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Voltage drop across diodes

  1. Aug 7, 2016 #1
    I want to make something clear. Most diodes have in their datasheet some current for a given treshold voltage. For example a led will pass 20mA current when 4volts are applied to it. Or a zener diode's minimum turn on current is 10mA when 5,6 volts are applied across it.
    Does this mean this litteraly means that the zener or the led will pass this current when the minimum turn on voltage is applied across them? I mean for example the led will pass 20mA when you apply 4volts to it. If you have 9volts you would need to add a current limiting resistor because the large voltage will push too big current through the diode and burn it. Is it some kind of refference or standart? In DC motor datasheets there are given two currents for a given nominal voltage. One at no load condition which is the smaller one and with load condition which is bigger. They are given so you can know in case you want to build a bridge for example. Isn't this the same thing? I need to know this because if I were to build a transistor zener voltage regulator I would need to know what current will the zener pass so I can calculate the required base resistor in order to give me the needed base voltage.
     
  2. jcsd
  3. Aug 7, 2016 #2

    davenn

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    That's a good start
    just keep in mind there are LED's that are rated for different voltages and currents
    always pays to check the datasheet



    Dave
     
  4. Aug 7, 2016 #3

    jim hardy

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    When dealing with diodes
    It is best to think of current as the controlled variable and voltage the observed variable,

    if you look more carefully at the datasheet for a zener they're telling you Vz has a % tolerance at Iz not the other way 'round. 5.6 volts at 10 ma was their goal when making the zener and you can buy them with typically 10% or 5% tolerance on that voltage.
    For LED's they are telling you approximately what voltage to expect at rated current of 20 ma.

    In this digital age we all want the world to be locked down and absolute but Mother Nature works in analog.

    You always set the zener current with your design. Choose an operating point above the knee and make sure current stays within device limits for all operating conditions of your circuit.

    old jim
     
  5. Aug 7, 2016 #4
    Thank you Dave, thank you Jim! I realy appreciate your help, but can I ask you one more question : When talking about zener diodes, we can clearly see from the I-V curve that below the zener treshold voltage (when talking about the reverse characteristic) there is still some leakage, thermaly generated I guess, current. Then at treshold voltage because of zener breakdown we get a very rapid increase of the zener current. We get the minimum zener current Izk, the test zener current Izt and the maximum allowed zener current that will be able to pass without the diode to reach the avalanche breakdown which will burn it. And now it is said that in cases when you want to use the zener for a voltage regulator, you must chose the zener current to be greater than Izk, but smaller than Izm. Smaller than Izm because we don't want to damage it. And greater than Izk because it won't regulate good if it's below it. Now that last thing, that if the zener current is smaller than the minimum zener current it won't be able to regulate as good, I can't understand. I've tried whole day writing in different forums and such, searching youtube and google for answer, but it looks like those sources offer only basic information. And so I couldn't find out and I still don't know how if the zener current is lower than the minimum zener current it won't be able to regulate as good. Can you help me on this one too?
     
  6. Aug 7, 2016 #5
    I am thinking, maybe its because of the internal ressistance each diode posseses. As the current decreaces and falls below Izk, less current through a constant ressistance means less voltage drop and so the zener voltage falls below the treshold voltage, thus messing with the output, causing it to be lesser.
     
  7. Aug 7, 2016 #6

    jim hardy

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    .
    it's because the zener (or avalanche) breakdown isn't a snap-action, it's somewhat gradual
    Wikipedia actually does a decent job
    https://en.wikipedia.org/wiki/Zener_diode


    but i thought this guy had a better diagram
    http://www.circuitstoday.com/pn-junction-breakdown-characteristics
    and you should peruse his article.

    zener1.jpg

    you want to avoid that nonlinearity around the knee .
    which might be under-emphasized even in this picture

    think for a moment ....EDIT oops i see i reversed my words again
    if you very slightly adjusted reverse current about some point, let's just say just a few microamps back and forth centered around a milliamp
    and measured the resulting fluctuation in voltage up and down

    errr,,, make that read
    if you very slightly adjusted reverse current about some point, let's just say just a few microamps back and forth up and down centered around a milliamp
    and measured the resulting fluctuation in voltage up and down back and forth
    end edit

    then divided Δvolts by Δamps, as we do in Ohm's law for DC
    you'd get a "dynamic resistance" to AC at that point on the curve
    and i trust it's obvious that'd be the slope of the curve at that point...

    Now look at this datasheet
    http://www.vishay.com/docs/85816/1n4728a.pdf

    and note they tell you what is "dynamic resistance" at two different currents
    upload_2016-8-7_20-43-5.png

    1N4782 at 1 ma DC, would give you a change of up to 400 max microvolts per microamp change in the DC current
    but at 76 ma DC, gives typically only about 10 microvolts per microamp change

    The smaller the slope the better the regulation
    but instead of "slope" they call it "Dynamic Resistance" (because that fits right in with 'small signal analysis' of circuits)

    They're telling you it regulates a lot better away from the knee.

    Hope it helps,
    old jim
     
    Last edited: Aug 8, 2016
  8. Aug 7, 2016 #7
    Thanks! Never would have thought it was so simple. Much thanks again!
     
  9. Aug 8, 2016 #8

    David Lewis

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    If you apply 4V to an average diode, enormous current will flow and it will burn out. As soon as a diode is biased into conduction it behaves like a short circuit. Its resistance is almost zero. However, if you put a current-limiting resistor in series with the diode then you're in good shape as long as the resistance is enough to keep the diode current at or below 20mA.
     
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