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Inner resistance: supplying correct current and voltage?

Discussion in 'Electronic Basics' started by Björn, Jul 30, 2003.

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  1. Björn

    Björn Guest

    A chip (a small digital radio reciever chip) should have 5 volts and
    4,5 mA according to its specifications. Now, if it had 1111 ohm of
    inner resistance, I would only need to connect it to a 5 volt battery,
    and the current would be 5/1111 = 0,0045. If the inner resistance had
    been smaller, I could've used external resistors to arrive at the
    target current and voltage. Not a problem.

    However, I measure the inner resistance of the reciever to 18000 ohm.
    Since 5/18000 = 0,00028 (0,28 mA) and 18000*0,0045 = 81 volt, I cannot
    achieve the specified voltage and current, can I?

    This is of course a very basic question. Anyone who has connected
    anything to anything must have had the same problem, and found a
    clever solution which unfortunately escapes me :) Should I maybe
    ignore the inner resistance I've measured (when the chip is off, of
    course) and just put 5 volt between the VCC and GND pins? But then,
    what's the point with specifying a current in the first place?
     
  2. No - you run the thing from the advertised 5 volts, and let the
    current fall where it may.
    The "inner resistance" you measure or calculate is not just resistance
    - it is actually the overall result of the internal circuit of the
    chip, including transistors which may react differently with different
    voltages applied to the part.

    When you measure the "resistance" of the chip with an ohmmeter, you
    will be applying much less than 5 volts to the circuit, so most of the
    transistors and diodes inside the chip will not be conducting.

    Yes. The part is designed to operate from 5 volts, and will draw
    whatever current it requires. According to your spec, the "typical"
    current will be 4.5 mA - but the actual current will vary with
    production variations in the devices, and probably also with what the
    device is doing at the time, and what it is driving.
    The current is specified so that the equipment designer who uses this
    part can determine how big a power supply he will need.
     
  3. Wade Hassler

    Wade Hassler Guest


    The current-consumption is specified so that you may size your power
    supply appropriately: it must provide enough current to run your
    receiver and
    any other loads, all at the right voltage.
    Wade Hassler
     
  4. Just hook the component to the specified voltage and it will draw the
    current it needs to operate by its self.

    Josh
     
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