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Designing a variable current circuit

  1. Aug 2, 2011 #1
    Ok here is my problem

    I have a system which is linked into a car. The system is an electrolysis unit designed to run at 5A. The system is wired in as per the 1st attached diagram.

    So positive and negative are taken from the battery and placed into a Ring Smartcom Towing relay (PN RCT460). This simply stops the unit from working unless the alternator is running (engine on). I can replace this by simply finding an ignition live wire and tapping into that. The live is then fed into the electrolysis unit where it does its thing.

    My problem I need resolving is that I have to tune each unit to the vehicle it is being installed on and this is currently done by changing the electrolyte strength to either allow more or less amps. This is a tricky and time consuming process.

    I want to fit a simple unit into the system that will allow me to have a single concentration of electrolyte and throttle back the current being sent to the unit so that I can fine tune the system by simply adjusting a screw or similar.

    Initially I thought a simple potentiometer would suffice until I read that they wont handle much more than 1W of power. At full power the system is using 69W at 13.8V (assumed alternator output).

    So I have done some searching and found diagram 2 which will hopefully work but it aint cheap to build and I am wondering if it is a little over engineered for what I need to do. I am also not sure of the power requirements on the resistors in the diagram and the possibility of removing LM117 as this is the most expensive part and possibly does not need to be there going off other diagrams I have found. http://www.national.com/mpf/LM/LM338.html#Overview" [Broken]. Also what do the capacitors do in this diagram???

    So are there any clever people out there that can help me in my quest to solve this apparently simple problem (in my head anyway although I am not anyway a qualified EE).

    Over to you guys and gals
     

    Attached Files:

    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 16, 2011 #2
    You need a switching power supply. Its not going to be an easy thing to do.
     
    Last edited by a moderator: Aug 17, 2011
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