Constant Current control cct. help please

Discussion in 'The Projects Forum' started by PhillT, Feb 27, 2010.

  1. PhillT

    Thread Starter New Member

    Feb 26, 2010
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    Although I am capable of building a project from a circuit diagram, I have not got the skill or education to design anything more than simplistic circuits, so I am here to ask for assistance from those with the education & skills.

    My project is to build a circuit that will result in a constant output current of 3.8ma supply to a varying resistive load. I have a 36 volt dc power supply unit (from an old printer) as the power source.

    The resistive load consists of two electrodes in an electrolyte which begins as Distilled Water, so very high resistance, which over time will reduce resistance as ions and particles increase in the electrolyte, but I want to maintain 3.8ma (well, as near as possible anyway) as the resistance of the load drops. This is vital to ensure the correct particle size and ratio of particles to ions in the solution. (Colloidal Silver)

    Any help in this regard will be greatly appreciated.
     
  2. PhillT

    Thread Starter New Member

    Feb 26, 2010
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    Far easier than I imagined.


    Cheers Alberto :D
     
  3. robert.e.lee

    New Member

    Aug 21, 2011
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    After doing some research on the antibacterial properties of colloidal silver, I decided to make some.

    Yet I haven't been able to find any information yet about to to keep a constant current while the resistance is getting lower and lower as time goes on.
    If you or some other knowledgeable person could post what I can do to keep the current from a AC to DC wall adapter constant it would be greatly appreciated.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I hope we're talking about topical application, not internal? Otherwise, I'd say this is too unsafe (eating the silver, not the circuitry) to discuss here.
     
  5. robert.e.lee

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    Aug 21, 2011
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    Topical or for cleaning\sterilizing purposes.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Yes, remember that bluish guy who took the healing properties of silver too seriously and chemistry too lightly? http://www.purestcolloids.com/blue-man.php
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Nevertheless, constant current source is as simple as a LM317 and two resistors. One resistor like this and another between out and gnd to maintain the 10mA minimal loading current, if your set current is lower than that.

    Tell us your input voltage and wanted current and I will help with the values.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, an LM317 needs 10mA to provide guaranteed regulation, but an LM317L only requires 5mA. You might be able to get by with just 3.8mA current and one 329 Ohm resistor connected from OUT to ADJ to get fairly decent regulation. However, if the water has really high resistance, you won't be able to apply enough voltage to any LM317 to get the current up to 3.8mA.

    You can get ~329.3 Ohms by wiring a 470 Ohm and a 1.1k Ohm resistor in parallel.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I tried a standard LM317 (TO220) recently and it regulated 1.25v ok with a 1k resistor, which was 1.25mA.

    So you should be able to use LM317 for 3.8mA with no problems using a 329 ohm (330 ohm) resistor.

    The "minimum 10mA" spec in the LM317 datasheet is only required to ensure proper regulation for the full range of operation of the device including transients etc.
     
  10. SgtWookie

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    Jul 17, 2007
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    Jack,
    None of the links you tried to post worked.
     
  11. robert.e.lee

    New Member

    Aug 21, 2011
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    sorry for the late reply, ohk so the AC->DC adapter I'm going to use (I'll pick it up today) is 30v, I think something like 530mA. Apperently I only need a few mA to get the electrolysis started like 1 -10, so this should be just right for this project.
    BTW I have next to no experience in electronics so if you could explain things in as common terms as you can I should have no problem. Thank you all for your time and help, I could never do this without it.

    Also I am going to test the current and voltage from a small solar cell that I have, I might try rigging up a setup with one or more of those as the power source, I'm just not sure if they can push enough power for it. I'll post the readings when I get them.
     
  12. SgtWookie

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    Jul 17, 2007
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    Most likely, the big problem you will face initially is not having enough voltage to obtain the desired current flow through the water. If the water is distilled and there are no impurities (minerals, metals, etc) dissolved in the water, it will be an insulator. You would have to have a fairly high voltage in order to get any current at all flowing through it.

    I = E/R, or in words, current through a resistance, is equal the voltage across the resistance. This is a part of Ohm's Law.
     
  13. robert.e.lee

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    Aug 21, 2011
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    So what if I start the reaction with the electrodes close then once there's some silver in the water move them farther apart.
     
  14. SgtWookie

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    Jul 17, 2007
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    You can try that and see if it works.
     
  15. robert.e.lee

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    Aug 21, 2011
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    Ohk so I just got the adapter its 30vdc .4A. I'm pretty sure that's too much current, but I may just put the electrodes (coins) farther apart.

    So to my understanding, the rate at which the silver particles come off the electrodes increases rapidly as the resistance drops. So what I am wondering is if I can make the rate stable vs. exponential.
     
  16. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    .4A is the maximum current the adapter is capable of. It will not produce that much if you don´t load it.
     
  17. robert.e.lee

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    Aug 21, 2011
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    So, say I wanted to limit the current to 5mA. How would I do that?
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Buy an LM317L, connect it like this and use a 250ohm resistor. Look here to see how to connect it, its on page 1.
     
  19. robert.e.lee

    New Member

    Aug 21, 2011
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    I got the LM317T they didn't have the L version, I don't know what the difference is. And if I=E/R then how will a 250 ohm resistor provide 5mA. .005mA=\=30V/250ohm. I'm sure that I'm missing somthing. Thanks for you time btw.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    The LM317 family of regulators has a reference voltage, called Vref in the datasheet. It's the difference between the OUT and ADJ terminals. The regulator attempts to keep that difference at 1.25v (nominally) by sourcing current from the OUT terminal.

    1.25v / 250 Ohms = 5mA.

    Vref can vary anywhere from 1.2v to 1.3v and still be within manufacturer's specifications. However, this is the easiest, lowest-component-count way to create a reasonably constant current source.
     
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