I have a 900 watts transformer from a microwave, this used to have a secondary voltage of around 2000 volts (for the magnetron) but I removed it. I wonder if by using 15 turn secondary that will give 12v (each turn gives 0.8 volt, tested) instead I can create a 75 amp (as 12x75 is 900) transformer and what cable diameter I should use, also whether to use single conductor or multi strand cable. Primary is 285 turn and capable of the 900w aka 3.91 amps it would take to achieve this. I have considered using solid copper bar max 6.5mm squared(insulated) that will fill the space left by the secondary that I removed is that feasible? I plan to run it at max 230v 2.5A(fuse) primary aka 575 watts, and after a bridge rectifier a 600w(12v 15A max input)dc to dc variable voltage and current converter for maximum control in a DIY power supply. Any advise on how I best can do this?

  • \$\begingroup\$ Does a transformer draw more current in the primary than the secondary demand warrants? \$\endgroup\$ – Anders_gbg Apr 7 '16 at 2:52
  • \$\begingroup\$ Transformers are not 100% efficient. Efficiency depends on the design and quality of manufacturing. Commercial power transformers have 3% loss give or take. If you provide your own transformers, the power company gives you that much of a discount since you are eating the transformer loss and not them. Your homebrew transformer may vary. All efficiency losses become heat. \$\endgroup\$ – Harper - Reinstate Monica Apr 7 '16 at 22:43

You should use magnet wire like the wire you removed and like the primary only larger. Larger sizes are available in square wire. The primary will not demand much more current than the turns ratio multiplied by the secondary current.

You should probably remove the magnetic shunt bar(s) from the transformer. The function of the magnetic shunts is to limit the primary current if a short circuit occurs on the output of the transformer. They do something like adding series resistance. They cause the secondary voltage to drop as the load increases. Some MOT project descriptions recommend their removal.

There may also be an extra secondary winding for the filament of the megnetron tube. That can also be removed.

Adding about 12 turns to the primary winding will reduce the extra current in the primary and increase the transformer efficiency.

You should probably use the largest wire that will fit comfortably in the space available. Using larger wire than is necessary will reduce the transformer losses and make it run cooler. Microwave ovens that are designed for home use normally operate for only a few minutes at a time a few times a day. To minimize cost, the transformer is not designed to carry full current continuously. The primary winding will git hot if it carries full current continuously, but it may help reduce the overall temperature if the secondary wire is larger than it would normally need to be to carry the desired current. Another answer suggests parallel connection of smaller wire rather than large wire that may be difficult to wind. That is a good idea, but if you gain extra winding space by removing the magnetic shunts, the winding does not need to be as tight to fit.

If you search the internet for microwave oven transformer (MOT) projects, you will probably find lots of advice. It would be a good idea to look at various YouTube and text descriptions to learn as much as possible.

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  • \$\begingroup\$ Why remove the metal bars between primary and secondary? Are they limiting capacity? Should I replace them with non magnetic isolation? \$\endgroup\$ – Anders_gbg Apr 7 '16 at 11:21
  • \$\begingroup\$ I tested with 3x 2.5mm2(3x 16 amp capacity) 15 turns gave me 13volt ac so I tock out one and got 12v ac. Then I added a bridge rectifier and got 11.2v dc, adding a 4800uf cap raised it to 17 volts. This was done unloaded as I did not have a suitable test load. I noticed on my oscilloscope that the rectifier and cap gave me a weird saw tooth wave form, I wanted stable 12v dc, what did I do wrong? Ps, the mot got very hot(not melting hot but to hot to hold) will different secondary wire reduce heat generation? \$\endgroup\$ – Anders_gbg Apr 7 '16 at 23:37
  • \$\begingroup\$ Is the scope AC coupled? What is the amplitude of the waveform? It sounds like you are looking at the ripple voltage. Do you have some load? How much? \$\endgroup\$ – Charles Cowie Apr 7 '16 at 23:49
  • \$\begingroup\$ I use a usb scope called "hantec 6022be" with a lap top, not the best I know but the best I could afford. I only tried with a 1M ohm resistor, being a bit weary of over loading my test circuit (bred board). Perhaps a 10w 12v light bulb would be safe to try, that should limit the current to less than 1 amp. Can the transformer have received some type of induction heating during my tests, as the cable was not hot but the metal/magnetic core was? \$\endgroup\$ – Anders_gbg Apr 9 '16 at 16:18
  • \$\begingroup\$ There are hysteresis and eddy-current losses in the iron that will heat it. If is heated very much with not much load, that is a sign that the primary voltage is too high or there are not enough primary turns. With 1M ohm load, there should be difficult to see any ripple voltage with 4800uf capacitor. I suspect a problem with the scope setup. 17 volts may be too much for a 12 V bulb even though automotive bulbs can see 15+. Do you have an AC ammeter? \$\endgroup\$ – Charles Cowie Apr 9 '16 at 17:57

Diameter of the round wire for the particular current is (roughly) 0.6*sqrt(current), for 75A it's going to be ~5 mm. Take 5 strands of 1 mm dia wire, should be easier to wind.

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  • \$\begingroup\$ You mean 25 strands. This can get confusing very fast. Think in terms of circular mil, square mm, or circular square mm. \$\endgroup\$ – Harper - Reinstate Monica Apr 7 '16 at 22:44

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