Fullscreen HTML5Med Res (??MB)HTML4

Mains Muzzler: a line filter for your computer If you suffer from a noisy mains supply and your computer often crashes, the Mains Muzzler could be the solution. It incorporates an effective filter for mains borne interference and a Varistor to clip dangerous spike voltages. By LEO SIMPSON Some computers are much more prone to interference than others. This can extend to machines which are ostensibly identical. In our own editorial office we have two Commodore PC-5 (IBM-compatible) machines, each fitted with 20 megabyte hard disc drives. If both machines are in use and a mains glitch occurs, one machine is much 32 SILICON CHIP more likely to reboot than the other. So it is not possible to predict with any certainty whether a particular machine will be prone to mains borne interference. If you do have a problem in this regard, a mains filter such as the Mains Muzzler can help. It is designed to attenuate high frequency signals, commonly refer- red to as "hash". It is most effective for signal frequencies of 500kHz and above. For low frequency signals it is not very effective but then neither is any other mains filter on the market. What won't it do? No mains filter can cope with short term reductions in the mains voltage which may last for a millisecond or so. If your lights dim or flicker momentarily when your fridge or washing machine turns on, or when someone turns on a vacuum cleaner, then that indicates that your mains voltage supply has dropped momentarily. This is because most appliance motors draw a heavy surge current at the moment of switch-on. Inevitably, the heavy surge current causes a drop in the mains supply from your switchboard and that drop in mains voltage may be enough to cause your computer to re-boot. There are three things you can do to prevent these short-term reductions in your mains supply. First and most expensive is to use an uninterruptible power supply. This will maintain the mains supply to your computer regardless of fluctuations or short term reductions in the line voltage supply from your electricity supply authority. An uninterruptible power supply (UPS) will even maintain the supply for a few minutes after a blackout occurs. This gives you enough time to save any material you have been working on and then close down your computer properly, parking the hard disc and so on. For businesses which cannot afford loss of computer data, an uninterruptible supply is a great idea. A less costly way of coping with short term reductions in the mains supply is to use a constant voltage transformer (also known as a "line conditioner"). This is a specially designed transformer which will regulate its output voltage within a small percentage range, for quite large variations in the input voltage. Such transformers are fairly expensive but they are effective. Another good way to insure against mains drop-outs is to have a separate "clean line" run from your switchboard to your computer. For a modest cost an electrician can run a separate circuit and this will be much less prone to interference from your domestic appliances. What about fluorescent lights? Then there is the problem of fluorescent lights. Does your computer often reboot when someone switches on a fluorescent light? It does, eh. And how old is the fluorescent light fitting? About 30 years old. Uh-huh. Well, you may be suffering from the dreaded "power factor correction capacitor" syndrome which we could abbreviate to the PFCC syndrome. "What's that?" you say, -~- A .01 250VAC 240VAC .01 250VAC SIOV S20K275 E .01 250VAC 1M 1W CASE N L2 L1, L2 : 13.5T,1 .25mm DOUBLE TOUGH ENAMELLED COPPER WIRE WOUNO IN ANTI-PHASE DN A NEOSID F9 FERRITE TOROID TYPE No.28-756-36 MAINS MUZZLER Fig.1: the circuit of the Mains Muzzler is a low pass filter. It passes the 50Hz mains supply while attenuating high frequency signals. The SIOV-S20K275 varistor attenuates large spike voltages. "Run that past me once more." Well older fluorescent light fittings, as fitted in older homes, were often intended for use in factories or office blocks. Because of this, they were fitted with power factor correction capacitors. These are usually about 3µF in capacitance and are wired directly across the mains supply, between Active and Neutral. The capacitors compensate for the lagging current through the iron-cored ballast inductor and thus "restore" the power factor to close to unity. The problem with power factor correction is that, when the mains voltage is first applied to the fluorescent light circuit , the capacitor is virtually a short circuit. Inevitably, this causes the mains voltage to drop drastically for a millisecond or so which is enough to cause some personal computers to reboot. Again, this problem cannot be cured by the Mains Muzzler or any other mains filter circuit, although the measures outlined above will work. Alternatively, you could decide to remove the power factor correction capacitor. This will not have any effect on the operation of your fluorescent light. It is just a matter or disconnecting the capacitor. They are generally housed in a large aluminium container and retained within the fluoro fitting by one or two screws. Removing it now might save you from a messy clean-up at some time in the future if the capacitor decides to spill its insides out. The capacitor should be disposed of without puncturing the can. The oil inside is one of the polychorinated biphenyls (PCBs) which are now regarded as environmentally hazardous. Incidentally, if you suspect that you have power factor correction capacitors in your fluorescent light fitting but you are not confident about mains wiring, have your local licensed electrician do the job of removal. Note also that some fluorescent lamps which do not have a starter employ a series capacitor for the starting function. In