GeeK ZonE

Winding Your Own Transformers

"Winding my own transformer? Why on earth would I do that!" is the first question out of most folks mouths when I talk to them about winding their own power transformers. The question makes sense too - why would you want to when there is perfectly good off-the-shelf trannies available through Hammond, Allied, Sowter, Turner Audio and a myriad of other well respected, quality manufacturers? Well, I will give you *my* resons for doing such and you may agree that the task is right for you too.

Cost effectiveness. Good transformers are hideously expensive. Even mid-grade transformers can be expensive when you calculate the shipping on fifteen pounds of iron across the country. Even if one were to trek an hour and go into the local big city to eliminate the shipping costs, the fuel/time/car wear and tear expenditure may even out expensive postal shipping and handling.

Recycling. It's good for the environment and good for the pocketbook. Most of us have plenty of iron on hand to do such a task. The iron is the most expensive part of a transformer to buy from scratch. Got an old clock radio? You have 10 watts worth of iron in the little radio transformer. Great for a small solid-state project or a vacuum tube bias supply. Do you have, or a neighbor has a poofed microwave? You have iron in there good for 500 watts continuous power - lotsa tube power supply options there. Tape player, boom box, old TV's - there's a supply of good iron almost everywhere in any household.

Custom voltages. Especially if we are playing with vacuum tubes, usually we want a nice, high voltage, low current power supply. High voltage transformers are getting harder and harder to find nowadays. Sometimes winding your own secondary is the only answer.

Knowledge. Learning the simplicity of a seeming daunting task can broaden our electronics knowledge and help us to avoid pitfalls of improper magnetics design. You also never know when a n00b will ask you "How do I....." and you can happily help them.

Quality. I agree that it will take a long time of basement winding to come to the equivalent quality of a Hammond or Sowter, but the possibility is there because you control 100% of the construction process. Example - most power transformers are rated for 2500 volts of isolation - you can make this 10,000 volts if you need to without paying someone an arm and a leg. Newcomers to transformer winding can beat the quality of your average transformer coming out of China/Taiwan on their second or third try.

Design Flexibility. You can turn that iron into pretty much anything. Features such as electrostatic shielding, multiple secondaries, weired taps on the windings and special insulation.

Satisfaction. The knowledge and bragging rights that you did it yourself!

An 18V tape deck transformer converted to a dual 130V and 12V secondary with electrostatic shield (note the grey drain wire near the yellow secondaries).

Things You Need to Know

Before I go sharing with you some transformer winding ways, there are a few basic things you need to know. Figuring a transformers properties, a few rules of thumb and of course, safety info.

Always, always keep first and foremost in mind that a transformer is a line operated and line isolating device. It's job beyond anything (unless it's an "autotransformer") is to keep you and others alive when they come in contact with anything conductive that comes out of what it supplies. Even if it's a plastic encased project, there will be a headphone jack, input probe, whatever that comes in contact with the DC supply of the innards. If those are not present, then perhaps a metal switch or screw could come into contact with a DC potential. Never defeat this purpose for any reason - you or perhaps a loved one, like a cat's life depends on this.

With this in mind, the important part to make sure of is in the primary winding isolation. The primary needs to be suitably isolated from the core and the secondary windings that go overtop of it. Most times, you will be keeping the original primary winding of the transformer. This will maintain the primary-core isolation integrity of the original design. Unless the transformer bobbin has been damaged in some way, leave it alone.

Use a lot of common sense - DO NOT ATTEMPT TO REWIND OR USE A HOMEWOUND TRANSFORMER IN A MEDICAL OR LIFE SUPPORT DEVICE!! The same goes for transformers used in the presence of explosive or flammable gasses - like oxygen.

Transformer voltages we read with voltmeters are in RMS. The peak voltage is much higher - 1.414 times higher to be exact. Keep this in mind when choosing hookup wire and calculating insulation thicknesses. Since powerline spikes can hit over 2500 volts, if you have a 10:1 step-up transformer (120V to 1,200V), your secondary spike can be 25,000 volts! Use a MOV or snubber in the primary of such step-up supplies.

