The rewound transformer for your power supply must meet the requirements of Australian Standard AS 3108, which requires that 3KV AC RMS can be applied without breakdown between primary and all secondaries, primary and frame, and secondary and frame. (For those of other than Australian nationality, find the test specs which govern transformer construction in your country.)
This very practical specification is the result of enormous experience, and has been written by the regulatory authorities to ensure that the user of a device such as a transformer or power supply is not electrocuted or injured for any reason.
ONLY FOOLS IGNORE SUCH REGULATIONS.....
This is definitely not just another piece of useless government legislation, and every effort must be made by YOU during the rewinding of your transformer to ensure this spec is met, and your efforts are electrically safe. In short, there are no excuses for shoddy workmanship and legally all devices connected to the mains and having outputs accessible to a user must meet this spec.
Before you start this job, remind yourself again that the supply is connected to the 240 volt AC mains and that mistakes can be fatal. For this reason, your workmanship must be first class. If you have any doubts about your abilities then either find someone who is qualified to inspect your work and tell you whether it is acceptable, or find a professional who will do the work for you. Remember also that the transformer core must be physically connected to the mains earth, and that the primary must be fused as per the circuit diagram.
THESE INSTRUCTIONS PRODUCE A TRANSFORMER WHICH WILL SUPPLY A MAXIMUM CONTINUOUS DC SECONDARY CURRENT OF 8 AMPS AND WILL POWER AN SSB TRANSMITTER WHICH GENERATES UP TO 100 WATTS PEP ON VOICE PEAKS (SUPPLY CURRENT PEAKS UP TO 20 AMPS, WITH AN AVERAGE CURRENT DEMAND OF LESS THAN 8 AMPS). FOR OTHER APPLICATIONS, SEE THE CAUTIONS IN THE ARTICLE SECTION "GENERAL COMMENTS".
THE SAME TRANSFORMER ASSEMBLY REWOUND WITH HEAVIER SECONDARY WIRE WILL SUPPLY 18 TO 20 AMPS DC CONTINUOUSLY, BUT THE 35 AMP BRIDGE RECTIFIER MUST BE REPLACED WITH MUCH HEAVIER DIODES.(SEE "ADDING MORE GRUNT" LATER IN THIS ARTICLE.
|Modifying a Microwave Transformer|
I used a transformer from a 750 watt Sharp unit but any transformer from a microwave oven having a larger power output can be used. The smaller units use 1.2 turns/volt meaning that the 18 volt secondary needs 22 turns. The larger units from 1kW 'nukers' have bigger cores and use 1 turn/volt (18 secondary turns). The problem with most modern microwave transformers is that the cores have been welded together and cannot be disassembled for rewinding. Some other method has to be found for quickly removing the secondary winding.
Now is the time to don your blue and white striped apron because the best way of doing this is with an old wood chisel and a large hammer (see photographs). As can be seen from the photos the secondary is removed by using the chisel to cut off the protruding C-section of copper on either side of the core. Work parallel to the surface of the laminations at surface level, alternately attacking the winding from either side. Prise off the bits of copper winding you cut through as you go. Be careful not to damage the smaller primary winding. When you have removed the protruding copper on both sides of the core, drive out the remaining plug of lacquer and copper from the lamination window using a 12mm square punch. Next remove the magnetron filament winding. Now, using the same square punch, remove the magnetic shunts on both sides of the window. These are small groups of I-shaped laminations which sit directly above the 240 volt primary. Clean up the window removing all loose insulation. Using a sharp Stanley knife, cut a couple of I-shaped pieces of 3mm thick craft wood or 3-ply of exactly the same width as the window. These are placed in the same position as the magnetic shunts just removed and force the primary and secondary windings to be well separated. Use the cardboard from an old manilla folder or heavy masking tape to line the rest of the window, making sure that anything which could damage the insulation on the secondary winding is very well covered. In particular, sharp edges must be turned into smooth radiuses by using lots of cardboard/tape.
Quickly wind a temporary secondary with 5 turns of any old plastic insulated wire, connect 240 volt to the primary, and measure the AC secondary voltage. Calculate the turns/volt and hence calculate the number of secondary turns you need for the 18 volt winding.
Remove the temporary secondary and wind the real secondary using standard plastic insulated 7 x 0.69mm wire. Make sure that the insulation on the wire you use is rated for continuous operation at 90 degrees centigrade or more (lower temperature ratings are not available these days anyway). The plastic insulation has an outside diameter of just a fraction under 4mm. Electricians use this wire in either single or 3-core form to wire 20 amp power outlets (white outer sheath). In the old imperial terms it is known as 7 strands of 0.026 inch dia. Copper. Another way of specifying this cable is by referring to the cross-sectional area of the copper, which is 2.5 square millimeters. You will need around 6 to 7 metres for the secondary. You can use any wire you like for the secondary, provided the insulation will take high temperatures and the cross sectional area is 2.5 square millimeters. Heavier wire will cause the bridge rectifier to fail because the peak currents will be too high. Smaller diameter wire will simply overheat. Do not use single strand wire which is almost impossible to wind neatly. Wires with 2.5 square millimeter copper cross sections are also available with many more than seven strands, and are very flexible and easy to wind. Wind the secondary neatly in layers, making sure that a minimum gap of 3 millimetres exists between it and any part of the primary winding. It may be necessary to bind some parts of the winding with tape to ensure this. The winding which results will deliver 18 volt no load or about 15 volt at full load. Add a couple of extra bits 3mm MDF or 3 ply across the width of the transformer so that the secondary cannot sag and touch the primary (see photos).
An aid to neatly winding the secondary is to cut some more bits of 3mm ply to exactly fit the height of the window. These can be used to force the turns to sit flat through the window as you wind each layer.
When all is finished, test the transformer. During testing include a 1 ohm 10 watt resistor in series with the primary. You will be astonished at the no load magnetizing current of your transformer, which will probably be around 2-3 amps (2-3 volts RMS across the 1 ohm resistor) This very high value is caused by the iron in the core spending a good part of a mains cycle in saturation. This technique means that the weight and cost of the transformer is minimised, but that it also has very high losses which necessitate fan cooling of the transformer assembly.
THERE ARE A NUMBER OF PHOTOGRAPHS BELOW SHOWING HOW THE TRANSFORMER IS REWOUND. VIEW THESE PHOTOGRAPHS AS YOU READ THE DESCRIPTION ABOVE