Experimental Low Voltage, Low Milliamperes Device

Discussion in 'Power Electronics' started by srodnesky, Aug 6, 2016.

  1. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    Hi, I am hoping to built a battery powered, wearable unit that precisely outputs a constant voltage over a selectable 1 to 6 volt range and a constant amperage over a selectable 1 to 6 milliamp range. Does this already exist? If not, any suggestions would be appreciated. Thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Not possible.

    First of all, you have to provide a quantitative definition of your meaning of "precisely". Secondly, all electronic devices are dependent on temperature by the very nature of solid state physics. Are you going to maintain the wearable unit to 24 ± 0.1°C or is the unit expected to operate in a wider temperature range, e.g. -40 to +55°C ?

    Now, you cannot have constant voltage and constant current at the same time. Decide which of the two you want. The other will be determined by Ohm's Law, i.e. I = V/R

    In other words, I and V are interrelated, depending on the value of R.
     
  3. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    Hi, thanks for taking the time to write to me.

    First, when I speak of wearable, I'm thinking more on the line of 'Walkman' wearable rather than 'Fitbit' wearable. That will hopefully simplify assembly temperature constraints.
    Second, the milliamp range would need to be the most precise "± 1 milliamp within the 1 to 6 milliamp range. The voltage needs to be as close to the various 1 to 6 volt steps as possible.
    Since the probes would be touching bare skin, that would be the resistance load that would need to be considered.
    Thanks for your feedback.
    Stephen
     
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  4. Papabravo

    Expert

    Feb 24, 2006
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    You still don't get the picture required by Ohm's law. Once again in words of one syllable or less. Pick either current or voltage to be constant. The other quantity will be determined by the load resistance. In other words you have only one choice -- not two.
     
  5. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    What I was trying to say was that CURRENT needs to be constant because of the narrow range needed. We can still target a voltage when plugging the resistant of the load (in this case the passing of current between two points a couple of inches apart on the skin) can't we?
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Further to Papabravo's post, skin resistance is very variable, so if you applied a constant voltage to the probes the current would also be very variable. Alternatively, if you tried to force a constant current through the skin then the voltage needed to do that would have to vary considerably.
    Be aware that passing any current through a person's skin could be dangerous to a particularly sensitive person or one with a cardiac pacemaker fitted.
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    So what are you expecting to do with this 1 - 6 volts and 1 - 6 mA DC in direct contact with your skin? Slowly electrolyze the minerals out of yourself? o_O
     
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  8. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    Perhaps I am not using the terminology correctly. One can readily purchase an AC to DC power supply with the following characteristics: output voltages: 3, 4.5, 6, 7.5, 9, 12V DC, output current: 800mA (regulated)
    However, for safety, using a 9 Volt power source could easily handle the final 1 - 6 volt output requirements. Where the commercial power supply offers 800mA current output, the unit would only need a maximum of 6mA.

    Perhaps I am not using the terminology correctly. One can readily purchase an AC to DC power supply with the following characteristics: output voltages: 3, 4.5, 6, 7.5, 9, 12V DC, output current: 800mA (regulated)
    However, for safety, using a 9 Volt power source could easily handle the final 1 - 6 volt output requirements. Where the commercial power supply offers 800mA current output, the unit would only need a maximum of 6mA.

    A voltage and current combination within those ranges post no harm to a person as long a they don't have a pacemaker or any other embedded electronic device.

    There are, however, so interesting experimental possibilities. At least interesting to me.
     
    Last edited by a moderator: Aug 7, 2016
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Iontophoresis would become a problem long before cardiac tetany.
     
  10. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    Thank you, but those are not concerns for me. This is just another tool for an area of research I have been doing off and on for a long time.

    With all due respect, I am not attempting to recruit you to undergo this research avenue. From you comments, I doubt if you have more than a passing interest. If I am wrong Dr Robert Becker's books "The Body Electric" and "Cross Currents" might provide a starting point.

    I was hoping there was someone who could supplement my foggy knowledge of circuit design.

    If not, thank you all for your time.
     
  11. hp1729

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    Nov 23, 2015
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    Design 801 low voltage low current.PNG

    You can get regulated voltage and current limitation. Something like this for a starting point. Powered from a 9 V battery. Skin resistance could be as low as 5,000 Ohms, so current is going to be less than 1 mA. But skin resistance determines actual current.
     