The existing primary-secondary insulation of low voltage transformers (conventional bobbin) will be suitable for up to 130 or so volts. If you are making a high voltage secondary, your concern now is secondary-primary voltage breakdown and the insulation will need to be beefed up a bit. This can be done with thick teflon tape found at your local plumbing store, glass tape found at some hardware stores and at all fibreglass manufacturing outlets or high-temprature electrical tape found in professional electrical supply stores. Regular electrical tape will not suffice, due to its temprature handling capability - usually 60C. or less. Use minimum 85C. or preferably 105C. rated electrical tape (which is usually glass based anyway).

As tempting as it might be, DO NOT reuse the enamel wire you pull off a transformer. This wire will have developed minute cracks and flakes of enamel will have come off during the unwinding process and can be very dangerous as either an electrocution or fire hazard. Also, this wire has undergone several heating/cooling cycles and will have been "tensilized" by machine winding. Most unsuitable for transformer rewinding.

If you do have to rewind the primary, always create a new bobbin for it. This can be done with cardboard. The thickness of the cardboard will determine the primary-core isolation voltage. Since powerline spikes can easily hit 2500V, use this figure as a design minimum. I'll touch more on bobbin creation later on in this article.

Next, you'll need to know the basic properties of the transformer - power handling and turns-per-volt rating. Power handling is easy to determine if you know the rated voltage and current of the secondary. A 12V at 1A secondary means 12VA (volt-amps) or roughly 12 watts of power. To determine the power handling of an ordinary power transformer when the secondary voltage or current is unknown, a rule of thumb is 10 watts per pound for a cool running tranny to 16 watts per pound if you design it for intermittent service, like a HAM transmitter.

Another accurate method of determining transformer power handling capability is through the transformer core cross sectional area. See the charts below for info.

Determining cross sectional area.


Image © The American Radio Relay League.

Here is a chart that roughly gives the VA rating of a transformer from its cross sectional area.


Image © The American Radio Relay League.

A more scientific way to determine VA capability is to power up the transformer on your bench and measure the no-load secondary voltage. Now add some resistance (big power resistors) across the secondary until the voltage drops by 30% for small transformers (<50 VA), 20% for medium sized transformers (50 to 100 VA) and 12-15% for big suckers (>>100VA). For high efficiency semi-toroidal transformers, all voltage drops will be roughly 1/2 of these listed. Make note of the voltage under load and use ohms law to discover VA rating:

* Rated secondary current = E/I = voltage under load/resistance in ohms
* VA rating in watts = I*R = current from above formula * voltage under load

Transformer disassembly

Now you know the VA rating of the transformer, you need to know its turns-per-volt (TPV) ratio. This will tell you how many turns to wind on your bobbin for a given voltage. This stage requires you to completely disassemble the transformer, lamination by lamination. First, write down the volts-under-load derived from the above test. Do not use the unloaded volts, or you will end up undershooting your target voltage on re-assembly.

Remove the housing and hardware (if any) and place them aside. You might have to peel off a bunch of paper crap that has had several years to settle in and get a good grip on things. Break them off with a screwdriver or sharp knife. Snip the secondary wires off as close to the winding pack as possible - this will keep them out of your way. However, protect the primary wires, as they can be reused if in good condition.

To take apart the laminations, use an extremely sharp knife that you don't mind ketting wrecked. A retractible blade knife from a dollar store is perfect for this step. Slice into the first and last two laminations all around the circumference of the transformer. Using pliers, pry these out. Don't worry that a couple get mangled, loosing a few laminations is par for the course and won't hurt the transformer operation at all. Once these are out, you'll easily be able to break the rest loose from the shellac that binds them. One, by one, the E-I core laminations will come out. This can take up to an hour on a large transformer for a practised hand, so put on a CD and make some coffee. Keep them in a safe place. Clean up the shellac flakies too - they are poison to children and pets if ingested.

