You’ve probably heard your potentiostat ‘click’ while running a cyclic voltammetry experiment or similar sweep methods. Have you ever wondered where that clicking comes from, and why it happens?
The clicking sound is made by a series of electromechanical relays (AKA switches) when they turn on or off to direct the flow of current (I) to a different shunt resistor. A shunt resistor is a specialized resistor with high accuracy and a low temperature coefficient. In most commercially-available potentiostats, current is not directly measured. Rather, current readings are calculated by dividing the voltage drop (V) across the shunt resistor by the resistance (R) of the shunt resistor.
I = V/R
Potentiostats use more than one shunt resistor for different magnitudes of current measurements. This is because a single shunt resistor cannot provide suitable accuracy for current measurements spanning orders of magnitude. It’s useful to know the number of shunt resistors used in a potentiostat by looking at the number of current ranges in the hardware specification datasheet provided by potentiostat manufacturers. For example, the Admiral Instruments Squidstat Plus potentiostat has 8 shunt resistors built in, which allow for 8 current ranges outlined below:
|Shunt Resistor||Current Range|
|1 Ω||97 mA to 1 A|
|10 Ω||9.7 mA to 97 mA|
|100 Ω||970 µA to 9.7 mA|
|1 kΩ||97 µA to 970 µA|
|10 kΩ||97 µA to 970 µA|
|100 kΩ||970 nA to 9.7 µA|
|1 MΩ||96 nA to 970 nA|
|10 MΩ||0 nA to 96 nA|
If one were to open a potentiostat, they would see a series of shunt resistors like those in this picture of the inside of a potentiostat:
Each shunt resistor has an accuracy within 0.1%. This means that the accuracy of measured current inside each range is within 0.1% of the maximum current in each range. For example, someone taking measurements within the top current range between 97 mA and 1 A can expect readings to be accurate to within ±1 mA. Conversely, at the lowest range between 0 nA and 96 nA, one can expect readings accurate to ±1 nA.
Given these constraints, one cannot expect nanoamp-level resolution if they are taking measurements between 97 mA and 1 A. On the other hand, measuring 1 A current in the 0 nA to 96 nA range will destroy the shunt resistor due to overheating. Therefore, all potentiostats use switches or relays to determine which shunt resistor to use depending upon the voltage drop across a shunt resistor when current flows. For example, the shunt resistor used in the 9.7 mA to 97 mA current range has a resistance of 10 ohm. If the voltage falls below 0.1 V (10 Ohm × 0.01 A), the relay directs the current to the next lower range (970 µA to 9.7 mA). If the voltage is above 1 V (10 Ohm × 0.1 A), the relay directs the current to the next upper range (97 mA to 1 A).
The most common relays are either ‘electromechanical’ or ‘solid-state.’ Electromechanical relays have mechanical parts that physically move, while the solid-state relays do not have any moving parts. The electromechanical relays click while switching. A ‘click’ means your potentiostat has an electromechanical relay and it is guiding current through different shunt resistors.
Below is a picture of what typical electromechanical relays look like inside a potentiostat:
The speed by which a potentiostat responds to a change in current range is determined by the slowest of its components. In a potentiostat with an electromechanical relay, the slowest component is the relay. Its response time is within a few milliseconds. These few milliseconds become very important if one is running an experiment where the current values change very fast over a wide range. This may be the case when one runs very fast cyclic voltammetry or a pulse experiment with very small periods. Another example would be testing electrochemical systems with very low impedance such as batteries, where a small voltage perturbation causes a substantial increase in current.
During such scenarios, if one chooses ‘autorange’ (i.e. the relays switches the current range automatically), you will lose data acquired during the first few milliseconds after switching. The exact amount of lost time depends on the response time of the relays. In addition, you might see noise in your data. To avoid data loss, experiments like this can be carried out by first preselecting a fixed current range so that relay switching does not occur.
Some potentiostats user interfaces make it very easy to preselect a current range to avoid this issue, whereas others may make it more complicated. The best rule-of-thumb is to base your selection on the highest current you expect to reach during the measurement. Using the current ranges of the Squidstat Plus as an example, a user expecting maximum readings of 1 mA would choose the 970 µA to 9.7 mA current range. You might also be fine selecting the 97 µA to 970 µA range, but you want to leave enough room in case the current is greater than expected. Preselecting a fixed current range is also helpful when running pulse experiments, where the time scale is on the order of milliseconds and the currents fall within different ranges.
The main limitation of selecting fixed current ranges is that there may be some loss in measurement accuracy. This is because the shunt resistor may be tasked with making measurements that are far lower than what its specified resistance value is designed for. Therefore, users should be careful when setting the parameters of their procedure; for example, select currents from the same range when running a pulse current experiment for best accuracy.
Lastly, it is important to remember that a user should avoid selecting a lower current range to measure high current, otherwise they risk destroying the internal circuits within their potentiostat due to rapid heating. Not all brands of potentiostats have built-in protections, so please be mindful!