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Diode ac and dc resistance?

  1. May 13, 2009 #1
    Diode ac and dc resistance??????

    here is my question ................saturation current of a silicon diode is 2uA.and forward current is 20mA...i want to find out the AC and DC resistances of the diode in 300k.....

    so i know that ,

    I(forward)=I(saturate)[exp(eV/nKT)-1]

    ..so by differentiating both sides w.r.t to V i got that dI/dV=I(forward)*e/nkt........here KT/e=26mv so by subbing than (and also n=1 when conducting through diode)i got dV/dI=1.3ohms .......
    so that is Ac resistance of the diode ............
    but i dont know how to find the DC resistance of the diode can anyone help me out
     
  2. jcsd
  3. May 13, 2009 #2

    madmike159

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    Re: Diode ac and dc resistance??????

    Are you sure that differentiation is right? doesn't look correct.
     
  4. May 13, 2009 #3
    Re: Diode ac and dc resistance??????

    i think its correct .......what i have done is after differentiation i have subb i(forward) to the term I(saturation)*exp(ev/nKT).....and im sure of that because i have done it before ..........but i dont know how to calculate DC resistance ...DC(resistance)=V/I .......am i correct ...........but i dont how to get a value for that ................
     
  5. May 14, 2009 #4

    madmike159

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    Re: Diode ac and dc resistance??????

    Yea sorry its correct. I thought doing that gave DC resistance not AC. Also do you know the foward voltage.
     
  6. May 14, 2009 #5
    Re: Diode ac and dc resistance??????

    No i dont ,,,,,,,,,but after referring to some of my books i got some idea ......though i dont have the forward voltage i might be able to calculate it using forward and reverse currents by subbing them in the above equation then can get forward voltage ............

    so i think DC(resistance)=V(forward)/I(forward)............
     
  7. May 14, 2009 #6
    Re: Diode ac and dc resistance??????

    can someone check this answer for me ....................here i forward is 20mA i saturated is 2uA and temp is 300k.............i got the answer as R(dc)=11.9 ohms ........am i correct ????????please someone check and tell me because in the MCQ question there is no such answer ..............:(
     
  8. May 14, 2009 #7

    madmike159

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    Re: Diode ac and dc resistance??????

    I'll ask my teacher. That sounds a bit high though.
     
  9. May 16, 2009 #8
    Re: Diode ac and dc resistance??????

    I calculated 11.97 ohms which agrees well enough with your answer.
     
  10. May 16, 2009 #9
    Re: Diode ac and dc resistance??????

    Thnaks dude !!!!!!!!!!it was a great help!!
     
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