Tens Source

Been contemplating it on and off for a while, breadboarded a little thing this weekend.

Note, this circuit produces waveforms typical of some TENS applications, but I in no way endorse its use as a medical device.

Schematic:

http://t3sl4.dnsdynamic.net/Images/TENS_Supply.png

If I were to make a device based on this, I think I would include the HV supply in the design (right now, I'm using this one), and run it from batteries or a medical grade source. I'd also add ESD protection on the input (reverse protection diode, input filter, TVS) and output (UF4007s, from GND, to each output, to +HV, and a TVS or MOV on the HV rail to catch overcharge).

Tim

Hi Tim,

I have a TENS machine and it uses isolation transformers in the outputs, even for battery useage. Perhaps for safety?

Cheers!

Not a bad idea... could easily add a small ferrite cored transformer, probably a couple hundred turns would do it. Could also avoid the HV supply if you use a step-up. Leakage and saturation limit what you can do with the waveform, but it may not be a problem, depending on just what waveforms are required.

Tim

They are really tiny. Reminded me of the IST's in late 70's/early 80's transistor radios.

The cores may be ferrite, as I can feel the heckuva sharp rise/fall time

yep ... drive an isolation type transformer .
this keeps the hv "on the other side" and provides additional safety .

some off those tens units i have messed with provide "multiple strike" pulses .
5 Hz seemed to be common .
some of the units did a ramp up/ramp down multiple strike .

i do not imagine that the waveform is super critical .

Far as I know, it's more about energy delivery, and keeping the voltage high enough to stimulate things.

Holding your fingers on the output of the above circuit, you can feel a tingle with the current control knob at about 10%, but on a larger, sweatier part of the body, like the arm, you can't feel much until 50 or 70% up. I think current density is the important part, which means lots more current is required, at about the same voltage or lower, for larger pads or greater separation.

Tim

Just an FYI for the TENS users. Don't put the leads on opposite sides of your head.

Don't ask my how I know that.

heh heh heh ... oooohhh , i smell a good story here !


"hey ! hold my soda and watch this !!"

I also read somewhere that excessive use (>20 mins at a time) can cause clots.

Perhaps someone heard of it too? Is this still holding or debunked?

Never heard of it, and can't imagine why it would (not directly anyway). Perhaps being immobile for long periods (a known cause of clots) while using a TENS?

I have heard of nerve damage being a possibility with some of the home-made units though.

[EDIT] FWIW...

My TENS unit (XFT-320) max output specs are 100V, 20mA, and 1-101 Hz. It also has different pulse pattern options (power/freq cycling, etc.).

However, the power ratings are either exaggerated, or else the pulse duty-cycle is pretty low. Testing with 67VAC (60Hz) at 16mA feels much more powerful than the TENS unit's highest setting.