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TL431 versus Zener

Discussion in 'Electronic Components' started by [email protected], Dec 23, 2003.

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  1. Guest

    Aside from the fact that the TL431 is adjustable, does
    it have any advantages over a Zener ?
     
  2. Well, I believe it has lower resistance, or in other words, the V-I
    curve is straighter. So it should give less variation in voltage as
    the current changes.

    But obviously the 431 costs more and can't handle as much current. So
    it depends on what you're using it for. If you're using it for a
    reference, then the 431 is a better choice. If you're using it for a
    Q & D regulated PS for something like an oscillator, then the zener is
    the common choice, especially if you use a zener that's close to 6V,
    where the tempco is at its minimum. But today a 78L05 or 78L08 is a
    convenient way to get a stable voltage at less than 100 mA, for an
    osc, for example.

    And now they (Maxim, LTC, etc.) have micropower references that need
    only a few tens or hundreds of microamps. If you try to run a zener
    at that low a current, you'll be lucky to get a fraction of its rated
    V.

    I bought a hundred 1N4734A 5.6V zeners on Ebay for $3.50 and a buck
    and a quarter shipping. That's a nickel apiece, a lot cheaper than
    the 431. :p


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  3. Well, in this URL below it's acting like an emitter follower. But
    that bad thing about it is that it's Class A, i.e. the current drain
    from the battery is high, and the same no matter what the speaker
    volume is. So it's wasteful of battery power. The first few circuits
    in this URL that use complementary output transistors are Class AB or
    B, so they take only a few mA idling current when the volume is very
    low. A better chip to use is the LM386, which can also be found on
    Ebay for cheap. It takes a minimum of parts, and can put our a half
    watt I believe, at 9V. But then these are really a radical departure
    from a true crystal radio. Even the single chip MK484 radio is
    getting a lot more exotic compared to an xtal radio.

    I ran a 50 foot (15M) wire out of my bedroom window and connected it
    to a crystal radio, and the music was free. But futile, because we
    lived a few miles from a 50 kW AM radio transmitter and another 5 kW a
    few more miles away. So about the only thing heard during the daytime
    was either of those, and usually a mixture of those, since the radio
    wasn't that selective. But at night the 50 kW shut down, and the 5 kW
    went to 500W, so it was a little bit better. But about the only thing
    I heard at night was KFI 640 AM, which is clear channel and can be
    heard all over the western U.S. at night.


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  4. Exador

    Exador Guest

    One advantage of the TL431 over a typical zener is that it is much more precise.
    Another is that it requires less current to operate.

    It may be slower than a zener, but I'm not sure about that.

    Mike
     
  5. I tested the parts. The LM386 at 46dB gain specified uses about 5mA
    quiescent current. The TL431, dependending on how you set it up, uses
    more. That is correct. I set it up to use about 7mA quiescent and get
    at least 40dB gain plus the advantage in my regen circuit of being a
    more efficient rectifier than a germanium diode. The two are about
    equal in performance as a small radio amplifier. The TL431 is not
    nearly as bad as you make it sound.

    The Tl431 is a bit noisier, will drive stereo headphones, but not an 8
    ohm load. But as a three terminal device folks are finding it very
    easy to solder into a circuit. The kit is designed for kids and they
    are having great success. So it not the ultmate answer to every small
    audio amplifier question, but it is a very good one.

    And I payed less than a nickel each for my TL431 parts.

    Bruce
    www.elmerdude.com
     
  6. Guest

    I may never build another general purpose audio amp
    from scratch again. For less than 5 bucks, you can
    get a kit of all parts for a LM386 amp or a TDA7052
    amp at Marlin Jones - KT 6017 and KT 6027 respectively.
    http://www.mpja.com/listitems.asp?dept=68&main=61

    I built both, and they work real well.

    That said, you've whet my appetite to play with the
    TL431 as an audio amp.
     
  7. Guest

    Thanks. I fiddled with a test circuit today, and the
    TL431 outperformed 2 zeners in series - a 5.1 and a 6.2
    by about 50 mV. But I reached the limit of my test
    setup, so the ~50 mV isn't a solid figure. The circuit
    was simple - the base of a 2N2904 is fed by the zeners
    or the TL431 circuit thru 1K to drop a relay out when
    voltage is reduced to 12.00. Trouble is, the supply I
    used is not that precise. In any event, in numerous
    tests I got worst case of 12.03 dropout with the TL431
    and 12.08 with the zeners, and the dropout voltage
    varied more often with the zeners, too.
     
  8. If you're talking about the followup I posted about what the circuits
    are and aren't, then I didn't try to make is sound good or bad. I was
    telling it like it is. If you want to use a TL431 for an amp, then
    it's just like an emitter follower. And it is running Class A, so
    there is the same current continuously thru it, no matter what volume.

    The LM386 is simple enough, and probably cheaper than the TL431,
    except if you buy it at Rat Shack. Of course at least you can get the
    LM386 at Rat shack, but not the TL431. But you can also buy two
    transistors, a PN2222 and PN2907 and build a Class B complementary
    circuit that draws very little power from the battery when the sound
    level is real low, yet can put out several tens of milliwatts to the
    speaker when the volume is increased. Like you say below, the TL431
    can't put out much power.

