Biasing

All Pearson Current Monitors use ferromagnetic cores which can become saturated by the dc component (average value) of the current (Idc), or by the current-time product (I•t) of the pulses. Since the output of the monitor is sustained by the changing flux level in the core, magnetic saturation will degrade performance. As a function of increasing Idc, the effective permeability of the core decreases, causing the droop rate and low-frequency cut-off point to increase. Also, the available flux swing is decreased, reducing the maximum viewable I•t. When viewing a pulse, the output voltage will drop to zero when the integrated value of current with respect to time causes the flux level to reach saturation. The monitor will recover after the applied current returns to zero and the flux returns to its remanent value.

Biasing can enable operation with a larger dc component of current and/or improve the maximum viewable current-time product. The objective of biasing is to reset the flux in the core to a value near the negative saturation level so that the maximum flux swing is available.

The I•t value given in the specification sheets is based on a flux swing from zero to saturation for all models. However, models indicated in the specification sheet with ** have high-permeability core material which has a residual induction of 0.6 to 0.8 of the saturation value. These models will need bias to obtain more than 0.2 to 0.4 of the rated I•t. With bias all models can achieve nearly twice the specified I•t, since the flux can be reset close to negative saturation.

For pulses with small I•t, the droop rate will increase as Idc approaches “Idc max” in the Bias Table. Idc max is the approximate level at which the droop rate will be doubled. In this situation, bias is used to cancel Idc and allow normal operation.

Bias current may be applied via an additional primary wire through the current monitor hole and should be of the opposite polarity to the expected signal. The correct value for the bias current is Idc, plus, for ** models, “Ibias” from the Bias Table. The bias current source must have enough resistance to avoid the effect of a shorted turn through the core.

Bias current may also be injected into the secondary winding via a “T” adapter connected to the output connector. The load at the instrument end of the cable should be high enough so that no significant bias current is diverted from the monitor. The source resistance of the bias current source must not load the output of the current monitor. For models with 50 ohm output resistance, a bias source resistance of 5000 ohms will result in a -1% shift in sensitivity. The amount of current to be applied with this method is that which was calculated above divided by “Ratio” from the Bias Table.

In a pure ac signal, Idc is zero and the I•t of the positive and negative parts of the cycle are equal. The “I/f” value from the specification sheet gives the maximum amplitude for a sine wave of frequency f. Biasing cannot improve performance for ac signals.