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

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What kind of meter is used in this EMF probe circuit, and how does the meter section work with AC or DC current?

It uses a small DC panel meter, specifically a 250 uA full-scale ammeter/D'Arsonval movement, not an AC meter [#21659867][#21659871] The two diodes rectify the signal so only positive pulses reach the meter, which makes the movement respond to the average DC component of the waveform rather than negative swings [#21659869][#21659905] One diode also gives the negative transitions a path to ground so the meter arm is protected from slamming back to zero [#21659869] In simulation, the meter reading should be taken after the diodes, because before rectification you will see the AC waveform and it can appear to average near zero [#21659880][#21659884] If you are seeing only tiny pA/fA currents, the thread suggests checking the excitation/coupling setup and the load values, because the original circuit expects a 9 V supply and a very sensitive 250 uA meter movement [#21659867][#21659880]

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#1 21659866 11 Oct 2011 13:56

Luca Tarpini Luca Tarpini

Anonymous

Post #1
21659866 11 Oct 2011 13:56

Hi, I'm a boy writing from Italy: sorry for my bad English!
I have some problems with one schematic that I found on a site; the
circuit is EMF PROBE WITH METER. (
http://www.zen22142.zen.co.uk/Circuits/Testgear/emmeter.htm).
In particular I don't understand the part of the circuit with the
meter: what the meter is? An amperometer? And how it works, with an AC
or DC current?

My big problem is that I have simulated the schematic with PSPICE but
I see on the meter a very small small current: pA or fA! I think isn't
possible but i don't understand where I can do the wrong thing. Any suggestions?
I can reply and describe what I do on PSPICE with more detail.

I very need of your help, I will be very gratefully if you ask me.

Greetings!
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#2 21659867 11 Oct 2011 14:06

Cody Miller Cody Miller

Anonymous

Post #2
21659867 11 Oct 2011 14:06

The site says it is an ammeter with a full scale of 250 uA.

I would suspect it is your source in the simulation. How are you generating your electromagnetic field in SPICE? Or are you just applying a signal in place of the inductor that is used as the field coupling element?
#3 21659868 11 Oct 2011 14:12

Luca Tarpini Luca Tarpini

Anonymous

Post #3
21659868 11 Oct 2011 14:12

I put an ideal sinusoidal voltage source coupling it with the inductor thanks to another inductor; briefly: I have put a transfomer and the sinusoidal source.

A question: what two diodes do? One diode and the capacitor makes the rectifying circuit and the other diode?
#4 21659869 11 Oct 2011 14:20

Mike Burr Mike Burr

Anonymous

Post #4
21659869 11 Oct 2011 14:20

The Diodes are provide only positive pulses to the D'Arsonval ammeter analog meter movement. The second diode, provides a path to ground for any negative transitioning signals to prevent the meter arm from slamming itself back to the peg on the 0. The meter movement doesn't take too much current to cause the arm to fully deflect.
#5 21659870 11 Oct 2011 14:42

Luca Tarpini Luca Tarpini

Anonymous

Post #5
21659870 11 Oct 2011 14:42

@ Cody: where do you read that it is an ammeter and not , for example, a dcmeter? On the page that I have linked I dont' find this information.

@->Mike<-: picoampere currents are very very low!

The source that I use is a sinusoidal source at 10kHz and an amplitude of 10mV. Can it work?
#6 21659871 11 Oct 2011 14:56

Cody Miller Cody Miller

Anonymous

Post #6
21659871 11 Oct 2011 14:56

Hi Luca,

I knew it was an ammeter because the post says "The meter is a small dc panel meter with a FSD of 250uA."

uA units gave it away.
#7 21659872 11 Oct 2011 15:03

Luca Tarpini Luca Tarpini

Anonymous

Post #7
21659872 11 Oct 2011 15:03

If I plot the current on the amperometer I see that it is also negative! It has fast oscillations with mean value=0.
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#8 21659873 11 Oct 2011 15:10

Mike Burr Mike Burr

Anonymous

Post #8
21659873 11 Oct 2011 15:10

Here's a source for a 250uA full scale meter.

http://andersmeters.com/Edgewise_Meter_010_Scale_250uA_--product--63.html

Depending on how you introduced the AC signal(with or without the 1mH inductor). It should give you a sufficient signal. If not then you can work with the gain of the amplifier stage to improve the gain. It should give you a linear growth for the range though with the inductor in place.

