I've put together a toroidal transformer using a core suitable for 1500VA. The primary coil is around 70 turns with the secondary coil being just one. When I plug it into the mains (110VAC), I just get a second of humming and the circuit breaker goes.

I'm guessing I'm missing something obvious but how do I stop the current from building up to the point it overloads. I'd like to keep it at 1kw continuous.

I've watched a number of YouTube videos, can't see what I'm doing differently.

Saturation flux density is 20350 gauss.

  • \$\begingroup\$ You better use a couple MOT’s cause your inductance is too low and you must not make R too high and the ferrite is saturating. It may be possible but not without some complete design parameters \$\endgroup\$ Commented Feb 8, 2019 at 17:46
  • \$\begingroup\$ What's your core's saturation field? \$\endgroup\$
    – Hearth
    Commented Feb 8, 2019 at 17:55
  • \$\begingroup\$ @Hearth assuming saturation flux density, that's 20350 gauss \$\endgroup\$
    – Chris
    Commented Feb 8, 2019 at 18:03
  • 1
    \$\begingroup\$ @Chris And what peak flux density do you calculate? \$\endgroup\$
    – Hearth
    Commented Feb 8, 2019 at 18:05
  • 1
    \$\begingroup\$ @Chris I couldn't say without much more information. I recommend doing the calculations to see if your core is saturating and how you might prevent it. \$\endgroup\$
    – Hearth
    Commented Feb 8, 2019 at 18:17

1 Answer 1


A toroid core intended for AC is rarely equipped with a gap (which would defeat the self-shielding property), so it may remain magnetized for a while after power is disconnected. In the case of a spot welder, with unknown secondary loading, saturation is highly likely to be a problem (very high inrush current) if the AC waveform is presented (the switch closed) at some random time.

Such a core just isn't right for this kind of many-short-bursts activation.


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