Kamis, 22 Maret 2012

power transistors

First things first. Let's start with T1, the main power transformer, which can be considered the heart of this circuit. I built T1 using a tape winding technique, and stacking four pairs of ferrite E cores to get the necessary magnetic capabilities. The manufacture of this transformer requires some handiwork, but it is worth the effort. I will describe the process in detail, so you can exactly follow the steps I took. If you prefer and have enough knowledge, of course you can go your own route.

Because four cores are stacked, there is no factory-made bobbin available for this transformer. So I made a paper bobbin. I wound the transformer using copper strips interleaved with Mylar sheet, because the thick wire necessary for the heavy current would be impossible to bend around the sharp corners of the bobbin. Instead of using a lot of thin wires in parallel, it is better to take this in a consequent manner and use copper tape. The whole assembly was sealed in epoxy resin and the magnetic cores glued in with epoxy. This is how you can do it:

First cut a piece of hardwood to serve as the winding core. As the center legs of the four stacked cores measure 62 x 12mm, this wood block must be 63mm wide and 12.5mm thick, to allow for some playroom. The length of the block can be around 100mm, or whatever you prefer. The height of the bobbin will be 28mm, so give theblock enough length to hold it with the bobbin in place. If you have a low speed lathe, winding machine, or similar, cut the wood block to such a length that you can mount it in the machine. I used a belt sander to bring the wood block to the exact dimensions. Try to be precise. If the block is too big, you will be wasting valuablewinding space, running the risk of not being able to fit the windings. And if the block comes out too small, your finished winding assembly may not fit the ferrite cores, making it unusable.

Now wrap the wood block in one layer of plastic film, of the kind used in the kitchen to preserve food. This material is an excellent demoulding agent. Cut a strip of strong packing paper, 28mm wide and about 1m long. Mix some 5-minute epoxy glue (I used the type sold in airplane modelling shops, which comes in good sized bottles), and apply a layer of epoxy to the paper strip. Now wind the strip very tightly around the plastic-wrapped wood block, to make the bobbin core. It will be about 6 layers of paper. Wrap another sheet of plastic film around your work, and press it between two wooden blocks hold together with rubber bands, so the long sides of the bobbin become flat and nice. Now get permission from your wife, mother, or whoever reigns in the kitchen, and place the assembly in the oven for about 15 minutes at 50øC. The epoxy sets much quicker and somewhat stronger at that temperature.

Now you will need some copper sheet 0.1mm thick, and some Mylar sheet of a similar thickness. Cut the copper in strips 22mm wide, and the Mylar in strips 28mm wide. If you can make long strips, say 2m, this is an advantage, otherwise you will have to solder individual copper strips together. In total, you will need about 7m of copper tape and slightly less Mylar tape.

When you are ready with this, your epoxy has had ample time to harden, so rescue your bobbin from the oven and go on. Take off the rubber bands, the outer wood blocks, and the outer plastic wrapping (don't worry if it doesn't come off completely). Do not remove the plastic wrapping that separates the bobbin from the wood. You now have your wrapped wooden core and the epoxy-paper bobbin on it.

Take a 60mm piece of #13 bare copper wire. Wrap the end of one of your copper strips around the wire, so that the wire protrudes only to one side from the copper sheet loop. Use a big soldering iron to flow some solder into the junction. Try to avoid getting solder on the outside, because this may later puncture through the Mylar insulation.

Now the winding starts. Position the copper wire on one narrow side of the bobbin, so that the copper strip is centered on the width of the bobbin, leaving 3mm room on each side. Stick the strip start to the bobbin with some thin adhesive tape. Position the start of one piece of Mylar strip so that it covers all the copper and is centered on the bobbin, and tape it in place. Now wind 15 turns of this copper-Mylar sandwich, as tightly as you can, keeping the Mylar aligned with the bobbin sides, and the copper nicely centered. Don't loose your grip, or the whole thing will spring apart. If your copper strip is not long enough, fix everything with rubber bands or a clamp, and solder another copper strip to the end of the short one, allowing 2mm of overlap. Before doing this, cut the first copper sheet to a length such that the joint will be on one of the narrow sides of the bobbin, because here you have space, while the wide sides will have to fit inside the ferrite core's window. If the Mylar strip runs out, just use adhesive tape to add another strip. Make the overlap 5mm, to avoid risk of creepage between the sheets, and also try to locate the joint on a narrow side of the bobbin.

When the 15 turns are complete, cut the copper strip to such a length that the second terminal will be on the same narrow side of the bobbin as the first terminal (the one you have already placed). Solder the second terminal (another 60mm piece of bare copper wire) to the strip, position it, and wind three or four layers of Mylar, to make a safe insulation between the primary and secondary.

If you think this is a messy business, you are right. But it's fun too! The secondary is just a little bit messier: It is wound with a five-layer sandwich! Four layers of copper and the Mylar topping layer. But it's only four turns total, so take a deep breath and do it!

First solder the four copper strips together around a piece of #13 copper wire. Don't be overly worried if the outcome is not very clean; mine was quite a mess too, and it worked well on the first try. Just be sure you don't create sharp edges or pointed solder mounds, because these may damage the insulation.

Now position the start of your secondary conductor in such a way that the pin will come out to the same side as those of the primary, but on the other narrow side of the coil assembly. So you will get a transformer that has its primary leads on one extreme and the secondary on the other, and will fit the printed circuit board nicely.

Wind two turns, solder the center tap wire between the four copper strips, wind the other two turns, solder the last pin, wind a finishing layer of Mylar and fix it in place with adhesive tape. Uff! This was the worst part. If you reached this spot, you will have no problem building the rest of this project!