    Last edited: Aug 6, 2016
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  12. Papabravo

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    Feb 24, 2006
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    What you have said is correct as far as it goes, but you are missing a crucial point. Just because a constant voltage supply is capable of delivering 800 mA does not mean that it will. For example a +12VDC supply will ONLY deliver 800 mA if the load is precisely 15Ω. Similarly, if the load resistance is 30 Ω the output current will be 400 mA. Is the picture starting to come into focus for you? If so great, and if not you need to stop trying to make the universe behave the way you want it to behave. It most certainly will not cooperate with you.
     
  13. hp1729

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    Nov 23, 2015
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    If the skin resistance is 10,000 ohms in order to get 2 mA through it you must apply 20 V. You can't force 2 mA through it ay a lower voltage.
     
  14. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    Yes, that is correct that 15 Ohms is the maximum load your 12 volt battery can handle and still deliver 0.8 amps, but we could still 'make the universe' behave and deliver 800 milliamps even if the end load is less than 15 Ohms by building compensating resistance into the circuit. Or by knowing the average load we will be encountering, we can plan for a reduced amperage output over the useful life of the power source. If you recall my target amperage range is only between 1 to 6 MILLIAMPS. If we assume that a good 9 Volt alkaline battery has an initial capacity of 500 Milliamps and peg the resistance at 1000 Ohms, we should have adequate power during at least the first half of the battery life.

    My question, is what circuit configuration can make that happen, while including a readout to track the output decay over the session (so I can know when to change the battery).

    Sorry, I answer the earlier post. The same answer basically applies to your post. I am assuming for this experimental series an average load (skin resistance) of 1,000 Ohms. The resistance of current trying to pass through (or even significantly deeply into) the body can go much higher of course. This is why the earlier concerns of Iontophoresis and/or cardiac tetany were misplaced. There are certainly protocols that can help bring the local skin resistance more in line with the 1,000 Ohm target (i.e.: salt water). Such concerns will be addressed once the equipment is available.

    Sorry, I was away from my computer for several hours and just discovered your post. THANK YOU SO MUCH.


    My target amperage range is between 1 to 6 MILLIAMPS. If we assume that a good 9 Volt alkaline battery has an initial capacity of 500 Milliamps and peg the resistance at 1000 Ohms, we should have adequate power during at least the first half of the battery life.

    My question, is what circuit configuration can make that happen, while including a readout to track the output decay over the session (so I can know when to change the battery).

    I'm aware that skin resistance can be MUCH higher, but the 1000 Ohm load should be enough and the inline current readout will provide realtime feedback regarding the effective output, which is determined by the combination of battery life cycle and circuit load (skin resistance).

    Thanks again for you help!
     
    Last edited by a moderator: Aug 7, 2016
  15. Papabravo

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    Feb 24, 2006
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    I can see now that no amount of logic and reason will dissuade you from this quest. So go do whatever you want and come back with a story of success or whatever else happens.
     
  16. tcmtech

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    Nov 4, 2013
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    Why? What purpose would that serve other than to make the load appear to be 15 ohms for no reason other than to use the full 800 mA capability of the power supply? o_O
     
  17. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    For heavens sake! I was merely extending the analogy used in earlier post that were trying to throw water on a simple technical request.
    For your information I am simply trying to replicate research that has been accomplished several times over the past 60 years. The papers in question state the volt and current settings used, but like most abstracts do not include detailed equipment composition.

    I have no idea why you and papabravo feel you have the right and/or responsibility to attempt to discourage me from pursuing this experiment. Please don't waste your time or mine.
     
    Last edited by a moderator: Aug 7, 2016
  18. hp1729

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    Nov 23, 2015
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    If you are measuring 1,000 ohms then you are in the ballpark. We can get back to the schematic suggested. How precise a voltage are you needing? A set voltage or adjustable? Set current or adjustable?
     
  19. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 801 low voltage low current adjustable.PNG
    Rough draft. adjustable.
     
  20. srodnesky

    Thread Starter New Member

    Aug 6, 2016
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    The various experiments used voltages between 2 and 6 volts and current between 2 and 6 milliamps. One experiment cited "2.4 milliamps at less than 3 volts." So adjustable voltage and amperage within the above specified ranges would be best if there was a way to insure accurate digital readouts during adjustments.

    Thanks
     
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