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Help needed with a diode and resistor formula

  1. Oct 27, 2011 #1
    This isn't a homework question but it is a formula from a book I have been using to study, so I'm posting it here.

    Here is the circuit and formula -

    diode resistor.JPG

    I don't understand why the voltage drop of 0.7 volts across the diode is being used as part of the formula to work out the current across R2 when R2 has it's own voltage drop across it. Can someone explain this to me please?
  2. jcsd
  3. Oct 27, 2011 #2
    I think that since the diode and resistor are in parallel, the voltage drop across them has to be the same. So since you know the voltage drop across the diode you can just extend that to R2.
  4. Oct 27, 2011 #3
    I don't see why the voltage drop across R2 and the diode would be the same.
  5. Oct 27, 2011 #4
    Well, the diode and the resistor are connected. The voltage at the pink point I added to your diagram can only be one value.

    diode resistor.JPG
  6. Oct 28, 2011 #5
    So the voltage drop through resistor 2 is the same as in the diode because they are in parallel?
  7. Oct 28, 2011 #6


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    Staff: Mentor

    Yes. ANY components that are in parallel all share the same potential drop.
  8. Oct 28, 2011 #7
    Thanks, well that clears things up for me.
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