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    How to control output current of a TENS machine

    Hello everybody, I'm working on a T.E.N.S. unit that has a maximum output voltage of 30Vpp (constant), my current system can generate different waveforms such as rectified sine wave, square wave, biphasic square wave, etc. Taking into consideration that the output voltage is "fixed" I want to regulate the output current from 0 up to 150 mA. The problem is that I dont have any idea about how to solve it, since I want to digitally control. I thinking in using a V to I converter but I don't really sure if it is going to work with huge Rload (human skin >~1Mohm). So any idea is highly appreciated. Thanks in advance.

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    Re: How to control output current of a TENS machine

    what your TENS frequency?
    Well you can use a switching regulator to control the voltage and current....
    IN this different mechanisms like Buck-boost, flodback etc are there to control the current.
    "Doubt grows with knowledge."
    "The best way to have a good idea is to have a lot of ideas"....Linus Pauling



    •   Alt17th January 2011, 09:45

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    Re: How to control output current of a TENS machine

    It won't work. Refer to the respective literature and parameters of professional medical devices. And don't forget, that some 10 mA can be letal when applying the wrong waveform with incorrect electrode placement. As a first guess, consider ohms law. Usually, maximum currents are calculated for 500 ohm electrode resistance, only achievable with respective electrolyte gel.



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    Re: How to control output current of a TENS machine

    amitjagtap, the operating frequency goes from 1 Hz up to 30 Hz. FvM, well about the current and voltage I was based-on a book named "Electrotheray using in Physiotherapy, Martin Rodriguez (Spanish Edition)" and stands that there are two ways of stimulate, using HV above 80 V with currents below 4 mA and in the other hand low voltage with high current below 200 mA (on ebay i found some one that can achieve up to 300 mA with an 16 V output). I think you are right about my 500 Ω question. So I need to built a V2I converter and make the needed calculation taking 500 Ω as maximum load? Thanks again for your replies.



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    Re: How to control output current of a TENS machine

    I'm not sure about the implementation details of the existing devices, but I would expect a constant current characteristics for precise operation. Of course only within in the given voltage limits.



    •   Alt17th January 2011, 18:18

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    Re: How to control output current of a TENS machine

    you can use a digitally controlled potential divider



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    Re: How to control output current of a TENS machine

    Hello, again. At this moment I think I figured out how to solve it. I found a paper that uses a current mirror (http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA410000) so making the needed modification I could achieve the current control. Thanks everyone for your assistance.



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    Re: How to control output current of a TENS machine

    Quote Originally Posted by efelnavarro09 View Post
    Hello, again. At this moment I think I figured out how to solve it. I found a paper that uses a current mirror (http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA410000) so making the needed modification I could achieve the current control. Thanks everyone for your assistance.
    Finally I make a kind of digipot, just like m.mokhtar recommends, using transistors and resistors just like I found in the L200 voltage regulator datasheet, well with the respective modifications for my needs. Thanks for your support.



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    Re: How to control output current of a TENS machine

    The resistance of a person's skin varies between people. Some people are sweating all the time so they have a low skin resistance. Other people have dry skin that has a high resistance almost like an insulator.

    So applying 30V to skin that is 200k ohms then the max current is only 30V/200k= 150uA which is 1000 times less than the 150mA you were talking about.



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    Re: How to control output current of a TENS machine

    So applying 30V to skin that is 200k ohms.
    As said, electrolyte gel is used with electrodes in eletrotherapy. This way a resistance down to 500 ohm is achievable. 150 mA however aren't under usual conditions (e.g. resosonable electrode size).



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    Re: How to control output current of a TENS machine

    If you apply a conductive electrolyte to dry skin then the electrolyte on the surface conducts, not the skin. Skin does not absorb the electrolyte unless the skin is deeply cut.

    Put a few hundred volts to deeply burn the skin then it conducts.



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    Re: How to control output current of a TENS machine

    Hi,


    I miss understood your question on my previous answer, 30 v will never allow you to get 150 ma, either to increase the frequency (the impedance of the human body reversely proportional to the frequency) or to increase your output voltage



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