I'm getting different answers watching the MOT (Microwave Oven Transformer) videos on YouTube. Some leave the shunt out, some leave it in, etc...

I have a MOT and want to make a 12VDC power source out of it. I saw one video where a welder uses a transformer and a shunt on a threaded slider to adjust the amperage.

Another has a lead on the winding that is shorter than the main winding and this gives a different voltage.

Q1. Can you have one secondary winding on a transformer and have different voltages from that one winding? I thought this is how 6/12V battery chargers work.

Q2. Do you need a shunt? What happens if you wind a transformer without any shunt and if you need to use a shunt, what difference does different shunts make?

Update: looks like a tap can be put in, so the only question left is about the shunt. Thanks.

  • \$\begingroup\$ Q1: Yes. It’s great to learn, but if you find any normal transformer with the correct primary voltage, I would suggest you use that over a MOT. \$\endgroup\$
    – winny
    Commented Dec 6, 2020 at 10:49

1 Answer 1


Do not use a magnetic shunt on a general purpose transformer. It increases the leakage inductance, which is what you want in a microwave oven power supply, but it will increase the output voltage droop when delivering a current.

If you're going to use a rewound MOT for a general purpose power supply, then put a few extra primary turns on. The MOT is designed to be cheap on materials, and be fan-cooled, so it skimps on core iron running it well into saturation, taking a magnetising current far higher than you would want for a normal power supply.

You can arrange to get x1 and x2 voltages from a single winding if you want, but it's less power efficient than having two output windings. The trick is to use a full bridge for the x1 output voltage, and a doubler for the x2 voltage. This arrangement was quite popular when making direct off-line equipment suitable for 120/240 V operation, but has fallen out of favour with the advent of 'universal' power supplies.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ "The trick is to use a full bridge for the x1 output voltage, and a doubler for the x2 voltage." I understand full bridge but what is a doubler? \$\endgroup\$
    – KarlJay
    Commented Dec 6, 2020 at 9:58
  • \$\begingroup\$ @KarlJay Added to my answer \$\endgroup\$
    – Neil_UK
    Commented Dec 6, 2020 at 13:50

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