Background: I need a transformer that can supply 6.3V (+/-1%, as per the heater's specs) and 25 A for a heater.

As a result, I decided to hack a MOT by breaking off the bottom laminations and removing the coils.

The plan is to use a variac (0-130 VAC, 1 kW max.) that will plug into a US standard single phase outlet (110-120 VAC, 50/60 Hz). The output of the variac will lead to the primary windings of the MOT.

For reference, the MOT core has a depth of 2 inches, a width of 3.5 inches, and a height of 3 inches.

For my application, I rewound the MOT such that the primary is made up of 17 turns of 18 AWG wire insulated for 300 VAC. The secondary is comprised of a single turn of 8 AWG wire. Both the primary and the secondary take up an equal length of the MOT core.

Since the bottom of the core was broken off, it is currently a bit loose but is attached to the rest of the core via gravity.

I used a zip tie to provide a 1 cm gap between the primary and secondary windings.

Here are a few pictures of the set up for reference:

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Question: Will this set up allow for me to produce a sufficient voltage (assuming I set the variac to around 107 VAC) and 25-30 A?

  • \$\begingroup\$ your heater seems remarkably fussy about supply voltage \$\endgroup\$ – Jasen Feb 21 at 4:38

no, that setup will not produce enough voltage (at mains frequency)

at mains frequency microwave oven transformers saturate at about 1 turn per volt, so you want a secondary with more than 6 turns.

Put the original primary back onto your transformer, and add 7 to 10 turns of thick copper wire, or magnet-wire "rope" for the secondary

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  • \$\begingroup\$ Would the secondary wires for a MOT be able to handle the 25 A current? Thank you. \$\endgroup\$ – Joshua Guertler Feb 21 at 4:42
  • \$\begingroup\$ if you twist many strands in parallel to make a rope it would be able to carry 25A, be sure that the ends of each strand are fully soldered \$\endgroup\$ – Jasen Feb 21 at 4:45
  • \$\begingroup\$ Is it acceptable for bare copper wires carrying 6.3 V to be in contact with each other in the secondary? The insulation may be too thick for my MOT. I'm planning to wrap it with electrical tape, but there might be some copper wiring that finds its way to another strand. \$\endgroup\$ – Joshua Guertler Feb 21 at 4:47
  • \$\begingroup\$ bare wirwes must be insulated (must not touch), the wires used in the original secondary have a clear insulating coating on them, this type of wire is called "magnet wire" \$\endgroup\$ – Jasen Feb 21 at 4:48
  • \$\begingroup\$ thats only for contact between turns of the secondary, it's ok to use stranded wire for the secondary so long as each turn of the secondary is insulated from the other turns. \$\endgroup\$ – Jasen Feb 21 at 4:50

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