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Newb question about voltage dividers

Discussion in 'General Electronics Discussion' started by jke, Oct 12, 2010.

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  1. jke

    jke

    4
    0
    Oct 12, 2010
    I have a very simple question about voltage dividers.

    Why bother with the 2nd resistor? Aren't you reducing the signal to the desired level with the first one? What's the point of creating the parallel circuit after the first resistor?

    Thanks.
     
  2. shrtrnd

    shrtrnd

    3,688
    457
    Jan 15, 2010
    You need to drop the full voltage across the two resistors.
    You pick off the voltage you want, between the two resistors, chosen for the voltage
    you want (when read between the two resistors).
     
  3. jke

    jke

    4
    0
    Oct 12, 2010
    Why is that? Aren't you dropping what you don't want across the first resistor and then utilizing the remainder in the load?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,267
    2,715
    Jan 21, 2010
    It will work with a single resistor if your load has a constant impedance -- your load then acts as the second resistor.

    However, in many practical cases, the load is of relatively high impedance and may not be constant over a frequency range (it may be AC coupled). The voltage divider has a lower impedance than the load so that the load does not significantly affect its behaviour.
     
  5. jke

    jke

    4
    0
    Oct 12, 2010
    OK, so to clarify, any unexpected spikes in voltage will be absorbed by the second resistor. The load may receive up to, but not exceeding the expected voltage.

    My remaining question: How does this matter for dc?

    Thanks again!
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,267
    2,715
    Jan 21, 2010
    No, a resistive divider simply divides a signal by a fixed ratio.

    If both resistors are equal, the voltage across the bottom resistor will be half of the voltage across the two resistors combined (ignoring the effect of any load).

    It works essentially the same way as a volume control does (since a volume control is typically wired as a resistive divider with the position of the control determining the ratio of the two resistances.

    For AC or DC a resistive divider does exactly the same thing.
     
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