REWIRING A MOT (Microwave Oven Transformer)

Aluminum windings are common. After seeing what you did with the copper strip I had my own idea to do it common kitchen aluminum foil. Several strip in parallel per turn, to achieve the thickness of your copper strip. It would have slightly higher resistance than your copper, assuming it's actually just aluminum and not something else.
 
strantor said:
Aluminum windings are common. After seeing what you did with the copper strip I had my own idea to do it common kitchen aluminum foil. Several strip in parallel per turn, to achieve the thickness of your copper strip. It would have slightly higher resistance than your copper, assuming it's actually just aluminum and not something else.
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Keep in mind that the resistivity of aluminum is about 50% higher than for copper. You would want to go down two wire gauges if you substituted aluminum for copper. Aluminum foils is quite thin, around 1 mil. To get equivalent current capability to Pete's .6mm copper, you would need somewhere around 40 layers of foil.

If it were me, I would use aluminum flashing from the DIY. It is about 8 - 10 mils thick and comes in rolls up to 50' long and in widths of 6" and 12" and is fairly reasonably priced. Actually less expensive than aluminum foil, pound for pound. I would pop it in the oven on high for a half an hour to anneal it. It takes around 650 -700ºF to anneal.

Copper flashing is also available from the same sources.
 
Digital multimeters are notoriously bad at measuring low resistances. Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage. I will apply something like 12 volts to a series string of a precision resistor and the unknown resistance and measure the voltage drop across the known resistor to calculate the current and measure the voltage drop across the unknown resistance. The current is Vprec./Rprec. and the unknown resistance is Vx x Rprec./Vprec. I would use a known resistance of something like 1 ohm so the current is equal to the voltage drop. I have a number of 50 watt 1% resistors which will take that kind of current overload for a short period. Even the low cost multimeters do a fairly good job of measuring millivolts.
 
RJSakowski said:
Digital multimeters are notoriously bad at measuring low resistances. Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage. ... Even the low cost multimeters do a fairly good job of measuring millivolts.
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Yes this is a really good way and I have done it also. DMM measures Ohms with .1Ω resolution, but measures millivolts to .001mV (.000001V) resolution, where with some 3rd grade math you can turn it into a .000001Ω value. If you do it often enough to where keeping up with a separate corded power supply and meter is cumbersome, you can get a cheapo milliohm meter. I have this one and its not the best but it's worth $82. Comes with kelvin clips and all. It's a chinese clone of a $600 meter
 
Petros, that's an absolutely wonderful and innovative solution you've come up with. Congratulations!
RJSakowski said:
Digital multimeters are notoriously bad at measuring low resistances. Without special instruments, the best way to get accurate low resistance measurements is to apply a known current and measure voltage. I will apply something like 12 volts to a series string of a precision resistor and the unknown resistance and measure the voltage drop across the known resistor to calculate the current and measure the voltage drop across the unknown resistance. The current is Vprec./Rprec. and the unknown resistance is Vx x Rprec./Vprec. I would use a known resistance of something like 1 ohm so the current is equal to the voltage drop. I have a number of 50 watt 1% resistors which will take that kind of current overload for a short period. Even the low cost multimeters do a fairly good job of measuring millivolts.
Click to expand...
Rick Sparber (a member of this forum) has done quite a bit of R&D on measuring low resistances and small changes in near-zero resistance, in connection with his design of touch-down probes for machine tools. He's also come up with a relatively simple stand-alone milliohm meter. It's based on a "free" Harbor Freight multimeter and a quad op amp. Here's a link to his article:
http://rick.sparber.org/electronics/ramp.pdf
 
hman said:
Petros, that's an absolutely wonderful and innovative solution you've come up with. Congratulations!

Rick Sparber (a member of this forum) has done quite a bit of R&D on measuring low resistances and small changes in near-zero resistance, in connection with his design of touch-down probes for machine tools. He's also come up with a relatively simple stand-alone milliohm meter. It's based on a "free" Harbor Freight multimeter and a quad op amp. Here's a link to his article:
http://rick.sparber.org/electronics/ramp.pdf
Click to expand...

Thank you for your kind words!
Rick's article is very impressive! It goes to the "to do list".
Petros
 
If you are able to save the secondary you can make a great de-magnetizer out of it.
I use about 24 volts AC on it but it will probably take much more.
Just slip the magnetized object through the center.
 
I just began rewinding a salvaged microwave transformer. Unfortunately, I am looking for 100 volts d.c. and figure that I will need about 80 turns. I am looking to have a a 1 KW transformer so I am winding with 12 AWG wire. Magnet wire is scarce as hen's teeth so I opted for using THHN plenum wire. It has two layers of insulation and a diameter of about 3mm. If I can keep the windings tight, I can get about 90 turns. I am about 10% into the winding.
 
As a rule of thumb, 10 AWG has an approximate resistance of 1 milli-ohm/ft. Resistance doubles for every three wire gauges so 13 AWG will have a resistance of 2 milli-ohms/ft and 7 AWG will have have a resistance of .5 milli-ohms/ft. Current carrying capacity will double when the resistance halves and vice versa.
 
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