Zener diode problem

Discussion in 'General Electronics Chat' started by surfdabbler, Jul 4, 2016.

  1. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
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    I am having issues with Zener diodes. First time, I assumed I must have a dud zener, but I have bought two more and they are doing the same thing. I'm trying to get a reference voltage for a voltage comparitor circuit, but the zener is not giving the correct reference voltage. I've take the zener out of circuit to figure out what's going on...

    My test circuit is pretty simple - V+ (11.5V), connected through resistor, then 3.3v Zener (reversed, with black line toward V+), then to gnd. At first, I was using a high resistor of 1Mohm, to get the reference voltage without drawing much current, but the zener voltage was very low, like 0.9V. I wondered whether the zener needed more current to achieve the required voltage, so I tried decreasing the resistor, and tried two different battery supplies, and got a big range of results...

    R zener v zener v
    (7.7V batt) (12.2V batt)
    1.2M 0.93v 1.01v
    400K 0.93v 1.01v
    10k 1.7v 1.8v
    150 3.07v
    50 3.5v

    So, I had to drop the resistor right down to get close to the right voltage, and at 50ohms, the zener voltage exceeded the spec voltage of 3.3v!

    Out of curiosity, I reversed the zener to be fwd biased, and repeated some of the tests, and the voltage varied between 5.5 to 6.5 through the same range, so fwd biasing seems to give a better result, as reasonably expected, but the reverse biased zener should be giving a more consistent 3.3V, right?

    The batteries were freshly charged, and I measured the battery voltages in circuit, so that should not be an issue. I have tested diodes from two different stores, and they are two different wattages (400mW, and 1W), and they are giving consistent results.

    What is going on?
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,995
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    I assume you haven't looked at the data sheet.
    Look there anf note the current used to specify the voltage rating of the zener and the dynamic resistance of the zener.
    Low voltage zeners have a high dynamic resistance and thus poor voltage regulation as the voltage varies significantly with current (called a soft "knee").

    Don't understand your results when forward biasing the zener.
    For that, its voltage should be comparable to a standard diode or about 0.7V.

    If you want a voltage reference that will operate at a low current (1ma) with a sharp knee, then use an IC reference, such as the 2.5V, TL431 (it can also be adjusted from 2.5V up with two resistors).
    For really low current you could use a 2.5V, LT1634 which can work well down to 10μA.
     
    Last edited: Jul 4, 2016
  3. DickCappels

    Moderator

    Aug 21, 2008
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    What is the part number of the Zener you are trying to use?
     
  4. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
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    Thanks for the update. Looking at my results (apologies for the formatting - it didn't come through, and I'm trying to edit that at the moment), I need to be looking at about 100ohms to get the correct voltage. at 7.7-3.3 = 4.4V, 100 ohms gives me 44mA, so should I assume my diode needs 44mA to operate correctly?
     
  5. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
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    My diode is a 1N746, and I also tried another one - will have to look up the other codes.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The data sheet shows that the 1N746 voltage is rated at 20mA.
     
  7. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
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    Hmm, I give up on trying to format my data!

    Thanks for the info. I had assumed that zeners were a lot better at their job, but I understand the 'soft knee' now, and I will adjust my circuit to aim for 20mA current through the zener. At this level it looks like the zener might be closer to around 3V, but at least the data makes sense now! Thanks for your help.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Higher voltage zeners, such as 5v-10V units have much sharper knees, with the 6.8V unit having the lowest dynamic resistance (sharpest knee).
     
  9. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
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    I adjusted the resistor to provide the correct current to the zener, and the whole circuit is now working perfectly! Thanks for your help!
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    Congrats on doing a successful exercise on Zener diodes. Yes the voltage across a Zener varies with the current through it. They are rated at a certain voltage at a given current. You can tweak the current to get a precise voltage. Using a constant current source instead of just a resistor gets you a pretty stable voltage.
     
  11. ScottWang

    Moderator

    Aug 23, 2012
    4,853
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    1N746_3V3ZenerDiode_Itest20mA_Imax110mA.gif
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,853
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  13. Bordodynov

    Active Member

    May 20, 2015
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    See

    REF.png
     
  14. hp1729

    Well-Known Member

    Nov 23, 2015
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    With that low a current the voltages will be off from expectations. Get the current up to about 1 mA (10K ohms or so) and you will get more reasonable readings.
     
  15. Bordodynov

    Active Member

    May 20, 2015
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    I took a resistor value of 1 meg as the author. Maybe his battery-powered and the currents of the order of one milliampere and longer is no suitable. There are available comparators and operational amplifiers with micro-ampere currents.
    REF1112.png
     
  16. hp1729

    Well-Known Member

    Nov 23, 2015
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    Ah, yes. I see.
     
  17. surfdabbler

    Thread Starter New Member

    Oct 7, 2013
    7
    0
    Thanks for all the updates. If I had to do the circuit over, I would certainly look into these improved voltage references, but I can live with the higher current for this particular application.
     
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