Overview
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 field strength 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 5206-8 is ideally suited for the calibration of magnetic field probes or sensors. Due to the high temperature proof Teflon-coated wire packet it is possible to generate magnetic fields up to approx. 1000 A/m (for a short time). Typical applications are magnetic immunity testing according to automotive standards or MIL-461F. When generating magnetic fields with Helmholtz coils the coil current is directly proportional to the magnetic field strength.
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 is usually considered as primary standard due to the easily calculable relation between current and field strength. If this relation should be controlled, a loop sensor or monitoring loop can be used to determine the actual field strength.
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 so on. All kinds 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.
Typical Test Setup for Immunity against magnetic fields
| Specifications | |
| Number of turns (per Coil) | 8 |
| Maximum Coil Current | 55 A, 5 min. |
| Nominal Coil Current | 34 A continuous |
| Coil Spacings | 300 mm |
| Maximum magnetic field strength | 1060 A/m, 5 min. |
| Nominal magnetic field strength | 650 A/m continuous |
| Magnetic field strength @ 1 A Coil Current | 19.23 A/m 145.68 dBµA/m |
| Current required for 1 A/m | 52 mA |
| Conversion Current Fieldstrength (Coil factor) | 25.7 dB/m |
| Coil diameter | 588 mm |
| Mechanical dimensions | 0.64m x 0.79m x 0.42m |
| Max. cubical shaped DuT | 32.5 x 32.5 x 32.5 cm |
| Terminals | 4 mm banana jacks Screw terminals |
| Usable frequency range | DC - 800 kHz |
| Inductance (per coil) | 91 µH |
| nductance (pair of coils) | 200 µH |
| Resistance (per coil) | 28 m? |
| Coil capacitance (pair of coils) | 49 pF |
| Resonance frequency (Pair of coils) | > 1500 kHz |
| Weight | 17 kg |
