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Salvage It! BY JULIAN EDGAR The good bits in microwave ovens You’ve got to be extremely careful with micro­ wave ovens – even a “dead” one can kill you! Here’s how to render it safe and salvage some very useful parts. F IND A HEAP OF discarded consumer goods and nine times out of ten there will be a microwave oven in there. Provided it’s free, there are plenty of small but useful components inside a microwave oven that are worth salvaging. However, there’s also a potentially fatal surprise for anyone who delves into the innards of a micro­wave oven without knowing what they’re doing. So how can you make sure you survive the encounter and what will you find in there that’s useful? Danger! Inside a microwave oven is a capacitor that may well be fully charged to well over 2000V. If so, that capacitor has enough Once you have connected the Active and Neutral pins of the power plug together (to discharge any capacitor on the mains side), cut the power cord off so the oven cannot possibly be plugged into the mains. This is a very important safety step. siliconchip.com.au stored energy to kill you, so it must be made harmless before you can proceed any further. In addition, there may also be other charged capacitors on the mains input just waiting to deliver a nasty shock. In some ovens, there is a bleed resistor across the high-voltage capacitor (or inside the capacitor) to discharge it after the oven is switched off. However, there is no guarantee that this resistor (if it’s there) has done its job as it may no longer be intact. If the bleed resistor has gone open circuit or if no bleed resistor is present, then the high-voltage capacitor could still easily have several thousand volts on it long after the oven has been switched off and disconnected from the wall socket. What’s more, it can retain this lethal charge for many months. That means that you have to assume that the capacitor is charged to a lethal voltage and must be safely discharged before you can remove any parts from the oven. You do that by first briefly shorting each capacitor terminal to chassis and then shorting the terminals directly together to make sure – all without touching anything. First, you will need two long, insulated screwdrivers suitable for working with high voltages (ie, 5000V or more), thick rubber gloves (eg, those used for handling acids, not the thin type used for washing up), a short length of heavy-duty insulated wire with well-insulated alligator clips at each end (see panel), thick rubbersoled shoes (or a thick rubber mat to stand on) and protective goggles (to protect your eyes from flying molten metal). On the left is the mains transformer and on right is the potentially lethal high voltage capacitor. You need to be able to access the terminals of the capacitor to be able to safely discharge it. Note that the thick heavily insulated cables seen here connect to the magnetron – discharging of the capacitor may also be achievable at the magnetron. December 2006  93 Use A Heavy-Duty Clip Lead Note that the light-duty clip lead shown in these photos is for demonstration purposes only. In practice, you should use a heavy-duty clip lead made from 10A mains wire (to insure adequate insulation and current rating), with wellinsulated alligator clips at either end. DO NOT use alligator clips with thin insulation. Before using the clip lead, use a multimeter switched to a low ohms range to ensure that the lead is intact – ie, there should be zero ohms between the two crocodile clips. Check the lead each time it is used, to ensure it is still intact. will often indicate its high working voltage (eg, 2100V AC). Wear rubber gloves & goggles To safely salvage the components from a microwave oven you must first discharge the high voltage capacitor that’s inside. To do so you’ll need two high voltage insulated screwdrivers (one shown here), thick rubber gloves of the sort sold for handling chemicals, and a heavy-duty jumper lead equipped with wellinsulated alligator clips (see panel). Additionally, you’ll need safety goggles and rubber soled shoes or a thick rubber mat. The first step is to connect the Active and Neutral pins of the power plug together using the alligator clip lead (be sure to wear the rubber gloves). This will discharge any capacitors across the mains input (but NOT the high-voltage capacitor). Alternatively, you can simply short all three pins on the plug to a metal plate. Once this has been done, cut off the cord so that the oven cannot be plugged into the mains. Next, remove the back of the microwave (don’t touch any of the parts or wiring) and locate the high-voltage capacitor. It will be in a metal canister near the transformer. Thick, heavilyinsulated leads will connect the capacitor, magnetron and transformer. In addition, the labelling on the capacitor Discharging the high voltage capacitor involves shorting each side of the capacitor (ie, each of its terminals) to earth, using a high-voltage insulated screwdriver, thick rubber gloves and a jumper lead equipped with alligator clips. Any exposed terminals on the magnetron should also be shorted to earth. 