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Why doesn't LED behave as a diode?

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Jayaram
2012-10-03
2012-12-15
  • Jayaram
    Jayaram
    2012-10-03

    I have noticed that I cannot use the LED as a normal rectifier. The LED conducts in both directions unlike a diode. Is it supposed to behave like that?

     
  •  magneto98
    magneto98
    2012-10-03

    Hi jayarams,

    Please post a schematic file that demonstrates the problem.

     
  • Jayaram
    Jayaram
    2012-10-04

    <Qucs Schematic 0.0.16>
    <Properties>
      <View=0,0,1044,800,1,0,0>
      <Grid=10,10,1>
      <DataSet=simple_rectifier.dat>
      <DataDisplay=simple_rectifier.dpl>
      <OpenDisplay=1>
      <Script=simple_rectifier.m>
      <RunScript=0>
      <showFrame=0>
      <FrameText0=Title>
      <FrameText1=Drawn By:>
      <FrameText2=Date:>
      <FrameText3=Revision:>
    </Properties>
    <Symbol>
    </Symbol>
    <Components>
      <.TR TR1 1 750 140 0 65 0 0 "lin" 1 "0" 1 "10 ms" 1 "1001" 0 "Trapezoidal" 0 "2" 0 "1 ns" 0 "1e-16" 0 "150" 0 "0.001" 0 "1 pA" 0 "1 uV" 0 "26.85" 0 "1e-3" 0 "1e-6" 0 "1" 0 "CroutLU" 0 "no" 0 "yes" 0 "0" 0>
      <R R1 1 430 210 15 -26 0 1 "1 k" 1 "26.85" 0 "0.0" 0 "0.0" 0 "26.85" 0 "US" 0>
      <GND * 1 250 280 0 0 0 0>
      <Lib D1 1 370 130 -26 13 0 0 "LEDs" 0 "red" 0>
      <Vac V1 1 250 230 18 -26 0 1 "12 V" 1 "100 Hz" 0 "0" 0 "0" 0>
    </Components>
    <Wires>
      <250 130 250 200 "" 0 0 0 "">
      <250 130 340 130 "" 0 0 0 "">
      <400 130 430 130 "" 0 0 0 "">
      <430 130 430 180 "" 0 0 0 "">
      <430 240 430 280 "" 0 0 0 "">
      <250 280 430 280 "" 0 0 0 "">
      <250 260 250 280 "" 0 0 0 "">
      <430 130 430 130 "output" 460 100 0 "">
    </Wires>
    <Diagrams>
    </Diagrams>
    <Paintings>
    </Paintings>

    Here is the .sch file using LED in place of diode

     
  •  magneto98
    magneto98
    2012-10-04

    Hi jayarams,

    If you click "Show Model" in the component libary, it shows for the red LED:

    .Def:LEDs_red _netC _netA
    Diode:D_red _netC _netA Is="1e-10 A" N="4.09" Cj0="3 pF" M="0.5" Vj="0.7 V" Fc="0.5" Cp="0.0 fF" Isr="0.0" Nr="2.0" Rs="0.1 Ohm" Tt="4.0 us" Temp="26.85" Kf="0.0" Af="1.0" Ffe="1.0" Bv="5" Ibv="10 uA" Xti="3.0" Eg="1.11" Tbv="0.0" Trs="0.0" Ttt1="0.0" Ttt2="0.0" Tm1="0.0" Tm2="0.0" Tnom="26.85" Area="1.0"
    R:Rleak _netA _netC R="50 MOhm" Temp="26.85"
    .Def:End

    The problem is the parameter "Reverse Breakdown voltage" / "Bv", which is set to 5V.

