how to measure AC current

[aliyesami]

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.

ok thanks Tony
a cold jug of beer next to the salt water jug will help also

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just noticed that the capacitor on the left hand side says just 0.1 .. is it 0.1 uf also ?

 

[Warpspeed]

Yes, 0.1uF (100nF).

Making that smaller will raise the whole frequency range. Its one of the "tweaks" you can do to match the probe to your seawater and microcontroller.

 

[FvM]

35 ppt is 53mS/cm

Yes, sorry for confusion. I was thinking of ppb...
The number refers to 20 ohms with K1 cell. So you have the opposite problem that the circuit must be able to measure very low AC resistance values.

 

[Warpspeed]

It can still work, but with a smaller exposed probe area.

The two jug solution sounds good !

 

[aliyesami]

what you mean by "smaller exposed area" ?
here is my circuit , two pins are not matching with the circuit given i.e CS and F , what should i do with those pins?

regards

 

[Warpspeed]

Pin 4 needs to be connected to pin 8, and both need to be supplied with +3.3v.

Less probe area will increase the resistance the probe sees for any given salinity.

 

[FvM]

You can expect 200 ohm AC resistance with a K10 probe and 50 mS/cm salt water. Means that the 390 ohm series resistor makes the dominant part of the overall resistance. But it can be eliminated in the instrument calibration.

 

[aliyesami]

You can expect 200 ohm AC resistance with a K10 probe and 50 mS/cm salt water. Means that the 390 ohm series resistor makes the dominant part of the overall resistance. But it can be eliminated in the instrument calibration.

sorry iam not sure I understand , you mean for calibration I should remove the 390 ohm resistor but for normal operatoin i have to keep it ?
how will that work ?

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i have two questions:
1- if the resistance is too low of salt water then shouldnt I be using K1 probe than K10 probe as its less sensitive?

2- what kind of output I will get on the output pin of 555 ?

thanks

 

[FvM]

No, you need to keep the resistor to protect the circuit and to limit the maximal frequency. But the frequency will be proportional to 1/(390 ohm + Rsensor). You can eliminate the 390 ohm effect in calculation.

 

[Warpspeed]

2- what kind of output I will get on the output pin of 555 ?

You will need a pullup resistor between pin 7 and +3.3v, (100K suggested).

The output on pin 7 will be a square wave 50% on, 50% off, and the frequency of this digital output signal will rise with increasing salinity.

Some software will then be required to count the incoming digital frequency over a fixed time interval.

 

[aliyesami]

You will need a pullup resistor between pin 7 and +3.3v, (100K suggested).

The output on pin 7 will be a square wave 50% on, 50% off, and the frequency of this digital output signal will rise with increasing salinity.

Some software will then be required to count the incoming digital frequency over a fixed time interval.

can I use frequency to voltage converter and then feed that analog voltage to the micro ADC ?

 

[KlausST]

Hi,

the easiest and maybe most precise way (depends on frequency) to measure frequency is to use the timer/counter periferal of a microcontroller.

Klaus

 

[Warpspeed]

Have to agree with Klaus.

If the frequency is fairly high, count the pulses over a fixed time interval.
If the frequency is low, easiest to measure the time interval between pulses.

Either way, your microcontroller will almost certainly have an internal counter timer that should be able to do either fairly easily.

That is what is so appealing about this 555 circuit.
Combined with a microcontroller, it requires absolutely minimal parts to convert conductivity into numbers with good resolution.

 

[aliyesami]

this probe uses BNC connector. I also have designed the PH circuit with the help of Texas Instrument's engineer and he designed the BNC connection in a way that is a bit difficult to understand . He connected the two BNC shell posts to signal ground pin and with a ground fill and did not connect it to circuit ground.
circuit works perfectly so I guess that's the way BNC connectors work.
Now I am trying to design this circuit the same way please see if its ok.
please advise

 

[aliyesami]

also what would be the frequency (low and high values) coming out of 555 for the salinity ranges I am after ?
I need this information to be able to use micro to capture and measure it.

thanks

 

[Audioguru]

Your IC has no part number on the schematic. I hope you will use a Cmos 555, not an ordinary 555.

 

[aliyesami]

Your IC has no part number on the schematic. I hope you will use a Cmos 555, not an ordinary 555.

its digi-key part number is 296-1336-1-ND
TLC555CDR , part desc says its linCMOS , is it same as CMOS ?

 

[aliyesami]

is it possible to find out the output frequency some rough lower n upper bounds ?

 

[Warpspeed]

Just test it with a jug of sea water.
Or mix up a known solution of common salt and water.
Trying to work it all out theoretically is too difficult and error prone.

 

[aliyesami]

hi Warspeed/Audioguru

this circuit (in post#34) is giving me upper frequency range from 7KHz - 12KHz .
is there a way to make this circuit produce 7KHz - 10Khz ?
please note that I don't want to trim the freq at 10Khz but rather shift the whole current frequency range a bit lower.
This is because I researched and all the current frequency to voltage converter chips in market have a max freq of 10KHz

thanks