I just came across this site looking for answers on some questions I have concerning microwave oven transformers (M.O.T.). I am currently going into my third year as an E.E. student and I am trying to design a DC stick welder using M.O.T. I have been researching this topic for a week or so, but haven't found all of the answers that I need. The question I currently have is, if I separated two M.O.T. cores, removed both of the I portions of the E.I. cores, and stacked the two E parts to make an E.E. core, would I need to rewind the primary coil or will it still be usable?

Because welding machines need low voltage-high current, the secondary is going to be rewound with a 4 AWG wire that has a current rating of 95A. Due to the thickness of the wire, it is impossible to get 20 turns in the M.O.T. core as it currently is. The diameter of the wire is 5/16" and will wrap side-by-side in the core twice. As the transformers currently sit it is possible to approximately get 4 layers of wraps for 8-9 total turns on a single E.I. M.O.T. core. To get an amperage low enough for the cable rating, I think I need approximately 20 turns. (At school I rarely go over 95 A using 1/8" rod and probably won't be using more than a 3/32" on my DIY welder. (welding sounded like a cool thing to do with my summer break)). However, if I combine two practically identical E cores from the M.O.T., I calculated that I can fit 22 turns around their cores with them put together.

From what I have put together, this sounds to me like a good idea to reduce saturation (still trying to figure out the physics behind this because our semester was shortened), copper loss, and to allow for the thicker gauge wire to fit in. I thought about using magnet wire, but to get to a 4 gauge there it appears that I would have to use a Litz wire. This would be my first time using magnet wire, but I did try to salvage some from a core that I cut open and twisted 8 16 gauge wires together to get an overall 7 gauge (it was too stiff to use in one core.) A side note on that, molten NaOH (sodium hydroxide) will do work on the enamel that coats the copper wire.

I have enough M.O.T.'s that I can stack two cores and have two transformers to run in parallel on the mains (120v), and series on the secondaries (unless someone wants to talk me into running the mains in series and plugging them up to a 240v outlet). I intend on using a 300A 1600V bridge rectifier to convert to DC then a choke/inductor and maybe 3 6000uF 50V Electrolytic Capacitors that are available to smooth out the ripple. Once I make it to this point I would like to install a power control circuit to control the power input to the primary coil variably limiting the output on the secondaries. Then I feel like a 555 timer would make a nice duty cycle but I've only watched two videos on that and still don't have anything to use it for yet.

Thanks in advance.

  • \$\begingroup\$ MOT stands for Microwave Oven Transformer. \$\endgroup\$
    – Hearth
    Jul 19, 2019 at 0:24
  • \$\begingroup\$ You should just ask a specific question and nor request "any help." Since there seems to be a fairly specific question in the midst of all of that commentary, I answered that and ignored the rest. \$\endgroup\$
    – user80875
    Jul 19, 2019 at 0:40
  • \$\begingroup\$ There are youtube videos doing this with mots - can’t be hard... See m.youtube.com/watch?v=tIlaxy9nUVc \$\endgroup\$
    – Solar Mike
    Jul 19, 2019 at 1:01
  • \$\begingroup\$ If driving from a 555, be very very very precise with the timing so you don't have an effective DC current. This will saturate and overheat your core after running for a little while, you can mitigate this by leaving a small gap when reconnecting your E-E's but at the cost of power output. \$\endgroup\$ Jul 19, 2019 at 1:18
  • \$\begingroup\$ @KentAltobelli dc welding is fine, so perhaps that could be an advantage... \$\endgroup\$
    – Solar Mike
    Jul 19, 2019 at 1:41

2 Answers 2


Stacking the two Es together to make an EE core will work, geometrically, but there are some practical gotchas to be aware of.

MOTs are built cheap to run hot, they are designed to operate well into saturation, drawing a huge magnetising current, to save on materials at the cost of power consumption. Consider the cooling fan to be part of the transformer system. While the original primary winding will work 'as well as it did', it didn't work very well originally. Consider using some of the extra winding space you've created to put on a few extra primary turns, to ease the maximum field down a bit and reduce your core heating.

You're nearly doubling the flux path length, which other things being equal will reduce the primary inductance and so increase the (already high) magnetising current. However the amount the core is saturated will have a bigger effect on primary inductance, so put those extra primary turns on to reduce the saturation.

Any air gap between the assembled Es will dramatically (air length has 1000x times the effect of iron length) reduce the primary inductance and so increase the (already high) magnetising current. Take a lot of care to not distort the Es as you separate them from the Is, sacrifice the Is to avoid damaging the welds that hold the Es together. Spend a lot of time grinding/linishing the mating faces flat (against abrasive paper backed with a sheet of glass) so they fit together well. Note that if they fit well over only (say) 70% of their surface, then unless you're running at only 70% of the saturation field those mating parts are going to saturate. If you're going to run near saturation, you need 100% of the mating surfaces flat. Don't stop linishing until they're flat.

  • 1
    \$\begingroup\$ Original laminations are often (usually) welded along the edges - at least to reduce hum and probably helps minimise airgap effects. || Very peripheral: One open E with 6V SLA on original primary makes a great lifting magnet. Over 100 kg on one I tried recently. Magnetic path through attracted item becomes important. Original uwave flat lamination plate works well. Thinner steel (even up to over 3/16" much less so. \$\endgroup\$
    – Russell McMahon
    Jul 19, 2019 at 8:06

The described transformer seems to have the same cross sectional iron area as the original with twice the original window area and a bit less than twice the original magnetic flux path. The original primary winding should be appropriate. The level of saturation will be unchanged since that is determined by the voltage, number of turns, frequency and iron area.

The original primary winding will have pretty close to the same maximum current. It should run slightly cooler because there is more iron area to dissipate the heat, but the heat losses generated in the copper will be unchanged. Rewinding the primary with the same turns of a larger wire would allow more current. It would be difficult to calculate a new current rating, but a reasonable estimate could probably be calculated.

Tightly joining the two halves and keeping them joined may be difficult.


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