94  Silicon Chip First, make a visual inspection to ensure you can access the capac­itor’s HV terminals and any exposed terminals on the back of the magnetron. DO NOT touch the capacitor’s terminals or any of the high-voltage wiring (this includes the wiring to the magnetron). If the capacitor’s terminals are hard to access and you don’t know what you’re doing, replace the cover immediately and quit while you’re ahead. Now, wearing the gloves, safety glasses and rubber soled shoes (or standing on the rubber mat), attach one alligator clip of the jumper lead to a good earth point (unpainted) on the metal chassis and the other alligator clip (at the other end of the lead) to the metal tip of one of the screwdrivers. Next, holding the screwdriver by its insulating handle, touch the tip of the screwdriver (complete with its attached alligator clip) to the exposed terminals of the magnetron and then to each of the capacitor’s terminals in turn. There may be a bright flash and a bang as the capacitor discharges, hence the need for the goggles. Of course, if the capacitor is not charged, nothing will happen (or, if nothing happens, it might not have discharged). Note: exercise extreme caution if you have to undo the bracket holding the capacitor in place is order to access its terminals. Also, in many cases, you will have to prise up the insulated clips on the capacitor’s terminals to expose them. Use an insulated screwdriver for this job and make sure you are wearing the rubber gloves. siliconchip.com.au Microwave Ovens Are Dangerous! Microwave ovens can be lethal devices. Never take the cover off a microwave oven and apply power or work on a live one. The high-voltage mains transformer and its associated high-voltage capacitor that power the magnetron can easily kill you. Silicon Chip Binders REAL VALUE AT In fact, the high-tension output from the transformer and the 3000V DC or so developed by the capacitor and diode rectifier circuit are considerably more dangerous than the 20-30,000V EHT in a colour TV set. That’s because the microwave high-tension supply is designed to supply real current! $13.95 PLUS P & P Even after it has been switched off and disconnected from the mains, a microwave oven circuit is extremely dangerous. The high-voltage capacitor can retain a lethal charge for quite some time (perhaps even months) after its last use. The moral here is simple – NEVER poke around inside a microwave oven (even a “dead” one) until the high-voltage capacitor has been safely discharged (see article). Do not even think of removing the cover of a microwave oven to salvage parts unless you are experienced, know exactly what you doing and fully understand the instructions given in this article for discharging the high-voltage capacitor. This is not a job for amateurs or for those with limited knowledge. People have been killed working on supposedly “dead” microwave ovens and salvaging a few parts is not worth your life! These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover When the high-voltage capacitor has been discharged, it should be removed and its terminals bridged, so that you know it is definitely discharged. Important: use a heavy-duty clip lead when discharging the capacitor – see panel on facing page. A lightduty clip lead like the one shown here has inadequate insulation for high voltage use and can easily fuse (they even go open-circuit in normal use). H Buy five and get them postage free! Price: $A13.95 plus $A7 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my Now wait a few minutes and then discharge the capacitor again. This step is necessary because the initial discharge may have only partially discharged the capacitor. Making sure By now the capacitor should have discharged but it might not have if there is a wiring fault in the oven (eg, if thewiring to the magnetron is open circuit), which is why the oven was discarded in the first place. siliconchip.com.au The only way to be sure is to directly bridge the capacitor’s terminals. That’s best done by first using your DMM to check the integrity of the shorting clip. You can then connect the clip lead between the two screwdriver tips and then simultaneously apply each screwdriver tip to the capacitor’s terminals to short them together (ie, via the attached clip lead). That done, unclip the jumper lead from the screwdrivers and connect it across the capacitor’s terminals.  Visa Card      Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ December 2006  95 ➋ ➊ ➌ ➎ ➏ ➍ ❾ ❼ ❽ Here are the parts typically salvageable from a microwave oven: (1) Magnetron – good magnets inside; (2) Transformer – it must be discarded; (3) 25W 240V bulb – good as a spare for your own oven; (4) Machine thread and self-tapping plated screws; (5) High power resistor; (6) 240V fan, (7) Microswitches – usually four in each oven; (8) 240V synchronous motor and gearbox with about 5 RPM output; (9) Temperature switches. You can then remove the HV capacitor and we strongly recommend that you solder a wire bridge across it’s terminals. Only now can you take off the gloves, goggles and rubber shoes. It cannot be over-emphasised that microwave ovens with charged internal HV capacitors are extremely dangerous! You absolutely must ensure that the HV capacitor is discharged