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A question about current

Discussion in 'Electronic Basics' started by Dana Raymond, Aug 30, 2003.

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  1. Dana Raymond

    Dana Raymond Guest

    If you are trying to connect your HDD Controller to some custom electronics,
    then, yes, 20mA is going to be a problem. Thats a pretty high current, so
    you will need to add a buffer of some sort. However, a 74HC04 will never
    present that kind of input load, so there's some confusion here. My question
    to you is: Why do you think the load is 20mA?

    To answer the 3.3V to 5V question can you provide the specific logic
    families (or part numbers) of the driver and the load? The answer is complex
    and is based on the kinds of families involved.

    Ioh is specifified as a negative number because current is flowing OUT of
    the pin, not in.

    Hope this helps.
    Dana Frank Raymond

  2. The minus in the current above simply indicates that the current when the
    output is LOW will be of opposite direction to the current when the output
    is HIGH. Another way of saying it would be that the HDD controller's output
    can "source" 2mA and can "sink" 6mA. Usually source currents are designated
    positive and sink currents negative, it seems that it's the other way around
    in your case, but you get the idea.

    Regarding the amount of current the inverter's input will draw, don't worry,
    it will be nowhere near 20mA! Logic inputs usually have high impedance,
    which means that they only require very little current to be driven (I think
    it's less than 2mA for TTL, only uA for CMOS). You can find this info in the
    data sheet of the IC/device you're using. For example, the input of your
    high-speed CMOS inverter will draw a maximum of 1uA = 0.001mA. Don't worry
    about it, nothing will get damaged.

    Again, nothing will get damaged, but it may not work. You have to make sure
    that the 5V input's HIGH and LOW voltage thresholds are within the 3.3V
    output's HIGH and LOW voltage levels. For TTL, the input thresholds are at
    least 2.0V for HIGH and at most 0.8V for LOW. For your HC inverter they
    depend on Vcc. Again, they can be found in the data sheet.

    Hope this helps.

    Costas Vlachos Email:
    SPAM-TRAPPED: Please remove "-X-" before replying
  3. Edward

    Edward Guest

    Hi everybody,

    I have a specification for a hard disk drive (HDD) controller and it has the
    following text:


    HIGH Voh = 4.6V ~ Vcc (Ioh = -2mA)
    LOW Vol = 0.6V (Iol = 6mA)

    My questions is, if the output signal from the HDD controller is attached to
    an input, which requires 20mA, could any damage be done the HDD controller?
    Am I right in assuming that an input would require a certain amount of
    current? As an example, lets say I connect the output signal from the HDD
    controller to an inverter (74HC04), I am assuming that the inverter will try
    and pull a certain amount of current from the HDD controller. Also, how
    come in the above text the Ioh parameter is minus 2mA, why is it minus and
    not positive?

    My second question is similar to the first. If I have a 3.3V output and I
    connect it to a 5V input, could any damage be done to the 3.3V system? I
    guess the answer is related to the first question.

    As you can probally tell from the above, I don't really know what im talking
    about so if you want me to expand on the above in anyway, just say.

    Thanks in advace,
  4. Fred Abse

    Fred Abse Guest

    Probably not, but it probably won't work because of voltage drop.

    Yes, but certainly well under 2mA. Microamps tops.

    Because it is OUT OF the terminal. Convention is that positive currents
    flow INTO a node.

    What it means is that the device will sink 6mA and source 2mA

    An input should be high impedance so it shouldn't draw enough current to do
    any harm, but the Voh of the 3.3V device might not be high enough to
    reliably cause the 5V device to change state. YMMV.

    What on earth that draws 20mA do you want to connect to an HDC?
  5. Fred Abse

    Fred Abse Guest

    A quick look at a Texas TTL databook shows the opposite.

    "Traditional" node-admittance network analysis uses the same convention,
    ie. to a node is positive, from a node is negative.
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