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Sch3mat1c
Posted: September 02, 2012 07:52 am
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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


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Geek
Posted: September 02, 2012 09:41 am
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Hi Tim,

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

Cheers!


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Sch3mat1c
Posted: September 02, 2012 10:35 pm
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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


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Geek
Posted: September 02, 2012 10:44 pm
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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 blink.gif


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CWB
Posted: September 03, 2012 01:25 am
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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 .


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Sch3mat1c
Posted: September 03, 2012 01:33 am
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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


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telomere
Posted: September 03, 2012 04:01 am
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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.

blush.gif


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CWB
Posted: September 03, 2012 10:56 am
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heh heh heh ... oooohhh , i smell a good story here !
laugh.gif laugh.gif

"hey ! hold my soda and watch this !!"
blink.gif wacko.gif


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Geek
Posted: September 03, 2012 11:29 am
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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?


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MacFromOK
Posted: September 03, 2012 11:00 pm
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QUOTE (Geek @ September 03, 2012 05:29 am)
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? dunno.gif

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

[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.


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