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I have a short (<1 s) pulse output that will be around 20 kV. Frequency is around 100-200 kHz. How would I make a coil that induces the biggest electromagnetic force (to radiate and mess with the internal circuitry of calculators).

My train of thought: According to Faradays law:

equation

(if you substitute dΦ with B*dA):

equation

if you substitute B with (U0I)/2pi*d, where you bring down 2pid to the denominator and fill in 1.26 *10^-6 for u0 (permeability of free space).

In conclusion this would mean that a bigger turns ratio N and a higher current along a shorter time would mean a higher EMF (logically).

But if you would look at the equation for the impedance you would not want a too high turn ratio because this would increase the inductance and thereby the impedance which would decrease the current.

Is this analysis correct? And what should you do with the inductor/coil to get a maximum EMF?

Schematic of used system:

Schematic of used system

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    \$\begingroup\$ Maximum EMF into what? What are you interfering with? At what frequencies? And, is this a good idea? \$\endgroup\$
    – Tim Williams
    Commented Dec 19, 2022 at 15:27
  • \$\begingroup\$ Why would this matter? Frequency wise it is in the around 100kHz-200kHz. If important, its function is to play with calculators and turn them off. \$\endgroup\$
    – Jelle
    Commented Dec 19, 2022 at 15:42
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    \$\begingroup\$ It matters because devices don't respond to EMF alone; you can put all the voltage you want next to something and, well, you can get a spark through the air and that'll do something, but otherwise it needn't do anything at all. Frequency matters because capacitive and inductive coupling have effect proportional to frequency. And frequency: is 100-200kHz the fundamental, repetition rate, or signal bandwidth? If this is a spark generator, the frequency can extend to 100s of MHz! \$\endgroup\$
    – Tim Williams
    Commented Dec 19, 2022 at 15:51
  • \$\begingroup\$ I am putting a capacitor and inductor at the output of the high voltage generator, so the resonance frequency will be customizable(depending on those values) and between 100 and 200 kHz. The HV generator will create an electromagnetic pulse (emp). This emp will cause a high emf which will be brought to the calculator via an inductor \$\endgroup\$
    – Jelle
    Commented Dec 19, 2022 at 16:15
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    \$\begingroup\$ You use EMF, EMI and EMP almost interchangeably. What's your end goal? \$\endgroup\$
    – winny
    Commented Dec 19, 2022 at 17:38

1 Answer 1

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EMI strong enough to mess with calculators within the same room is also highly illegal for you to produce - you'd be breaking multiple regulations all at once. That's where this pursuit should end.

It's not only about unlicensed transmitted power density, but also about interfering with other users of the spectrum, and potentially exceeding safe human exposure limits to RF.

Never mind the potential for you to get hurt while testing the circuit, operating it without covers in place, etc.

In other words: if you're thinking of doing it anywhere in the world where such laws are enforced, you'll be opening yourself and also your employer up to lots of liability. If this is a school environment with any sort of health and safety oversight, as an employee you might be fired or seriously reprimanded for even seriously suggesting such a stunt. As a student, you might be expelled, and this would be the lightest consequence perhaps.

Good luck if someone manages to bring about a successful civil case that their health/income potential was affected by such a stunt. Is disabling calculators worth getting bankrupted for?

As for demonstrating the effects of EMI legally: any old audio effect or amplifier will probably rectify and output GSM time-division transmit envelope, so a basic GSM/GPRS ("2.5G") phone will do for that. Or a nearby walkie-talkie in CW mode, where the carrier is 100% modulated by Morse code.

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  • \$\begingroup\$ Thanks for the legal advice(and your concern), but your comment is not fully correct. As you mentioned the legality of project depends on the country. This magnetic field that will be induced will be on such small proportions that it will not interfere with other users. Even if, I am using a faraday cage and proceed this project with caution. On top of this, the rf exposure will be for short periods of time and not continuous. \$\endgroup\$
    – Jelle
    Commented Dec 19, 2022 at 21:59
  • \$\begingroup\$ "This magnetic field that will be induced will be on such small proportions that it will not interfere with other users." Do you have any means to measure it, in bandwidth wide enough to be sure you aren't missing anything? And, have you tested how strong of a field does your chosen calculator require? Anything with modern CPUs in it (like graphing calculators) will be immune to operation directly adjacent to a cell phone transmitting at full power - that's up to a couple of Watts peak. I have an old $1 solar cell calculator and it doesn't care much about a 10W UHF walkie-talkie next to it. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Dec 19, 2022 at 22:52
  • \$\begingroup\$ It is certainly very possible that some particular calculator models will be much more sensitive than the average one, but even then it's a bit of a shoot in the dark unless you have an RF lab to experiment. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Dec 19, 2022 at 22:56
  • \$\begingroup\$ Maybe to help picture it better, this has a lot in common with an tesla coil, which to my knowledge is not illegal in my country and a lot of people experiment/play with this. This project is on a way lower power scale than a Tesla coil. As noted further in my response, a faraday cage will be used to severely limit/ block the (small amount) RF. Thank you for your concern again but it is not necessary \$\endgroup\$
    – Jelle
    Commented Dec 19, 2022 at 23:02
  • \$\begingroup\$ Aside from legality according to local and international law, assistance with designing and building a potentially destructive and dangerous device (especially since the stated purpose is to interfere with other electronic devices), it probably goes against site policy. \$\endgroup\$
    – PStechPaul
    Commented Dec 20, 2022 at 19:45

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