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Zener Minimum Current?

Discussion in 'Electronic Basics' started by Brian, Mar 1, 2005.

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  1. Brian

    Brian Guest

    In a simple voltage regulator circuit, using a resistor and zener diode. The
    formula for the resistor in series with the zener diode is:

    R = [Vin(max) - Vz] / [Iz(min) + IL(max)]

    My question is, how do you calculate or find what the minimum zener current
    should be? I don't find this in the data sheets for zener diodes.

    Thanks,

    Brian
     
  2. There is a 'test current' given in the datasheets. That's the best one
    to use if you want the zener to be near it's rated value.

    --
    Regards,
    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.
     
  3. Brian

    Brian Guest

    So in a worst case situation, if say the input voltage varied from 10
    volts to 20 volts and the zener volts was 9 volts (with a load current of 20
    milliamps), you would use the zener test current for the Iz(min)? I know,
    there is a better way to do this than with a zener diode, but for
    information sake I would like to know the right way to do this.
    Thanks,

    Brian
     
  4. john jardine

    john jardine Guest

    The minimum zener current is that current which you want to still flow
    through the zener when a maximum load current is being taken.
    Zeners are pretty crap at low currents anyway and work better with at least
    1/2 ma through them at all times.
    It's easiest just to make the minimum current about (say) 10% of the maximum
    load current.
    This way, the load can increase a bit without the zener voltage falling
    over.
    i.e. Resistor is = [Vin(max) - Vz] / [1.1 x IL(max)]
    regards
    john
     
  5. John Larkin

    John Larkin Guest

    Resistor-zener regulators are numerically challenged. In your example,
    the drop across the input resistor varies over an 11:1 range, and it
    has to be able to supply the load on the low end, never mind a min
    zener current. So the worst-case input current will be at least 11x
    the max load current, or 220 mA. At 20 volts in, the zener dissipates
    almost 2 watts and the resistor more. Efficiency is insane and things
    will fry.

    Oh, 1 mA is a good min zener current.

    John
     
  6. I think your forumla is wrong.

    You need to think about what's happening. A zener diode is a shunt
    regulator. It will gobble up as much current as it can if the voltage
    across it is above it's rated value. Thus, your resistor is there to
    prevent a virtual short circuit between Vin and Vz.

    One main problem with shunt regulators is that they don't work very well
    if the input voltage drops below their design value (I guess this is
    true of all regulators)

    Because of this, if you want regulation, you want to make sure the
    voltage is always above Vz, given your required load current. If the
    current is maximum (IL + Iz(min)) then the voltage drop across the
    resistor is R * (IL + Iz(min)). Since the zener must have at least it's
    rated voltage across it to work, so the input voltage must be *at least*
    Vin = Vz + R * (IL + Iz(min)).

    What this means is that the formula should really call for Vin(min).

    As an example, suppose your input voltage ranges between 10V to 20V, and
    you have a 5.1V zener. It's Iz(min) is 10mA, and your load is 20mA.
    Then, using your formula,

    R = (20 - 5.1)/(30mA) = 496 ohms.

    However, if you use that with your 20mA load at 10V, then, even if the
    zener stops drawing any current (which it will do), your resistor will
    drop 9.93V! This only leaves you 0.07V. So, you aren't regulating.

    However, if you use

    R = (10 - 5.1)/(30mA) = 163.3 ohms

    then at 20V, your zener will still be regulating (although the voltage
    across your load will be a bit higher, since the zener voltage will rise
    a bit with more current). It will suck up the excess current. If your
    load is using 20mA, then it'll take in the extra 70mA.

    Thus, using Vin(min) in the forumla, your regulator will be regulating
    when input is between 10V and 20V.

    One other parameter you need to complete your design of the zener
    regulator is the power rating of the zener. You need to ensure that the
    power rating of the resistor and zener are within their maximums. And
    you need to consider temperature effects on the zener. However, you
    didn't ask about that.

    Usually, using a linear regulator that has temperature compensation and
    current limiting built in is a better bet. However, there are times when
    a shunt regulator is the only way to go.

    --
    Regards,
    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.
     
  7. Chris

    Chris Guest

    Good morning, Brian. The minimum zener current is Iz(k), or the zener
    knee current. It is specified on zener data sheets.

    For instance, my Fairchild data sheet for the 1N4740 (10V zener) lists
    an Iz(k) for the 1N4740 of 0.25mA. If I was using this zener, I would
    need real justification to go below 1mA minimum current -- zeners just
    don't work very well near the knee.

    Good luck
    Chris
     
  8. John Fields

    John Fields Guest

     
  9. Jamie

    Jamie Guest

    i normally tried to have the Zener at 50% rated current while the load
    is on the circuit.
    there should be a Knee current spec'ed some where, this is the lowest
    suggested current to operate the diode in. I(k) or something like that.
    i think it depends on who makes the docs, i have also seen it referred
    to as valley current.
     
  10. Brian

    Brian Guest

    You are right about the formula being wrong. I meant to put Vin(min).
    After seeing all the different answers (which I really appreciate), I did
    some more digging into data sheets for zener diodes. I did find one data
    sheet for 5 watt zeners, which had curves for zener volts versus zener
    currents. On these curves, it showed that there wasn't a lot of change in
    zener voltage (for zener currents above 1 milliamps), on zener diodes from 6
    volts and above. On zener diodes below 6 volts, zener current was very
    important. On zener diodes below 6 volts, using the test current shown was
    very important (which I found to be, the zener current required for it to
    dissipate 1/4 of its' wattage rating).
    Thanks,

    Brian
     
  11. peterken

    peterken Guest

    One better and more stable way is to set the zener using a current source
    since when Iz varies Vz also will vary a bit
    For the best current in a reference....
    Usually I aim for a zener 5V1 @ 10mA since these are most stable if set this
    way
     
  12. mike

    mike Guest

    A Zener is a poor choice for this application.
    You've picked a set of numbers that are a challenge for most simple
    regulators. IF you can tolerate a minimum of 2V or so between min in
    and out, use a 3-terminal regulator. Or a more expensive LDO one that
    will run on 1V differential.

    Soapbox alert...

    You don't give clues to the application. There are some common issues
    ignored by newbies.

    Zeners have a resistive term.
    Zeners have a temperature coefficient...sometimes significant.
    Zeners below 5.1V have a very soft knee, often.
    Input voltage can have ripple that puts the minimum trough voltage
    well below what you read on a meter.
    Sometimes you can reduce the min/max current ratio by using a PTC
    resistor. An incandescent light bulb makes an interesting PTC.
    Plot some V/I curves for 12V-28V light bulbs and see if that helps.
    mike

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  13. jsmith

    jsmith Guest

    First of all, most low voltage 1/2W zeners have a test current of 20ma.
    Above 10-12V the test current begins to decrease. The value is easily
    checked out in most catalogs or on line. Since you know the B+ use ohms law
    to select a resistor value that will allow the test current to flow through
    the zener. Whammo, you've got it. To verify the stability, test your circuit
    by varying the voltage across the desired range using a bench power supply.
    Remember the rule, if you double the voltage across the resistor you have
    selected, know that the current will also double, and the wattage will
    increase by four times. Useful rule of thumb to always be aware of.



     
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