HHS 5204-12

The Helmholtz-Coils are especially designed to generate precisely defined magnetic fields from DC to the upper end of the audio frequency range and beyond. The generated fields are in a strongly linear relation to the coil current. The fieldstrength can be calculated exactly by analytical (or numerical) methods, based on the coils' geometry, the number of turns and the coil current. Therefore, the HHS 5204-36 is ideally suited for the calibration of magnetic field probes or sensors. Due to the high temperature proof Teflon wire packet it is possible to generate magnetic fields up to approx. 3000 A/m for short times. Typical applications are magnetic immunity testing according to automotive standards or MIL STD 461. When generating magnetic fields with Helmholtz coils, the coil current is directly proportional to the magnetic fieldstrength. The calibration of the magnetic field is finally traceable to a current measurement (or to a voltage drop at a known resistor). The Helmholtz Coil itself does not require a calibration. Alternatively, a small loop sensor can be used to determine the actual fieldstrength.

Installation:

The Helmholtz-Coils should be installed on a desk in a sufficiently large seperation from sources of unintentional magnetic fields, e.g. transformers in power supplies, conductors carrying high currents, computer monitors, loudspeakers, cathode ray tubes (CRT) and more All kind of magnetic material (e.g. steel, Nickel, Cobalt) shold be removed from the near surrounding of the coil. The wires which are used to connect the current source with the Helmholtz-Coil should be twisted to avoid an unwanted injection of magnetic flux.

The coil terminals are assigned with the characters A, B, C and D. The generator (current source, audio-amplifier) is connected to the terminals A and C, the terminals B and D are connected with the short cable supplied with the coil.

An additional verification can be done by measuring the magnetic fieldstrength between the coils. Assuming a wrong connection, the fieldstrength decays very sharply in the center between the coils, because the fields compensate each other.

The Helmholtz coils can be operated at three different coil separations. The 'classic' operation mode has a coil spacing of 200 mm, which results in a maximum fieldstrength at medium field homogenity (0.5 dB) and smallest EuT-volume. The mean spacing of 250 mm yields a medium fieldstrength, best field uniformity (0.36 dB) and medium EuT-volume. The largest spacing yields low fieldstrength, a large EuT volume with a slightly reduced field uniformity of 1.24 dB.

The following drawing shows the HHS 5204-12 with the medium coil spacing of 250 mm. The diagram at the right part of the drawing indicates the contributions of each single coil and the resulting sum - fieldstrength caused by a coil current of 1 A.

Fieldstrength Determination:

There are two methods to determine the actual magnetic fieldstrength:

1. Determination of the coil current

• Current transformer clamp
• Measuring the voltage drop across a well-known resistor
• Direct current measurement

2. Determination of the fieldstrength using a field monitoring loop

The direct current measurement has the disadvantage that the measurement equipment itself heats up, which leads to increased measurement uncertainty or even destruction. The use of a calibrated current transformer clamp has two advantages: it is floating (potential isolation between measuring circuitry and amplifier output circuitry) and without thermal stress. In cases where the voltage drop across a known resistor (e.g. 100 mW / 20 W up to 14 A or 10 mW for highest currents) is measured, it is essential to provide sufficient cooling and potential isolation of mains driven voltmeters.

The determination of the magnetic field-strength using a sensor loop (field monitoring loop) allows also potential isolation without temperature stress.

Measurement uncertainties of less than 0.5 dB (typ. < +/- 0.3 dB) are achievable without problems.

HHS 5204-36

Application:

The Helmholtz-Coils are especially designed to generate precisely defined magnetic fields from DC to the upper end of the audio frequency range and beyond. The generated fields are in a strongly linear relation to the coil current. The fieldstrength can be calculated exactly by analytical (or numerical) methods, based on the coils' geometry, the number of turns and the coil current. Therefore, the HHS 5204-36 is ideally suited for the calibration of magnetic field probes or sensors. Due to the high temperature proof Teflon wire packet it is possible to generate magnetic fields up to approx. 3000 A/m for short times. Typical applications are magnetic immunity testing according to automotive standards or MIL STD 461. When generating magnetic fields with Helmholtz coils, the coil current is directly proportional to the magnetic fieldstrength. The calibration of the magnetic field is finally traceable to a current measurement (or to a voltage drop at a known resistor). The Helmholtz Coil itself does not require a calibration. Alternatively, a small loop sensor can be used to determine the actual fieldstrength.

Installation:

The Helmholtz-Coils should be installed on a desk in a sufficiently large separation from sources of unintentional magnetic fields, e.g. transformers in power supplies, conductors carrying high currents, computer monitors, loudspeakers, cathode ray tubes (CRT) and more All kind of magnetic material (e.g. steel, Nickel, Cobalt) should be removed from the near surrounding of the coil. The wires which are used to connect the current source with the Helmholtz-Coil should be twisted to avoid an unwanted injection of magnetic flux.

The coil terminals are assigned with the characters A, B, C and D. The generator (current source, audio-amplifier ) is connected to the terminals A and C, the terminals B and D are connected with the short cable supplied with the coil.

An additional verification can be done by measuring the magnetic fieldstrength between the coils. Assuming a wrong connection, the fieldstrength decays very sharply in the center between the coils, because the fields compensate each other.

The Helmholtz coils can be operated at three different coil separations. The 'classic' operation mode has a coil spacing of 200 mm, which results in a maximum fieldstrength at medium field homogenity (0.5 dB) and smallest EuT-volume. The mean spacing of 250 mm yields a medium fieldstrength, best field uniformity (0.36 dB) and medium EuT-volume. The largest spacing yields low fieldstrength, a large EuT volume with a slightly reduced field uniformity of 1.24 dB.

The  following  drawing   shows  the HHS 5204-36 with the medium coil spacing of 250 mm. The diagram at the right part of the drawing indicates the contributions of each single coil and the resulting sum -fieldstrength caused by a coil current of 1 A.

Fieldstrength Determination:

There are two methods to determine the actual magnetic fieldstrength:

1. Determination of the coil current

• Current transformer clamp
• Measuring the voltage drop across a well-known resistor
• Direct current measurement

2. Determination of the fieldstrength using a field monitoring loop

The direct current measurement has the disadvantage that the measurement equipment itself heats up, which leads to increased measurement uncertainty or even destruction. The use of a calibrated current transformer clamp has two advantages: it is floating (potential isolation between measuring circuitry and amplifier output circuitry) and without thermal stress. In cases where the voltage drop across a known resistor (e.g. 100 mW / 20 W up to 14 A or 10 mW for highest currents) is measured, it is essential to provide sufficient cooling and potential isolation of mains driven voltmeters.

The determination of the magnetic field-strength using a sensor loop (field monitoring loop) allows also potential isolation without temperature stress.

Measurement uncertainties of less than 0.5 dB (typ. < +/- 0.3 dB) are achievable without problems

• Узнайте по тел.: +7 (495) 105 96 88
• Напишите нам письмо: info@micro-electronics.ru