    But here's a circuit that uses a single AA cell, and drives a pair of
    stereo earphones. Check it out.
    http://www.redcircuits.com/Page38.htm
    The one I built draws a half dozen mA from the AA cell, and the
    compression doesn't do much, but the volume is adequate. The output
    stage is about the same complexity as the TL431. Notice how they
    don't use the headphone ground, but drive the two earphone elements in
    series. This helps increase the impedance.

    --
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    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
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  9. For Gp amps, that's true, but for some amps, building your own can
    help cut down the quiescent current and match the amp output to higher
    speaker impedances such as headphones, which may be 150, 300 or more
    ohms.

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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  10. Don't be afraid to use the TL431 itself as the comparator. In other
    words, connect the relay between the 'cathode' of the Tl431 and the
    supply, with a spike clamping diode across the relay coil. Then just
    divide the supply voltage down to the 2.5V with two resistors or a
    pot, with the wiper connected to the adjust terminal of the TL431.


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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  11. Guest

    Good advice (as usual). I changed relays - the first one drew
    about 160 mA, the new one draws about 70 - and fiddled. Your
    idea works. But I'm leery of using just a pot - I put a fixed
    resistor between the pot and + - I dunno what would happen to
    the 431 if the adj terminal was connected directly to +. Anyway,
    that worked. The reason I was using the transistor was to
    supply the bigger relay, because max for the 431 is 100 mA.

    The adjustment on the pot is damn touchy - with the values I
    fiddled with, a 1 ohm swing on the pot makes a lot of difference.
    (I'm calling 100 mv a lot of difference). I may have to table
    further experiments until I can dig up a 20 turn pot at
    500 ohms - I can series resistors with it to get it to the
    sweet spot. I can also fiddle with the fixed R.

    Sigh. I think I'm going to have to build a more precise
    power supply, too, in order to get more solid data. I do
    not have precise control below 100 mV, and it looks like
    I need precision down to 1 mV for these tests. Is there a
    simple "add-on" circuit I can use?
     
  12. Guest

    Well, I re-thought this - I only need adjustment precision,
    not "power supply precision". I don't care about no load
    to full load variation for these tests. I watch the meter
    while I lower the supply voltage until the relay drops
    out. All I have to do is record the last "good" voltage -
    the voltage at which the relay is energized. Doesn't matter
    what the voltage increases to when the load is removed. To
    that end, I performed surgery on the supply today, and added
    a 100K resistor and 50 K pot to the adjustment circuit. That
    allows me to adjust the supply with a lot more precision.
    The modified circuit:
    20K
    ---+---/\/\/\/-+----------------+--- To regulator
    | ^ | |
    o | | |
    Sw / +----+ +-------+
    o | |
    | v |
    +---/\/\/\/\------/\/\/\/\---+
    100K 50K

    To use it, I crank the 50 K pot for max voltage, then
    adjust the 20K pot (coarse adjustment) to about 12.15.
    Then, with the 50K pot (fine adjustment) I can crank
    the voltage down to ~ 11.90

    The effect is like having a fixed resistance in series
    with a ~200 ohm pot totaling about 9.2K when set for
    12.15 on the coarse pot.

    Hopefully, I'll be able to get back to the TL431 vs Zener
    experiment tomorrow, now that I can adjust the supply
    with better precision.
     
  13. I've bought a couple HP 6214 and 6216 PSes on ebay, and they have a
    dual concentric pot with coarse and fine V adj. The coarse is a 5k
    pot, and the fine is a 50 ohm pot in series with it. The fine does
    almost nothing, unless the situation is like what you're doing. ;-)

    Don't forget to include some bypass caps to minimize the noise and
    maybe add some delay or hysteresis.

    I just realized that you might have the pot on the lower end of the
    divider. If so, then you're adjusting only 2.5V, which may cause it
    to be real tetchy. Might be better to put the pot in the high side,
    with a series resistor.


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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  14. Guest

    I guess that's essentially what I've done to my supply, in
    terms of function. Your 2 pots are 100:1 - my "effective
    pot" gives me about 200 ohms or ~ 50:1 at 12 V (the 20 K
    pot is about 10K at 12 volts). Mathematically it's effectively
    over 900 ohms in series with 20 K when the 20 K pot is at max.

    I was thinking of you when messing with my power supply, and
    wondering if those supplies you bought had a fine and coarse
    adjustment.
    Yup - the TL431 circuit uses a pot on the low side - 500 ohm
    pot in series with 1K, and the resistor to the high side is
    1K. When I adjust to get 12.00 V, the pot reads 208 ohms.
    Its a big wirewound pot. I tried the high side originally,
    with a different pot and a series resistor - that was before
    I modified the supply. I'll have to try that again. But
    I get very good results out of the TL431 circuit. I can
    set it right on the gnat's ass. I haven't gotten near as
    good results with the zener - but I have more tests to do.

    I have to use the xsistor with the zener, but the 431 can
    drive a relay directly. However, I'm going to test it
    again with the transistor circuit and original relay so that
    I can switch between the zener and the 431 to get a more valid
    comparison. That's the way I had it originally, and there
    was not as much difference between the two devices
    that way as there was in today's test. Actually, I couldn't
    get the zener to perform as well as it had in the past
    today using an npn. Gonna go back to the original
    xsistor & circuit.

    The original circuit that got me wondering about this
    is long since designed - I'm having too much fun with
    the experiments to finish that project now. Besides,
    that project is now a matter of designing the physical
    layout, and I don't enjoy doing that.
     
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