Once that is done, peel off the tape/paper wrap over the secondary. Keep the primary wires protected from bending/breaking. However, if they are just too un weildy, snip off the primary wires right at the junction of enameled copper/insulated wire. Tape them and bend them in toward the bobbin where the core was. If they break, this means they were too old or damaged to be reused again safely anyway.

Start unwinding the secondary windings, keeping an accurate count of how many are on there. Once you have done this, divide the amount of turns by the loaded secondary voltage and you have the TPV for that core - this is very important as that TPV rating will be for that core only. Different cores may have different ratings. The higher the quality of the core iron, the lower the TPV will be. Transformers average 8 or more TPV for clock-radio sized transformers to 2 TPV or less for 150W or larger power transformers.

Example - you unwound a transformer with a loded voltage of 11 volts. There were 86 turns of wire on the secondary. 86/11=7.8. Therefore the TPV for your core will be 7.8 turns for every volt you want on for the secondary. You can also calculate the amount of turns the primary has too if you want. 120*7.8=936 - there will be approximately 936 turns of wire on the primary. So if you want to make a 1:1 isolation unit - wind 936 turns of wire for your secondary. If you want a 300-0-300 transformer, you would use 4680 turns tapped at 2340 turns. But, this is unrealistic as 1) wire losses would severely limit the current to a low value and 2) are you sure you could fit that many turns on a transformer with a 7.8 TPV rating? Doubtful.

Choosing the Correct Wire

You may have some enamelled copper wire (often called "magnet wire") on hand or you may have to go get some. Just what size of wire do you need? That depends on 1) your required current derived from the VA rating and voltage and 2) how many turns will be on the bobbin. The more turns, or the longer the wire, the larger in diameter it should be above that specified of its rated capacity to offset losses and make your transformer run cooler. Remember, what isn't turned into output, is turned into heat!

For power transformers, choose the wire that closely suits your expected maximum current draw, then go one size larger. Example - you expect to draw 100mA from your supply. The closes representative is 30 gague (always go over, than under). Use 28 gague for your transformer. But, that's just conservative ol' me. You might want to go 30 gague, but you may be disappointed - especially if the transformer is for a class-A amplifier or other constant-current draw supplies ;-)

Below is a little chart to help you select the wire you might need. I have included the common sizes from 14 to 40 AWG. However, the industry is turning more and more to metric, because it is an international standard. So metric sizes are given as well (it's easier to use in calculations too!).

Information derived from the ARRL Radio Amateurs Handbook and are based on 700 circular mils/amp capacity.
TPI is based on enamel and is approximate as coating thickness differs between manufacturers.

Make sure when choosing a gague of wire, that you have enough room on your core for the amount of wire you wish to wind. If you have so many turns, or you need such thick wire to meet your current requirements that the bobbin won't fit back on the core - you need a bigger core, plain and simple.

Before we leave off this subject and get into the fun of winding, consider the quality of enamel wire that you buy. If you get your wire from an electronic supply house that sells small spools especially for transformer or electric motor winding, this is great. But there are many grades and coatings available. Some are low temprature (<60C.) or low voltage enamel that are completely unsuitable for transformers. They are usually sold as hobby magnet wire and are ok for RF coils and such in radio equipment or school projects, but are DANGEROUS if used in a line operated device. Random lots of such magnet wire can be found in stores like Radio Shack. They buy from the lowest bidder, so one pack of wire might be very crappy for anything but coils and the next pack might contain the best stuff available bought surplus - you never know. Don't take the chance. A motor rewinding shop MAY sell in small quantities retail, especially in remote towns.

