Help With EMF Probe with Meter

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!

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?

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?

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.

@ 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?

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.

If I plot the current on the amperometer I see that it is also negative! It has fast oscillations with mean value=0.

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.

So, what te plot is showing??

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.

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

EDIT: Satisfyng, not 'satisfactory'

No suggestions, mike?

Can't you upload a picture of the schematics you used for your simulation, please?

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.

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.

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.

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?

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.

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?

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)

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?

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)

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.

Sorry I don't understand what you mean speaking about 'roll of' and ' Minor for the most part, but a good starting point'.

Briefly: why the existence of the roll-of should be a good starting point??

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.

Ok, I imagined this considerations but I've prefer you wrote it first!

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.

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!