Correct, the plotting isn't showing the drop created by changing the +/- signal to just a + signal.
#9 21659874 11 Oct 2011 15:26

Luca Tarpini Luca Tarpini

Anonymous

Post #9
21659874 11 Oct 2011 15:26

So, what te plot is showing??
#10 21659875 11 Oct 2011 15:47

Mike Burr Mike Burr

Anonymous

Post #10
21659875 11 Oct 2011 15:47

It would depend on where you are taking the measurements, and also what you have in place for the meter. I'm simulating it currently and I see about 400uA, @about 500mV. If I measure across the meter then I see the AC signal deviations, but I've got a 1nOhm resistor to simulate the meter resistor.
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#11 21659876 11 Oct 2011 15:59

Luca Tarpini Luca Tarpini

Anonymous

Post #11
21659876 11 Oct 2011 15:59

I'm using the IPROBE component of SPICE. So, is your simulation satisfactory?
How simulated the source in conclusion?

Take a look to my circuit in cattura.png, please
#12 21659877 11 Oct 2011 16:01

Luca Tarpini Luca Tarpini

Anonymous

Post #12
21659877 11 Oct 2011 16:01

EDIT: Satisfyng, not 'satisfactory'
#13 21659878 11 Oct 2011 16:41

Luca Tarpini Luca Tarpini

Anonymous

Post #13
21659878 11 Oct 2011 16:41

No suggestions, mike?
#14 21659879 11 Oct 2011 17:14

Luca Tarpini Luca Tarpini

Anonymous

Post #14
21659879 11 Oct 2011 17:14

Can't you upload a picture of the schematics you used for your simulation, please?
#15 21659880 11 Oct 2011 19:10

Mike Burr Mike Burr

Anonymous

Post #15
21659880 11 Oct 2011 19:10

Sorry, looks like my last post didn't make it. Looking at your schematic, you have the 75 ohm load to simulate a speaker. Usually they are 4-6 ohms. This could be providing some loading of the op-amp output. Second, put the 1mh inductor in place of the transformer and tie your input parallel across the inductor. It also looks like your measuring the output in front of the diodes, however the measurement should be done after, to get the representation. The circuit is identical except for the resistor and transformer. Another question concerns the power supply. The original schematic specified a 9v supply.
#16 21659881 12 Oct 2011 11:32

Luca Tarpini Luca Tarpini

Anonymous

Post #16
21659881 12 Oct 2011 11:32

It doesn't work.
Take a look to my new schematic: PSPICE forced me to add a resistor parallel to the inductor (I choose 0.1Ohm); I put the Resistor to simulate the meter but PSPICE sai to me that simulation failed with 1nOhm, so I must put a value of 10nOhm.

What type of analysis do you take? I do a transient analysis, is iy correct?
And what source do you use: I use a sinusoidal source a 10kHz and an amplitude of 500mA.
#17 21659882 12 Oct 2011 14:32

Mike Burr Mike Burr

Anonymous

Post #17
21659882 12 Oct 2011 14:32

I am using Multi-Sim, so it has a continuous running simulation mode. I'm building the circuit with Qucs, and Gschem and will attempt to see if they operate differently. Physically it is showing the output, but it might be missing it statically.
#18 21659883 13 Oct 2011 11:37

Luca Tarpini Luca Tarpini

Anonymous

Post #18
21659883 13 Oct 2011 11:37

Ok, thanks!

