FAQ
TL;DR: It’s a 250 µA full-scale DC panel ammeter driven by a rectified EMF probe; “The meter is a small dc panel meter with a FSD of 250uA.” [Elektroda, Cody Miller, post #21659871]
Why it matters: If you know the meter type and rectification, you can simulate, build, and troubleshoot EMF probes with confidence. This FAQ is for hobbyists and students asking how the EMF probe’s meter is chosen, wired, and interpreted.
Quick Facts
- Meter spec: DC panel ammeter, full-scale deflection (FSD) 250 µA. [Elektroda, Cody Miller, post #21659871]
- Supply: single 9 V battery shown in the original schematic. [Elektroda, Mike Burr, post #21659880]
- Sensor: 1 mH inductor used as the magnetic field pickup. [Elektroda, Mike Burr, post #21659880]
- Frequency behavior: usable ~10 Hz–1–2 kHz; noticeable roll‑off near 10 kHz. [Elektroda, Luca Tarpini, post #21659886]
- Rectifier/protection: two diodes pass positive pulses and shunt negatives to protect the meter. [Elektroda, Mike Burr, post #21659869]
What kind of meter does this EMF probe use?
It uses a small DC panel ammeter with a 250 µA full‑scale deflection (FSD). That means 250 µA drives the pointer to the end of the scale. “uA units gave it away.” [Elektroda, Cody Miller, post #21659871]
Is the meter measuring AC or DC?
The probe rectifies the pickup signal with diodes, so the meter sees a DC level proportional to field strength. Negative swings are shunted, preventing the pointer from slamming below zero. [Elektroda, Mike Burr, post #21659869]
How do the two diodes around the meter actually work?
One diode and the capacitor form a rectifier that passes positive pulses to the meter. The second diode provides a return path for negative transitions, protecting the delicate D’Arsonval movement from reverse current. [Elektroda, Mike Burr, post #21659869]
Why do I see tiny pA/fA currents in SPICE?
Your source or coupling may be wrong. Drive the pickup coil via a realistic coupled inductor and measure current after the rectifier. Ensure the meter is modeled as a small resistance path, not an ideal probe. [Elektroda, Mike Burr, post #21659875]
My plot shows current going negative—did I wire it wrong?
You likely measured before the diodes. Measure across the meter, after rectification. The display before the diodes will look bipolar, but the meter side should be unipolar. [Elektroda, Mike Burr, post #21659880]
What frequency range does this circuit respond to?
Builders observed solid response from about 10 Hz up to roughly 1–2 kHz, with reduced amplitude and lower average reading near 10 kHz due to roll‑off. [Elektroda, Luca Tarpini, post #21659886]
I see 500–600 µA in simulation. Will the 250 µA meter saturate?
Yes, at the meter terminals that exceeds FSD and will over‑deflect. Real coils often pick up µV‑level signals, so practical current can be lower than aggressive simulations suggest. [Elektroda, Mike Burr, post #21659903]
What does “roll‑off” mean here?
Roll‑off is gradual attenuation as frequency rises. The probe’s R‑C network and amplifier bandwidth reduce high‑frequency gain, so 10 kHz shows a smaller, more zero‑centered average. [Elektroda, Mike Burr, post #21659892]
Is the transistor stage necessary, or can the op‑amp drive the meter?
One participant notes the op‑amp can drive the 250 µA meter directly. The added transistor isn’t strictly required for meter drive in this design. [Elektroda, DAVID CUTHBERT, post #21659909]
Should I use a single‑supply or dual‑supply op‑amp?
The schematic shows a single 9 V supply. A single‑supply audio op‑amp can behave better here than a dual‑supply part like the LF351 run from a single rail. [Elektroda, Mike Burr, post #21659898]
Why does SPICE force me to add resistors across inductors or use tiny series resistances?
That’s a convergence aid. Adding a small resistor in parallel with the inductor, or a tiny meter resistance, helps the solver. It doesn’t represent ideal behavior; confirm by measuring after the diodes. [Elektroda, Luca Tarpini, post #21659881]
How should I generate the EM field for simulation?
Use a sinusoidal source coupled through a transformer model into the 1 mH pickup. Drive reasonable amplitudes and observe current after the rectifier into the meter model. [Elektroda, Luca Tarpini, post #21659868]
What analysis type should I run?
Use transient (time‑domain) analysis to see rectified meter current rise and settle. Simulators like MultiSim also show oscilloscope traces across the meter versus time. [Elektroda, Mike Burr, post #21659884]
What power source do I need to build it?
A 9 V battery is sufficient for the original schematic. Wire it with correct polarity and decouple the op‑amp supply as recommended. [Elektroda, Mike Burr, post #21659880]
Is there a more stable version of the EMF probe?
Yes. A second version improves gain control and isolates the meter and audio outputs. It was suggested for better stability during testing. [Elektroda, Mike Burr, post #21659901]
Quick 3‑step: how do I wire and read the meter?
- Place the meter in series after the rectifier diode and smoothing capacitor.
- Observe polarity so current drives upscale.
- Calibrate so 250 µA equals full‑scale under a known field. [Elektroda, Mike Burr, post #21659869]
Although I'm thinking I might actually get the EMF meter done this weekend and have a chance to try it out. should be interesting to do. 