Show HN: 32V TENS device from built from scratch under $100
littlemountainman.github.io
littlemountainman.github.io
I reverse engineered some existing device, there are TONS of safety measures.
At least: current limiting resistor, transformer, voltage/current feedback, GND isolation, MCU protection.
After replicate all of these, I still not brave enough to try it myself, I just find it too dangerous.
PS: TENS device is fun. article seems like a bait.
Has anyone ever seen something like a TENS shirt or shorts that has the pads built-in?
> Each electrode channel is tied to a finger pad
but the layout shows the finger pads are all tied together. What am I missing?
That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...
Below 2,5V is usually when you don't wanna use them anymore
That being said, system designer might choose higher cut-off point, since:
1) charge/discharge curve is S-shaped. There is very little energy in that last few millivolts;
2) battery (protection) circuit, and/or battery itself probably have some small leakage current. However minuscule, over months/years on a shelf, even some nano-amps of leakage will add up. If you want device to survive that, you have to factor this in, so that rest cell voltage still stays above safety threshold even after storage.
Also, "Li-ion" is quite a wide category. Don't use arbitrary voltage as a fast rule. Look up datasheet, or characterize actual cell you use. For some[2], disconnecting at 3.6V would mean leaving 50% of capacity unused. For other[3], that would be a reasonable, if somewhat conservative threshold.
[1] https://docs.rs-online.com/080b/A700000007848112.pdf
[2] https://www.murata.com/-/media/webrenewal/products/batteries...
[3] https://ntrs.nasa.gov/api/citations/20140005830/downloads/20... (page 4)
3.0V is considered basically the highest voltage. Most chemistries suggest even lower, 2.8, or even 2.5 in some situations assuming you can control the cutoff carefully. Perfectly safe to do so. You only start to have issues when you’re south of 2.5 without a load.
Most advanced battery usages let the cells drop even below that during heavy load.
This is not right (3.6v certainly is and can be cut off depending on device and battery).
One thing you are not considering is discharge after the cut off. Fuel gauge, protection circuitry, the cut off circuitry and battery itself has some discharge.
So you don’t want to have the cut off being too low because then the battery is permanently dead after not using it for X period of time.
You want to leave some margin there.
Depending on product, battery chemistry and design I have seen cut-off at 3.0-3.6v.
The margin is already there at 3.0V. You can still recharge batteries discharged below 3.0V. It just becomes dicey below ~2.5V.
It really depends on application, battery size and leakage. In consumer world of electronics for example there’s an often requirement to make sure device turns on after being on a shelf for 1/2 - 2 years.
Then when you do the math it ends up needing to set the limit to 3-3.6v.
>The margin is already there at 3.0V. You can still recharge batteries discharged below 3.0V. It just becomes dicey below ~2.5V.
The margin isn’t big enough for some products. Furthermore some of the more leading edge batteries (in terms of energy density) have higher leakage which requires having more margin.