Fullscreen HTML5Med Res (??MB)HTML4

Another reader’s approach to exposing photo PC Boards. . . Cook ’em in the old microwave! I decided to build the UV exposure box for photo-sensitive PC boards, as published in the November 2007 SILICON CHIP. The first thought I had was to use the timer module in an old microwave oven, instead of building the timer design used in the project. Then I thought: “Why not use the whole oven?” W hy not, indeed? I wouldn’t have the lid of the original design to clamp the transparency down onto the PC board but this could be overcome by making a platter and using a sheet of glass to apply enough weight to ensure intimate contact. When activated, the platter-motor of the oven, slowly rotates the PC board pattern within the UV light, providing a very even exposure of your pattern. I can obtain very even and consistent results with transparency film and about 30-40 seconds exposure time 84  Silicon Chip (although I tend to pre-heat the tubes for five minutes first). There is a significant difference in both light output and colour temperature between a just-turned-on tube and one which has been on for a few minutes. One big advantage of using an old microwave oven: just as when the oven was an oven, when the time is up, the oven beeps loudly at you, to tell you that your board has finished cooking exposing! By Graeme Rixon So I bought a secondhand microwave oven for $5 from my local refuse recycle centre, in a “working but no glass turntable” state. If your council has kerbside cleanups, you may well find one that’s about $5 cheaper. By far the majority of microwave ovens are discarded because a 30c high-voltage diode has blown. But we don’t even need that because we aren’t interested in the cooking side of things. The only things you need to check, when looking for an oven to convert, is that (a) the timer and its display works siliconchip.com.au WARNING: MICROWAVE OVENS CAN BE LETHAL! A microwave oven is a very dangerous device. Even service personnel who are very familiar with them will tell you they are not their favourite equipment to work on. A microwave oven operates at a potential of several thousand volts and contact with any part of the high voltage circuitry can be (and indeed has been) fatal. Even a microwave oven which has been turned off for some time may have enough energy stored to give a potentially lethal shock. NEVER operate a microwave oven without the cover on and secured and without the door interlocks operating. This article is intended for those who are experienced with mains device wiring in general and microwave ovens in particular. Follow exactly the steps outlined in this article regarding making the microwave oven safe before, and as you modify it. and (b) that the platter motor turns. It does not matter if the oven has a glass platter or not – I made my own from a sheet of MDF and an old speaker. Stripping the oven Stop! Don’t go in like a bull at a gate and start cutting wiring willynilly. If you don’t heed the warnings further on, that could be as far as you get. Forever. First step is to physically cut off the mains power lead outside the case so that no-one, yourself included, can possibly energise the oven without its cover on. OK, now that should render it harmless, right? Don’t you believe it! Even when not connected to power; even when it hasn’t been powered up for some time, a microwave oven can bite! Make no mistake – contact with the high voltage capacitor, even with the oven off, can be fatal – you have been warned. The high voltage capacitor must be safely discharged before you proceed any further. SILICON CHIP published an article in the August 2003 issue (pages 84-87) on how to re-use the timer module from a microwave oven and this covers the safety concerns well. The oven was carefully stripped of the magnetron, high-voltage transformer and capacitor, platter rollers, cooling fan, and oven light – these were simply discarded. All mains wiring was removed, so that a custom wiring loom could be installed. Once you have removed the capacitor, do not simply throw it in the bin without a shorting link across its terminals, as the voltage across these capacitors can regenerate over time. Maybe it’s not enough to kill but if the terminals are contacted with it is enough to give you, or the poor rubbish man or woman a bad jolt (especially with wet hands). To discard the capacitor, I re-used a simple spade-to-spade jumper lead from the original oven wiring and installed it across the capacitor terminals, before disposing of the unit. Failing this, a piece of bare copper wire twisted firmly around the terminals will do. With all the HT wiring, etc, out of the way, you can set about actually building the UV oven. We make use of the existing oven timer module, as more often then not, it contains its own power supply