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What to do with 0.5 V and 25 mA?

Discussion in 'General Electronics Discussion' started by thmsclrk, Jul 12, 2013.

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  1. thmsclrk

    thmsclrk

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    Jul 12, 2013
    Dear all,
    I'm actually a biologist/chemist by trade and have built a DC fuel cell for demonstrations that can generate about 0.5 V and 25 mA. At the moment we wire this to a multimeter to show a current, but I'd like to use something else. I know there are 2 mA LEDs out there but I need a way to boost the voltage without resorting to building new cells or quickly rewiring capacitors from parallel to series. I hear there are joule thieves that might do this but the parts seem very difficult to find.
    Any suggestions about what I might do with this (other than bin it) would be fantastic!
     
  2. BobK

    BobK

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    Jan 5, 2010
    I built a Joule Thief from parts in my cabinet, so, not hard to find at all. The only questionable one is the ferrite torroid, which I reclaimed from a random inductor I had taken out of something.

    Bob
     
  3. thmsclrk

    thmsclrk

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    Jul 12, 2013
    Thanks for the quick response bob! I try a joule thief with a toroid, but it didn't produce any current, probably because I didn't have the right transistor that could operate with this current. Any suggestions which might be the best components to build for this current/voltage?
    Thanks,
    Tom
     
  4. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Pretty much any general purpose NPN will do. I used a 2n3904. Did you try reversing one of the transformer windings connections? They have to phased correctly in order for it to work. Also, did you try using a 1.5V battery? It might not work at 0.5V. 25ma should be plenty to operate it.

    Bob
     
  5. thmsclrk

    thmsclrk

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    Jul 12, 2013
    Thanks Bob, you've hit my problem - I want to try and get all the power from the fuel cell, which means that I need to get the voltage up to over 1.5 V from the rather poor 0.5 V supplied from a single cell. It cost about £400 to build the current prototype so I don't really want make more..
    Tom
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    One possibility is to arrange a push button switch so that it reverses the polarity of the DC, and then pass this (now 1V ptp) square wave through a multiplier made with schottky diodes).

    You *might* be able to generate a high enough voltage to light a led. You'll have to keep pressing the switch really quickly though.

    edit: and interesting possibility is to create SMPS that will operate from the output voltage it generates and to use this to boost the voltage of your battery. Whilst this would almost certainly require a "boost" to start it, it *might* continue to operate as long as the load on it was low. I had a thread around here somewhere about a SMPS boost regulator made using a CMOS hex schmitt trigger, Some of these are specified to quite low voltages, and probably operate to even lower voltages. Coupled with a mosfet with a very low Vgs(th) you *might* be able to make something.
     
    Last edited: Jul 13, 2013
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    Here it is... https://www.electronicspoint.com/sm...t-trigger-oscillator-t251353.html#post1486958

    You could try to run the chip from Vout rather than Vin.

    A momentary contact switch across the mosfet would give you a "starter". Several components and I dare say some values would need to change.

    You may be able to run it open loop and use a LED (maybe a white one) as a shunt voltage regulator.

    It's a cute enough idea that I might try it :)
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That's a good question. The fundamental problem is that 0.5V is not enough for most circuits to make any use of. Linear Technology have developed some ICs that are specifically designed to operate from extremely low voltages. Some of them might be of use to you. See http://www.linear.com/products/energy_harvesting
     
  9. thmsclrk

    thmsclrk

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    Jul 12, 2013
    Thanks to you all - there does seem to be some opportunites in the links you provided. The 'LTC3108-1 - Ultralow Voltage Step-Up Converter and Power Manager' seems like it might have potential.
    Following the peltier idea I found links to this ultralow power joule thief made by Magpwr. He used 4 JFET transistors in parallel. If I could get some of these then I might be on to something. Also need a bigger torroid.

    http://www.overunity.com/13175/25mv...g-our-body-heat-free-energy-247/#.UeOvh1PahOs.

    Tom
     
  10. gorgon

    gorgon

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    Jun 6, 2011
    Why not use two fuel cells in series to generate 1.0V @25mA. This voltage should be easier to use for your demo.
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    He answered that:
     
  12. thmsclrk

    thmsclrk

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    Jul 12, 2013
    Hi Gorgon,

    It cost around £400 to build this prototype, most of it engineering time. It is to show how power can be created from bacterial metabolism. I admit if I had my time/money again I'd build it as two cells in series. Hindsight is a wonderful thing..

    Thanks,
    Tom

    Edit: Thanks KrisBlueNZ - you're faster on the keyboard than me.
     
    Last edited: Jul 15, 2013
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