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431 shunt regulator help

Discussion in 'Electronic Design' started by Jim, Feb 18, 2009.

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  1. Jim

    Jim Guest

    trying to drop 12 vdc to 5vdc for an extended scale A/D, I would like
    to use a TI431 to sense anything above 9 or 10 volts, feed it into my
    uC for display on a digital meter. I am a software guy with limited HW
    experience so here are my questions, they all pertain to a simple
    shunt regulator circuit, eg; current resistor on Vin. Ref tapped into
    voltage divider (R1, R2)

    with a regulated input voltage that of 12v and a uC A/D pin that can
    accept 5v, what resistors do I need for the current resistor and the
    voltage dividers in order to capture the top 5 volts (lets say 9v and

    If you would rather "teach me to fish" I will accept a formula that is
    written in 8th grade terms ; )

    Any help
  2. Guest

    The only useful help we can offer is the suggestion to go for a less
    ambitious project.

    If you want to drop the 12V to 5V to get a regulated 5V supply for
    your A/D converter power supply, the TL431 is a poor choice of
    regulator. The LM6805 will do a perfectly adequate job and leaves you
    less room to screw up.

    You can scale down the 0-12V input range to 0V to 5V with a simple
    voltage divider. If you were to tell us which A/D converter you were
    planning on using (or identify the processor of which it forms a part)
    we could suggest specific resistor values.

    If you want to expand the range 9V to 12V to fill the 0V to 5V range
    of your A/D converter, life gets a little more complicated. If you
    told us quite a lot more about what you think you are trying to do we
    could certainly come up with something - but granting the expertise
    you are exhibiting here, it is less than certain that you would be
    able to get it to work.
  3. Jim

    Jim Guest

    Thanks for the reply, what I am doing is making a DVM to read the
    12vdc output voltage of a power supply. Due to grounding issues, a DVM
    cannot be powered by the supply it is measuring unless it is driven by
    a microcontroller type device. My plan is to use an analog in pin on
    an arduino board which can accept an input of up to 5 volts and
    converts this input into 1023 digital steps (it can source or sink
    25ma) Since the power supply output is regulated, I see no need in
    monitoring voltage under, lets say, 10v. By using a zener, rather than
    simply a voltage divider, this allows me to expand the DVM scale and
    get a finer resolution on the voltage that matters.

    My idea is to create a simple shunt regulator to get me there. I
    realize that there is a minor non-linearity close to the zener
    voltage, however, if I set the voltage around 10, by the time the
    voltage reaches the range that I really care about, lets say
    11.75-13.00 it should be linear again.

    Now I suppose I could just go out and get a 10v zener, but thats not
    the point. I want to play, I want to do something new and learn...I
    mean I could have just bought a power supply, or a dvm. Im sure you
    know how it is.
  4. tomrei

    tomrei Guest

    Hi Jim,

    glad your are interested in electronics.

    i can think of a few ways of doing this.

    1, you can clamp the ground return of your ADC to 10V, so your ADC
    will sitting on top of 10v and sample 10-15V.

    2, use a differential amplifier.
    use a resistor and zener in series from supply to get stable 10v, then
    put both supply and zener 10v to the inputs of the differential
    amplifier. the differential voltage will be amplified and be the input
    to ADC. this is a good way, but trouble is you'll need +/-1515V for
    the OP AMP supply.

    3, quality resistor and high resolution ADC. personally i think this
    is the best choice. seem like your ADC is about 10bits resolution.
    i'll suggest a 12bits standard one, it's simple enough part. higher
    resolution ones may be hard to setup.
    you can use high quality resistors to divide the input voltage by say,
    3. the voltage range you are interested are around 11-13V, so the band
    is 2V, divide by 3, gives you 0.666V.
    for a 12 bit ADC of input 0-5V, that gives you 2^12=4096 samples
    5v/4096=1.22mV per sample.
    log545/log2=9 bits

    so with this setup, your 11-13v measurement resolution is about
    9bits.which is not bad.

    hope it helps.

    yours Ren
  5. James Arthur

    James Arthur Guest

    Is this what you're suggesting? ...

    (view diagram in Courier font)

    Vin >----------O-------.
    9-15v | |
    | .-.
    | | | R1
    | | |
    | '-'
    | |
    ,---' | R1-R2 set TL431 to desired
    TL431 / \<-----O voltage drop (9 volts).
    --- |
    | |
    | .-.
    | | | R2
    | | |
    | '-'
    | |
    | | R3 R3-R4 scale remaining 0-6volt signal to
    | | 0-5v, AND, provide bias for TL431.
    | ___
    O--------------|___|----> Vout 0-5v
    .-. R5
    | | R4 (protects a/d input
    | | against faults)

    That's a handful of 1% resistors--not terribly accurate, but
    maybe good enough. Suitability depends on your needs. What are
    they? (accuracy, power consumption, calibration...)

