at the circuit Design by OTTO PRIMOJ Did you see the laser featured in the November issue but decided against it because you wanted more power. Now here's your chance: a laser power supply which will drive tubes with up to 10 milliwatts rating. 80 SILICON CHIP + 1 2 V O - - - - - - - - - - - - - - - - - - - - - -.....- - - - - a ~ - - - - . 470 + 16VWI 05 06 - - - - - - - - - , - I I - - - H.....+-+N-,1-+__.,,¼---0+V TO LASER TUBE .,. +OV ZD1 5.1V 56n G 10k LASER POWER SUPPLY Fig.1: the circuit is essentially a DC to DC converter with feedback via optocoupler IC2. The 555 timer drives Qt which switches transformer Tl. The transformer output then feeds a Cockcroft-Walton rectifier/multiplier stage. shown in Fig.1. As you can see, there is quite a bit more to it than the circuit featured in our November issue although there are some similarities. First, the circuit is essentially a DC to DC converter, stepping up the nominal 12 volts DC input to the thousands of volts needed to fire and run the laser tube. The circuit has a major refinement though, because it has negative feedback to adjust the high voltage output, depending on whether the tube has fired or not. modes of operation of the 555. In the start mode, it delivers a frequency of around 13kHz with a pulse duty cycle of about 70%, as determined by the setting of trim pot VR1. Normally though, it stays in the start mode for only a few milliseconds after which the laser tube fires. When that happens, current passes through the tube and back through the negative return side of the high voltage rectifier circuitry and transformer secondary. 555 timer Optocoupler control The core of the circuit is ICl, a 555 timer, and Ql which is a BUZ10 Mosfet. ICl is connected as an astable timer and its output pulses from pin 3 drive the gate of the Mosfet transistor, Ql, via a 560 resistor. This Mosfet then pulses the transformer Tl which steps up to high voltage AC on its secondary side. These high voltage pulses are rectified and further stepped up by a Cockcroft-Walton rectifier/multiplier circuit comprising diodes D3-D10 and the associated .0lµF/3kV capacitors. The output voltage from the secondary side of the transformer is controlled by varying the frequency and pulse width of the signal to the gate of the Mosfet. In effect, there are two In doing so, the laser tube current must pass through the light emitting FACING PAGE: the laser tube and its power supply. The holes drilled in the plastic tube let you see when the tube is operating. diode of ICZ and the associated 1000 resistor (see the lefthand side of the circuit of Fig.1). ICZ is an optocoupler and when current passes through its LED section (pins 2 & 1), its Triac section, between pins 6 & 4, conducts. This brings trimpot VRZ into play and so the frequency of the pulses from ICl rises markedly, to around Z0kHz or so. Also, the duty cycle is now close to 50% (set by diodes D1 & DZ) so that the efficiency of the inverter is close to optimum. Hence, the inverter (consisting of ICl and Ql) provides a lot of herbs to the laser to start it and then is throttled back to sustain the laser discharge at a set current level, in this case, around 5 milliamps. Current drain of the DC-DC con- The laser tube in its naked form: even though it is quite big, its rating is modest at 4 to 5 milliwatts. The anode is at the right hand end. JANUARY 1991 81 verter, with the laser tube running , is around 1.6 to 1.8 amps. This can be supplied by a 12V battery or DC power supply. Construction A number of prototypes were built for this article, two of which are shown here. The first is the simpler .01.t7ffliFH!.I .o3,\ Jiic:::o,i~ I- PARTS LIST 1 SmW laser tube 1 plastic utility case , 130 x 68 x 41 mm (Altronics H-0203) 1 PC board, 105 x 59mm , (cut to suit plastic case) 1.ferrite transformer core, Philips FX3720 or Siemens B66 337G-X 127 1 plastic former to suit transformer, Philips DT 2723 or Siemens B66 272-C-1001T1, with prewound secondary (see text) 1 TO-220 mini heatsink (for 01) 1 405mm length of plastic tube, 50mm O.D. 1 40mm-length of 15 x 10mm Handitube 2 metres of EHT cable (see text) 1 plastic cable clamp 1 nylon screw and nut (to suit cable clamp) 1 1.5-metre length of 0.8mm enamelled copper wire Semiconductors 1 5_55 timer (IC1) 1 MOC3021 optocoupled Triac (IC2) 1 BUZ10 or BUZ71 Mosfet (01) 2 1N914 silicon diodes (D1, D2) 8 1N4007 silicon diodes (D3 to • D10) 1 5.1 volt zener diode (2D1) Fig.2: the wiring diagram of the laser power supply. Note that an additional ballast resistor (2 x 22kQ 2W in parallel) is mounted at the anode of the laser tube, in series with the 68kQ resistor from the power supply board (the scheme is similar to that used for the laser described in the November 1990 issue). of the two and is the one which we'll highlight. It is built in two