AC current source ? any idea for soil resistivity purpose

[Overspeed]

Hello

As I want to make some trials of soil resistivity measurement during summer time

I have a square wave inverter circuit with a frequency around 130 Hz ( can be adjusted by software )  with an output voltage of 70 Volt ( I can change the transformer ) but 70V look enough

I need a circuit able to generate a courant source of 10 mA ou 20 mA , I have no knowledge in AC current source , does someone can provide an idea of circuit ?
OR
Perhaps it s possible to simply limit the current at the input of the inverter ? as that a DC current ?

Thanks in advance
OS

[moffy]

The diodes have been chosen for convenience, but this circuit should work, it turns a DC current limit into an AC one, just make sure the current limit can drop the full voltage safely.

[Overspeed]

Hello

Thanks but it s seem that change AC to DC , I want to keep AC current to avoid soil electrical problem .

I link a schematic of the 2 solutions I plan and two possible circuit to control the current at the DC level

let me know if this circuit can work without transformer problem in the inverter

Regards
OS

[tszaboo]

That's a lot of electronics. You can just capacitive couple a PWM signal. It's not going to be sine wave, but who cares? The chemical process doesn't.

[moffy]

Hello

Thanks but it s seem that change AC to DC , I want to keep AC current to avoid soil electrical problem .

Regards
OS

If you look at the trace you have AC in, then current limited AC out, that is the green trace. The four diodes just switch the direction of the current source in sync with the AC input.

[Sensorcat]

If you want to measure the electric properties of soil, there's no reason to have 70V between the electrodes used. A few volts are absolutely sufficient. Perhaps you came to the conclusion that you need 70V because you have a large electrode distance in mind? Better have them only about 1 cm apart, because that way, you can keep the electrodes small, have a moderate, easy to measure impedance, and have full control over the soil sample you are measuring, plus some confidence that this sample is quite homogeneous.

I wrote impedance, not resistance, because it is preferred to measure with pure AC, no DC component, to avoid corrosion and a flow of ions that, on the long run, changes the soil sample. Doing so, you can conveniently model the soil as a parallel resistor and capacitor. By measuring amplitude and phase, both components can be identified. The electrodes can even be covered with an insulator if this is carried out properly.

[Overspeed]

Hello

Thanks but it s seem that change AC to DC , I want to keep AC current to avoid soil electrical problem .

Regards
OS

If you look at the trace you have AC in, then current limited AC out, that is the green trace. The four diodes just switch the direction of the current source in sync with the AC input.

Hello
Ok as that a simple circuit I will test it , no fancy IC

Regards
OS

[Overspeed]

That's a lot of electronics. You can just capacitive couple a PWM signal. It's not going to be sine wave, but who cares? The chemical process doesn't.

Hello

On all the soil resistivity tester I have checked they use AC , some with low voltage but most of them with quite high voltage , I use PWM with a non symmetrical duty for other resistivity tests but for soil I have always read that better and safer to use AC as AC cancel the soil charge

the circuit posted are not complicated and can easy allow to set a current value

Regards
OS

[Overspeed]

If you want to measure the electric properties of soil, there's no reason to have 70V between the electrodes used. A few volts are absolutely sufficient. Perhaps you came to the conclusion that you need 70V because you have a large electrode distance in mind? Better have them only about 1 cm apart, because that way, you can keep the electrodes small, have a moderate, easy to measure impedance, and have full control over the soil sample you are measuring, plus some confidence that this sample is quite homogeneous.

I wrote impedance, not resistance, because it is preferred to measure with pure AC, no DC component, to avoid corrosion and a flow of ions that, on the long run, changes the soil sample. Doing so, you can conveniently model the soil as a parallel resistor and capacitor. By measuring amplitude and phase, both components can be identified. The electrodes can even be covered with an insulator if this is carried out properly.

Hello
Yes that long distance ( 1 meter pitch ) purpose is to locate foundation of a old building in a field , my inverter can modified easily to reduce voltage in case of need .

My first goal is to built a working system '' quick and dirty '' without the need to run complex design and improve it after I have a batch of current sensor with various amp op I can use for free , so just need to control the current

I agree with impedance / resistance as that AC and not DC , i will measure variation in one batch of measurement and not really absolute values as the soil humidity is not a constant .

Last generation equipment allow to measure induced polarisation too .

Regards
OS

[Sensorcat]

... purpose is to locate foundation of a old building in a field , ...

In that case, ground-penetrating radar is what you need.

[Overspeed]

... purpose is to locate foundation of a old building in a field , ...

In that case, ground-penetrating radar is what you need.

Hello
Not really electrical resistivity is used by a lot of companies for various purposes ( archaeology , mineral search . water search ) that a powerful mean  , not spectacular as GPR but in most case interesting and powerful as that easier to go deeper or to cover very large surface

Technologies allow now tomography and induced polarization but also inversion software
https://resipy.org/gallery/nb_02dc-topo.html

That also far more cheaper than a GPR

Regards
OS

[Kleinstein]

For safety reasons I would keep the voltage / current a little lower. E.g. less than 40 V / 5 mA.
One may not need a true current source. A simple bridge ciruit with voltage source and resistor could be enough. The nonlinear response should be easy to correct.

For looking into the depth the soil resistivity makes absolute sense, especially with more electrodes to measure the portential.

One may consider to use more than 130 Hz to get a bit more separation from 50 Hz main to allow easier filtering, though a 50 Hz notch should aready work well the 130 Hz.

It looks like an interesting project.

[Overspeed]

For safety reasons I would keep the voltage / current a little lower. E.g. less than 40 V / 5 mA.
One may not need a true current source. A simple bridge ciruit with voltage source and resistor could be enough. The nonlinear response should be easy to correct.

For looking into the depth the soil resistivity makes absolute sense, especially with more electrodes to measure the portential.

One may consider to use more than 130 Hz to get a bit more separation from 50 Hz main to allow easier filtering, though a 50 Hz notch should aready work well the 130 Hz.

It looks like an interesting project.

Hello
2 pins resistivity use high current but close distance , 4 pins (wires ) soil resistivity use quite high voltage and low current , the rule is no body is in the measuring field and all equipment are battery powered so floating to the earth .

I have a 36V DC source I can link it to my pulsed mosfet generator ( not inverter ) and make some tests of resistance , main problem with soil is the fact that keep the electrical charge so pure DC is not the best to make measurement and more measurement in a chain / string of spike

I link a doc with most common resistivity value

Regards
OS

[moffy]

I designed a moisture meter several years ago and AC was preferred because of battery type EMFs being generated by the electrode in the soil, it also helps to reduce erosion of the electrodes. But I just used an AC coupled 5V square wave and synchronous demodulation for detection.

[Overspeed]

I designed a moisture meter several years ago and AC was preferred because of battery type EMFs being generated by the electrode in the soil, it also helps to reduce erosion of the electrodes. But I just used an AC coupled 5V square wave and synchronous demodulation for detection.

Hello
I agree on electrode erosion saving but do you have a circuit for the synchronous demodulation ? here I use a 4 wire configauration so that will fit after voltage amplifier by a op amp ? or you connect straight on the output voltage ?

Regards
OS

[moffy]

The following reference shows a single op amp synchronous demodulator: https://electronics.stackexchange.com/questions/437940/synchronous-demodulation-with-single-supply-op-amps

[Kleinstein]

If the interfering signal is not very large one can do the synchronous detection in the digital domain. So read the AC signal fast enough with an ADC of sufficient resolution (SD-ADC chips like MCP356x got relatively cheap) and than do the math.