Winding

If you are winding a low voltage secondary, this step can be done with handwinding as opposed to using a winding jig. For higher voltages, you really should use a jig for not only a neater job, but to keep track of how many turns you have. There's nothing more frustrating than going, "588...589...590...(wife interrupts with task list)...uhhhh...ummmm.....SHIT!" :-p

There are two basic types of windings - the scramble wind and the even layer wind. The scramble wind is common on split-bobbin types of cores that we see a lot of nowadays. The split-bobbin is usually made of plastic and can be found on any type of power transformer, high voltage, low voltage, high current, low current. They are the easiest to rewind because the primary is physically seperated and no electrostatic shield is needed. All you have to do is keep track of your turns, then finish. Sometimes the scramble-wind can be found on low voltage, low current transformers with traditional bobbins.

For the standard bobbin, an even-layer technique is best. This requires more effort, but the quality of transformer is much higher. For standard bobbins, you should really add an electrostatic shield between the primary windings and your new secondary. This shield serves multiple purposes. It provides an effective isolation statically (the transformer principle is a magnetic isolator only), it reduces HF noise from being coupled to the secondary winding and it will shunt to ground line voltage should all hell break loose in the primary-secondary insulation and blow your fuse (you DO have a fuse in your power circuit, right?). Once you have a bare primary, if the insulation is still good, just add your electrostatic shield over it, insulate it with a quality tape previously mentioned or a thick layer of paper. Two layers of non-acidic 24# bond laser paper is good for all but very high (>500V) voltages. The figure below shows how to make an electrostatic shield.

[UPDATE] I have just learned from Pat at Turner Audio that a LV winding, like say a small 6.3V winding for filaments that has *one lead grounded* placed between the primary and secondary windings will also be an effective electrostatic shield.

Paper and cardboard has a puncture voltage of about 200V/mil (a mil is .001 of an inch), so 100V/mil is a good working rating. Use this guideline for calculating any paper or cardboard based insulation.

Applying the electrostatic shield to a conventional transformer.

The electrostatic shield should be high enough to come within 1/8 of an inch from the top and bottom of the bobbin. You have to also allow for an air gap, of about 1/4 of an inch, as the ES shield should NOT make a full circle around the primary. The ES shield itself is a turn around the core and generates a voltage. Usually less than a volt. Because of its cross-sectional area though, very high current can be induced into it - up to 100 amps or more! If there was no air gap, it would be a shorted turn. This shorted turn will make lots of heat, smoke and probably - fire.

If you are using copper foil, solder the drain wire to the foil. If you use aluminum foil (like I do) strip about three inches of insulation off the drain wire and make a zig-zag pattern with the bare copper, with its height about 3/4 that of the shield foil. Now tape this to the foil really, really tight. The compression of the tape and the secondary windings will be enough to keep it in place. When installing the transformer, the drain wire should always be connected to an earth ground. Finish the ES shield with the same level of insulation you would use for the primary-ES shield. Time to add the secondary.

Leave yourself about two inches of tail-wire and begin your even-layer secondary over the ES shield insulation no less than 1/8 of an inch from the end of the bobbin and finish the layer at the same distance. Make sure your secondary wires will be on the opposite sides of the primary wires. When you reach the other end of the bobbin, tack the wire with some tape and add a layer of insulation over this winding. Use of thick teflon plumbing tape is great for this because teflon has great electrical qualities, excellent heat-handling characteristics and will self-form over the previous winding and adheres to itself well. continue winding up and own until you have achieved your desired number of turns. Once you have added the final layer of insulation, snip the wire, leaving yourself another 2 inches of tail-wire.

Proper winding of the secondary.

If you are winding multiple secondaries, begin as above, but be sure to label the first secondary with sticky taps, black marker or ideally, use a different color of enamel wire. Once finished, place a piece of thin cardboard onto the side with the secondary wires and tape it there. Strip and tin your desired color hookup wire that you will use to exit the transformer to your project. Using 240 to 320 grit sandpaper (180 grit for large wire), strip the enamel off the secondary wires within 1/4 of an inch to where they enter/exit the bobbin. Tin them too and solder them securely to the hookup wire. Once done for both wires, tape them (individually, NOT both with one piece) with electrical tape to the cardboard as shown below.

