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post for September 4, 2014

A few posts ago (this post) I started breadboarding the Wien Bridge Oscillator.  I was having trouble getting it to work…I think I’ve got it working now and can move on to where all the action happens – the second op amp.  Speaking of op amps, as I googled about, I found a pdf that discusses the terms used on an op amp data sheet.  YIPPEE!  That will be helpful.

# The Goal

The goal of this post is to bread board the E.C. circuit – at least get past the Wien Bridge Oscillator stage!

# Thanks to Those That Went Before

• Chris Gammell – Chris guides me through learning about this stuff.  You know when you have a great teacher?  How that one person makes such a difference?  Chris is that kind of teacher/guide.  I learned A LOT from his Contextual Electronics courses.

# The Problem

While the steps and sub circuits are correct in the earlier post, the +/-V power source for the rails of the op amp were not.  I totally messed this up.  I had +V at +5V and -V at 0V.  I completely ignored -V – which of course doesn’t make sense because this is an AC signal.  Yet, when I went back to LTSpice to model the circuit with the same floating ground I used in the Healthy pH Shield (discussed in this post), the Wien Bridge Oscillator did not work.  Hmmm….DOH!!!!

# The Solution

The solution is to split the power supply into +2.5V and -2.5V.  The AC signal  fits within this range since the Vpp is between .9V and 1V.  And as commonly the case – splitting power involves setting up a circuit that uses a voltage divider: the op amp is needed because the load of the E.C. circuit will draw some of the voltage away from the voltage divider.  This means the virtual ground will be lower than .45V.  I discuss this in an earlier post.

## Caveat Using LTSpice

Note the op amp using in the LTSpice model above is the LT1006.  This is important for the LTSpice model to work.  A lot of parameters go into defining an op amp in LTSpice…I’m not sure how they all coexist and what changes affect what.  I do know I was using a single supply for the op amp.  The LT1006 is modeled to be a single supply.  The model would not work with op amps that assumed -V.

## The Size of the Resistors

The size of the resistors (R2 and R13) is relative to how much current is needed (V=IR).  The more current, the more energy it takes to run the circuit (P=IV).   Looking at the sub circuit in which voltage goes from +5V to 0V, the amount of resistance is R2 + R13.  So if R2=R3=1KΩ and V=5V, I = 5V/2KΩ = .0025A (2.5mA).  Using R2=R3=1Ω, I = 2.5A.  Using R2=R3=100KΩ I = .000025A (25uA).

## The Amount of Current – Why 1KΩ Resistors?

How much current will be needed for the E.C. circuit?  The biggest current draw are the three op amps.  Let’s say I choose a quad op amp like the MCP6244 (data sheet).  The data sheet notes each op amp uses a supply current of 50uA.  It goes on to note if an op amp is not used – which is the case in this circuit, the amount of supply current used will be minimized.  So 50X4 = 200uA = .2mA.  Even adding in the additional parts, an available current of 2.5mA will be more than enough.

# Wien Bridge Bread Board (Revisited)

I set up the circuit using the same wiring I had before (see the images in this post).  This time I added the split power supply circuit to provide VGnd. I had a TL072 op amp from an earlier prototype so I’m using it in the VGnd circuit.  The TL072 pins (from this data sheet): Here’s the AC Signal I measured at the Wien Bridge op amp’s output pin: Finally – YIPPEE!!! Looks like the breadboard worked.  Since DC coupling is on and VGND = 2.5V, 0V on the scope display is really at 2.5V relative to the +5V power supply.  The Vpp is pretty much the same as the LTSpice model came up with.

With AC coupling turned on the AC signal is centered at 0V: Now the signal is centered on 2.5V = 0V and the Vpp is .9V instead of 1V.  I do not see the Vpp difference as significant.  I am thinking changing state then choosing Auto to automatically adjust the signal reentered the signal.  Perhaps Auto is something I should stay away from.  For now, I just note it…

# What’s Next

I’m relieved that progress has been made on the bread board prototype.  Now I want to add in the all important/exciting next step – getting an ECv measurement.

As always, thank you and please find many things to smile about.