What you have now is a springy, messy coil assembly that will fall apart if you let it go. You have to seal it. This is easy to do:

Wrap your two wooden blocks, the same you used to press the bobbin, in plastic film. Place them against the sides of the coil assembly, and apply hard pressure, using a clamp or a lot of rubber bands, so that the long sides of the coil straighten out completely, and any slack is displaced to the narrow sides. Now mix a fair quantity of epoxy glue, place the coil assembly so that the pins face up, and let the epoxy run into the coil. Continue supplying epoxy until it starts to set. If it drips out from the other side, no problem. Just don't do this work over your uncle's persian rug. When the epoxy doesn't flow any longer, turn over the coil assembly, mix a new batch of epoxy, and fill the other side completely, forming a smooth surface. As the downside is now sealed, the epoxy will not flow out there. And when this epoxy has set, turn the assembly over again, mix epoxy, and apply it to form a smooth surface there. The idea is to replace all the air between copper and Mylar sheets by epoxy, and specially to fill the room left by the copper strip, which is narrower than the Mylar. This filling is necessary both for mechanical and for electric safety reasons.

Now convince your kitchen's monarch that this devilish thing will add a nice scent to the next apple pie, and place it in the oven again. Let the epoxy harden completely, then remove the coil from the oven, remove the clamp, rubber bands, wooden blocks, wooden core and all remains of plastic film.. And now comes the big WOW!!! You will be surprised how your messy and springy assembly changed into a very robust, hard, strong and nice coil!

Now test-fit the ferrite cores. See if they can be installed easily, so that each pair of facing E cores gets in intimate contact without pressing on the winding. If everything is right, the winding should have some playroom in the assembled core. But it is easy to get too much epoxy on the coil. If this happened to you, just take a file and work the epoxy down so that it doesn't disturb the ferrite. The ferrite core MUST close properly, otherwise you will later burn out the power transistors

When the sizes fit, prepare some epoxy (again...!), apply a very thin layer to all contact faces of the ferrite cores and mount them onto the coil assembly. You can hold them in place with adhesive tape until the epoxy sets. If you dare interrupting in the kitchen for a third time, use the oven to quick up the hardening! The last thing you have to do is bending the copper wires into the proper shape to fit the printed circuit board holes. Be sure that on the secondary winding the center tap is actually in the center position! The polarity of the other pins doesn't matter. This completes
the manufacture of T1.

All the other transformers and coils are just child's play after making T1. The current sense transformer T2 has a lot of turns, but there is absolutely no need to wind them nicely side-by-side. You can use a winding machine with turns counter, or you can just wind it by hand. Get some #36 or other thin enameled wire, solder the end of it to one of the extreme pins of the EE24-25-B bobbin, and wind 100 turns. Don't worry if your winding is criss-cross and ugly, and don't feel guilty if you loose count and wind a few turns more or less. As long as you don't overdo it, it will just affect the position of VR1 when you align the completed power supply later. Solder the wire tothe center pin of the same side, then wind another 100 turns in the same sense. Solder to the other extreme pin on the same bobbin side, and apply one or two layers of Mylar, just to protect the thin wire.

Now take a piece of #15 plastic insulated cable, wind one single turn over the Mylar and solder the two ends of the cable to the two extreme pins of the other side of the bobbin. It doesn't matter which end goes to which side. Install the EA77-250 core with a small amount of epoxy cement, and T2 is ready.

T3 is made using the same type of bobbin and core as T2. First you wind 26 turns of #27 enameled wire. The 26 turns fit nicely in a single layer. Wind a one layer of Mylar sheet, then put on the next 26 turns. Bring the wires out to one side of the bobbin in such a way that they will not be too close to the other windings. Wind 3 layers of Mylar tape, to give a safe insulation between primary and secondary. Now, wind 8 turns of #20 wire, and solder the ends to the bobbin pins. Look at the printed circuit board drawing to understand which wire to solder to which pin. Wind a single layer of Mylar, then make the other 8-turn winding over the first one. This will leave a space at one side of the bobbin which is big enough to take the single turn of #15 plastic insulated cable, which completes the assembly. Now glue the core in place with epoxy cement, and T3 is ready!

L2 is wound on an Amidon T-157-26 iron powder toroid core. As it is too difficult to bend thick wire through a toroid, and tape winding it is not practical either, I chose to make this coil with 10 pieces of #16 enameled wire in parallel. Cut the wires to about 1.5m length, and twist them together. Then insert the bundle into the core, and starting from the middle of the wire bundle, wind 7 turns, using half of the core's circumference. Now wind the other 7 turns, starting from the middle towards the other end of the wire bundle. The 15th turn is the one you made when you inserted the wire bundle into the core! You will not be able to make a beautiful, nice winding, as the total of 150 wires passing through the toroid is too much to fit them in a single layer. But this doesn't matter at all, as long as you get 15 loops of the wire bundle through the toroid's hole, and there is plenty of space for this.

To make L3 you must first get a suitable core. I used a fraction of an old ferrite antenna rod, which broke into several pieces when I let it fall down... So, this is the recipe: Take an old ferrite antenna rod of about 10mm diameter, throw it out of the window, go pick up the pieces and select one that is about 50mm long! If you live in a 40th floor, better don't use the window method. Instead, break the rod in some more controlled way. You can't wind L3 on a bag of ferrite dust!
The winding itself is easy enough: Just wind 10 bifiliar turns of #11 enameled wire. This wire is quite stiff, but it is still no problem to handle. I suggest you wind the coil on a 10mm drill bit, then spring it open and place it on the ferrite core. Otherwise the ferrite could be cracked. Fix the core in the winding with some epoxy. Bend the wires in such a way that all four of them look down with the core straight up. That's the position L3 is mounted on the PCB.

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