controlling and measuring current

Discussion in 'General Electronics Chat' started by gary74, Oct 9, 2008.

  1. gary74

    Thread Starter Member

    Sep 1, 2008
    this is a bit confusing. i am setting up a hydroxy cell and am getting a starting reading of 10 ohms of resistnce through my electrolyte.. if it heats up I understand that the resistance drops, but if i read the resistance after the cell has been energized and then turned off.. i get resistance in only one direction. Why? Its almost like it is becoming a battery or capacitor and retaining a very mild charge.. Still I am drawing too much current at times and need to control the current without changing the electrolytic mixture. Are there FET transistors that are large enough to control 40 amps with a forward voltage of 12v or less on the gate? Aren't FET the transistors that are voltage controlled current control devices?
  2. mik3

    Senior Member

    Feb 4, 2008
    Yes there are but you want just to switch the current on/off or make a constant current source or to vary the current linearly?
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    Your cell might read in only one direction because of deposition on the plates. The electrolyte gets ionized, and the components get deposited on the cell plates. They work a bit like the coatings on the foil in electrolytic capacitors.

    Electrolysis cell resistance effectively goes down because chemical reactions double in speed for each 10 deg. C rise in temperature. I doubt if the actual resistance changes significantly.

    There are FET's able to handle vastly more than 40 amps. That 12 volts on the gate should be considered a minimum voltage to place a power FET into saturation.

    You might find this thread interesting -
    Last edited: Oct 9, 2008
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    I hate to nit-pick, beenthere, but just for the record, saturation has a different meaning in MOSFETs than it does in BJTs. See the graph in the Wikipedia entry on MOSFETs.
  5. gary74

    Thread Starter Member

    Sep 1, 2008
    i want to control the current regardless how low the resistance goes so that from a 120 amp 12v battery i never draw more than 40 amps, actually alittle less... i was thinking that with the right paramaters a lug fitted fet and using the 12v source for the gate control i could set the max current flow to 40 amps or less depending on the resistance or lack there of. the resistance changes as the cell heats and the electrolizer gets active.
  6. gary74

    Thread Starter Member

    Sep 1, 2008
    i am using an old catalog from allied electronics and probably just don't know what i am looking for.. but FET seems to be the thing i want... one rates i suppose at perhaps 120 amps and then adjust it down with the supply votage to allow 40 am-s max. i really need just to find a sec sheet on their max am potential and all first.. i have not found what i need to read yet on the net.. i am sure its there.. just time and patience get limited. Bright note however after all the stuff i have experimented with i have seen this hydro cell working on a vehicle and the guy that has shown me has not even got the most efficient system of cells and plates. he is getting about \48% increase in fuel economy on gas.. it sorta surprosed me with no more gas than he is creating. This is a mn with several different vehicles running it in martinsville va
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    That is not quite the way to go about managing the current. You really should read that thread I gave you the link to. Especially if you are attempting to use that PWM circuit built around an LM324 op amp. That circuit can't work as advertised. But one of our contributors has come up with one that will work.

    If you see only a few bubbles and hear a claim of 48% gas mileage increase, get a tight grip on your wallet. That is not even remotely possible. People claiming to have efficiencies greater than what physics says is possible are running a scam to take your money.
  8. SgtWookie


    Jul 17, 2007
    Interesting piece of information... so if you applied 12.6V across your 10 Ohm cell, Ohm's Law says I=E/R, so 12.6V/10 Ohms should result in 1.26A current.

    Apparently, once current is applied, the resistance drops. I hadn't expected that!
    Power MOSFETs are best utilized as switches; either ON or OFF. If you attempt to operate them in the linear region (like a variable resistor) they will dissipate (waste) a great deal of power in the form of heat. If not heat-sinked properly, they will quickly burn up.

    There are some schematics floating around on the Web that use PWM (Pulse Width Modulation) to try to limit the current; the few I've seen fail to use an inductor. As a result, they are limiting their average current, but during the non-powered period they are not producing gas, and during the powered period they are providing far too much current and heating up the electrolyte.

    The way to control the current efficiently is to charge an inductor (get current flowing through it) using a MOSFET that has it's gate controlled using PWM (Pulse Width Modulation) along with a current sensing resistor to monitor how much current is flowing through the cell. It's possible to use the current sense resistor to maintain the desired current automatically.

    See the link Beenthere posted. It's a rather long thread.
  9. John Luciani

    Active Member

    Apr 3, 2007
    What you want is a constant-current source or sink.
    For a schematic of a current sink scroll down to the electronic
    load section at

    The problem you will have is power dissipation in the MOSFETs.
    In order keep the current constant the op-amp
    will change the Rds(on) of the MOSFET. As the resistance in your
    cell decreases the MOSFET resistance has to increase to maintain
    constant current. Your thermal design has to be able to handle the
    maximum power dissipation.

    Fortunately the current-sink circuit is low-cost and you can parallel
    multiple cells. The current sink does waste energy. A current source
    would cost quite a bit more but would drastically reduce the waste.

    (* jcl *)