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Is a current sink the opposite of a current source? — Parallax Forums

Is a current sink the opposite of a current source?

po2lepo2le Posts: 76
edited 2008-10-10 23:55 in General Discussion
Is a current sink the opposite of a current source?· In other words, a 25ma sink will draw 25ma no matter what the voltage applied?· As opposed to·a 25ma current source which provides 25ma to a circuit.confused.gif

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-10 23:01
    A current sink connects the output point to ground. A current source connects the output point to the power source.

    The amount of current depends on what's available. A 25mA sink will draw 25mA if that's available. If the source can only supply 15mA, that's what will flow. For that matter, if the source can supply more than 25mA, the Stamp or Propeller I/O pin will draw as much as is available until it burns out.
  • po2lepo2le Posts: 76
    edited 2008-10-10 23:25
    I'm thinking of mesh analysis, where a current source can be converted to a voltage source and vica-versa. Is this a different situation? It has been a number of years since I've worked with these things, and its taxing my memory.

    In mesh analysis, could a current sink be shown as one of the internal currents? (small letter i's). For instance, a current source in parallel with two resistors, with a voltage applied across all three. In this case you would have three loop currents, could one of them be considered the current sink?

    po2l
  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-10 23:33
    I think you're complicating the situation. You could download the Microchip PIC 16C57 reference manual which shows a functional schematic of the I/O pin structure used in the BS2. You'll notice that, in output mode, it consists essentially of two MOSFETs, one to Vdd, the other to Vss, only one of which will be turned on at a time.
  • po2lepo2le Posts: 76
    edited 2008-10-10 23:45
    When the pic16f57 data sheet states that the max. current sink of the input ports (pins) of that chip is 25ma, does that mean at a particular input voltage?
    Mike Green said...
    (The amount of current depends on what's available. A 25mA sink will draw 25mA if that's available. If the source can only supply 15mA, that's what will flow. For that matter, if the source can supply more than 25mA, the Stamp or Propeller I/O pin will draw as much as is available until it burns out.)
    In other words, that 25ma sink is not a constant, but just a maximum under which the input currents must be kept without burning out the stamp.

    How does one calculate the output impedence of a previous ckt. in order to match the pin input ?
    Maybe I'm making this way to complicated.

    po2l
  • po2lepo2le Posts: 76
    edited 2008-10-10 23:50
    Sorry, I am making this much more complicated than need be. But, I'm relearning a few things. Thanks for the help.

    po2l (Patrick)
  • Mike GreenMike Green Posts: 23,101
    edited 2008-10-10 23:55
    You're mixing up input mode and output mode. What I've described is the output pin structure. When the upper MOSFET is turned on, there's a low impedance connection between the I/O pin and Vdd. When the lower MOSFET is turned on, there's a low impedance connection between the I/O pin and Vss. When neither MOSFET is turned on, the I/O becomes a high impedance defined mostly by the leakage through the MOSFETs and the reverse connected protective diodes connected between the I/O pin and the power rails (Vdd and Vss). There's also an MOSFET gate that provides the actual input to the microprocessor. Look at the diagram in the Microchip datasheet.
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