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TENS Under the Hood: Maximum Voltage

05/20/2020 • Shai Gozani M.D., Ph.D.

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This post is the first in a series exploring how TENS (Transcutaneous Electrical Nerve Stimulation) devices work. Our intention is to go “under the hood” of these valuable pain relief devices to help you understand their operation, and we will explore key technical specifications and operating principles. We hope this information is useful when deciding on a TENS device for your particular needs.

In this post we address one of the most important technical attributes of a TENS device, maximum voltage. Read on to learn what it is, and why it’s important in obtaining effective pain relief.

Nerve stimulation

TENS provides pain relief by passing an electric current between electrodes placed on the skin.  The current flows across the skin underneath the electrodes and through the body where it stimulates nearby nerves.  This current initiates nerve impulses, called action potentials, that travel to your spinal cord and brain to activate a complex neurological circuit that produces pain relief.

Why maximum voltage matters?

How does a TENS device produce the stimulation current?  Although the flow of current through the electrode and skin is complex, the basic concept can be understood using Ohm’s law that you may remember from high school physics:

            Voltage = Current x Resistance

Resistance is the tendency of your skin to impede electric current.  Dry skin has a higher resistance than hydrated skin because the dry skin acts as a barrier to current flow.  Ohm’s law tells us that as the resistance increases more voltage is required to generate the same current.

Most people need between 15 and 50 milliamps (a milliamp is 1000th of an amp) for a strong but comfortable stimulation to produce pain relief.  Let’s assume you need 25 milliamps and have healthy and well hydrated skin and therefore a resistance of 1000 ohms.  Well then you need (25/1000 x 1000) = 25 volts, which most OTC TENS devices can provide.

But what if you need 50 milliamps for pain relief and have dry skin with a resistance of 2000 ohms?  Now you need (50/1000*2000) = 100 volts.   Few OTC devices can generate such a high voltage.  A larger maximum voltage can be critical, providing flexibility for cases needing higher stimulation current and having greater skin resistance.

Differences among TENS devices

Portable TENS devices are powered by small batteries that only provide a few volts; for example, 2 AA batteries create 3 volts.  Therefore, the devices must boost the low battery voltage to a high nerve stimulation voltage, requiring a specialized electronic circuit.  Basic TENS devices have simple circuits that typically generate no more than 40-60 volts.  Prescription devices may provide 100 volts.  Quell has an advanced stimulation circuit based on a proprietary neurostimulation microchip that reliably generates up to 120 volts, which is 2-3 times higher than most OTC TENS devices.

If you would like to read more posts in the TENS Under the Hood series, please click here.

©2020 NeuroMetrix, Inc. All Rights Reserved.

About The Author
Shai Gozani M.D., Ph.D.

Dr. Gozani is an expert in non-invasive neurostimulation. He received his M.D. degree from Harvard Medical School and the Harvard-MIT Division of Health Science and Technology. Dr. Gozani earned his Ph.D. in Neurobiology, M.S. in Electrical Engineering & Computer Science and B.A. in Computer Science from the University of California at Berkeley. Following his studies, Dr. Gozani conducted post-doctoral research at Harvard Medical School and MIT. He holds 36 U.S. patents and has authored over 30 articles in scientific and clinical peer-reviewed journals. Dr. Gozani is founder, president and CEO of NeuroMetrix, Inc.; which designs and manufactures Quell.

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