TENS Under the Hood: Trip Conditions

07/08/2020 • Shai Gozani M.D., Ph.D.

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This is the seventh in a series of posts that explore how TENS devices work.  Our intention is to go “under the hood” of these useful pain relief devices to help you understand their operation.  We explore key technical specifications and operating principles. We hope this information is valuable when deciding on a TENS device for your particular needs.

TENS devices are most often characterized by their stimulation specifications such as maximum voltage (TENS Under the Hood: Maximum Voltage), which is understandable given that nerve activation directly drives pain relief.  However, device safety features are equally important, despite garnering far less attention.  Most TENS devices have trip conditions, which act like the circuit breakers in your home that trip to protect against an excess current surge.  Read on to learn about trip conditions found in TENS devices and the role they play in keeping you safe during TENS therapy.

Open Circuit

Detection of an open circuit is a standard trip condition.  An open circuit occurs when the flow of electric current from the device, into one of the electrodes, through the body, into the other electrode and back to the device is broken.  The most likely cause is an electrode coming off the skin or the device and electrode disconnecting.  It is essential for a TENS device to halt stimulation if an open circuit occurs.  The reason is that the user may accidently “close the circuit” if they touch the electrode or device resulting in a painful shock.  All TENS devices should provide this trip condition.


A trip condition that is found in some TENS devices, such as Quell®, detects when the stimulation pulse is delivering more current than intended, called an overload.  This is an important safety feature to prevent painful and potentially dangerous nerve stimulation.   The cause of an overload is usually a defect in the electronic circuit that controls stimulation.  A common reason for this defect is damage due to a “static shock,” which is technically described as electrostatic discharge (ESD).

Electrode Peel

A novel trip condition found only in Quell devices is detection of electrode peeling.  It is not uncommon for electrodes to peel from the skin, particularly after prolonged use.  Electrode peeling may also occur if a TENS device is worn during sleep due to frequent body position changes and the lack of awareness by the asleep user.  Electrode peeling reduces the contact area between the electrode and the skin.  This can make stimulation painful, and in extreme cases, can lead to skin damage.  Quell implements a patented electrode peel detector that automatically halts stimulation if the area between the electrode and the skin decreases to a critical level.  Quell is the only TENS device specifically cleared by the FDA for use during sleep.

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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