TENS Under the Hood: Intensity Control
This is the third in a series of posts that explore how TENS devices work. Our intention is to go “under the hood” of these important pain relief devices to help you understand their operation. We explore key technical specifications and operating principles. Our hope is that this information is useful to you when evaluating TENS devices.
Stimulation intensity, which determines how strong TENS feels, has the greatest influence on pain relief among all TENS parameters. An intensity that is not felt will not provide pain relief. Scientific studies and clinical experience suggest that effective pain relief occurs at an intensity that is perceived as “strong but comfortable.” In this post we will discuss how TENS devices control stimulation intensity.
How is Intensity Controlled?
The most important control on a TENS device is for intensity. It is analogous to the volume control on your car radio or smartphone which determines loudness. Originally, TENS devices had an analog knob that the user turned to increase or decrease intensity. Now most TENS devices are digital and use up and down buttons. Quell® goes further with algorithms that automatically manage intensity, so the user only needs to occasionally adjust with a smartphone app.
What actually happens when the stimulation intensity is increased or decreased? Generally speaking, either the current passing through the electrodes or the voltage applied to the electrodes is changed. The former is called current regulation and the latter is called voltage regulation. Does it matter which one is used? In fact, there is an important difference because current rather than voltage is the determining factor for nerve stimulation.
To explore further, we revisit Ohm’s law introduced in the first blog (TENS Under the Hood: Maximum Voltage).
Voltage = Current x Resistance
Since current is the driver of nerve stimulation, it is useful to re-arrange Ohm’s law as follows:
Current = Voltage / Resistance
The equation above shows that current depends on both voltage and resistance. Recall that the resistance is determined by the user’s skin and by the interface between their skin and the electrode, both of which may change day to day, and even hour to hour. A current regulated TENS device automatically sets the voltage to produce the desired current irrespective of the resistance. If the resistance increases then the device compensates by increasing the voltage, and if the resistance decreases the device adjusts the voltage down. In either case, the current remains at the intended value.
A voltage regulated TENS device controls the voltage rather than the current. As a result, changes in resistance will lead to changes in current even if the voltage is unchanged. If the resistance increases then the current decreases and if the resistance decreases then the current increases. This may cause unstable nerve stimulation and can impact pain relief. At a minimum, it is inconvenient to have to “hunt” for the right intensity with every use due to changes in resistance. Moreover, current regulation is safer than voltage regulation because the current will not spike if the resistance quickly drops.
Design of Current and Voltage Regulated TENS
Since the benefits are clear, why don’t all TENS devices utilize current regulation? The reason is that reliable current regulation is technically challenging and requires specialized electronic circuits. As a result, most TENS devices use some variant of voltage regulation. When TENS therapy is used occasionally for short periods, the repeated inconvenience of finding an effective intensity is acceptable. In more demanding applications, such as Quell, where users typically wear the device for 8-12 hours a day, stable stimulation current is required. For this reason, Quell utilizes sophisticated current regulation.
If you would like to read more posts in the TENS Under the Hood series, please click here.
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