and transformer. All you have to do is connect the 240V AC mains to it, to have a fully functional timer, complete with output switching relay. All the features of the oven timer are used, such as the door-switch, which will stop the time and shutoff the tubes, if the door to the oven is opened while the tubes are running – the exact same concept that shuts off the magnetron when the oven was an oven. The tubes are 15W blacklight-white tubes (NEC type FL15BL) and they are fitted in rubber bushes, fixed to holes bored through the oven cavity. The bushes prevent the tubes breaking if the oven gets a bump. With the pilot holes for the tube placement marked and drilled, you then have to cut the large holes for the plugs to fit in the oven wall. I drilled the holes in the very oldfashioned way of marking the size of the hole for the plug using the pilot hole as a centre reference, and then drilling a series of small holes These two photos show the wiring between the starters (bottom of pics) and the tubes above. This wiring is the same as shown in the November 2007 article. Can you spot the really bad mistake? Using green/gold wire for ANYTHING but earth wiring is a big no-no, despite the clear warning sign printed on the side of the oven. Always remember that green/ gold is only for Earth, brown is only for Active and blue is only for Neutral. siliconchip.com.au October 2008  85 Inside the old microwave oven showing the positioning of the 15W blacklight fluoro tubes through the oven wall. The ballasts were fitted where the old high voltage transformer and capacitor were originally located and completely new mains wiring loom fitted. The timer has its own transformer; this is left intact and is used to switch the fluoros and turntable motor on and off. At the end of the chosen time, the “oven” dings to tell you the board is properly “cooked”! Once again, note our comments on the use of green/gold wire – it must not be used for anything but an earth connection. around the edge of the line, then using a metal cutting drill-bit to bridge all the small holes. The rough edges of the holes were filed a little, to take away the sharp points but there is no need to make the holes perfect – the rubber bushes will protect the glass of the tubes from any sharp edges from the holes. If you are prepared to sacrifice a hole-saw blade, one of these could be used to cut the holes instead. But as the walls of the oven are usually stainless steel, be prepared to blunt the hole-saw very quickly. Still, it will save you a significant amount of time. The rubber bushes I used are intended for use as body plugs. They are commonly used in the vehicle industry to seal holes in car bodies. The ones I used were 45mm in diameter and cost about $5 each. When purchased, these plugs are all rubber, and a suitably-sized hole has to be cut in each one, to allow the tube to pass through it. I made a “punch” to cut my holes out of an 11/16 socket from an old socket set, sharpened around the edge with a bench grinder. You then line up the socket and plug in a medium-sized vise (with a wood block behind the plug), and tighten up the vise hard to cut the hole through the plug. Ideally, you want a hole in the plug which is slightly smaller then the diameter of the tube, so that when the tube is inserted, it is a nice firm fit in the plug. Two of these plugs will hold the tube firmly in place within An old speaker frame is used to raise the 30cm MDF “platten” up closer to the UV tubes, for shorter exposure times. The speaker even comes with suitable mounting holes! 86  Silicon Chip the oven cavity, and protect the tube glass at the same time. My 11/16 “punch” gave me a nearperfect 24mm hole in the plug, making the tube a nice, firm-but-not-too-tight fit. Fit the plugs to the oven first, then insert the tube through them. Smear a little Vaseline around the inside edge of the hole in the plug, to aid in fitting the tube. If the turntable is missing (they usually are!) one can made from a circle of 12mm thick MDF - I traced around an old LP record to get the 12” size I wanted. Tracing around the centre hole in the record also gave me an accurate centre reference for the wood disc. The normal oven turntable position is really a bit too far away from the UV And here’s a shot showing how it all fits in. Actual distance between the tubes and PC board is not critical – about right is about right! siliconchip.com.au tubes, so I mounted mine on an old 8-inch speaker chassis. This was then mounted to the old oven platter-motor spigot. Find a suitable speaker – you can remove the cone and coil if you want to but it won’t matter if you don’t. A hole is then drilled directly through the magnet core and the platter-motor spigot is bolted to the speaker chassis magnet. That done, the speaker chassis is lined up and centred on the MDF