parts: a plastic tube containing the laser and a separate plastic case to contain the DC-DC converter. Inside.the plastic case is a PC board w hich accommodates the inverter transformer and the rest of the components . Before assembling any components onto the board, we suggest you check it for any faults in the copper pattern or undrilled holes. These should be fixed before going any further This done, check that it will fit securely into the base of the specified plastic u tility box. This is important because it is desirable that no screws be used to mount the board into the box, especially at the high voltage end of the PC board. That point confirmed, we suggest that you then wind the transformer. Winding the secondary winding, which goes on first, is not an easy job. There are 700 turns of 0.2mm enamelled copper wire required. These must be carefully layer wou nd, with Mylar insulation between each layer. Fortunately, kits for this project will be supplied with the transformer secondary already prewound. All that you will have to do is to add the primary winding, which is 15 turns of 0.8mm enamelled copper wire. The start and finish of the prinary winding should come out of the former on the opposite side to the secondary winding terminations. Make sure that the primary and Capacitors 1 470µF 16VW electrolytic 1 .039µF metallised polyester 1 .01 µF metallised polyester 4 .01 µF 3kV ceramic Resistors (0.25W, 5%) 1 68kQ SW wirewou nd 2 22kQ 2W (on laser tube) 110kQ 1150!2 1 100!2 256!2 1 2kn cermet trim pot (VR 1) 1 2kQ multiturn trimpot (VR2) 82 SILICON CHIP Inside the laser power supply case, showing the general arrangement of the components. Be careful not to work on this equipment when it is powered up, as the circuit generates very high voltages to fire the laser tube. Inside the deluxe version of the laser showing the arrangement of the mains power supply and the motor driven mirrors to deflect the beam. The laser was compliantly mounted inside metal clamps using silicone sealant. Note the finned heatsink for the Mosfet switching transistor. secondary winding start and finish leads are thoroughly scraped clean and tinned with solder. The transformer is then installed on the PC board. The lower core half is placed onto the board, then the wound former placed into it and the top core half placed over that. Finally, a long screw is passed through the whole assembly and fixed with a nut and nylon washer to secure it. Do not over-tighten the nut, otherwise you run the risk of the core cracking or being distorted which will increase the current drain. Solder in the four transformer leads and then proceed with assembly of the remaining components, except for the Mosfet, Ql. This is mounted on the outside of the plastic box, together with its own TO-220 heatsink. Note that the eight high voltage rectifier diodes are mounted in pairs, This is an end-on view of the case for the simpler version, showing how Mosfet Qt is mounted. The heatsink is necessary to ensure adequate cooling. with their flying ends covered with a short length of plastic sleeving. You will need to drill four holes in the plastic box, two in each end. At the low voltage end of the case, you will need a single 3mm hole to mount the transistor and its heatsink and a 10mm hole for the transistor leads. At the other end of the case, you will need a 3mm hole to mount the cable clamp and a 6mm hole for the high voltage output cable. Mounting the laser As mentioned above, this version of the laser uses a simple presentation 'with the tube itself housed in a length of 50mm OD plastic tubing. This is fitted with Perspex end pieces, with a hole drilled in the output end, to offer :p.o obstruction to the laser light. The laser tube is compliantly mounted in the plastic tube by means of short lengths of 15mm (ID) by 10mm (wall thickness) Handitube, a foam rubber commonly used for lagging hot water pipes, padding for roof racks and so on. A short length is placed over each end of the laser tube before it is put into the plastic tube. A nice touch with the plastic tube is to drill a number of holes along its length. This is not necessary for ventilation but lets you see the tube "lazing" away. It is also a good idea to spray the tube and its endpieces matte black, for the sake of appearance. The EHT cable from the power supply to the laser tube was merely coax cable in the prototype but two lengths ofEHT cable will be supplied with kits. An additional ballast resistor, in the form of two paralleled ZZkQ 2 watt resistors, needs to be attached to the laser anode, before the cable is connected. Setting up When all the circuitry is assembled and connected (and checked for errors!), you are ready to set the laser power supply. Set trimpot VRZ for maximum resistance by rotating the screw adjustment