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Kinda cool. Per cm, I believe is per square cm. Voltage is also per area. INMO, this really needs to be a constant current source with a very small compliance voltage. If out of compliance. their needs to be an indication. I have designed current sources in the 100 mA range and have designed I-V converters in the sub nA to mA range.
This design isn't easy. I did design a biasable 4 terminal I-V converter, but without a hard compliance limit. I see where this is important here. Battery life is yet another issue.
It's a gismo that could place a voltage from -10 to +10V annd measure currents from 0.01 mA FS to 100 mA FS in 4 ranges. 4T means it could compensate for voltage drops in the leads. This is necessary for currents >1 mA.
The reason why is that it doesn't regulate. Yes it is a current source into a fixed load. As the silver depletes the system needs some way of telling you that. Basically the voltage increases. Skin resistance usually varies all over the map. I think, about 10 mA across the heart is fatal.
I just measured dry skin and I got about 2 M ohms. Same for tooth to gum. I got about 1 m ohm on my tounge. It varies wildly. The silver should lower that.
Anyway, what's the maximum area?
In reality, you need to do an experiment and find out what the compliance would have to be. The experiment would be the silver contraption and an ohmmeter. Then you have a chance of designing the circuit. Electrode size, I think would change the voltage dependency. I;d look up the patents referenced. patents.google.com
You need a reference to set the current. A way to reduce the current when the voltage gets too high and a way to detect that your out of compliance: Probably current to low.
An OP Amp is short for an Operational Amplifier. In short, it amplifies the differnce of its inverting and non-inverting terminals by a huge number. That device can be used to create adders, subtractors, integrators. I-V converters etc. Ahence. The term operational.
In your thumbnail, the device in the triangle is an OP Amp. IC techniques allow for nearly
Perfect matching of resistors by laser trimming. Matching of components is extremely
important for some applications.
The pic with the AD8603 is probably the way to go. The system would probably have to operate on plus and minus 2.5V power supply. Operating near ground is tough without what's called split supplies. +-2.5 is not common, but since the currents are low, one can use a switching regulator to get to 5 V and then use a power supply splitter IC to create the ground in the center.
With the right reference and choice of resistors, you should easily be able to achieve 100-200 nA. Printed Circuit Board layout at these levels is critical.
The LM10 **broken link removed** would probably be a good choice for the reference to set the current.
So, with a few parts, you have a current source that meets your needs with a +-2.5 V compliance (give or take)
This one **broken link removed** used an input of 2V to 3.6 volts.
You now have a basis for some sort of design. What's missing is how to set the compliance voltage and indicate out of compliance. This is where it gets tricky and I need to think some more.
At the very least, you'll need fixed references for the two voltage limits, again using LM10's.
A question then arises as to what to do when the voltage limit is exceeded?
1. disconnect
2. disconnect after a certain amount of time in the condition.
3. Reduce the current, so the voltage is in the required range. Indicate that the circuit has dome so.
4. Just indicate that it's out of range. LED, Buzzer etc.
Actually I think the voltage in this design is less important than the amp. I think it can't go over 200 nanoamp or it wouldn't work, but I guess it could go over 1V.
Modeling in a version of SPICE such as LTSpice would be very useful, but the models must be available. This is not something that I have done before. I tried, briefly once.
One idea would be to turn the setpoint input into an adder/subtractor or sink/source. It would take away from the current setpoint if the upper voltage was exceeded.
It might just turn off if the voltage gets too low (below 0.3V), latch and indicate via LED. Pushing an overide button would aid set up. A small connector would be available to attach a battery operated voltmeter to measure voltage and current and battery voltage
That's my idea at this point. A battery test push button wouldn't hurt.
The power source is an issue. The CR2032 coin cells might work. These are <$0.50 when bought at the right place and $5.00 at Radio Shack. I buy 20 at a time.
If I had my druthers and this was a commercial product, I'd might think about using a low power processor like the MSP430 to measure and interface to a display. The latter has a VERY VERY steep learning curve.
So, anyway, that's the basis of a "thinking out loud" design.
The circuit I once designed ( the 4 terminal I-V converter) is similar. It controlled voltage and if the converter overranged even momentarily blinked a LED whose color was red or green depending on polarity. It had a crude current source/sink (+-50 mA) as well. It operated at the high end of current measurement with low voltages and could have a nearly 0 volt voltage drop at the device. There were lots of issues in the design. I did not do voltage compliance issues.
This is low current/low power with compliance. I think I said, I put together a system that could measure the resistance of a piece of paper.
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