Biasing to increase the ∫idt value of the monitor
The presence of the high permeability magnetic core and the finite values of load and wire resistance limit the current-time product (∫idt) a monitor can measure before its output abruptly drops towards zero. This drop occurs when the core material saturates and its permeability approaches the permeability of air. High current-time products can be achieved by increasing the core cross section or by selecting a model with lower sensitivity.

For unipolar pulses, another method exists in biasing. With biasing, a separate current-carrying conductor is inserted into the monitor opening, as shown in the figure below, and a DC current flowing in the opposite direction of the primary current is applied.

Depending on the core material and the selected model, an increase in the current-time product, ∫idt, on bias current is shown in the graph below for IPC's standard L models. This graph shows that for a relatively modest DC current of 200 mA, the current-time product can be increased by more than a factor of 2.

For monitors with larger core diameter, such as the H, B or C models, correspondingly larger currents must be applied to achieve the same improvement.

When applying a bias current it is important that the impedance of the biasing circuit is high enough not to interfere with the measurement of the primary pulse. To achieve this a resistance that is at least as high as the table below indicates must be inserted in the biasing circuit. The biasing current can be increased by looping several biasing turns through the monitor. In this case, the minimum resistance values shown in the table must be multiplied by the square of the number of the bias turns.

For example, for our CM-10-L, which has a sensitivity of 0.1 V/A, a minimum resistance of 66 milliohm must be present in the biasing circuit if one primary turn is used. On the other hand, if 10 primary turns are used, the minimum resistance must be 100 times as high, i.e. 6.6Ω for the above example.

With the minimum resistance in place, the monitor will read 0.5% low. Doubling Rmin would reduce the error by a factor of two, and the monitor would read only 0.25% low.

Another type of biasing may be used by applying the bias current to the secondary of the current transformer. When using this method it is important that the impedance of the bias circuit is much higher than the load impedance of the current transformer. IPC would be happy to assist in specific applications.

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