Analyzing and Understanding Waveforms on a TENS Unit

[JustMeHere]

Probably need to have your tens machine hooked up to a pork butt or something to measure it.  My understanding is the measure the current and keep it at a safe level.

[mzzj]

I don't know anything about TENS or this circuit but 300 v pk-pk as measured by a scope probe sounds reasonable.  It takes a fair bit of voltage to get through the skin even for a small current.  However the available current / energy will be limited to a (hopefully) safe level.  In actual use the voltage will probably be lower due to the loading by the user.

You can't just program an AWG to make the waveform you see, that would be unsafe.

Resistance gets suprisingly low with the proper tens pads. 500-1000ohms per limb is good guess with large 2*2” pads. I have measured less than 500 ohms in some cases.
IEC 60601-2-10 Standard calls for 500 ohm test load and staggering 500vpp open circuit voltage. Typical cheapo home unit delivers around 100vp or 200vpp.

Above mentioned standard has further rules on output current and pulse energy. Allowed limits are much higher than one might initially think.

Thank you for all the data points. After reading your post I decided to check the resistance with the pads.   The pads measure 1.5 inches square.   I placed the pads on my skin within 6 inches of one another, similar to the way that they would be utilized.   I got a reading of 600 K ohms (plugging the multimeter probes into the back of the pads).  So my results are not lining up with your claims.  What is it that I am doing wrong in my measurement's?

skin resistance is quite a bit non-linear and there is weird capacitive part to it.
For more meaningfull results you need to use signal that is closer to actual tens output.

In pracrtice you can put a 100r shunt resistor in series with the tens output and your butt and work out the impedance from measured current and voltage.

[JJ_023]

Probably need to have your tens machine hooked up to a pork butt or something to measure it.  My understanding is the measure the current and keep it at a safe level.

I already measured the current and posted the results in the beginning of the thread. 

[JJ_023]

I don't know anything about TENS or this circuit but 300 v pk-pk as measured by a scope probe sounds reasonable.  It takes a fair bit of voltage to get through the skin even for a small current.  However the available current / energy will be limited to a (hopefully) safe level.  In actual use the voltage will probably be lower due to the loading by the user.

You can't just program an AWG to make the waveform you see, that would be unsafe.

Resistance gets suprisingly low with the proper tens pads. 500-1000ohms per limb is good guess with large 2*2” pads. I have measured less than 500 ohms in some cases.
IEC 60601-2-10 Standard calls for 500 ohm test load and staggering 500vpp open circuit voltage. Typical cheapo home unit delivers around 100vp or 200vpp.

Above mentioned standard has further rules on output current and pulse energy. Allowed limits are much higher than one might initially think.

Thank you for all the data points. After reading your post I decided to check the resistance with the pads.   The pads measure 1.5 inches square.   I placed the pads on my skin within 6 inches of one another, similar to the way that they would be utilized.   I got a reading of 600 K ohms (plugging the multimeter probes into the back of the pads).  So my results are not lining up with your claims.  What is it that I am doing wrong in my measurement's?

skin resistance is quite a bit non-linear and there is weird capacitive part to it.
For more meaningfull results you need to use signal that is closer to actual tens output.

In pracrtice you can put a 100r shunt resistor in series with the tens output and your butt and work out the impedance from measured current and voltage.

Thank you for your comments.   What exactly do you mean by "need to use signal that is closer to actual tens output"?  I am using a TENS  machine.  Maybe you can elaborate and keep in mind that I am a novice.

When you put a shunt resistor of such low value the unit just shuts down.  I am not sure what the cut off point is for the unit not shutting down but low R  values shut the unit completely down.

[TimFox]

"Shunt" has two meanings, which I think you are confusing.
If you put a "shunt resistor" in series with the load (your body), then measuring the voltage across it with a meter or 'scope gives you the current through the load.  The shunt resistor should be a value lower than the actual load.
"Shunt" also means parallel, so placing a large resistor in parallel with the output as a load can simulate your body resistance to  measure the output voltage under load.
Your measurement of 600 k ohms body resistance may be too high.

[JJ_023]

"Shunt" has two meanings, which I think you are confusing.
If you put a "shunt resistor" in series with the load (your body), then measuring the voltage across it with a meter or 'scope gives you the current through the load.  The shunt resistor should be a value lower than the actual load.
"Shunt" also means parallel, so placing a large resistor in parallel with the output as a load can simulate your body resistance to  measure the output voltage under load.
Your measurement of 600 k ohms body resistance may be too high.

Thank you for explaining.  I have a short resistor I can put in series and give it a go.   What I am confused about is "Your measurement of 600 k ohms body resistance may be too high".  How could this be too high if it is what it is.  Meaning that's what my body measured.

[thm_w]

It can be too high if your skin is dry, the pads are not clean, the pads are not pressed down firmly, or are too far apart, or in the wrong place on your body. etc.

https://tensunits.com/blogs/blog/tens-unit-placement-101

[JJ_023]

It can be too high if your skin is dry, the pads are not clean, the pads are not pressed down firmly, or are too far apart, or in the wrong place on your body. etc.

https://tensunits.com/blogs/blog/tens-unit-placement-101

Interesting read thank you.  I usually clean the pads with rubbing alcohol and try to press them down and hold them when taking a resistance measurement.