TENS Under the Hood: Powering TENS Devices
This is the eighth 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 rapidly consume battery charge because they stimulate nerves non-invasively across the skin for up to several hours a day or longer. As discussed in our first post (TENS Under the Hood: Maximum Voltage), skin has a high resistance to current flow and therefore nerve stimulation requires high voltage and substantial electric power. Our topic today covers how TENS devices are powered to overcome this challenge.
We start by discussing power conservation. Most of the power consumed by a TENS device is attributable to nerve stimulation. For now, we will ignore power consumed by other important TENS device components such as the microprocessor and the display. The voltage used to generate the nerve stimulation current is much higher than the battery voltage (for example 80 volts versus 3 volts). That means the battery voltage must be increased or “stepped-up” by the TENS device. Circuitry that increases the voltage also affects the current, so the current drawn on the high voltage side of the circuit will be different than the current drawn on the low voltage side. To calculate the current on the low voltage, or battery side, we rely on the fact that power is conserved across the circuit boundary. That means the electric power provided by the battery equals the electric power used by the high voltage side of the device plus losses due to inefficiencies such as generation of heat. Electric power is defined as the product of voltage and current. This leads us to the principle of power conservation:
stimulation voltage x stimulation current = efficiency x battery voltage x battery current
In other words, the power used to stimulate nerves equals the power produced by the battery, subject to an efficiency factor (typically between 0.5 and 0.7). When evaluating battery life, we are primarily concerned with the battery current so we re-write the power conservation equation as:
battery current = (stimulation voltage x stimulation current) / (efficiency x battery voltage)
As we can see from this equation, the greater the stimulation voltage and stimulation current and the lower the efficiency, the more current that will be drawn from the battery. The reason we focus on battery current is that it determines how long a battery will last before needing to be replaced or recharged. Battery capacity is rated in milliamp hours, abbreviated mAh. For example, 2 AA batteries in series have a battery voltage of 3 volts and a battery capacity of about 2000 mAh.
We will use an example to demonstrate how battery current and battery capacity interact to determine battery life. Let’s say that nerve stimulation requires a stimulation voltage of 50 volts and stimulation current of 10 milliamps (stimulation current and the intensity of the stimulation pulse are proportional but the relationship is a bit complicated and so won’t be covered in this post), and the TENS device has an efficiency of 0.5. Under these conditions, battery current = (50 volts x 10 milliamps)/(0.5 x 3 volts) = 333 milliamps, and the 2 AA batteries will provide about 2000/333 = 6 hours of stimulation. We see that battery life is maximized by keeping the stimulation voltage as low as possible and efficiency as high as possible. These are both important attributes of TENS circuit design. Quell® utilizes a novel voltage control algorithm that minimizes the stimulation voltage needed to stimulate nerves.
Different Ways to Power a TENS Device
There are three ways to power a TENS device: replaceable batteries (like most toys), an embedded rechargeable battery (like your mobile phone) and AC power (like your TV). OTC TENS devices are powered by replaceable or rechargeable batteries. The only TENS devices that use AC power are professional units used by health care professionals in their clinic.
The most attractive option for consumers is an embedded rechargeable battery, such as in Quell, because it relieves the user of the cost and inconvenience of regularly changing the batteries. The reason that not all TENS devices uses an embedded rechargeable battery is that this approach requires specialized electronic circuits to safely charge and operate, which increase complexity and cost.
If you would like to read more posts in the TENS Under the Hood series, please click here.
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