disc and simply screwed on as if you were mounting the speaker in the normal way, making use of the speaker chassis mounting holes. The turntable assembly is then pushed onto the platter-motor driveshaft, in the middle of the bottom of the oven cavity. A Nylon washer smeared with Vaseline on both sides is placed between the platter spigot and the oven floor itself, to prevent binding. The PC board is placed facing upwards on the “turntable” and the transparency pattern then laid on top, A small sheet of 6mm thick glass then placed on top of that, to hold the pattern in close-contact with the board. This requires that you cut the normal A4 sized transparency to size but I only make small boards anyway – I have never needed to make A4-sized boards, so this is no real problem (for me). The electrical system is essentially the same concept as that published in the November 2007 issue of SILICON CHIP, except that the tubes are 15W types, and the ballasts are 30W type. I bought the tubes for $20 per tube, delivered by mail. Most lighting specialists will either have these tubes or be able to get them Finally, a shot with the lamps lit. The door interlock has been defeated for this photo but the interlock should be used so that when the door is open, the UV tubes go out. So are these tubes dangerous? No, not in normal use but you wouldn’t want kids staring into them for long periods. . . for you (they’re used in electronic bug killers to attract the bugs). The two ballasts are mounted to the base of the oven, where the highvoltage transformer and capacitor were. The starters are on the base of the oven, on the other side of the oven cavity. The starter sockets and ballasts were stripped from old light fittings. The connections to the tube terminals were made using 16 PC boardmount terminal blocks. As it happened, the terminal shape was round and fitted perfectly the connections to the tube filaments. The primary reason for choosing these was that I needed to bring the connections from the tubes out at right-angles to the tubes themselves, for neatness – and especially on the starter-side of the oven cavity, as there is not much room there. Each terminal was soldered to a 10A wire, by inserting the pin of the terminal into the centre of the strands of the wire, then soldering, so that the wire is connected centrally to the terminal pin, instead of soldered alongside. This produces a much neater looking connection between the terminal and the wire, when it is heatshrunk, over a side-by-side soldered connection. Wire was obtained by buying a 10-metre 10A extension cord, cutting off the plug and about three metres of cable (this becomes the oven power cord!), and the socket end of the extension lead right at the end. The socket is disposed of. The three cores of the extension cable ripped from the sheath provided about seven metres of blue, brown and yellow/green 10A wire with which to wire up the oven, at minimum cost. If you elect to use hookup wire instead, make sure it is rated at 250V AC or higher (ordinary electronic hookup wire is generally rated at 100-150V so is not suitable), hence the reason for using wires stripped from a mains extension cord. All mains terminal blocks in the oven had small pieces of insulating plastic underneath them to offer a little extra protection against a stray strand perhaps touching the oven case. The same plastic was also installed under all the starter sockets. The plastic came from an old DVD case, which was simSC ply cut up with big scissors. into MOTORS/CONTROL? Electric Motors and Drives – by Austin Hughes Fills the gap between textbooks and handbooks. Intended for nonspecialist users; explores all of the widely-used motor $ 55 types. Practical Variable Speed Drives – by Malcolm Barnes An essential reference for engineers and anyone who wishes to or use variable $ 94 design speed drives. AC Machines – by Jim Lowe Applicable to Australian trade-level courses including NE10, NE12 and parts of NE30. Covers all types of AC motors. $ 66 DVD Players and Drives – by KF Ibrahim DVD technology and applications with emphasis on design, maintenance and repair. Iideal for engineers, technicians, students, instal$ 85 lation and sales staff. There’s something to suit every microcontroller motor/control master maestroininthe the SILICON CHIP reference bookshop: see the bookshop pages in this issue Robot Builder’s Cookbook – by Jim Lowe For the first-time builder, advanced builder and robotics student. Provides a painless explanation $ 63 of robot programming. Switching Power Supplies – by Sanjaya Maniktala Theoretical and practical aspects of controlling EMI in switching power supplies. Includes bonus $ CD-ROM. 106 ! Audio ! RF ! Digital ! Analog ! TV ! Video ! Power Control ! Motors ! Robots ! Drives ! Op Amps ! Satellite siliconchip.com.au October 2008  87