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I'm thinking of getting an auto transformer and rectify it to make mains isolated 240V 8A DC power supply. I would like to know if its possible to rewind an old microwave transformer to 1:1 isolating transformer with maximum output of 2kW. I know that the waveform wouldn't be smooth so i'll be using beefy 450V 2000uF capacitor for rectification.

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    \$\begingroup\$ Did the microwave transformer come from a 2000 watt oven? \$\endgroup\$ – Andy aka Jan 26 at 10:26
  • \$\begingroup\$ Yeah the MOT is capable of 2kw \$\endgroup\$ – Teslacoiler11 Jan 27 at 0:52
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The following is largely complementary to Dave Tweed's answer

  • The welds are generally along the surface and can be removed with an angle grinder "fairly easily". The laminations can be rebuilt using a clamp to hold the core together. Epoxy helps prevent lamination chatter but DO NOT introduce any additional air gap (or "epoxy gap") between laminations.

  • The magnetic shunt must be removed if the transformer is to have anything like normal regulation. Otherwise the transformer has an intentionally designd "droop" in its load characteristic.

  • Microwave ovens tend to run the iron rather hard - well up the saturation curve.
    Adding a relatively small number of extra turns greatly reduces the magnetising current.

  • It may not be true in all cases but in at least some transformers the mains primary on its bobbin comes off easily as a complete unit. If the secondary is then removed - destructively or otherwise the available space left from the secondary & shunt would allow two identical primaries to be added.


To get 2 kW you'll need two typically sized transformers - or primaries from 4 identical ones installed on two cores. These can then be connected with primaries in parallel and secondaries in parallel.

If you add extra turns to the primaries and not the secondaries you'll get a degree of stepdown which will reduce the DC level that youd otherwise get. A sinewave will peak rectify to 1.414 the RMS ac value or in this case 240 x 1.414 =~ 340 V. Under load this will be somewhat lower and mean DC depends on the degree of filtering.

With 2000 uF filtering at 8A you'd probably get 10-20V of ripple. V drop across a hald cycle at 8A =
Vdrop = t x i / c = 0.01s x 8A / 0.002F
= 40V.
That assumes instantaneous peak charging of the capacitor at the start of each half cycle - which would not be the case. But it gives you a feel for the order of ripple voltage involved.

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  • \$\begingroup\$ Ok so running it at higher current significantly increases the ripple voltage. I have two 450V 2000uF capacitors so connecting both in parallel should decrease the ripple voltage by half. I would only use this power supply under 100V DC so it shouldn't really be a problem. \$\endgroup\$ – Teslacoiler11 Jan 27 at 1:06
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Possible, but not easy to rewind, because microwave oven transformers are welded shut.

a 1KVA transformer can be used as an autotransformer to step 120V 16A up to 240V 8A.

but then you say "isolated" - an autotransformer won't make isolated power.

and you say "240V DC" , you don't make 240V DC from 240V AC.

So, the answer is no. Parts of your plan need further refinement.

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  • \$\begingroup\$ The welds are generally along the surface and can be removed with an angle grinder "fairly easily". | The magnetic shunt must be removed if the transformer is to have anything like normal regulation. | They tend to run the iron rather hard - adding a relatively small number of extra turns greatly reduces the magnetising current. | It may not be true in all cases but in at least some cases the mains primary comes off easily as a complete unit. If the secondary is removed - destructively or otherwise the available space would allow two identical primaries to be added. \$\endgroup\$ – Russell McMahon Jan 26 at 12:59
  • \$\begingroup\$ 240 volts and 8 amps is 1.92 kVA almost double what you recommend. \$\endgroup\$ – Andy aka Jan 26 at 13:02
  • \$\begingroup\$ that's why a 1KVA autotransformer would work to add the extra 120V \$\endgroup\$ – Jasen Jan 27 at 7:45

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