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how to measure AC current

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aliyesami

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i need to measure resistivity between two points using AC current , what kind of circuit I would need for this ?

thanks
 

Different solution, depends on the complexity and price. For example, you take some shunt resistor and sample the voltage with an adc, and use a microcontroller to calculate the current. Please provide more informations, you need an analog or digital signal?
 

johnjoe said:
Different solution, depends on the complexity and price. For example, you take some shunt resistor and sample the voltage with an adc, and use a microcontroller to calculate the current. Please provide more informations, you need an analog or digital signal?
Click to expand...
I prefer analog signal as I have a microcontroller where I can read analog values.
I need it to measure the resistivity between 30-36 ppt of salt water.
 

There are two possible ways to measure ac resistance (conductance),

Apply an ac voltage across a pair of electrodes and measure the current, or feed an ac current through the probes and measure the voltage across the probe.
Either method should work, its really a matter of personal choice.
 

Hi,

What voltage, current and frequency ranges?
What waveform?

Klaus
 

aliyesami said:
i need to measure resistivity between two points using AC current , what kind of circuit I would need for this ?

thanks
Click to expand...
Poking around the internet I just found a very simple ac conductivity circuit using a cmos 555 that outputs changes in frequency with changes in conductance, ideal for driving a micro !
https://emesystems.com/OL2mhos.htm

There is probably a lot of other really good stuff out there with a bit of searching...
 
voltage can be anywhere from 3v to 12v max , any frequency and any waveform will do .

- - - Updated - - -

Warpspeed said:
Poking around the internet I just found a very simple ac conductivity circuit using a cmos 555 that outputs changes in frequency with changes in conductance, ideal for driving a micro !
https://emesystems.com/OL2mhos.htm

There is probably a lot of other really good stuff out there with a bit of searching...
Click to expand...

this solution looks good, but what changes i have to do to be able to measure salt water ?
also I am not understand the role of the regulator there
 

Warpspeed said:
Poking around the internet I just found a very simple ac conductivity circuit using a cmos 555 that outputs changes in frequency with changes in conductance, ideal for driving a micro !
https://emesystems.com/OL2mhos.htm
Click to expand...
The 555 has DC pulses as its output. The DC causes electrolysis of the probes which corrodes them. Symmetrical AC should be used as it being used in this thread.
 

aliyesami said:
This solution looks good, but what changes i have to do to be able to measure salt water ?
also I am not understand the role of the regulator there
Click to expand...
You will only need to develop a probe with appropriate exposed area in contact with the water to place the conductivity within a reasonable range for the circuit to operate.
If its almost pure clean water, a larger probe surface area would be required than if measuring really heavy salty brine. This will need to be done experimentally.

The voltage regulator ensures that the 555 chip receives a constant operating voltage as the battery ages.

Mr Audioguru,
The circuit is a simple oscillator in which the timing capacitor charges and discharges through the impedance of the water between the electrodes.
First one way, then the other, so its an alternating current.
The 555 changes state at 1/3 and 2/3 the supply voltage so the output waveform will always be be an equal 50% 50% duty cycle.

There is no residual dc, as the article explains, the very large 2.2uF coupling capacitors ensure that the current flowing between the probes will be symmetrical and alternating in either direction.

Its all extremely clever and SIMPLE !
 

i have done enough mapping and calibration for my PH probe so thats not a problem as long as I can get a range of voltages out of the conductivity probe i can translate it to anything.
I will be using these commercial probes ,what is K10 and K1 iam seeing with these probes? someone told me its the "cell size" and effects the current reading of the probe, can you tell me exactly how will it effect me in interfacing this to any circuit ?

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please take a look at the circuit, did i correctly remove the part since I will be using my own 3.3v power supply from the board ?
also i am not understanding where do i get the output from this circuit to interface to the micro ?
cond_circuit.jpg
 

Pin 7 of the 555 chip called "disch" is an open drain output that pulls low each half cycle.
You will need a pull up resistor from pin 7 to 3.3v, and that will then produce a 3.3v digital signal for your microcontroller.
 
the conductivity of a liquid is notional the conductance from the opposite faces of a 1cm cube, this is a K1 probe. If the faces are 1/10 cm, 1mm apart then the probe gives a ten times higher output, i.e. K10.
As tap water is about 1.5K (600 micro mhos) then saline would be a great deal lower, say 10-> 100 ohms, depending on how salty it is. Just a rough figure to do your design around.

Frank
 
Last edited:
30 - 36 ppt salinity has been mentioned above in the thread, referring to a very low conductivity around 0.05 µS/cm, respectively 20 MOhm with K1 cell. The conductivity is below the range of pure water in equilibrium with air (dissolved CO2 causes some conductivity). If the ppt numbers are correct, this isn't a simple measurement problem.
 

Having dealt with "fresh" de-ionised water, I have never found it to be lower then .5 micro mhos, is this the limit for water with CO2 dissolved in it? Or was our water bought from a power station a bit nasty? It was used for steam cooling klystrons, in a good system it reached 2.2 micromhos, and we had to change it when it exceeded 10.
Frank
 

FvM said:
30 - 36 ppt salinity has been mentioned above in the thread, referring to a very low conductivity around 0.05 µS/cm, respectively 20 MOhm with K1 cell. The conductivity is below the range of pure water in equilibrium with air (dissolved CO2 causes some conductivity). If the ppt numbers are correct, this isn't a simple measurement problem.
Click to expand...

hi Frank

35 ppt is 53mS/cm (i think "m" here stands or milli and not micro" )
are you saying that this circuit cant measure sea water ?

- - - Updated - - -

as Chunky pointed out the differences between K1 and K10 probes , how will this circuit react to each one ?
do I have to do any modifications depending on which probe i use?

regards
 

The above circuit is very flexible, and it should certainly be able to be made to work very well with sea water.
It works basically in the exact same fashion as this:
circ130.gif

Although the 555 circuit is probably better than an HC14, because it has much more well defined switching thresholds.

Anyhow, some time ago just out of curiosity I was investigating how best to build very wide range VCO, and for this I used an H11F1 fet opto isolator, an HC14 and a 47 pf timing capacitor.
It could be swept continuously from well below 1Hz to well over 10 Mhz by controlling the led current in the H11F1. Absolutely amazing.

I do not know how much resistance was actually there to make a 47pF capacitor oscillate at about 0.3 Hz, but it must have been well up in the thousands of Meg ohms.

I am sure that the above cmos 555 oscillator circuit can be made to oscillate (in the audio range) over a very wide range of salinity by tweaking the resistor values and the timing capacitor value.
 
Last edited:

all good news . I bought a K10 conductivity probe so I will put the circuit together and start testing.
any care i need to take in the board layout in terms of component placement?
 

It should all be very non critical as far as layout goes.

As the output is going to be a frequency, you might need to think about the upper and lower frequency limits that your microcontroller can comfortably handle, and just do some testing with a jug full of ordinary sea water to start with.
 

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