Since the monitors are transformers they will not be able to measure DC current. However, DC current that is present will influence the droop and low frequency characteristic of the monitors. This DC current can be either a part of the main signal or it can be carried in a separate wire through the monitor opening. The figure below demonstrates the change in the Lower 3 dB frequency due to a DC current for our standard L models. From the figure one can see that approximately a 100-fold increase in the Lower 3 dB frequency can be expected with a 5 Amp DC current. Analagous changes occur for different models. The droop is similarly influenced, but the ratios are somewhat less than shown below. This change in droop and lower frequency characteristics is directly proportional to a change in permeability of the core material. No harm is done to the monitor when operated under these conditions, but the monitor loses usefulness when the DC level is so high that the Lower 3 dB point or the droop becomes too high to be acceptable for a particular application. The DC current does not change the high frequency response of the monitor.
The increase in droop and Lower 3 dB frequency is only present while the DC is applied. For example, if one would apply bias current in the form of unipolar pulses, the effect would only be present during the pulse period and not during the period between pulses, with the exception of a short transition period after pulse termination, typically less than 20 ms.