this case the capacitor must be not be removed otherwise the light will not work. ]ANUARY1989 33 CORD CLAMP GROMMET Neutral to discharge the capacitors when the power is disconnected. It will also discharge any mains capacitors in the computer's power supply. Why did we put the resistor in? It can prevent a shock from the pins of the IEC mains socket on computers. These pins are inevitably exposed when the detachable cord is removed. N MAINS CORD EARTH GREEN/YELLOW ~ FRON {E Construction A © ,e <at>~ 12mm x 48A NYLON SCREWS AND NUTS \ A I © ©~ I 1• .o, GROMMETS (E;f \ ~ ~I © Fig.2: wiring details for the Mains Muzzler. All wiring to the PCB and to the mains sockets must be run using 250V AC-rated cable. Note that Nylon screws and nuts are used to secure the mains socket that sits above the inductor. This is to prevent the possibility of shorts. Muzzler circuit The circuit of the Mains Muzzler is a low pass filter. It passes the 50Hz mains supply while attenuating high frequency signals. It has a .01µF 250V AC capacitor connected directly across the Active and Neutral lines. These then pass through inductors 11 and 12 which are both wound on a ferrite toroid. After the inductors, both Active and Neutral lines are bypassed to 34 SILICON CHIP the earth line via .01µF 250V AC capacitors. To protect the computer against large spike voltages, a Siemens Varistor, type SIOV-S20K275, is connected between Active and Neutral. Normally open-circuit, this device becomes a very low impedance (ie, a short circuit) to any large voltage spikes. Finally, a 1MO 1W resistor is connected between Active and We built our prototype Mains Muzzler into an attractive extruded aluminium case with . folded aluminium cover. This is finished in blue and grey powdercoat enamel which stands up well to knocks and scuffs. The case is just long enough to comfortably accommodate four surface mounting mains sockets (as made by Ring-Grip or HPM). While not presently stocked by parts retailers, the Betacom case can be obtained directly from the Australian distributors Temple Smith Australia Pty Ltd, 2-12 Harp Street, Campsie (PO Box 196), NSW 2194. Phone (02) 78 3436. In other states, phone Melbourne (03) 781 1013; Adelaide (08) 365 1000; Perth (09) 272 7122; and Brisbane (07) 252 7466. The case is available by mail order for $24.95 plus $4.00 for packing and postage. Payments may be made by cheque or credit card. Alternatively, you could use a large diecast aluminium box although the mains sockets will have to be staggered (ie, not mounted in a straight line) to fit on the lid. A suitable diecast case is available from most parts retailers (eg, Jaycar Cat HB-5016). All the filter components are mounted on a printed circuit board measuring 70 x 120mm (code SC10101891). This fits easily into either of the specified cases. Assembling the board is quite a straightforward process although there is a little work to be done in winding the ferrite toroid. This has two windings, each 13.5 turns of 1.25mm double-tough enamelled copper wire. These are wound on as shown in the accompanying diagram and photo. Before installing the toroid on the PARTS LIST The PCB is mounted inside the case on 6mm standoffs. Note that all mains wiring to the sockets passes through 8mm rubber grommets. The pen points to one of the two Nylon screws used to secure one of the mains sockets. 1 extruded aluminium case with cover, 230 x 11 0 x 30mm (Betacom Cat IC4-3); or one aluminium diecast case, 190 x 110 x 60mm (Jaycar Cat HB-5446 or equivalent) 1 printed circuit board, 120 x 70mm, code SC10101891 1 moulded 3-pin plug and 3-core mains cord 4 surface mount 3-pin mains sockets 1 cordgrip grommet 1 Neosid F9 ferrite toroid, 31.5mm OD, 19mm ID, 12 .5mm high, type 28-756-36 1 SI0V-S20K275 varistor (Siemens) 3 .01µF 250VAC capacitors (Philips 2222-330-41103 or Wima MP3) 1 1 MO 1W resistor 2 metres 1.25mm-dia doubletough enamelled copper wire 1 2 8mm rubber grommets 3 solder lugs 4 6mm standoffs 6 4mm x 1 2mm roundhead screws & nuts (to secure mains sockets) 2' 4mm x 1 2mm Nylon roundhead screws & nuts 4 3mm x 1 2mm countersunk screws & nuts (to secure PCB) 4 3mm shakeproof washers M'iscellaneous SC10101891 Fig.3: you can use this full-size pattern to etch your own PCB. printed board, the ends of the coils must be carefully scraped clean of enamel and then tinned with solder. After pushing the four tinned leads through the board holes, crimp them over to mechanically hold the toroid down on to the board. Then the leads can be soldered. By the way, the holes for the toroid leads can be drilled with a No 55 drill or a 1/16-inch drill. The holes for the various input and output mains leads to the board should also be drilled with a 1/16-inch drill; Before mounting the four surface mount mains sockets on the lid of the case you will need to do a certain amount of drilling. Each socket requires two 1/8-inch diameter holes for the mounting screws and three 8mm holes for the mains leads. These latter holes are fitted with 8mm grommets to prevent chafing of the lead insulation. One of the mains sockets mounts just above the toroid and the screws are likely to interfere with the inductor windings. To prevent any possibility of damage to the 250VAC-rated hookup wire, four rubber feet (if Betacom case not used). windings, the screws in question should be of Nylon. All the wiring inside the Mains Muzzler should be run using wire with 250VAC-rated insulation. Three solder lugs are used to terminate the earth wires for the mains cord, from the printed board and from the earth line to the mains socket. The printed board is mounted inside the case using 6mm metal standoffs, screws and nuts plus shakeproof washers. Use your multimeter (switched to a low Ohms range) to check all the wiring before using the completed Mains Muzzler. ~ JANUARY 1989 35