|TRANSFORMER CONSTRUCTION DETAILS|
The rewound transformer for your modified switch mode 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.) As the transformer core is ferrite (which is an insulator) and has no surround that is connected to anything, this boils down to applying 3KV RMS between the primary and secondary windings.
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, including bad construction practices and mains surges.
ONLY FOOLS IGNORE SUCH REGULATIONS
This is definitely not just another piece of useless government legislation, and every effort must be made 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. Donít kill yourself (or ruin precious equipment) in your attempt to save five minutes!
To meet specification, standard transformer practice demands that the interwinding insulation is capable of taking this voltage, and that an air gap of at least 6mm exist between any part of the primary and secondary (Air at sea level breaks down at around 20 volts per thou.)
Dealing with the interwinding insulation first, yellow polyester tape Furon CHR M54 (part No. 452140250 - from Australian Isola Materials SA) is 2.5 thou thick and has a dielectric strength of 3500 volts per layer.
Two layers are therefore necessary to truly meet spec. between primary and secondary, and the double layer also provides an additional margin of safety to cover the case of a pinhole existing in the tape. It should also be remembered that the tape is very thin and can be easily penetrated by small particles which may adhere to the adhesive during application. This problem is overcome by beefing up the primary/secondary insulation with two overlapping layers of 4 thou thick mylar film as used for the manufacture of overhead projector transparencies. This material is first class insulation, as it has excellent voltage breakdown properties and will take the very high temperatures found within a photocopier on the heat sealing drum, which melts pvc powder into the surface of either paper or an OHP transparency (well in excess of 200 deg. C.). It is also readily available in small quantities from your local stationer.
The second area of concern is the 6mm primary/secondary gap mentioned above. It must be realized that air is trapped within the layers of a transformer and when placed under electrical stress, this air ionizes forming the basis for an interwinding arc. As this ionized air cannot easily escape, the situation is dangerous and necessitates a wide gap. The breakdown which is most likely to occur is the one where an arc jumps from the edge of the primary winding, through the gap between the edge of the insulation and the bobbin, and back to the edge of the secondary winding. This means that both the primary and secondary winding layers must start and finish more than 3mm in from the edge of the insulation. Any wire which is brought from the winding to a termination on the bobbin through this safety gap must have insulation over it which will take at least 1.5 KV RMS so that the safety gap is not compromised. This forces the use of thin walled Teflon (or similar high melting point plastic) tube over the winding wire between the winding and terminal to provide this additional insulation, as the lacquer on most winding wires cannot be relied upon for more than 100 volts of insulation.
THERE ARE A NUMBER OF PHOTOGRAPHS BELOW SHOWING HOW, LAYER BY LAYER, THE TRANSFORMER IS CONSTRUCTED. VIEW THESE PHOTOGRAPHS AS YOU READ THE DESCRIPTION FOLLOWING.
WINDING THE TRANSFORMER
The existing transformer must first be stripped down. Immerse it in standard paint stripper overnight and it will literally fall apart. The bobbin, which is made of bakelite, is totally unaffected by this brutal process. Use rubber gloves (and eye protection) and before disassembling the transformer,remove the paint stripper from all parts using a toothbrush, hot water and soap. Remove all windings, counting the number of primary turns and measuring the wire gauge used. On the standard transformer used in the ST-230 series power supplies, these numbers are 40 turns and 0.63mm dia. Carefully remove and save the small pieces of Teflon tube used to provide additional insulation between a winding and its terminal. This stuff is virtually unobtainable in small quantities and is vital in constructing a safe transformer. Save the copper shim from the Faraday shield and the 5V 20 amp winding. Some of this copper will be used to construct a new Faraday shield for the new transformer. Thoroughly clean all cores and the bobbin, paying particular attention to the cleanliness of the mating core surfaces. The assembled transformer must have no air gap. DISCARD ALL COPPER WIRE.
Rewind the primary using 40 turns of 0.63mm dia Solderite wire. The first layer is 22 turns, followed by 2 turns of polytape, followed by 18 turns of wire. Teflon tube must be used (as specified above) on the winding ends between the winding and termination. As each layer is completed, fill the safety gap at either end of the winding with paper masking tape cut to the correct width. Sufficient paper tape should be added so that the paper tape height matches that of the wire. This allows the primary/secondary insulation to sit nice and flat. Finish off with another two layers of polytape. By the way, two layers of insulation means exactly what it says i.e two full turns of insulation plus a small overlap. These overlaps should be placed adjacent to the transformer terminals, so that the precious window space in the ferrite core is preserved for windings rather than filled with excess insulation.
The Faraday shield is the next item to be constructed. First cut a piece of copper shim 16mm wide and of sufficient length to go one and a sixth times around the completed primary. Remove all tape, clean, and using minimum solder attach a fine copper wire to one end of this shim. Use a piece of Teflon tube to insulate the wire between the Faraday shield and the termination (which is the terminal on which the primary finished). Next, cut a piece of polytape three times the length of the copper shim and lay the copper shim, terminating wire and insulation at one end of, and vertically central on the tape. Fold the polytape over the assembly thus trapping the Teflon tube and insulating both sides of the Faraday shield. Again all parts of the Faraday shield must have an insulated safety border of at least 3mm. Stick the Faraday shield into position over the primary and terminate it on the appropriate primary terminal making sure that a shorted transformer turn is not formed in the process.
Wind on two and a quarter layers (2 layers with overlap), of 4 thou thick mylar cut to width to exactly fit the bobbin. Do NOT use any film other than Teflon or Mylar for this most important insulating layer. PVC and other low melting point plastic films will flow under pressure at high temperatures, eventually leaving zero insulation thickness.
Now wind the first half of the new secondary winding. This comprises of 5 trifilar turns of 1mm dia. insulated copper wire, i.e. 3 paralleled windings of 5 turns. Study the photographs carefully. Note particularly the 3mm safety gaps, insulation at the start and finish of the winding, and the terminals at which the winding starts. Two wires are terminated on one terminal whilst the third wire is terminated on a single terminal. These terminals are connected in common on the switch mode power supply PCB. Allow sufficient length of wire for a 200mm flying lead length from the top of the transformer. When the winding is complete fix it in position with two layers of paper masking tape.
Now complete the second part of the secondary. Again study the photographs noting the points previously made in para 5. Also note that winding continues in the same direction from the flying lead centre tap. Again the winding is 5 trifilar turns of 1mm dia. copper and finishes with 2 wires terminating on one terminal and one on the second terminal. It is probably best wound in reverse, starting at the finish terminals and ending at the centre tap. When the winding is completed cover it with a further 2 turns of masking tape. Paper masking tape is used as insulation for the low voltage secondaries because unlike mylar or polytape, it conforms well to uneven surfaces. But note that its electrical properties are far inferior and it is only useful for low voltage work. The compliance of the masking tape can be used to actually shape and position the secondary windings so that the 3mm safety gaps are assured and visible from outside the transformer.
Insert the cores into the bobbin and tape them together using at least 2 turns of 10mm wide paper masking tape. Make sure the core halves align well and that there is no included material between the cores to create an air gap.
Immerse the entire assembly for 20 minutes in polyurethane varnish. Remove the assembly and hang it until it stops dripping. Bake it at 100 deg. C for 5 hours to cure the varnish. Your transformer is now complete. If you have access to a 3KV breakdown tester, test your transformer for primary/secondary breakdown. If you canít do this, your first class insulating materials and workmanship are your only assurance of electrical safety.