clockwise until it clicks. Set VR1 fully anticlockwise, then adjust VR1 until the tube just fires. Measure the DC voltage across the 100Q resistor and adjust VRZ until the voltage is 550 millivolts. This corresponds to a tube current of 5.5 milliamps. By the way, there is a choice of two tubes for this project. One is rated at Where to get the kit A full kit of parts for the laser described here (mounted in a plastic tube) is available from Oatley Electronics who also own the design copyright on the printed circuit board. The laser power supply kit, complete with plastic utility box, PC board, and transformer with prewound secondary, is available for $59.90. Oatley Electronics have two tubes to suit the power supply: a 2-3mW tube at $177 and a 4-5mW tube (same physical size) at $259.00. Alternatively, Oatley Electronics can do a package deal comprising the laser power supply kit as described plus the 2-3mW laser tube, plastic tube and all hardware, for $240. Certified postage on any of the above items is $5.00. For further information, contact Oatley Electronics at PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985. JANUARY 1991 83 'MUSIC-ON-HOLD' KIT Comercially this unit could cost 1000's of dollars. Simply press the 'Hold' button and your caller is able to listen to your selection of radio music or tape. Press the release button to resume the conversation. When the line is captured there is an audible pulser and a flashing LED, to remind you. Connects across your existing phone line. ~ As featured in fllMl!!!!f September '90 SPECIAL JANUARY PRICE $39.95 ,~ . Price does not include radio. sec approved 12V DC plugpack $14.90. LASER PRODUCT ACCESSORIES X·Y SCANNER with Dielectric Mirrors for 633nm (He-Ne). These units work great for Beam Positioning Impedance: 8 ohms & Low Speed Scanning. Input Power: 2 Watts Angle of Deflection: ± 12° Frequency Response: DC • 120 Hz ~--~·,, ONL V $220.00 POLYGON SCANNER comes complete with motor and driver. Has LED indicator which lights when the motor speed is locked at 4667 RPM. The deflection is 90° and the mirror is 8 sided coated aluminium. Input Voltage: 18 · 24 VDC (Fuull Speed) ~ Input Current: 200mA Sweep Frequency: 622.3 Hz Q . Q ONL V $129.50 COLLIMATOR which can be used to reduce the laser beam width $79.00 TV TRIPLER The deluxe version of the laser project was built into an aluminium case fitted with a small fan for cooling. The keyswitch was fitted to stop unauthorised use. 2-3mW while the other is rated at 4-5m W. Both tubes are identical in appearance and required current. The difference in rating comes about from the quality of the mirrors and the precision of their adjustment, which affects the total light output. Deluxe version Used in a well known late model TV, should also be a suitable replacement for many other units. WE HAVE A SMALL QUANTITY AT A SPECIAL PRICE OF ONL V $12.90 ea SEE IN THE DARK! with our INFRA RED NIGHT VIEWER . We have good stock of the 6032A tubes. This is your opportunity to build a prod· uct which commercially cost 1000's of dollars at a fraction of the cost. We supply the electronic kit, a 6032A Image Intensifier tube, and the case components for only. As mentioned earlier, two versions of this laser have been built. We'll make a few comments on the more deluxe version now. It was housed in a substantial case made from sheet aluminium. The case houses a mains power supply which provides 12 volts DC to the laser power supply board. The photos show the details. Also included in the case are a pair of motors at the business end of the laser tube. These drive compliantly mounted alumininised mirrors which deflect the beam. By feeding the motors with different levels of DC they run at different speeds and thereby generate all sorts of interesting Lissajous patterns - great for a simple laser light show. SC ONLY $339.00 BRAND NEW TUBES The 6032A tubes are BRAND NEW and perform well with a good infra red torch. Torch and Lenses are not supplied NICAD BATTERY PACKS HIGH QUALITY JAPANESE 500mAH penlight batteries for under $2.00 ea. These are 7.2V multipurpose "Six Packs" that you can easily reduce in size, or dismantle to individual cells. BULK ENQUIRIES WELCOME. $10.90 per Pack These will not last at this price so get your order in quickly 1.5V to 9V Convertor in 'Silicon Chip' November 1990, uses only 3 components and has multipurose a lications, $7.99, a bar ain at this rice. OATLEY ELECTRONICS fiiiii!!!iil l--1 84 rili:5J PO BOX 89, OATLEY, NSW 2223 Telephone: (02) 579 4985 ~ Fax No: (02) 570 7910 Certified p&p $4-$6 Aust. NZ add $2 SILICON CHIP A closer view of the motors and their compliantly mounted aluminised mirrors which are used to deflect the beam. By operating the motors at different speeds, you can use the laser to generate all sorts of interesting patterns.