My question relates to pulsed induction (PI) metal detection in particular.

It is my understanding that in a DC circuit an inductor acts as a conductor with little to no energy stored in a magnetic field. You will usually see them in DC circuits to help attenuate any high frequency noise components.

With a PI metal inductor, a short pulse of current for a few µs is supplied to a coil of wire. This pulse generates a magnetic field around the coil. The magnetic field around the coil excites eddy currents on the surface of surrounding metals which in turn induces a small magnetic field around these materials also.

When the current supply is removed from the coil (e.g. the end of the pulse) the magnetic field collapses and back-EMF is generated. The length of time it takes for the back-EMF to settle is determined by the amount of surrounding metal, as it will take a short amount of time after the field around the coil collapses for the induced magnetic field around the materials to collapse and these interfere with the EMF present in the coil.

How is a magnetic field generated and maintained during a pulse around an inductor? As far as the inductor is concerned, during that time when the pulse is settled, isn't it effectively just the same as DC?

Here is a little bit more on the subject of PI if needed.

  • \$\begingroup\$ When a sinusoid reaches it's peak and shallows out. is it settled then? You might argue no but you could also argue yes. How is one to know something has actually settled unless you know what's coming next? The inductor certainly doesn't know what is coming up next. The problem is "settled" is vague and subject to human perception of time. Better to just think about it in terms of what the inductor actually cares about: rates of change. \$\endgroup\$ – DKNguyen Oct 1 '20 at 15:54
  • \$\begingroup\$ OK, so I can understand how a magnetic field is generated around an inductor when there IS a rate of change with a pulse, i.e. the rise and fall of the signal. There are a huge number of sine components there, and there is a rate of change. But between the rise and fall, observed with a suitable time resolution, a pulse is just a square wave. Why doesn't the magnetic field collapse between the rise and fall? Or would it but the pulses are so short that it just doesn't have the time to decay without removing the current source? \$\endgroup\$ – ChrisD91 Oct 1 '20 at 15:55
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    \$\begingroup\$ Are you aware that a DC current still produces a magnetic field (think a coil of wire around a nail and connected to a battery)? Any current flow produces a magnetic field, but in order induce a current to flow in something with that magnetic field it needs to be changing. If that changing magnetic field is coming from an electromagnet, then you need to be varying the current through the electromagnet. \$\endgroup\$ – DKNguyen Oct 1 '20 at 16:05
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    \$\begingroup\$ Yes, i think I understand. If you were to wrap wire around a nail in your example and connect it to a battery, you'd have effectively built an electromagnet. \$\endgroup\$ – ChrisD91 Oct 1 '20 at 16:09
  • \$\begingroup\$ In other words, all currents produce magnetic fields, but only changing magnetic fields induce currents. \$\endgroup\$ – DKNguyen Oct 1 '20 at 19:54

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