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Why are my LTSPICE results so different from the 1N4148 datasheet and real life?

Discussion in 'General Electronics Discussion' started by makiyoung, Jun 13, 2017.

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  1. makiyoung

    makiyoung

    3
    0
    Jun 13, 2017
    I plotted the IV curve for a 1N4148 diode and obtained this. The simulation shows about 60 mA of current at 0.8 V while the datasheet shows about 30 mA. I also checked this datasheet and it also shows 30 mA. In real life I measure 26.4 mA at 0.8 volts.

    Considering this is supposed to be a model of that actual diode, the result seems rather far off.
     
  2. Alec_t

    Alec_t

    2,950
    803
    Jul 7, 2015
    Did you mean to add links or attach something?
    Interesting though. At Vfwd=0.8V Vishay's datasheet shows a spread from 9mA to 20mA, NXP's datasheet shows ~40mA and Fairchild's shows 30mA. So on balance it looks like the Spice model is wrong.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    The slope of the curve is very steep. I would be happy that it's within cooee of the result you're getting. The value is also likely to be pretty temperature dependent too.

    (Within cooee is similar to within a bull's roar, but less than a packed lunch and a waterbag. Probably closer than Woop-woop unless you're already well beyond the black stump)
     
  4. Audioguru

    Audioguru

    3,260
    701
    Sep 24, 2016
    Why do you need the sim to show one voltage/current that you cannot buy? You get whatever they have because they cannot be made to all be the same.
    Are you trying to match a diode to the base-emitter of a transistor or to another diode? You can't unless you buy many, test them all and sort them.

    The text in the datasheet shows the range of voltage and current. The curves in the datasheet shows only a "typical" one that you can't buy and many are minimum and others are maximum.

    Maybe the datasheet measures the forward voltage immediately and before the diode heats but maybe the sim waits for the temperature of the heating diode to finish rising.
     
    Arouse1973 and Harald Kapp like this.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,628
    Nov 17, 2011
    e.g. NXP:
    upload_2017-6-14_8-15-59.png
    max. forward voltage even at 10 mA is 1V.
    So much for tolerances.
     
    Arouse1973 likes this.
  6. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    I make mine 0.758 mV at 30 mA at 25 deg C.
    Thanks
    Adam

    Diode Drop.PNG
     
  7. Audioguru

    Audioguru

    3,260
    701
    Sep 24, 2016
    Is 25 degrees C the temp of the chip or the temp of the ambient air?
    But we do not want the forward voltage of your diode because it is different for most 1N4148 diodes.They cannot all be made exactly the same, they have a range of forward voltage.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    11,445
    2,628
    Nov 17, 2011
    Even LTSPICE offers two different models:
    • 1N4148
    • MMSD4148
    These will deliver different characteristics. Some models fpr 1N4148 from different vendors that you can find on the internet have again other parameter values.

    It's a good thing you do not take the simulation result as a given truth but you question this result. I hope more users of simulators were that thorough.
     
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