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post for September 2, 2014

I’ve been studying electronics since last January.  There is so much I don’t know…it takes many (many, many) iterations before I truly understand something.  This is certainly true when it comes to the EC Circuit

# The Goal

The goals of this post  include:

• insert more context into the stages of an E.C. Circuit.  I discussed my original interpretation (this post) based on SparkysWidgets miniEC and the discussion on the product page.
• Figure out the E.C. Value based on the voltage value read by the ADC.

# Thanks to Those That Went Before

I realize I thank the same people.  This is because they have contributed the most to the knowledge I have gained that has allowed me to make this post.  It is important for me to be thankful each day in my electronics learning process to those that graciously share their knowledge.

• SparkysWidgets – A major ramp up in my learnings about an E.C. circuit and the electronics behind it came from the miniEC effort and the gracious sharing of knowledge.
• Chris Gammell – Chris is an amazing mentor/guide.  His Contextual Electronics courses have been instrumental in super charging my knowledge in electronics.  I just wish he had more course offerings.
• The person behind this post – The post seems to be written awhile back and the author has moved on to other interests.  The information about how an E.C. circuit works is very instructive.  Thank you!

# The E.C. Circuit

I’ve gone through this a few times…but what the heck, each time I re-explain, aspects of the circuit become more clear…so here I go again.  Here is an image of an E.C. Circuit: An AC signal needs to be generated.  This is done by using a Wien Bridge Oscillator.  The AC Signal has a Vpp ~= .9V.  The AC Signal is then shrunk to ~= .2Vpp.  This is to minimize the amount of voltage that is injected into the bath.  Step 3 is where the action is.  The shrunken AC signal is fed into the non-inverting input of the 2nd op amp.  The probe is hooked up to the inverting input, acting as a variable resistor.  The gain loop magnifies the reading so that it stands the best chance of being accurately read by the ADC.  The gain is variable because the probe – acting as a variable resistor (R0 in the LTSpice model) – will change the gain by 1 + R9/R0.

The next step is to calculate the E.C.

# Calculating E.C.

Now there is enough information to back calculate the value R0 is at when a ECv is read.  Here are the simulation results for ECv when R0 goes from 200Ω to 1,300Ω in 100Ω increments: The voltage when R0 = 200 is ~2.24V.  When R0 = 1300Ω, the voltage ~= .435.

Applying:

• Gain = VOut/Vin
• Gain = 1 + R9/R0
We know VOut (ECv) and R9.  We’ll be figuring out R0.  To get to Vin is a constant value that can be measured by taking E.C. readings from a  known concentration. For example, if the known concentration was at .005S, then R0 = 200Ω.  If R0 = 200Ω then Gain = 1+R9/R0 = 1 + 4000/200 = 21.  21 = VOut/Vin  Vin = 2.24V/21 = .107V.  It’s probably a good idea to average values across a few known concentrations.  Once the Vin is know, it is a constant value for all calculations.

So we know:
• Vin = .107V
• R9 = 4K
Step 1: calculate the Gain based on vOut/VIn.
Step 2: Use the Gain value calculated in Step 1 to figure out the value of R0.  Gain from Step 1= 1 + 4K/R0
Step 3: Translate to Siemens by taking 1/R0.  Some examples are given in the table below based on the above image:

 ECv (vOut) Gain=  vOut/vIn R0(Ω) E.C (S) 2.24 20.93 200.7 .005 1.53 14.3 300.8 .003 .435 4 1,333 .0007