IPC's Current Monitors
IPC's current monitors are wide band current transformers that measure ac or pulse currents with an oscilloscope or other voltage-sensing instrument. By passing the current-carrying conductor or charged particle beam through the monitor opening, the monitor will sense the associated magnetic field and accurately measure the current without requiring a direct connection. IPC's current monitors convert the primary current into a proportional voltage signal, which is then displayed on the scope or voltmeter.

IPC's wide band monitors are available over a wide range of sensitivities, currents, and frequencies: from 0.01mV/amp to 10V/amp, from micro-Amperes to Mega-Amperes, and from 30 milli-Hertz to 100 Mega-Hertz.

Magnetically coupled current monitors can read currents to very low frequencies but are intrinsically incapable of reading DC currents. Consequently, any DC currents will not be displayed on the scope, and repetitive unipolar pulses will appear on both sides of the zero line. Positive pulses will have a negative baseline and vice versa.

External Termination, Cable, and Connectors
A 50Ω coaxial cable, terminated into a 50Ω coaxial feed-through termination at the scope, should connect the monitor to the scope. The sensitivity expressed in output volts per primary amp always assumes a 50Ω termination at the scope. Some other suppliers suggest the use of different terminations depending on the frequency of the applied signal, which results in different output sensitivities. To avoid any possible errors and confusion, all of our standard monitors use 50Ω as termination resistance on the scope. IPC's monitors typically use female BNC connectors with the exception of the M & T models, which use a short section of cable and a male BNC connector. UHF, type N, or other connectors can be used at the customer's request.

Accuracy and Scope Selection
Each IPC monitor is individually calibrated to a maximum output error of +0.5%, -0.0% at one particular frequency, typically 400Hz. For best accuracy, it is important that a precision 50Ω feed-through termination is used at the scope. For a bandwidth extending from a decade above the lower 3 dB limit and a decade below the upper 3 dB limit the response is substantially uniform. At the lower frequency end, the sensitivity decreases steadily and drops off at the rate of approximately 6 dB per octave. In contrast, at the higher frequency end it is quite common that the response oscillates before exceeding the 3 dB limit. IPC monitors incorporate means to make the response less sensitive to the location of the primary conductor within the monitor opening. However, for best high frequency response it is still advisable to center the monitor around the conductor. If a monitor is to be used at one particular frequency, IPC would be willing to calibrate the monitor to that frequency.

For best high frequency response, it is important that the scope or other voltage-sensing instrument has a bandwidth that is at least four times higher than the highest signal frequency.

Electrostatic Shielding and Grounding
All IPC monitors are encased in a conducting shield to minimize electrostatic pickup. For the standard models, one end of the secondary, the case, and the connector are tied to the same potential, and the monitor is grounded at the scope. To avoid ground loops it is advisable to ground the monitor only through the coax cable and provide insulation between the monitor and ground. For high voltage applications where a flash-over between monitor and current-carrying conductor is a possibility, we recommend that the monitor itself be grounded via two 1/4-20 threaded holes in the mounting bracket. In this case an insulating connector should be specified to provide electrical insulation between the monitor winding and ground. For conditions of extreme electrical noise, double-shielding can be supplied.

Temperature and Environmental Considerations
IPC's monitors are not strongly influenced by temperature. For example, a 10°C variation in temperature will change the sensitivity by less than 0.1%. The monitors can be used in vacuum, atmospheric, and compressed air, as well as in insulating liquids.

Additional Information

High Voltage Considerations
IPC's current monitors are well-suited to measuring currents on high voltage conductors. In most instances, it is easiest to use the insulation of the high voltage cable to provide the required electrical hold-off strength. For very high voltages, one can select a monitor with a large center hole and attach conducting toroidal rings on each side of the monitor to eliminate corona emanating from the monitor. For pulse application, immersing the monitor in an insulating fluid is often a desirable option. For best hold-off strength, the conductor diameter should be approximately one third of the hole diameter. For additional service, please call the factory.

Transient Limitation
Our published data provide information on maximum peak current and RMS current. Typically there is a large difference between the two. This suggests that the peak current could be observed only for very short periods of time before overheating would set in. Fortunately for unipolar pulses overheating is not a problem because saturation limits the secondary current before overheating can take place.

For bi-directional signals, such as AC waves, overheating may be a problem. However for frequencies below 60Hz ,saturation will again limit overheating. For higher frequencies, on the other hand, the I_peak/f value of the applied signal can become smaller than the listed AC saturation value. Under this condition, saturation no longer limits the current, and overheating can become a serious problem. In this case, it is important that the listed RMS rating of the monitor is not exceeded. If short-time operation at currents exceeding the RMS rating is necessary, it is important to limit the exposure time so that a critical (I²t)cr product is not exceeded. This critical (I²t)cr value depends on the model selected. For example, for IPC's CM-10-L model, the (I²t)cr value is 2.5 x 10^5 A²s. For an applied current of 1 kA, the maximum on-time would therefore be 2.5 x 10^5/10^6 = 0.25s or 250ms before serious damage could be expected. IPC would be pleased to provide (I²t)cr values for different models if one wishes to use a monitor above its rated RMS current level.

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