Boltek EFM-100, Installation And Operator'S Manual

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C H A P T E R 1 - I N T R O D U C T I O N
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The EFM-100’s electric field mill senses electric field by repeated exposing and shielding a
series of sense electrodes.
An electric field mill uses a mechanical chopper to alternately shield and expose several
sense plates to an electric field. When the sense plates are exposed to the electric field an
electric charge is drawn from ground to the plates through a sense resistor. When the
sense plates are shielded from the field the charge flows back to ground, again through the
sense resistor. This moving charge is an electric current which is measured as an AC
voltage across the sense resistor. The size of the voltage is proportional to the size of the
electric field applied to the plates.
Figure 3: Field Mill Block Diagram
Charge flowing onto and off of the sense electrodes will develop a voltage across the sense
resistor. This voltage is amplified and fed into an analog switch along with an out of phase
version of the signal.