What is the difference between a voltage control and current controlled stimulator?
We often get asked to explain the difference between a tens device - which is a voltage controlled output and our tVNS which is a current controlled output. They sound the same so how are they different?
Here is everything you need to know to understand the difference.
The first thing to understand is that the sensation you feel is the flow of current. It is the current flow that provides the tickle, scratch or pin sensation that you feel on your skin.
When you increase the intensity and the sensation increases that is because more current is flowing.
The corollary to this is that you do not feel the voltage.
There is no such thing as the flow of voltage. You are not passing a voltage through your vagus nerve to stimulate it.
The flow of current is controlled by the voltage applied and the resistance encountered.
This is a popular image used to explain this concept that you may remember from school.
Figure 1. Ohms Law of Nerve Stimulators
As illustrated, the amount of current that will flow - the Amps or intensity - depends on how much voltage is pushing and how much resistance is impeding that flow.
When you increase the voltage, the intensity will increase as more current is flowing.
So the voltage is very important to how much current will flow, but of equal importance is the resistance.
For any given voltage, the resistance will determine how much current will be able to flow. When the voltage stays the same, the current is inversely proportional to the resistance.
If you have a low resistance then you will get a high current.
If you have a high resistance then you will get a low current.
So to recap the points so far
So is the resistance a constant fixed value in vagus nerve stimulation?
The resistance in a piece of copper wire is generally fixed and changes only a little with heat. The resistance of living biological tissue is extremely dynamic and changes constantly both second by second, day to day and season to season.
There are also massive differences from person to person.
The resistance is very much not fixed and is in fact constantly changing.
You may have tried electronic body composition scales. These work through biolelectrical impedance analysis. When you stand on the sensors they pass an electric current through your whole body. Based on the measured resistance and some look up tables they can, using your weight, make estimates for your body composition.
These measurements are averages across your whole body and you can see they change from day to day.
Figure 2. Changes in Resistance mean changes in Current flow
So what does this mean?
Going back to our understanding that the current flow is controlled by the voltage and the resistance, if the resistance is constantly changing, then the current flow must be constantly changing too. The current flow will increase when the resistance drops and decrease when the resistance rises.
What this means for a voltage controlled device?
A voltage controlled device will allow the user to set the output voltage. But as we see the current will actually be controlled by the changes in resistance.
When you are only in control of the voltage, the resistance is in control of the current..
A current controlled device lets the user set the intensity, not the voltage.
It then measures the resistance and outputs the exact required voltage to reach that intensity.
As the resistance changes it constantly changes the applied voltage.
A current controlled device will always output the same current at each intensity level. It will give the same intensity person to person, session to session, second to second, even millisecond to millisecond.
This is why current control is the gold standard for electrical stimulation.
It is accurate, precise and repeatable.
Providing current control in a device is a more technically challenging and expensive than simple voltage control. Precision components are required to measure the resistance and then very quickly alter the voltage.
tENS is a voltage controlled device. It was designed for stimulating muscles and passing electricity across a much larger body area.
When you are dealing with the longest cranial nerve in your body, the superhighway for so many functions, it is important to control the current that is applied to it. Using a tENS device does not allow for this control.