    This schematic shows the differences:

    <Qucs Schematic 0.0.16>
    <Properties>
      <View=190,-185,1685,713,0.754975,0,12>
      <Grid=10,10,1>
      <DataSet=simple_rectifier.dat>
      <DataDisplay=simple_rectifier.dpl>
      <OpenDisplay=0>
      <Script=simple_rectifier.m>
      <RunScript=0>
      <showFrame=0>
      <FrameText0=Title>
      <FrameText1=Drawn By:>
      <FrameText2=Date:>
      <FrameText3=Revision:>
    </Properties>
    <Symbol>
    </Symbol>
    <Components>
      <GND * 1 260 340 0 0 0 0>
      <Vac V1 1 260 290 18 -26 0 1 "12 V" 1 "100 Hz" 0 "0" 0 "0" 0>
      <R R1 1 440 230 15 -26 0 1 "1 k" 1 "26.85" 0 "0.0" 0 "0.0" 0 "26.85" 0 "US" 0>
      <Lib D1 0 380 190 -26 13 1 2 "LEDs" 0 "red" 0>
      <IProbe Pr1 1 440 300 16 -26 1 3>
      <.TR TR1 1 240 470 0 73 0 0 "lin" 1 "0" 1 "10 ms" 1 "1001" 1 "Trapezoidal" 0 "2" 0 "1 ns" 0 "1e-16" 0 "150" 0 "0.001" 0 "1 pA" 0 "1 uV" 0 "26.85" 0 "1e-3" 0 "1e-6" 0 "1" 0 "CroutLU" 0 "no" 0 "yes" 0 "0" 0>
      <Diode D2 1 370 -90 84 -43 0 2 "1e-15 A" 0 "1" 0 "10 fF" 0 "0.5" 0 "0.7 V" 0 "0.5" 0 "0.0 fF" 0 "0.0" 0 "2.0" 0 "0.0 Ohm" 0 "0.0 ps" 0 "0" 0 "0.0" 0 "1.0" 0 "1.0" 0 "5" 1 "1 mA" 0 "26.85" 0 "3.0" 0 "1.11" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "26.85" 0 "1.0" 0 "normal" 0>
      <Diode D3 0 370 60 84 -43 0 2 "1e-15 A" 0 "1" 0 "10 fF" 0 "0.5" 0 "0.7 V" 0 "0.5" 0 "0.0 fF" 0 "0.0" 0 "2.0" 0 "0.0 Ohm" 0 "0.0 ps" 0 "0" 0 "0.0" 0 "1.0" 0 "1.0" 0 "0" 1 "1 mA" 0 "26.85" 0 "3.0" 0 "1.11" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "0.0" 0 "26.85" 0 "1.0" 0 "normal" 0>
    </Components>
    <Wires>
      <260 190 260 260 "" 0 0 0 "">
      <260 190 290 190 "" 0 0 0 "">
      <410 190 440 190 "" 0 0 0 "">
      <260 320 260 340 "" 0 0 0 "">
      <290 190 350 190 "" 0 0 0 "">
      <440 190 440 200 "" 0 0 0 "">
      <260 340 440 340 "" 0 0 0 "">
      <440 260 440 270 "" 0 0 0 "">
      <440 330 440 340 "" 0 0 0 "">
      <290 60 290 190 "" 0 0 0 "">
      <440 60 440 190 "" 0 0 0 "">
      <290 -90 290 60 "" 0 0 0 "">
      <290 -90 340 -90 "" 0 0 0 "">
      <440 -90 440 60 "" 0 0 0 "">
      <400 -90 440 -90 "" 0 0 0 "">
      <290 60 340 60 "" 0 0 0 "">
      <400 60 440 60 "" 0 0 0 "">
      <260 190 260 190 "V_in" 230 140 0 "">
      <440 190 440 190 "V_out" 470 160 0 "">
    </Wires>
    <Diagrams>
      <Rect 620 587 944 618 3 #c0c0c0 1 00 1 0 0.0005 0.01 1 -14.4 2 14.4 1 -0.00876719 0.002 0.0130405 315 0 225 "" "" "">
    <"V_in.Vt" #ff0000 0 3 0 0 0>
    <"Pr1.It" #ff00ff 0 3 0 0 1>
    <"V_out.Vt" #ff00ff 0 3 0 0 0>
      </Rect>
    </Diagrams>
    <Paintings>
    </Paintings>

    or get it here:

    http://snipt.org/vVga0

    Still, in case of a reverse breakdown I would expect a much higher current then in forward directions.

    Cheers
    Carsten