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zener trouble

Discussion in 'Electronic Basics' started by Erik Durling, Oct 28, 2004.

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  1. Erik Durling

    Erik Durling Guest

    We're a couple of students doing an electronics project. We are to
    construct a battery indicator circuit. We had a finished circuit on
    paper and we had simulated it with Multisim 7. But when we tested the
    circuit in real life, it didn't work at all like we had planned (or
    according to simulation). The circuit was based upon the idea of
    zenerdiodes having a constant voltage drop, and not conducting if the
    circuit isn't able to supply the needed voltage across it. But the
    zener-diodes doesn't stop conducting at all, and the voltage drop is
    far from constant.

    This is the main circuit:

    The zener diodes used in real life are: BZX55 and not BZC55, but that
    ought not change anything (right?).

    We did a DC Transfer Analysis in the simulator and got three nice
    curves showing the voltage at the three collectors (compared to
    ground) as the voltage from the source goes from 0-4.5 volts, and
    everything seems right there:

    So, the problem we're having with our real circuit is that the LEDs
    never turn off (the transistors are always saturated). The zener
    diodes doesn't stop conducting when the voltage across them goes under
    their specified value.

    What's our mistake/misconception?

    Do the zeners only operate correctly at a certain current?
  2. Zeners conduct at all voltages, but the current is very small when the
    voltage is under the zener voltage.
    (It is actually an exponential relationship between voltage and current)

    With your circuit the hFE of the transistors amplify the base current
    very much, so the little current through the zeners is enough to turn on
    the LEDs.
    You should design a better circuit.
    The one you have is based on faulty assumptions.

    (The reason why it worked in multisim is probably that is uses simplified
    simulation which does not work in real life when you create a design
    based on using components in a non-regular way.)

    Zeners are made to provide a fairly constant voltage, not current.
    Do not use zeners to control current.
  3. John Larkin

    John Larkin Guest

    Low voltage zeners, below maybe 5 volts, are terrible. Their
    conduction knee is sloppy as hell, which your simulator probably
    doesn't model accurately. And your open bases are *very* sensitive to
    small leakages; open bases are considered bad form by most designers.

    Add b-e resistors to divert some of the leakage current, and maybe go
    to bandgap pseudo-zeners (LM4040 types) which behave much better.

    But I'm impressed that you're actually verifying your simulations by
    experiment; some people just quit when the sim works.

  4. How low does the battery voltage have to go before the LEDs turn
    essentially off? I am suspicious that you may have the zeners

    That said, such low voltage zeners are not really switches that
    suddenly conduct when the voltage across them rises above the zener
    knee voltage, but are exponential devices, much like forward biased
    junctions. In fact, you may get a better knee out of a green LED,
    forward biased. To improve the switching effect, you could load
    (parallel) the base emitter junction with some resistance to divert
    zener current while keeping the base ot emitter voltage below about .5
    volts up to the supply voltage that you want the LED to light.

    But using a comparator (comparing your divider voltage to a fixed
    reference instead of a transistor sensing zener current) would give a
    lot cleaner threshold.
  5. John Larkin

    John Larkin Guest

    This brings up some interesting issues:

    Why would anyone simulate a circuit this simple?

    If something this simple doesn't simulate correctly, what hope is
    there for a serious circuit?

    Did your instructor deliberately give you a circuit that would
    simulate correctly but not work in real life? If you're very lucky,

    Why not look at a zener data sheet?

    When you built it and it didn't work, why did you resort to a
    newsgroup for help? Why not measure and analyze the voltages and
    currents and figure out what's actually happening? The methodology
    here is: fiddle with the simulation until it works; then fiddle with
    the circuit until it works, or ask for help; no thinking required.

  6. Fred Bartoli

    Fred Bartoli Guest

    This is the better case.
    Some other people run into prod when the sim works.

    Anyone having some nice story about direct run from sim to prod ?

    On the contrary, I once did the expertise of a GTO firing hybrid that had
    probably been designed with the pencil/paper method and not enough tested.
    Few of them had the interesting feature of being permanently on at elevated
    As those were for railways traction, I've been nice fireworks stories.
    2 hours simulation work sorted the pb out. Then 2 more weeks to convince the
    customers staff.
  7. You made some wrong assumptions about the zeners. They indeed need some
    current for the function you want them to fulfil but there is more. One
    thing you can do to get a better insight is making a real life I-V plot of
    (one of) the zeners you used. Pay special attention to the part below zener
    voltage. You will measure microamps but that's still current. Multiplied by
    beta (or Hfe) will give enough current through the LED to make it light.
    Keep in mind that although a general purpose LED likes a 20mA for optimal
    funtioning most of them start to light already at 1mA.

