Custom current monitors

Start by defining the expected peak current, pulse duration, repetition rate and 10–90% rise time. Both the peak-current rating and the I·t limit must remain above the application values, because a monitor can satisfy the peak rating yet still saturate when the pulse lasts too long. Choose a usable rise-time specification shorter than the signal rise time, then estimate flat-top deviation by multiplying droop in %/ms by the pulse duration in milliseconds. Check that the selected sensitivity produces a practical voltage without overdriving the output or measuring instrument. Finally, confirm that the conductor, insulation and required clearances fit through the stated aperture.

No, steady-state DC is not reproduced because the output depends on changing magnetic flux in the current-transformer core. A DC component or repetitive unipolar pulse train can progressively bias the core, increasing droop and the low-frequency cut-off while reducing the available I·t capacity. Biasing can sometimes be introduced through additional turns in the aperture or through a coaxial T-adaptor to reset the operating flux point. Individual DC-bias limits are not included in the custom-range data, so they require application-specific confirmation. In practice, selection should account for average current and pulse repetition as well as peak amplitude and pulse width.

The stated sensitivity applies with a high-impedance input, typically around 1 MΩ in parallel with 20 pF. The monitor generally behaves as a voltage source with approximately 50 Ω series impedance, so a 50 Ω instrument termination forms a two-to-one voltage divider. A model rated at 1 V/A will therefore produce approximately 0.5 V/A at the terminated input. Matched termination can improve pulse-edge fidelity when the connecting cable is electrically long compared with the fastest signal component. The reduced sensitivity must be included in current calculations, calibration settings and instrument scaling.

Standard-package models suit installations where the conductor can be passed through a closed aperture during assembly. Clamp-on models separate into two sections, allowing placement around an existing conductor, although their plug-and-socket contacts must be handled carefully when opening and closing the unit. Thin models are intended for locations where axial installation space is restricted, while double-shielded variants can help reduce electric-field coupling. Large-aperture constructions may be suitable for high-voltage conductors when the required insulation and clearances are provided. Selection should consider aperture dimensions, conductor routing, access, shielding, earthing and mechanical support together.

Droop describes the initial downward slope of the output during a constant-current portion of the pulse. As a practical approximation, multiply the specified droop in %/ms by the pulse duration in milliseconds to estimate the output change across the flat top. A model rated at 0.02%/ms would produce approximately 0.02% deviation during a 1 ms pulse, while a high-droop model is intended for much shorter events. Lower droop normally corresponds to a lower low-frequency cut-off, but it can involve trade-offs in rise time, physical size or sensitivity. Model selection should therefore be based on the actual pulse width rather than peak current alone.

Usable rise time is the 10–90% current transition that produces no more than 10% initial overshoot or ringing in the output. The selected monitor should have a stated usable rise time shorter than the rise time of the signal being measured. Across the range, listed values extend from 1.5 to 2,000 ns, while high-frequency −3 dB points extend from 0.2 to 250 MHz. The −3 dB bandwidth alone does not fully describe pulse behaviour, so the time-domain rise specification should also be checked. Faster models may trade low-frequency response, I·t capacity or RMS-current capability for improved edge reproduction.

For sinusoidal signals, check the low and high −3 dB points, maximum RMS current and I/f limit. The signal frequency must remain within the stated bandwidth, while RMS current must remain below the thermal rating. Divide the expected peak current by the lowest operating frequency and compare the result with the model’s I/f value; for example, an I/f rating of 3 A/Hz permits 300 A peak at 100 Hz, subject to the RMS limit. The listed range covers low-frequency points from 0.03 to 100,000 Hz and upper points from 0.2 to 250 MHz. Always evaluate the lowest operating frequency because the I/f demand increases as frequency falls.

The housing is conductive and its painted finish does not provide electrical insulation. The monitor must never be mounted, removed or opened around an energised conductor, and the conductor insulation must withstand the full voltage difference between the circuit and the housing. In single-shielded constructions, the shield is normally connected to the output connector and coaxial braid, which can establish a path to the instrument earth. Double-shielded models provide separate outer-shield earthing options through their mounting arrangement. The complete installation should therefore be checked for insulation, clearance, earthing, cable routing and accessible conductive parts before energisation.