    James Arthur
  6. Jim

    Jim Guest

    yes, although I hadnt thought of R3, R4, R6, I was just going to
    adjust the zener to around 9 or 10 and then just "assume" it would
    never go 5v above that. I see that R3,4,5, supply a little insurance.
    So how would I figure the resistor sizes.

    Thank you to all of the rest of the solutions above, I was hoping to
    avoid op-amps and the like only because my aim was to get the zener
    thing dialed in. Also I have been on Usenet since it began, so I didnt
    take offense to any perceived slight, I have a thick skin when asking
    for free advice.
  7. Jim

    Jim Guest

    By the way, as far as suitability, I dont care about current draw, its
    going on a 30 amp supply, accuracy and calibration? It would be nice
    if the display was +/- .01 volt, although I havent played with the a/d
    on the arduino before.
  8. Guest

    John Fields seems to think that practical advice is nasty - if I give
    it. I notice that he didn't bother to contribute his own particular
    insight on the problem, so he may be less nasty than I am. but also
    appreciably less helpful.

    If you really want a suppressed zero scale, a zener isn't a
    particularly nice way to do it; it's hard to get a tolerance of the
    zener voltage that is is less than 2%, and the knee is never very

    There are are some cheap, close tolerance voltage references around -
    the Linear Technology LT1009 2.500V reference offers +/-0.2% for a
    couple of bucks, and Farnell is now offering a Texas Instrument second
    source part for even less. Zetex has got into the act with an +/-0.5%
    SOT-23 version of the LM4040 for little more than one dollar - the
    LM4040C50FTA does 5.00V and the LM4040C25FTA does 2.50V.

    You'd need a summing amplifier - probably two - and some precision
    resistors to get from 0V to 12V in to a 0V to 5V output that covered
    the 10V to 12V range on the power supply. The voltage rails available
    make quite a difference to the ways in which you might do the job. and
    you have to worry about what happens to the output to the A/D
    converter input pin when the power supply is outside the range 10V to
    12V (or whatever you want to look at).

    Like I said in my first response, when we know more about the circuit
    we can make more explicit suggestions.
  9. Guest

    John Larkin is not only thin-skinned in his own right, but also
    amazingly tender about the sensitivities of unfamiliar posters. Like
    John Fields, this sudden (and somewhat unexpected) burst of
    consideration for the OP's fragile feelings didn't stretch far enough
    to generate any advice on solving the technical problem, but John is a
    busy man, desperately trying to keep up his cash flow while the
    economy slides out from under him.
  10. Guest

    Using the +12V supply (that you are measuring) as the the source of
    the +7V reference (via a voltage divider) used to be the kind of "drop-
    off" that excited a certain measure of hilarity in the places where I
    used to work (all those years ago).

    Since Jim is even older than I am, it's less funny than it used to be.
    Maybe it is as well that John Fileds and John Larkin didn't run the
    risk of exposing their own decaying neurones.
  11. ehsjr

    ehsjr Guest

    V = Vref * (1 + R1/R2) Vref is a characteristic of the
    431, and is about 2.5 volts. So, if you make R1 2.6K and
    R2 1K, you get ~9V.

    Alternatively, you can use a 5K 10 turn pot in place
    of R1 and R2 and adjust it to get your 9 volts.

  12. James Arthur

    James Arthur Guest

    Pick R1-R2 for, say, a 9v drop, per the data sheet formula.

    (R1 + R2)
    V = Vref * ---------

    Then you'd make the impedance of that R1-R2 low enough to
    drive the TL431 input's bias current without significant
    error. I(bias) = 4uA. That gives you R1 and R2.

    Next you'd pick a series combination of R3-R4 that provides the
    TL431 with at least 1mA of idle current wherever you want it
    to work accurately. The LMV431 is better--it'll do this job on
    80uA and has just 0.5uA input current (Vref = 1.24v for that
    part though--don't forget this when calculating R1-R2).

    R5 isn't critical.

    Note that this circuit craps out at low Vin since, ultimately,
    R3-R4 won't provide the TL431 with enough bias current for the
    IC to regulate accurately.

    You could improve that by dropping less voltage across the
    TL431, leaving some drop across R3-R4, and removing the
    offset voltage in software, and/or by using the LMV431 instead
    of the TL431.

