Serious Transformer Problem!

Discussion in 'General Electronics Chat' started by jegonz, Apr 14, 2008.

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  1. jegonz

    Thread Starter Member

    Jan 25, 2008
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    Hello, I wrote already some questions about transformers before, but still can´t get to what I need...So I think I have a serious mis-conception in all this. So please if anyone can help me to understand, I´ll be grateful.
    The thing is that I have a square wave like the attached one... this is an input of 9Volts peak, Burst variable frequency of 1KHz to 6KHz, and a burst duty cycle of around 30 to 50%, being the burst frequency from 30 to 70Hz, and I need a similar waveform in the other side of the transformer but with 90Vpeak and 0.1Apeak (or more)... the spike has to be small also.
    The load is the human skin :cool: , that behaves like a serial RC circuit... and the impedance is approx. 500ohms (this value is and approx of the rms voltage and current mesuare in the electrodes plug to the skin)
    I tryied an audio transformer OMG-3, 3W(1:24) with good results, but not enough since the voltage drops a lot when put into the load, so the maximun voltage obtain is around 50Vpeak. Since the secondary has high resistance, and the load is low resistance, then the power is not enough to give the 90V i need... I think this is already a mess, so please if you understand anything that im saying, please let me know!:(
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    From an former Biomedical Electronics Tech...What are you trying to do? Not the electronics side...the clinical side.

    Ken
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
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    Impedance matching can indeed be a frustrating process at times.:rolleyes:

    For a 500 Ohm load at 90Vp, current will be 180mAp. Input current at 9Vp will need to be 1.8Ap. With a50% duty cycle, your supply must provide more than 8W. Is your supply big enough?

    To minimize spikes, try a small capacitor across the transformer secondary.
     
  4. mik3ca

    Active Member

    Feb 11, 2007
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    You could try RC coupling instead of transformer coupling. I think transformer coupling uses more power. I use RC coupling all the time.

    It is the capacitor I use that separates stages, not transformers.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You realize that what you're attempting has life-threatening implications, correct?

    With those kinds of signal levels, you could cause ventricular fibrillation if the current path happened to be through someone's chest cavity.
     
  6. mik3ca

    Active Member

    Feb 11, 2007
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    I will agree. 90V is a lot for voltage.
    Why do you need 90V?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Agreed, that is a lethal voltage level to apply to a person.
     
  8. recca02

    Senior Member

    Apr 2, 2007
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    Well, he never said the human skin belonging to an alive human:p:eek:.

    Kono denki wa kiken dessu!(did I get that right?)
     
  9. RaoulDuke

    New Member

    Apr 11, 2008
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    Making a TENS device....or EMS?

    I think I still have a schematic of a TENS/EMS device I designed a couple of years ago. If I find it at home later I will post it.

    The design used a FET and an audio transformer, and a PIC to make several types of waveforms for different treatment protocols.
     
  10. jegonz

    Thread Starter Member

    Jan 25, 2008
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    jeje, thanks for all the replies. Im doing Extracellular Muscle nerve stimulation, to help hemiplegic patients recover some motion that can help them to walk. I know this voltages are really big and dangerous if you are using it in a wrong way. But most of the devices that are in the market use up to 100V. Because we need to bypass the skin impedance to reach the nerve termination with around 100mA to create action potentials, therefore movement with enough torque and velocity. Of course I'm talking about 90Vpeak, in a pulse that maybe is around 43us to 0.48mS, and the burst is of 30% of a 30 to 70Hz PWM, that means that the total average voltage elicit is small, but is the peak what I need to fix to create more torque.
    The thing is that what Im trying to do is a portable device, so from a 9V 1500mAh battery (3A max) I'm trying to get the desire voltages and currents, but the problem like thingmaker said, impedance matching problems is difficult. I thought of using capacitors (like a capacitor pumps), but that might be dangerous because if not in use, the electrodes will have really high voltage that can discharge on you all in the same moment if you touch it and then something really wrong might happen, so I will have to make a discharge circuit making it more complex. Of course, if that what i need i will have to do it...

    I did one putting normal 9V batteries in series to get 54V, and it gave good results... but for a portable device, no very good. Besides normal bateries are 200mAh, so they are no good. The problem is that now that we are using the transformer, the output shape is not that square, so this means that even if I get the 50V peak, the total voltage in the small time is less that 50Vpeak, so the current also.

    Now, Why do I need 90V... because the system controls the output voltage and I want a large range of voltage to operate. Maybe at the end I will need less than that, but for now I need to test, so that is why I'm aiming 90V. Anyways, don't worry I test the ciurcuits outside in phantoms before plug in it to me or anyone else, not planning to kill anyone! :rolleyes:

    Now for normal people, this is too high, I can make a normal leg move with less than 20V, 10mA, so a small transformer is enough. But for hemiplegic that body caracteristics change, so we need more current and more voltage, the last test I made to a hemiplegic person I reached 50Vpeak I got a good movement, but not enough.

    For RaoulDuke> It will be very generous of you if you can share with me that info. :)

    And to recca02, you got it right! ;) omedetou!

    thanks to everyone!
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I suggest that further discussions about this type of thing would be better taken off the forums.

    Such experimentation, IMHO, should only be conducted under the direct supervision of one who has earned a medical degree, and who is a currently licensed practitioner.

    While much good may become of such experimentation, this is not a realm for laypersons who may be tempted to dabble.

    I request that this thread be closed to further discussion.
     
  12. jegonz

    Thread Starter Member

    Jan 25, 2008
    19
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    I know what you mean by that, but at least for me I have a degree in biomedical engineer and another in control engineer, also any of the experiments I do are supervised by rehabilitation certify doctors. So the thing is that I have a problem in the design, due to lack of knowledge and practice I have in the transformer/power area, and spite of a lot of research I have made I can't get to answer. And sadly I don't know anyone that have the knowledge and practice in this area. That is why I came for help in this forum, so maybe some one would be able to help me. In the botton line the problem I have is a circuit design problem, not an application problem. But of course the application is important to make a design.

    So, if the thread is blocked, then I will understand no problem at all. Anyways thanks to all the people that help me. :)
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    I understand; however this forum is open , and your application is one not to be trifled with by other than highly trained professionals.
     
  14. jegonz

    Thread Starter Member

    Jan 25, 2008
    19
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    yes that is true. So will continue my research! anyhow, Thanks for the help! :)
     
  15. thingmaker3

    Retired Moderator

    May 16, 2005
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    I'm of two minds about this. I'd like to continue the discussion, but I understand Sgt. Wookie's concerns.

    I'm the only Moderator awake this late, but we should have a decision within the next 8 hours or so.

    I'll err on the side of caution, and lock the thread pending review.

    Jegonz, please check back again to see if the moderating team decides to open the thread again.
     
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