by Harry Lythall - SM0VPO


How many times have you needed a transformer for a "special" need? It happens to me all the time, especially when building DC-AC or DC-DC inverters and converters. At the moment I am needing a 20-watt transformer that gives me 250-0-250v @ 50mA plus 6.3vAC @ 2 Amperes. They are available, but not to suit my pocket. I don't mind paying up to SEK90 ($15, or £7). The transformer I want is available from ELFA, but it is 10 times the price. There are cheap transformers available from abroad, but the postage becomes a problem due to the weight.

I gave up re-winding transformers about 10 years ago because I could not get those cheap Spanish 230v-115v auto-transformers that were perfect for rewinding. But in English we say "Needs must when the devil drives", which means that I have scratched my head a few more times and found a practical method. This is what I will share with you now.

Here in Sweden, we have a company called "Kjell & Company". they sell a range of transformers, which at first glance seem like all the usual crap on the market - doped in varnish, making them unusable for rewinding.

Fear not, for I have found a way, and it is both do-able and has a few nice spin-offs. All you need is one of these transformers.

The source transformers I use


The first thing to do is to dismantle the transformer into its component parts. You do not need to remove the wire from the coils, but you must take pains not to damage the coils. You are going to need them.

The transformer is shrouded with a 2-part metal mounting bracket, or shroud. Lift the tags at the bottom and straighten them out. The bottom plate can be removed and the transformer removed from the mounting bracket.

Next you have to remove the laminations without damaging them. The problem is that they are all gunged-up with varnish. But you can use a thin hobby knife: the sort you use when making model aeroplanes. You can press this under the top lamination to separate it from the rest of the laminations. You basically need to break the varnish seal.

Use a modellers knife to separate the laminations

When you have done this, put the transformer in a vise and use a screwdriver and hammer to gently tap the first lamination out. You may have to destroy the first, but with a little care it can be removed without even scratching the surface.

Use a screwdriver to tap-out the first laminate

Now remove all the remaining laminations: separate with a knife, push the knife round the sides and down the middle. There are two types of lamination here, these are in the form of an "E" and an "I". The transformer is assembled with "EI" inserted in alternate directions. Save them all, and try not to bend them.

The dissasembled transformer

Key to illustration:
1 - Mounting bracket - two items
2 - "E" laminations - about 30 of them
3 - "I" laminations - also about 30 of them
4 - Plastic coil support - mounts the coils rigid to prevent vibrations
5 - The mains 230v (115v) main winding - to be reused
6 - 12-0-12v winding - to be replaced

One small point here, the laminations need to be insulated from each other. The cheaper transformers are NOT, so you need to spray them with varnish, or paint, on one side. If the laminations do short then you can get a "shorted turn" in the final transformer. It will be noisy, and get hot.

If you use the same transformers I have used then you can also prise the coils away from the inner plastic holder. Each coil is a separate module. In principle you can combine these modules to make new transformers, for example 230v to 6-0-6v, 6-0-6v to 12-0-12v, or 12-0-12v to 24-0-24v, etc.

Plastic coil support holds the coil modules

You could dismantle the low-voltage coil and re-use the former, but I chose to copy the former dimensions and make a new former. I used 0.5mm copper-clad board, from which I etched away the copper. On one of the side-pieces I made a few lands of copper to connect the coils to. This is very practical.

Evaluating Your Transformer

Temporarily wind 100 turns of wire on your new former

You can make a simple winding tool by cutting a block of wood to fit in the center of the coils, and passing a bolt through the wood. Use a metal washer or plate at the ends to holes the former in position. Next count the number of turns you get at the chuck when you turn the handle one revolution. I get 51 turns for 13 turns of the handle = 3.92308 turns per revolution of the handle.

Transformer winding tool using hand-drill

Loosely assemble the transformer with the mains 230v (115v) coil and the new 100-turn coil. For this test it is easier to insert all the "E"s in one direction and use the mounting bracket to keep the "I"s together in place. Connect the transformer to the AC-mains supply.

Transformer test coil assembled and ready for testing

Measure the output voltage from the transformer. In my case it was 16vAC. This means that I have 100 / 16 turns per volt = 6 turns per volt. So now I know that I need 6 turns multiplied by the voltage I want, plus a couple of extra turns.

Coil Calculation

Now that you know how many turns per volt, and you already know the voltages you want, all you need to do is work out how many turns of wire you need and how thick wire you will need for the current and space available. Not that there will always be typically 10% to 20% wasted space if you do not wind in perfect close-spaced layers. You can do this for low-voltage windings, but when more than about 500 turns are needed you just simply pile-wind, trying to keep the layers as even as possible.

This table will give you an idea as to the size of wire you need. If you cannot get enough turns with a wire that is just thick enough, then you need to down-grade the turns per volt and rewind the primary. But with commercial transformers the size/space should always work out for you.

Diameter (mm)AWGI-maximum (mA)Length per 100g reel

Copper wire table - based upon 3-Amperes per square millimeter

Note that copper wire in air will carry 2 or 3 time this amount, but when in a plastic case, or wound into a coil, then heat cannot escape. As a result it is customary to downgrade to 3 Amperes per square millimetre cross-sectional area. Military specifications are even lower than this figure.