Preaparing to finish the secondary.

To add strain relief to the secondary wires, tape another piece of thin cardboard ontop of the soldered wires, then bend the wires around that piece and wrap a layer of electrical tape over that assembly, going completely around the windings.

Finishing the secondary.

If (as earlier mentioned) you had to snip the primary windings because of damaged hookup wire or whatever, finish them the same as above.

Now is the time you'll want to do a continuity test. Get an ohm meter and look for a resistance, roughly equal to the length of wire you used (see the wire selection table to calculate approx. resistance). If this is fine (within 20%), continue. If there is no reading, something is open. Perhaps you didn't strip/tin the wires good enough? This is the most likely cause as good enamel is tough as leather and a bugger to strip and tin properly. God forbid one of the wires broke - you'll have to unwind it and do it all over again. But, what if the reading is well below the expected? Before you panic, use a microcaliper to measure the wire diameter - perhaps you got a size larger wire mislabelled a size smaller. That has happened to me before. Maybe you have some shorts in there. If so, a piece of debris may have got in the windings and shorted the wires out, or you have really crappy wire. If the latter is found to be the case (flaky enamel), get a refund and have the retailer alert the manufacturer, as they may have cranked out a bad batch. It's possible the wire was just too old too. Exchange it for fresh stock.

Transformer Reassembly

If you are here, congratulations! You have finished the most tedious and time consuming part of rewinding your transformer! Now for the really boring part - reassembling all those E and I lamination - yuck!

Bring out that box of loose laminations that you set aside from disassembling the transformer. Clean off all the bits and pieces of old shellac and if you find rust spots, remove them with a rotary hand tool or sandpaper.

The laminations need to be electrically isolated from one another (the shorted turn analogy comes into play here) and glued together unless you want a lot of noise, vibration, hum and excess magnetic interference from you freshly rewound tranny. Origionally, the transformer was hot-dipped in shellac and oven bake cured to satify these requirements. I really don't recommend this at home because it's a PITA (Pain In The Ass), smelly and quite toxic. The home rewinder has two easy and effective alternatives - microcrystalline wax and clear nail polish. Yes, NAIL polish :-)

Microcrystalline waxes can be obtained from candle making supply houses. They have a melt point ranging from 180 to 240F (the candle industry still uses F.) Get the higher one. Melt it in a pot, soak your laminations in it, completely covering them, remove each one with pliers and gently wipe with a non-dusty towel or cloth and let each piece cool. Reassemble transformer. I find this method also a PITA. I use nail polish :-p

Your local drugstore carries clear nail polish for real cheap. The glossy stuff is best. In Canada, each tube of teeny-bopper cheap stuff costs about $0.50 - so grab a case. One tube will last for two 20W transformers all the way to two tubes for a 250W big dude. Begin assembling the E and I laminations as shown below, placing a layer of nail polish between. Make sure the I laminations do not fully touch the ends of the E laminations either. Nail polish is flexible even when dry, tough, heat resistant, water resistant, UV resistant, electrically non-conductive even when wet and best of all - you can raid your wife's junkbox for it, if you run out!

Stacking the E and I lams alternately.

Because of the thickness of the nail polish (or wax), you may not be able to get all the lams in. Ifd you have up to three spare lams, don't worry about it. If you have more, try compressing the lams together with C-clamps on each corner. If you are using the nail polish method, wipe off the ooze. Put some more lams in. Once all the lams (or as many that want to go) are in, use a small hammer and tap all the I lams so they are even on the surface. At this point, they will not short with the E lams, because of nail polish ooze or wax buildup (here's a rare case where wax buildup is good!). If you for some reason, have all the lams in place and the bobbin assembly is loose, use paper or cardboard shims to hold it in proper place and tight. If it is loose, it will rattle arround during operation, decrasing your transformers life and be very bloody annoying.