I don't uinderstand one thing: you see your result in domain of the time or of the frequency?
When you plot the results (the current in the meter, for example) what do you have on the X axis?
#19 21659884 13 Oct 2011 11:40

Mike Burr Mike Burr

Anonymous

Post #19
21659884 13 Oct 2011 11:40

I'm using a meter to indicate the current. It's measured over time. I'm also plotting the voltage across the meter on an o-scope inside of the simulation, and it monitors the rise with time also.
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#20 21659885 13 Oct 2011 12:37

Luca Tarpini Luca Tarpini

Anonymous

Post #20
21659885 13 Oct 2011 12:37

Thanks for all, Mike. I think the circuit work good: in the meter from 10Hz to about 1kHz I always find an alternate current with an amplitude of about 500u. At 10kHz the results going worse but I think it's normal.

Are you in the same opinion with me?
#21 21659886 13 Oct 2011 12:43

Luca Tarpini Luca Tarpini

Anonymous

Post #21
21659886 13 Oct 2011 12:43

I reply but there was a problem.

I think that finally the circuit works good: from 10Hz to about 1-2kHz I see in the meter an AC current with an ampliutde of about 500uA. At the frequency of 10kHz the circuit go worse: the current has less amplitude and its mean value is more near to zero (is it normal? look the picture)
#22 21659887 13 Oct 2011 12:44

Luca Tarpini Luca Tarpini

Anonymous

Post #22
21659887 13 Oct 2011 12:44

Thanks for all, Mike. I think the circuit work good: in the meter from 10Hz to about 1kHz I always find an alternate current with an amplitude of about 500u. At 10kHz the results going worse but I think it’s normal.

Are you in the same opinion with me?
#23 21659888 13 Oct 2011 12:59

Luca Tarpini Luca Tarpini

Anonymous

Post #23
21659888 13 Oct 2011 12:59

I reply but there was a problem.
I think that finally the circuit works good: from 10Hz to about 1-2kHz I see in the meter an AC current with an ampliutde of about 500uA. At the frequency of 10kHz the circuit go worse: the current has less amplitude and its mean value is more near to zero (is it normal? look the picture)
#24 21659889 13 Oct 2011 13:17

Mike Burr Mike Burr

Anonymous

Post #24
21659889 13 Oct 2011 13:17

I did notice a bit of roll off when I played with the frequencies. Minor for the most part, but a good starting point. I will have to build it from parts and see what I can accomplish with bench supplies etc.
#25 21659890 13 Oct 2011 13:24

Luca Tarpini Luca Tarpini

Anonymous

Post #25
21659890 13 Oct 2011 13:24

Sorry I don't understand what you mean speaking about 'roll of' and ' Minor for the most part, but a good starting point'.
#26 21659891 13 Oct 2011 13:33

Luca Tarpini Luca Tarpini

Anonymous

Post #26
21659891 13 Oct 2011 13:33

Briefly: why the existence of the roll-of should be a good starting point??
#27 21659892 13 Oct 2011 13:35

Mike Burr Mike Burr

Anonymous

Post #27
21659892 13 Oct 2011 13:35

Roll off is the attenuation of the signal as the frequency goes higher. It's not an immediate drop in output, but a slower drop. Adjusting the capacitors and resistors in the circuit will give different filtering effects at different frequencies.

Because the circuit is a very basic design, there are going to be some limits to it's accuracy. The deviations experienced in modeling may or may not show in actual operation.

In theory you should be able to create filters that will ignore frequencies below mains, or above 10kHz. But it will take a little time working with the circuit to find which ones are causing the majority of the filtering of the higher frequencies.
#28 21659893 13 Oct 2011 14:23

Luca Tarpini Luca Tarpini

Anonymous

Post #28
21659893 13 Oct 2011 14:23

Ok, I imagined this considerations but I've prefer you wrote it first!
#29 21659894 13 Oct 2011 14:28

Mike Burr Mike Burr

Anonymous

Post #29
21659894 13 Oct 2011 14:28

Sometimes I forget to make things easily understandable. Usually happens when I'm working on other projects. 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.
#30 21659895 13 Oct 2011 14:37

Luca Tarpini Luca Tarpini

Anonymous

Post #30
21659895 13 Oct 2011 14:37

Yes, I think so!
I need more time to find the opamp and the BJT, about ten days.
Let me know if you find some interesting tips!
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Topic summary

✨ 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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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]

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