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I wound my own coil (air-core) using 22AWG enameled magnet wire. The diameter is 3cm and the length is 1cm. I wound 36 loops which resulted in 3 layers of wire on the spool. I am pulsing current too quickly to use my ammeter so I measured the current indirectly by measuring the resistance and voltage of the coil with an oscilloscope.

I wired a resistor with known value in series with the coil and measured the voltages across the resistor and the coil. I made sure to measure the voltages after the inductive impedance decayed to virtually zero. I calculated a resistance of 0.284ohms, which is close to the calculated value based on the coil dimensions/wire length.

I removed the resistor in series with the coil. The voltage across the coil was 6.22V, giving a current of about 22A. When I use these values to calculate the magnetic field, I get almost 100mT, yet the field I measure is less than 1mT. I purchased a proper magnetic field sensor which I verified works. I used this online calculator to verify as well: https://www.omnicalculator.com/physics/solenoid-magnetic-field

The power supply is comprised of 2x 3.7V lithium ion batteries (7.4V) which are rated for 35A. I have verified that they can indeed supply up to at least 20A, possibly greater. They have been fully charged.

Is there an obvious reason the field is so weak? Where should I begin looking to find the culprit? Any help is appreciated.

Parker Lloyd
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  • Try checking measurement of a magnetic field when supplied with a steady, easier to measure, current. Walk before you run. – Neil_UK Apr 26 '23 at 05:50
  • Unfortunately I cannot. The PCB I made to drive the coil requires 8V. The pulsed current is far greater than the max rated current for 22AWG wire. The coil temperature stays low when the time averaged current is below the maximum. I accidentally melted a previous coil with constant current. – Parker Lloyd Apr 28 '23 at 02:10

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