Theory of Operation
The potentiostat / galvanostat
takes advantage of the relatively slow electrode kinetics of most
electrochemical energy cells, using a modified voltage divider and a digital to analog (DAC) /
digitally variable resistor (DVR) / analog to digital (ADC) feedback loop (Figure 1). Through
the
use of a high speed microcontroller (Atmel168), the feedback loop polls at a rate over 1 kHz.
The potentiostatic feedback is triggered by a user's request for a given potential across the cell.
The relay closes, and the ADC measures the actual potentia
l across the cell, and the
microcontroller increments the DAC in the direction that will adjust the cell to the desired
potential. The DVR set between the ADC and DAC determines the current via Ohm's law:
Equation 1
Which relates to:
Equation 2
If the
DAC is adjusted to its maximum or minimum, the microcontroller will reduce the resistance
of the DVR to allow more current to pass according to Equation 2. Alternatively, if the difference
between the DAC and the ADC is insubstantial the microcontroller wi
ll increment the DVR to
increase the resistance, thereby reducing the current through the cell. The speed at which this
feedback loop occurs is only limited by the microcontroller, and most modern microcontrollers
can complete this loop in fewer than 500 m
icroS. After each loop the microcontroller may report
the setting of the DAC and the DVR, and the reading of the ADC from which a computer
program can compute the current instantaneously via Equation 2.
Circuit Schematic
In the case of the galvanostatic
loop, the user enters a desired current, and the feedback loop
optimizes around a difference between the ADC and the DAC rather than the ADC alone. To
read an open circuit potential, the microcontroller opens the relay and the ADC measures the
potential.
S
ource:
http://steingart.ccny.cuny.edu/ardustat

theory
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