I'm trying to create an electromagnet from a microwave oven transformer. The secondary coil will be removed, and the top of the transformer (The I in the E-I transformers) will be sawed off (to increase field strength).

My DC power supplies can only supply low currents (12 V @ 5 A, 20 V @ 2 A) since it's DC, so I thought that AC current might be better. I'm not that familiar with using AC. The problem right now is that I'm unsure whether removing the secondary coil and cutting the top off will cause a short circuit issue.

This is because I had read somewhere that the impedence comes from trying to change the magnetic field of the iron core (in between the cycles). With part of the core removed, would that cause any problems?

To clarify further: The only thing keeping the primary coil from short circuiting is the flactuating magnetic field (trying to change the magnetic field of the iron core). So with part of the core removed (only the I part of EI), would that cause any issues?

Thank you for your time. Any related articles/resources would also be helpful.

Note: I'm using 240 VAC @ 50Hz.

  • \$\begingroup\$ Just curious: where are you getting a dual-frequency supply of 50/60 Hz? \$\endgroup\$
    – Transistor
    Aug 12, 2017 at 15:31
  • \$\begingroup\$ Oops, I was looking at the rating for my power board. It is actually 50Hz in Australia. \$\endgroup\$
    – user159672
    Aug 12, 2017 at 16:34

2 Answers 2


Opening up the core will drastically reduce the inductance of the transformer, which will result in drastically increased current for a given AC input voltage. Applying 240V to a transformer with the core opened up may well result in smoke.

If you must do this get yourself a variac so that you can gradually increase the voltage while measuring the current to ensure it doesn't get excessively high.

  • \$\begingroup\$ may well result in smoke I would be really surprised if it didn't, in any transformer that's not impedance protected - and those are small power. I've destroyed a transformer I was hand-winding once. I was tired and was tweaking the number of turns between powering it up to measure stuff. The last time I powered it up, I forgot to clamp the core on it. Took seconds for the magnet wire insulation to become useless :/ \$\endgroup\$ Jun 19, 2022 at 17:46

Using AC you can be sure that the "electromagnet" will not be a short circuit as it has some pretty high inductance. But would the electromagnet powered by AC have any use for your problem? Maybe you could describe more how you will use the electromagnet.

Feeding the electromagnet with DC certainly means that the only thing limiting the current is the coil resistance. In practice, your power supply will fail to deliver such a high current and you won't get near the expected magnetic field strenghts and even probably damage your power supply.

  • \$\begingroup\$ My aim is just to make an electromagnet as strong as possible for learning. It seems that the only limiting factor (currently) is the capability of the DC power supply to deliver a high current, (as you suggested), which is why I'm looking towards AC. To confirm, even without the iron core, the inductance with AC will still be enough from keeping it short circuiting? And I'm guessing that with part of the iron core removed, the current might be higher due to a weaker magnetic field? \$\endgroup\$
    – user159672
    Aug 12, 2017 at 16:31
  • \$\begingroup\$ Use a 6 volt lantern battery. Experiment. Have fun. \$\endgroup\$ Aug 12, 2017 at 17:40
  • \$\begingroup\$ I am wondering if an alternating magnetic field will be of any use for your experiment. What you said is correct, a winding without an iron core has a lower inductance therefor the impadance is lowered too. \$\endgroup\$ Aug 12, 2017 at 18:41
  • \$\begingroup\$ WARNING - without the top bar (I) of the core, air gap will be huge, inductance will be very low, so AC currents will be destructively high. So DON'T try powering from AC mains. Measure DC resistance, that will help you plan a suitable DC supply ... a car battery would certainly be capable of high currents ... whether too high or not, can't tell without more info. \$\endgroup\$
    – user16324
    Aug 12, 2017 at 20:29
  • \$\begingroup\$ @BrianDrummond Using a battery (drill or car) seems like a good idea, they seem to be able to deliver high DC Current. The primary coil itself is around #14 gauge, and is around 0.8 to 0.9 Ohms. Would AC be safer if I add a 10A breaker? Here's a video for reference, I think he uses 220 VAC (in comments): youtube.com/watch?v=wzXRFp0DDrU \$\endgroup\$
    – user159672
    Aug 13, 2017 at 1:59

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