I need 250-0-250v plus 6.3v. The cross-sectional area of the center of the transformer inside the coil is determined by the power required. But we already know this since I chose a 36-Watt component. My former is 11mm wide and 6mm deep at the inside where the wire will be. I therefore have a space budget of 66mm≤. I need 250-0-250v @ 50mA, which is 25mA per 250v section. I can use 33mm≤ of the space budget for the HT winding, and the other 33mm≤ for the LT winding.

250-0-250v HT WINDING

At 6 turns per volt, I need two windings of 250v, or 500v X 6t/v = 3000 turns.

This means that with a space budget I need to get at least 3000 / 33 = 91 turns in every square mm of space. At 100 turns per millimetre I can select enamelled copper magnet wire with a diameter of 0.1mm. To all intents and purposes, the wire is square. You are not going to wind the coils in perfect layers. 0.1mm Diameter wire just happens to be sufficient to carry 25mA (at 3A per square millimter). Perfect! the HT coil looks like this:

1500 turns + 1500 turns using 0.1mm Diameter wire.


At 6 turns per volt I need one winding of 6.3v X 6t/v = 18 turns (use 2 extra) = 20 turns. According to the copper wire tables, 0.85mm Diameter will carry the 2 Amperes, but I have space for 1mm Diameter enamelled copper magnet wire. This will mean two nice neat layers of wire and take 2 X 11 = 22mm≤ from my space budget. 1mm Diameter wire will also deliver up to 2.5 Amperes. Perfect! the LT coil looks like this:

20 turns using 1.0mm Diameter wire.

Coil Winding

I have 100s of those cheap'n nasty pencils you get every time you go to a lecture or conference. These are perfect for coil winding. Empty out the lead and cut out the guts that grip the lead. Now you can pass the 0.1mm Dia. wire though this when you are winding. In this way you can put the wire where you want it, and prevent the occasional turn slipping over the cheek of the former. For the 1mm Dia. wire you can do this manually.

Winding a coil using the pencil tool

Begin by threading the wire through the empty propelling pencil, then through a hole in the cheek of the former. Clean and solder the end of the wire to the copper connection terminal. Start winding. In my case I need to turn the drill handle 1500 / 3.92308 = 383 turns.

It is important with high-voltage windings that the wire is built up in uniform layers. In this way you will avoid having high voltages between adjacent layers. This could be a source of failure. But if you try to build up the winding evenly from side-to side as you wind then it will be quite adequate.

When you have finished the first HV coil, cut off the wire and thread the end through a hole in the former cheek. Clean and solder the wire to the second solder pad. Put one layer of masking tape over the coil. The tape should be 1mm wider than the coil former so that it laps up the side of the former.

Connect the next wire end to the second pad and wind the next 1500 turns in exactly the same way, and in the same direction as the first coil. Finish off by connecting the coil to the third solder-pad. Put one layer of masking tape over the coil.

The 6.3v LT winding is wound over the first coil. Start by passing the end of the wire though a hole in the former cheek. Clean and solder the wire to the first LT connection pad. Wind the 20 turns as neatly as possible in two layers. Pass the end of the wire through another hole in the former cheek. Clean and solder the wire to the second LT connection pad.

Testing The Coil

Temporarily assemble the transformer with ONLY the 230v (115v) mains coil. Pass AC 50Hz (60Hz) through the coil from another low voltage transformer, for example 24v is perfect. Measure the voltage across the coil and the AC current drawn by the coil.

Temporary assembly for testing

Now re-assemble the transformer with the 230v (115v) mains coil AND your newly wound secondary coil, as shown above. Check each coil for DC continuity (resistance) and that there are no short circuits to chassis, or between coils. Pass AC 50Hz (60Hz) through the coil from the low voltage transformer, just as before. The primary coil should draw about the same AC current as before. If the current is significantly higher then you have probably got a shorted turn in your new coil. Do not worry about all the vibrations, this will stop when the transformer is finally assembled, properly.

If you have plenty of time, or you are re-using old magnet wire, then you should do this test after each coil winding has been wound. That way you can catch a fault at an early stage and it can save you a lot of re-winding.

Finally, and assuming all is well, strip down the transformer then re-assemble it properly, inserting the "E"s from alternate sides. This is the way it was assembled when you first took it apart. The "I"s you push into the vacant slots in between the ends of the "E"s. The last couple of "E"s are a bit hard to fit in, but you need to straighten up all the laminations and compress them a little.

If you needed to varnish or paint the laminations, then you must be careful to insert them all with the paint-side uppermost. Another point is that it will not be possible to fit them all in, but save the unused laminations for your next transformer. After re-building three or four transformers you may have enought left-over laminations to build another transformer from all the left-overs :-)


Well, that was not so bad. If you followed the steps you will find that it is easy tp re-wind a transformer of the type I used. These transformers cost as little as SEK19 ($3, £1.5) each for the 5-Watt versions. One 100G roll of enamelled magnet wire costs SEK60 ($10, £5) and will be sufficient for about 5x 250v windings on the 36 Watt transformer.

If you want a speaker transformer for valves, then the square-root of the impedance ratio for the Anode:Speaker will give you the turns (voltage) ratio. The 230v (115v) anode coil is already wound for you. The only difference is that all the "E"s should be fidded from the same side of the coil, as was used for the test. When you assemble the transformer, put a thin sheet of photo-copy paper between the bank of "E"s and the "I"s so that there is a small 0.1mm gap. This will prevent the transformer core being magnetically "saturated" with the anode standing DC.

Well I hope that you have learned something useful with this information. Very best regards from Harry - SM0VPO

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