This master class will be a little controversial and will cause more than one disparate opinion. I want to share how to make a powerful rectifier from a microwave oven transformer - a power supply for the voltage I need.
Very often, microwaves break down and are thrown into the trash. I recently had another one break and I decided to give a second life to its transformer.
The transformer there is a step-up transformer and usually converts 220 V into the high voltage of 2000-2500 V necessary to excite the magnetron.
I have seen many people convert these transformers to either a resistance welder or an arc welder. But I have never seen powerful power supplies made from it.
After all, the transformer is very powerful, about 900 W, and this is not small. In general, I will show you how to rewind the transformer to the voltage you need.
Disassembling a transformer from a microwave oven
Typically a microwave transformer contains three windings.The most numerous, wound with the thinnest wire, is the boost, secondary, whose output is 2000-2500 V. We don’t need it, we will remove it. The second winding, thicker, with less wire compared to the secondary, is a 220 V network winding. Also, between these two massive windings, there is the smallest one, which consists of several turns of wire. This is a low-voltage winding of approximately 6-15 V, which supplies voltage to the magnetron filament.
Cutting off the seams of the magnetic circuit
It is necessary to cut down the seams that hold the “W”-shaped and “I”-shaped plates together. The seams of the Chinese manufacturer are not as strong as they seem. You can cut them down with a grinder or even split them with a chisel and hammer. I used an angle grinder, it is a humane method.
Removing the coils
We remove all the coils. If they are very firmly seated, tap them gently with a rubber hammer. We only need the 220 V winding; we remove the rest. We put back the primary winding at 220 V and place it down the “W”-shaped core.
Secondary winding calculation
Now we need to calculate the number of turns of the secondary winding. To do this you need to find out the transformation coefficient. Typically, in such transformers it is equal to one, therefore one turn of wire will produce one volt. But this is not always the case and you need to double-check it.
We take any wire and wind 10 turns of wire onto the core. Then we assemble the core and clamp it with a clamp so that it does not fall apart. Be sure to supply 220 V to the primary winding through a fuse. And at this time we measure the voltage at the output of the 10-turn winding. In theory it should be 10 V.If not, then the transformation ratio is not the same as usual and you need to do calculations to calculate the voltage for your winding. All this is not difficult, fifth grade mathematics.
I have two transformers available. I will make one for 500 V, the other for 36 V. You can make it for any other voltage.
Winding a 500 V transformer coil
The transformation ratio of my copy is one to one. And in order to wind a 500 V winding, I need to make 500 turns of wire on the coil accordingly. We take the wire.
Of course not like that, but wound on a drum. We estimate the current strength and the volume of the coil. From these values we select the wire diameter.
This is a simple device I put together for winding a coil. The core itself is made of wood, the sides are made of plexiglass. You can attach it to a drill or screwdriver.
I wound it, assembled it, connected it. I measure the output voltage, I almost got it - 513 V, which is acceptable for me.
36V transformer
The 36 V winding can also be wound manually by taking the appropriate wire. To dress and straighten the winding on the core, you can use such wedges, see photo.
After the winding is completely stretched, place tightly compressed paper into the holes formed after removing the wedges. This is my primitive way. Then I recommend impregnating the winding with epoxy, otherwise it will make a lot of noise.
Work on mistakes
I rewound the winding to make it tighter and more powerful. To do this, I wound it with double wire, instead of one thick one. I'll connect them at the end.
Once all the windings are secured, it's time to assemble the transformer core. To do this, we secure the entire structure with a clamp and arc weld the same places as before.There is no need to make a thick seam, everything should look as it was.
Next, for my rectifier I will need:
I will load the rectifier at 20 A, naturally the diode bridge needs to be installed on the radiator.
Also, if you use a metal case like me, then do not forget to ground it.
About security
Be careful when connecting the transformer, never rush and double check everything. Connect the transformer only through a fuse to avoid possible short circuit. Do not touch live parts while the transformer is operating.
Also, when processing metal, be sure to be careful and use eye protection.
Remember that you do all actions at your own peril and risk!
All the best!
Original article in English