    For a circuit like this I would use one zener or bandgap as a reference
    voltage. A comparator (an LM339 contains four of them) and a voltage
    divider. The reference voltage can be used for all three comparators. The
    voltage divider will set the voltage at which the comparator will switch.

    Below I give an idea to make something like that using discretes. Beware
    it's just an idea.

    | | |
    .-. .-. .-.
    | | | | | |
    | | | | | |
    '-' '-' '-'
    | | |
    | | |
    | | V LED
    | | -
    | | |
    | | |
    | | |/
    +-------|------| Si
    | | |>
    | | |
    | +---->|--+
    | | Si |
    .-. | .-.
    | | - | |
    | | \ Z | |
    '-' ^ '-'
    | | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    petrus bitbyter
  8. John Larkin

    John Larkin Guest

  9. me

    me Guest

    But I'm impressed that you're actually verifying your simulations by
    Don't be. If they didn't have to build it to demonstrate it in the lab
    they would have called it good after the simulation.
  10. John Larkin

    John Larkin Guest

    I resently had a tour of the EE department at Cornell University. I
    figure that computer screens outnumber oscilloscope screens by roughly

  11. Erik Durling

    Erik Durling Guest

    I think I explained the purpose of the zener improperly. We have it
    there to increase the potential needed to saturate the transistor so
    it needs a smaller change in the source to turn off the LED.
    But the "leakage" was still the big problem eitherway.


    Would it work better by connecting ordinary diodes in series?

    What's meant by "open bases"?

    Thanks! That made a huge difference.

    The fresh-voltage is about 4.5, then one LED turn off every 0.5 volt.
    Though we have potentiometers connected between the two resistors in
    the picture (and the values differ a little), but I left that out on
    purpose, those values aren't that important for the "problem".

    Isn't that the point with zeners (that they're backwards)?

    That was what we planned to use at first, but they themselves needed a
    higher voltage than the battery was able to provide (less than 3 volts
    at minimum).


    Cause we wanted to know what components we needed before we spent our

    Actually, our instructor doesn't know shit about electronics (he
    thinks he does though).

    We trusted in the simulator (won't do that again =) )
    newsgroup for help?

    Cause we didn't have access to anyone competent to help us.
  12. Well. Here is a funny thing. I just simulated a circuit with a variable
    voltage source across a 1k resistor, connected to a 3.3V zener, and then
    to ground. The voltage source goes from 0 to 10V in 10 seconds. Oddly,
    the junction between the resistor and zener starts out at 2.3V, and goes
    up slowly from there. Thus, it appears that spice believes that the
    zener diode is a voltage source This happens for both circuitmaker and
    LTSpice. There is actually current flowing backwards through the
    resistor into the voltage source until the dc source gets to equal the
    'voltage source' of the zener. I'd say that the spice implementation of
    zener diodes of low value is seriously flawed when used with a voltage
    lower than the actual zener voltage minimum.

    Here is the spice netlist:
    * node0--[V1=PWL]--node3--[R1=1k]--node4--[XD1=1N5226B]--node0--GND
    *Spice netlist for Circuit: C:\CM60S\Circuits\UNTITLED.CKT
    V1 3 0 DC 0 PWL( 0 0 10 10)
    XD1 0 4 X1N5226B
    R1 4 3 1k
    ..SAVE V(3) V(4) @v1[p] v1#branch @r1[p] @r1
    *BKGND=RGB 0 0 0

    * Selected Circuit Analyses :
    ..TRAN 20m 10 0 20m

    * Models/Subcircuits Used:

    *1N5226B 3.3V 500mW Si pkg:DIODE0.4 A,K
    ..SUBCKT X1N5226B 1 2
    D1 1 2 DF
    DZ 3 1 DR
    VZ 2 3 2.536
    ..MODEL DF D (IS=2.51N RS=84M N=1.7 CJO=182P VJ=0.75 M=0.33 TT=50.1N)
    ..MODEL DR D (IS=5.59M RS=8.4 N=15)
    ..ENDS X1N5226B

