EMF Probe With Meter Circuit: What Type of Meter Is Used and How Does It Work?

The discussion centers on understanding and simulating an EMF probe circuit with a meter, specifically clarifying the type of meter used and its operation. The meter is identified as a small DC panel ammeter with a full scale deflection (FSD) of 250 µA, measuring DC current resulting from rectified AC signals. The circuit uses diodes and a capacitor to rectify the AC signal induced by an electromagnetic field into a DC current suitable for the D'Arsonval analog meter movement. Simulation challenges include observing extremely low currents (picoamperes or femtoamperes) and negative oscillations in the meter current, attributed to simulation setup issues such as source modeling and measurement points. Proper simulation requires placing the inductor as the field coupling element, using a sinusoidal source at appropriate amplitude and frequency, and measuring current after the diode rectification stage. Frequency response shows the circuit performs well from 10 Hz to about 1-2 kHz, with roll-off and reduced meter current amplitude at higher frequencies like 10 kHz, which is expected due to circuit filtering effects. Practical considerations include using a 9 V power supply, selecting suitable op-amps (e.g., LF351 or alternatives like LM386 for single supply operation), and understanding that the meter current should be a DC level after rectification to avoid meter needle oscillation. A second, more stable version of the probe circuit with improved gain control and isolation is also referenced. The discussion emphasizes trial and error in coil design and component selection to optimize sensitivity and meter response.
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