If the transformer was encased in a housing like my 130V/12V example near the top of the page, do a quick dynamic test (plug it in and see if it works), then you can reinsert it, using a generous amount of nail polish, or tape if you used the wax method, to keep the tranny from rattling around in the case. The nail polish smell can take several weeks to go away, depending on the size of the transformer. At this point, pat yourself on the back - you're done!

Creating Bobbins

If you wish to wind yourself a custom bobbin to replace an old, damaged one or need to rewind the primary windings because of age, it's as simple to do as the secondary circuit, but more emphasis has to put on safety here. First, select some cardboard with the correct thickness for your needs. As mentioned previously, a safe working voltage for paper is 100V/mil of thickness. Take into account a 2500V powerline spike and the thickness should be at least 25 mils, or .025 inch. For simplicity and safety sake, standard 1/32 inch (.031) card stock can be used.

First, measure the amount of cardboard that you'll need. Do this by placing an E lam on a piece of cardboard and make a strip the height of the centre post on the lam. This will assure that the transformer will go back together properly. Next, stack all the E lams together, compress them loosely with C-clamps (or good size grip-clamps) and wrap the strip around centre and trim it where they just meet and tape it (Scotch tape is OK here) as illustrated below.

Exaggerated detail of measuring the cardboard strip for bobbin fit.

Now, make some ends for the bobbin. While this isn't necessary, it makes winding a whole lot easier. Cut two flat cardboard square/rectangles with an outer width equal to the inside with of the gaps on either side of the centre post (it'll have to slide inside there) and cut out an opening in the middle the size for a tight fit OVER your bobbin that you just made. Slide those over the ends (you don't want to add length to your already perfectly fitting bobbin) and glue with FABRIC glue (available at fabric stores and craft departments). Run a bead of it on both sides and smooth with your finger. Why Fabric glue? It's moisture proof, fairly heat proof, flexible, tough and non-flammable. Details below.

Gluing the ends on the bobbin.

Completed conventional bobbin.

If you would rather make a split-bobbin for extra isolation, eliminating the stage of an ES shield or are lazy and want to scramble-wind, make a third endpiece and create a split-bobbin like below.

A split-bobbin.

Trim your bobbin ends so that it slides nice and smoothly onto the transformer core E stack and this will guarantee a perfect fit. Unless you are like me where you could measure thirty times with a micrometer and still be out an inch (:-p) Use the transformer reassembly instructions outlined previously.

Rewinding the Primary

Once all the safety precautions have been observed, rewinding the primary is no more daunting than winding a secondary. However, one must pay closer attention to the wire tables above and figure in losses. A good conventional transformer is 92% efficient while toroids can be 98% efficient. Since this efficiency has a lot to do with both the grade of iron used in the core and construction techniques, you may have a transformer that is only ~85% efficient - the rest gets turned into heat. Good practise is to calculate your primary wire size and add 20%. Again, this may bring you to the next larger wire.

Tucking a little thermal fuse into ontop of the primary winding and covering it with tape is a good idea too. An 85C. thermal fuse is common and if the transformer heats up too much, it will open. But for small (<50W) transformers, this is usually unecessary and impractical due to size constrainsts.

Given that there is a more stable line voltage and basically only two of them (120 and 240) worldwide, the number of turns to use will be fair simple. Use the same TPV figure used on the secondary. But what if you have some raw iron and don't know the TPV figure? Here's some rules of thumb - 10W to 25W transformers, use 1000 turns. 25W to 50W, 800 turns. 50W to 100W, 500 turns. 100W to 150W short stack, 400 turns; long stack, 300 turns. I have seen some 100W transformers use 255 turns though (good iron!). The better grade of iron, the lower TPV you'll need. For 50Hz operation, add up to 25% to these figures. I have no experience with units above 150W yet, so you'll have to wait for those figures ;-)

Conclusion

Rewinding or even making from scratch your own transformers isn't as fearsome of a task as it sounds. All it takes is some simple math, common sense and a LOT of patience. It can be fun, is very rewarding and practical.

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