Based on my search and simulation, I need to generate an AC with 100mA ~ 1A with around 10kHz ~ 100kHz frequency. The higher the frequency and current, the less I need to denoise on the sensing toroid. I don't think the voltage matters a lot here, I think anything around 3V ~ 12V should work.
I wonder what are my options for generating this AC signal?
The two ways I know of right now:
Use a motor driver like L298N to generate a high current (~600mA) square wave (i.e., PWM), then filter it with a low pass RC filter.
Well it depends on the inductance of your toroid! However if you think 12V 1A would work that is less than 12W (I wont do the sums) so the amp above would cope.
Of course the article does not identify the toroid other than the SHAPE of the coil. If it has a core, what material is the core? Some low frequency toroid core material is coiled steel similar to transformer lamination steel. Other core material may be specific mix of iron powder or other semi-magnetic material.
Remember that high current MEANS large wire diameter and may be very difficult to form into a toroid or to wind on a core.
If you attempt to build this device, be prepared to wind a lot of different toroids.
The whole point about a toroidal a core is it doesn’t radiate so how will a second toroidal coil pick anything up? That is why they are used as transformers in a Hi-Fi setup.
That link to the research paper is only the summery and it doesn’t mention a toroidal at all.
Correcting my answer.... Apparently, this relies on eddy currents in a conductive fluid causing magnetic coupling between two toroids.
They must be entirely submerged, including between the two toroids. The liquid acts as a lossy coupling turn, as if you put a single turn of wire with high resistance through both toroids.
Don Lancaster developed a method of PWM to generate sine waves using a special PWM that tends to only generate higher, more easily filtered harmonics. I'd use that to generate the sine wave, using an H bridge and pi filter into the transmit coil.
I need to generate an AC with 100mA ~ 1A ... I don't think the voltage matters a lot here, I think anything around 3V ~ 12V should work.
I didn't mean to express that I will aim for the highest current just immediately. Rather, I will experiment with a bunch of setups to see what is the best tradeoff between energy efficiency, cost, and easiness of denoising.
That audio amplifier looks very powerful indeed.
I just ordered a cheaper, smaller power opamp according to TI's guide https://www.ti.com/product/ALM2402F-Q. I will see how this one works and gradually move to that audio amplifier.
Yes. With higher amperes, I might need to get better quality enameled wire. WIth my current setup, the power is not that different from a cheap DC motor, so I think I'm probably ok for now...
Isn't the H-bridge idea is basically similar to my first method:
*Use a motor driver like L298N to generate a high current (~600mA) square wave (i.e., PWM), then filter it with a low pass RC filter.
I also realized that to achieve the frequency range I want, I probably need a motor driver (basically an H-bridge) with higher switching frequency, such as TB6612.
The "induced current" in the image in fact is the induced magnetic field. That field is concentrated in the core and any magnetic material (iron...) nearby. The water sits in the kind of "air gap" known from certain classic transformers, closing the gap more or less efficiently. Put another core in parallel to the inner one and you can wind on it another receiver coil with strong coupling to the primary core. The outer core can be used for the first tests "on land". Move it gradually away from the primary core to reduce the coupling and inductance to determine the electrical limits of your setup. Note that too strong coupling can lead to saturation of the core, increasing the current in the primary coil only.
For the wave shape I'd try a simple rectangle instead of a sine, eventually shaped mildly by an RC low pass filter. Use a scope to check the primary and secondary wave shape.
I interpreted the image as a cylindric core with two coils on it. Did I fully misunderstand the toroidal approach as measuring the conductivity of the water?
If you look close at that image, that's two toroids instead of coils. The point is that we want to induce current within the liquid to test it's resistance.
If that were two coils, it'd be measuring the magnetic permeablility of the liquid. And I imagine that to be super hard since that will be some very very small constant for any typical liquid such as sea/salt water.
My problem is, every time I see a project like this, I want to build one. I wonder how sensitive it can be made to be, in a home lab. IE, sufficient to measure steam distilled water?