before proceeding. What ever you do, don’t remove the capacitor and leave it where some unsuspecting person might pick it up! Salvaging useful parts Most people don’t immediately associate a microwave oven with big strong magnets and microswitches. However, along with quite a few other bits and pieces, you’ll find these in virtually all salvaged microwave ovens. After you’ve discharged and re96  Silicon Chip moved the high-voltage capacitor, you can remove the two nearby large components – the mains transformer and the magnetron. Unfortunately, despite initial appearances, the transformer isn’t worth keeping. The transformer is a stepup design that multiplies the 240V applied to the primary about eight times, thus giving around 2000V on the secondary. Therefore, it would theoretically appear that if 240V is applied to what was previously the HV secondary, about 30V will be available on what was previously the primary. However, these transformers use a secondary that is earthed at the transformer frame, so the secondary MUST NOT be connected to mains power. Interestingly, on these transformers, there’s also another secondary that looks suitable for a low voltage output when mains power is connected to the original primary. However, that would leave 2000V across on the terminals next door – potentially lethal. In short, throw the transformer in the rubbish bin (so no-one else is tempted to use it). It’s too dangerous to use in any other application. By contrast, the magnetron is worth salvaging – no, not to make a ray gun to shoot the neighbour’s cat but for the extremely powerful disc shaped magnets that are inside. These can be used wherever you need strong magnets. One good application is to glue a magnet to the base of a plastic or metal dish in which to keep nuts and bolts. That’s especially useful when you’re disassembling a piece of equipment – even if you knock over the dish, the nuts and bolts won’t go anywhere. Note that these magnets can be fractured, leaving very sharp edges – so be careful when handling them. While you’re in this section of the oven you can remove the temperature switches. All ovens have one and some have two. These are normally-closed (NC) designs that go open circuit if an over-temperature condition occurs. These can be salvaged for use in other equipment. Often the trip temperature is written on the switches – 95°C and 125°C are common. You’ll also find several microswitches (usually four). These snap-action switches are used in the door interlock mechanism and usually have a high current rating; eg, 16A at 250V AC. Don’t forget the fan All microwave ovens use an internal fan. These 240V fans are not enclosed (ie, the motor terminals and windings are exposed) but they are fine if mounted inside a case, with suitable insulation applied to the terminals. Even if you don’t have a use for a 240V fan, you’ll find the blade assembly easily pulls off the shaft and can just as easily be pushed onto the shaft of a small DC motor. In fact, my personal desk fan comprises an exmicrowave fan-blade assembly powered by an ex-VCR DC motor, mounted on a small salvaged in-line power supply. It is quiet, compact and very effective – and each summer I marvel at the longevity of the motor which in its two lives must have done an awful lot of revolutions! What else? So what else will you find in the siliconchip.com.au Two disc-shaped magnets like these are inside the magnetron. As shown, they’re very strong and so have lots of uses. oven? Well, in addition to the above “goodies”, you will find a high-power ceramic resistor (typically 20W and 800W), a compact 25W 240V light bulb (best put aside as a spare for your own microwave oven) and the turntable motor. The latter is a compact and enclosed 240V AC synchronous unit that uses an internal geartrain to provide an output speed of about 5 RPM. This is ideal for use wherever you want something to slowly rotate (a spit on a barbecue perhaps?). It can also be mechanically driven backwards, to act as a compact high-voltage alternator; eg, for use as a hand-cranked LED torch (see Salvage It! for January 2006). The touchpad/LCD/timer assem- bly can also be removed and put to use – see “A Digital Timer for Less than $20” in the August 2003 issue of SILICON CHIP. Conclusion The parts salvaged from a microwave can be used in lots of other applications. These parts include switches, magnets, fans, high-power resistors and the turntable motor. They take up very little storage space and can be quickly and easily salvaged. But whatever you do, make sure that you first carefully and safely discharge the high-voltage capacitor – we want you to be around to salvage lots of SC other goodies in the future! Custom-made Lithium Ion, NiCd and NiMH battery packs Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you salvage the high-quality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! NiCd/NiMH Smart Chargers www.batterybook.com (08) 9302 5444 Li-Ion Smart Chargers 2400mAh NiMH AA cells siliconchip.com.au High-quality single cell chargers with independent channels. Charge any combination of NiCd & NiMH AA and AAA cells Ask for a free copy of the Battery Book catalogue or visit www.batterybook.com December 2006  97