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current limit?

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  • toolguy500
    yall, what do i do to limit current in a circuit? i have about a 12v source i wanna connect to about a 1 ohm load, limit to about 5A and have the supply stay
    Message 1 of 4 , Nov 1, 2002
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      yall,

      what do i do to limit current in a circuit?

      i have about a 12v source i wanna connect to about a 1 ohm load,
      limit to about 5A and have the supply stay around 12v.

      suggestions?

      thanx,

      jim
    • Ned Konz
      ... Depends on how much efficiency you want. If you don t care about power or heat, you could use a linear circuit (like two power transistors or an LM338) to
      Message 2 of 4 , Nov 1, 2002
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        On Friday 01 November 2002 12:43 pm, toolguy500 wrote:

        > what do i do to limit current in a circuit?
        >
        > i have about a 12v source i wanna connect to about a 1 ohm load,
        > limit to about 5A and have the supply stay around 12v.
        >
        > suggestions?

        Depends on how much efficiency you want. If you don't care about power
        or heat, you could use a linear circuit (like two power transistors
        or an LM338) to drop the 7V. But you'd be dissipating 35W of heat:
        this is more than the 25W you're putting into the load!

        Much higher efficiency would be to use a switching power supply that
        could give you 5V at that current.

        Do you really need current limiting, or just a 5V supply? That is, is
        this a resistive load where you could use a fixed voltage?

        --
        Ned Konz
        http://bike-nomad.com
        GPG key ID: BEEA7EFE
      • Dairyo Gokan/Studio NAND L.L.C.
        Hi, There is no way limit current 5A for 1 ohm load with keeping supply voltage 12 volts. To limit current 5A for 1 ohm load, you must reduce supply voltage 5
        Message 3 of 4 , Nov 1, 2002
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          Hi,

          There is no way limit current 5A for 1 ohm load with keeping
          supply voltage 12 volts. To limit current 5A for 1 ohm load,
          you must reduce supply voltage 5 volts or less.

          The physical laws and phenomenons are fair for everything in
          anywhere.

          To import a current limiter to your project, you have to detect
          load current and feedback it to the voltage regulator to control
          output voltage.

          Generally, a resistor with low resistance value is used to detect
          the load current. By measureing the voltage drop between both sides
          of the resistor, It is possible to know that how much current is
          flowing in the closed circuit.

          +---------------------------------------------------------------------+
          | From : Dairyo Gokan mailto:gokan@... |
          | Org. : Studio NAND L.L.C. http://www.studio-nand.com |
          +---------------------------------------------------------------------+
          ----- Original Message -----
          From: "toolguy500" <toolguy500@...>
          To: <SeattleRobotics@yahoogroups.com>
          Sent: Friday, November 01, 2002 12:43 PM
          Subject: [SeattleRobotics] current limit?


          > yall,
          >
          > what do i do to limit current in a circuit?
          >
          > i have about a 12v source i wanna connect to about a 1 ohm load,
          > limit to about 5A and have the supply stay around 12v.
          >
          > suggestions?
        • Mark Medonis
          Well, you have a couple of options. You can add a 1.4 ohm resistor in series, giving you 2.4 ohms total. Ohm s law says V = I * R, so 12V / 5A = 2.4 ohms. Or,
          Message 4 of 4 , Nov 1, 2002
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            Well, you have a couple of options. You can add a 1.4 ohm resistor in
            series, giving you 2.4 ohms total. Ohm's law says V = I * R, so 12V / 5A =
            2.4 ohms.

            Or, you could use a bipolar resistor and a VERY big heat sink, adjust the
            base voltage to get the current level you want. The transistor acts like a
            variable resistance in that case, although you can set it to a particular
            current level independent of supply voltage.

            A FET can do this trick as well.

            Now, if your load was inductive, you could use PWM to control the current.
            This is the most efficient method by far. But for a purely resistive load,
            it doesn't really work. You would get 12A, then 0, then 12A, then 0.

            Mark Medonis


            > From: "toolguy500" <toolguy500@...>
            > Date: Fri, 01 Nov 2002 20:43:04 -0000
            > To: SeattleRobotics@yahoogroups.com
            > Subject: [SeattleRobotics] current limit?
            >
            > yall,
            >
            > what do i do to limit current in a circuit?
            >
            > i have about a 12v source i wanna connect to about a 1 ohm load,
            > limit to about 5A and have the supply stay around 12v.
            >
            > suggestions?
            >
            > thanx,
            >
            > jim
            >
            >
            >
            > Visit the SRS Website at http://www.seattlerobotics.org
            >
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