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I just bought a Duxtop Pro portable induction hob and tested it out with an oversized pan with a little water in it. I expected it to not be totally efficient considering it was hanging over the sides a little bit but what I didn't expect was for the hot spot to be one single ring in the center about 4" in diameter. Not a disk but literally a ring. Is that possibly a defect or is this normal?

EDIT: I uploaded a better picture with a cast iron pan that fits perfectly within the larger ring for max pan size. Since it's black it's easier to see the ring of bubbles where it's heating up. I've written the manufacturer but just figured maybe this is a normal thing experienced people would recognize immediately. Or, conversely, that it's not at all normal.

cast iron pan full of water on induction hob with ring of bubbles

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  • …other than it's too small for the pan? This sound like something you should be asking the manufacturer.
    – Tetsujin
    Aug 7, 2022 at 8:20
  • I did send it to them too but am expecting slow response. I wasn't sure if this fell into the category of something that is completely obviously wrong (or normal!) for people that had seen lots of induction burners in use. I expected a disk of bubbles rather than a ring. Tried using one that fit correctly and it's the same ring effect. Aug 7, 2022 at 18:28

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There is no plausible way I know of for an induction cooktop to accidentally heat only a small area. Given the physics of the thing, it’s all or nothing. What you’re looking at there is a low-quality induction cooktop, from a manufacturer who decided to not make a high-quality induction cooktop.

.....Buuuuut, just to set the right expectations, an iron pan is going to show a cooktop at its worst. Cast iron has low thermal conductivity. It’ll make hotspots their most hot-spotty, compared to an aluminum or copper pan. If you’d put an aluminum-core pan on that burner, you’d see a convex disc of boiling.

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  • You put an aluminum or copper pan on an induction burner and it's going to refuse to start because aluminum or copper is non-magnetic...so you need a fancy induction sandwich base, minimum, to make that an option.
    – Ecnerwal
    Aug 8, 2022 at 20:32
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    @Ecnerwal Aluminum core. See, e.g. All-Clad.
    – Sneftel
    Aug 8, 2022 at 22:28
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Yes, it is absolutely normal. The makers of these cooktops are constrained by two important points:

  • Cost. The smaller the induction coil they use, the cheaper the production
  • Safety. They have to restrict the minimum size of the vessel to completely cover the coil. If they were to use a, say, 20 cm coil, you wouldn't be able to use pans with <20 cm bottom diameter (which is not the same number as the pan size in the store!)

So, if you want to know what size the spot will be, you can read it up in the documentation as the minimum size of pan bottom that is compatible with the cooktop. The last time I looked these things up, 12 cm was pretty much universal.

It is also common to see a ring of bubbles rather than a disc; I am not entirely sure why, but I suspect that it is again technically easier and cheaper to make a ring-shaped coil than a dense spiral-shaped one.

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