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Topic: creating a controllable tens machine (Read 10 times) previous topic - next topic

tomwalker0472

Hi Retroplayer,

Thanks again for the help - I did miss it.

I just tried to use it and when I enter 5v to 40v it tells me to change to valid inputs. Does it only support step down conversion? I couldnt find anything in the help section.

Retroplayer

I'll have to play with it again. When I posted the link, I had played with it then and got it to work for what is needed here but I don't remember all the settings I had chosen.

dc42

I investigated Arduino-controlled TENS last year. After some internet research I determined that it needs short (less than 1ms) pulses of constant current, up to about 30mA @ up to about 100V. The pulses are supposed to be alternating polarity to prevent electrolysis. In a multi-channel TENS unit, all the channels should be isolated from each other, since you don't want current flowing between different pairs of electrodes.

I came to the conclusion that the easiest solution was to use a transformer for each pair of electrodes (thereby providing both isolation and voltage step-up), and to feed the primary of the transformer with a constant-current pulse (alternating the polarity) from an H-bridge. It so happens that constant-current H-bridge chips are produced for controlling the shutters of digital cameras, and these chips can switch up to 500mA. If the H-bridge is powered from a 9V battery (which is what almost all the commercial TENS units recommended by the UK NHS appear to use as the power source), up to about 7V can be put across the transformer primary. A transformer with a turns ratio of about 12:1 will then allow the required voltage and current to be generated.

I knocked up a prototype, using 5 mosfets to switch in different current setting resistors (giving 31 intensity levels). For the transformer, I tried using small mains transformers in reverse (which turned out to have too much resistance), designed a home-wound ferrite-cored transformer (which I never got round to winding), and ended up using an audio transformer that did the job well (although it was heavier than ideal for a wearable unit).

If you are interested, drop me a PM and I'll search for the schematic.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu

#13
Apr 11, 2013, 05:45 am Last Edit: Apr 11, 2013, 05:47 am by sonnyyu Reason: 1
Quote
After some internet research I determined that it needs short (less than 1ms) pulses of constant current, up to about 30mA @ up to about 100V.


What is the duration time?

Quote
designed a home-wound ferrite-cored transformer (which I never got round to winding)


Can I use this one Newark #67R8826 as ferrite-cored as sample to order custom made transformer?

Newark ferrite-cored

I have no clue which one is good. but that one cost most.

Here is the sample image of custom made transformer.




sonnyyu


If you are interested, drop me a PM and I'll search for the schematic.


I drop you a PM.

Quote
What kind of pads do you use ?


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