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  13. John Fields

    John Fields Guest

    I've read some of the replies to your original post, above, but your
    last post seems to show that you're still pretty much in the dark, so
    here goes:

    If you hook up this very simple circuit:

    | |
    [R1] [VOLTMETER]
    | |

    You'll notice that if you start with +VIN at 0V and then make it more
    and more positive, the voltage across R1 will start to increase long
    before +VIN gets to 4.3V. The reason for that is because the Zener
    isn't a perfect switch and it will start allowing current to flow
    through itself before its Zener voltage is reached. Zeners are
    designed to be shunt regulators, and if you look at a data sheet for a
    Zener you'll find that the Zener voltage is only guaranteed to be
    within a certain range of voltages if the current through the Zener is
    the "test current", usually 20mA for 1/2 watt diodes with a Zener
    voltage of 12V or less.

    Now, if we take a look at your circuit:

    | |
    | [LED]
    [1K] |
    | [150R]
    | |
    | C
    +--[<ZENER]--B Q1
    | E
    [10K] |
    | |

    we can see that as VIN starts to go more and more positive, more and
    more current will start to flow through the base-to-emitter junction
    of the transistor, just because the Zener isn't a switch and will
    start to conduct well below its Zener voltage. If you have a
    transistor with a reasonably high beta (100 to 300) then the current
    which is flowing through the Zener (and also through the b-e
    junction)will cause a collector current to flow which will be 100 to
    300 times higher than that. So, even if you're way below the Zener's
    knee and you have, say, 10µA of reverse current flowing, the collector
    current will be somewhere between 1mA and 3mA, which will be enough to
    light the LED through that 150 ohm resistor.

    If what you're trying to do is build something with LEDs which light
    sequentially as the supply voltage increases, then you need to use
    three comparators, a voltage reference, three LEDs and a handful of

    Want a schematic?
  14. These work down to 2.7 volts:
  15. Erik Durling

    Erik Durling Guest

    We understand this would be easier to build with comparators and
    voltage reference components, but we really want to build it using
    only discreete components.
  16. John Larkin

    John Larkin Guest

    A base-emitter resistor will help a lot.

  17. john jardine

    john jardine Guest

    Something like this has quite a clean switch-on action when the battery
    drops below 3.0V.
    Tr1 compares the fixed 2.5V supply with a divided down version of the
    battery voltage and starts turning on when its base voltage goes below 1.9V.
    The "stable 2.5V" can come from a zener or a reference.
    (the 1k and 10k at Tr2 are there just to kill any leakage currents)

    Battery Volts
    | Stable 2.5V
    | ,---------,
    .-. | |
    | | | .-.
    5k6| | | | |
    '-' | | |47
    | |< Tr1 '-'
    +---------| BC556 |
    | |\ |
    .-. | V Led
    | | .-. -
    10k| | | | |
    '-' | | |
    | '-'1k |/ Tr2
    | +-------| BC546
    | .-. |>
    | | | |
    | | | |
    | '-'10k |
    | | |
    === === ===

    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  18. Bill Bowden

    Bill Bowden Guest

    Maybe you can just put the LEDs in the emitter side of the
    transistor, so they light at some base voltage set by a
    couple resistors? No zener needed since the LEDs light
    at around 2 volts. Something like this:

    + Battery +
    | |
    | \
    | / R3
    \ \
    R1 / |
    \ C
    | |/
    +-------B| NPN
    | |\
    \ E
    R2 / |
    \ LED
    | |

  19. As I've pointed out elsewhere, Zener models appear to suck.

    Here is a simple circuit that will do what you need:

    | |
    | |
    e |
    .---b [1k]
    | c |
    | | |
    | | |
    {10k] [1k] |
    | | e
    | o-----b
    | | c
    zener | |
    | [10k] LED
    | | |

    When Vin is above the zener voltage + a bit, the left transistor will
    conduct. If its conducting, then the base of the right transistor will
    be higher than the emitter + 0.7, that transistor will be off, turning
    the LED off.

    When the voltage drops below the zener voltage, the left PNP transitor
    will turn off, causing the right PNP transistor to turn 'sharply' on. As
    the voltage continues to decay, the LED will remain on, but will get dimmer.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  20. CBarn24050

    CBarn24050 Guest

    Subject: Re: zener trouble
    Hi, if you look at the datasheet you will see why it doesn't work. Why you
    would even consider simulating such a circiut is beyond me.
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