    We'd usually just use an op-amp difference amplifier for
    this job:

    R1 R2
    Vin >----\/\/\/----O----\/\/\/---.
    | |
    | ===
    | |\ GND
    | >---------O--\/\/\/--> Vout
    .---|-/ | R5
    | |/ | (a/d protection)
    | |
    +9v >---/\/\/\----O----/\/\/\------'
    R3 R4

    R1=R2=R3=R4, precision resistors.

    If you want a floating DVM supply for one of those little LCD DVM
    modules, just make a little floating supply, like the cap-coupled
    charge-pump thingie at the bottom of this web page:

    James Arthur
  13. Jim

    Jim Guest

  14. Jim

    Jim Guest

    thank you very much
  15. Jim

    Jim Guest

    Thank you all for the advice and entertainment. This place as a
    certain neighborhood bar feeling about it.
  16. krw

    krw Guest

    Right. It's not "work" Slowman's a piece of.
  17. Jim

    Jim Guest

    well, thank you all again...last call
  18. Guest

    But then again. neither does John, on the evidence available.
    Since it's worth nothing to anybody else, there's not a lot of

    I've spent some time (and a little money) trying to find Peter
    Baxandall's original paper, on "Class-D oscillators",

    Baxandall, P.J, Proc I.E.E 106, B, 748 (1959)

    without any success. My little brother (who has convenient accesss to
    university libraries that ought to have copies of the journal) reports
    " it wasn't in their complete 1959 Part B collection. From some
    checking of citations, I've worked out it is in a supplement of
    presumably the May 1959 Part B issue."

    The late Tony Williams, who was the source of the reference, isn't
    available for advice.
  19. Guest

    Version 4
    SHEET 1 880 680
    WIRE 192 16 -288 16
    WIRE 368 16 192 16
    WIRE 192 64 192 16
    WIRE 368 144 368 16
    WIRE 192 160 192 144
    WIRE 336 160 192 160
    WIRE 544 176 400 176
    WIRE -288 192 -288 16
    WIRE -128 192 -288 192
    WIRE 336 192 64 192
    WIRE -288 256 -288 192
    WIRE 192 272 192 160
    WIRE 256 272 192 272
    WIRE 400 272 336 272
    WIRE 544 272 544 176
    WIRE 544 272 480 272
    WIRE -128 288 -176 288
    WIRE 192 384 192 272
    WIRE 416 384 192 384
    WIRE 544 384 544 272
    WIRE 544 384 480 384
    WIRE 192 448 192 384
    WIRE -288 560 -288 336
    WIRE -176 560 -176 288
    WIRE -176 560 -288 560
    WIRE 192 560 192 528
    WIRE 192 560 -176 560
    WIRE 368 560 368 208
    WIRE 368 560 192 560
    WIRE -288 592 -288 560
    FLAG -288 592 0
    SYMBOL References\\LT1021-5 -32 240 R0
    SYMATTR InstName U1
    SYMBOL Opamps\\LT1006A 368 112 R0
    SYMATTR InstName U2
    SYMBOL voltage -288 240 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 12
    SYMBOL res 176 48 R0
    SYMATTR InstName R1
    SYMATTR Value 10k
    SYMATTR SpiceLine tol=0,1
    SYMBOL res 176 432 R0
    SYMATTR InstName R2
    SYMATTR Value 10k
    SYMATTR SpiceLine tol=0.1
    SYMBOL res 496 256 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R3
    SYMATTR Value 124k
    SYMATTR SpiceLine tol=0.1%
    SYMBOL res 352 256 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R4
    SYMATTR Value 1k
    SYMATTR SpiceLine tol=1
    SYMBOL cap 480 368 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C1
    SYMATTR Value 0.1µ

    The LT1021-5 is a ridiculous over-kill for the application, but it is
    in the LTSpice library while the cheaper LM4040C50FTA I mentioned
    earlier isn't.

    I've also left out the 100nF bypass capacitor that even the LT1006
    single supply op amp should have.

    Almost any other low off-set single supply op amp that could survive
    the peak supply voltage would do as well as the (elderly) LT1006.

    I've followed Jim Thompson in figuring that the OP wants to digitise
    the supply voltage range from 10V (which would give 5V at the output
    of the LT1006 in this circuit) to 14V (which would give 0V at the
    output of the LT1006, if the LT1006 could pull right down to the 0V
    rail, which it can't, since it can only pull 40uA down to about 50mV).

    The 0.1% resistors are E96 values and available ex-stock from Farnell,
    some of them as single parts (though for many you have to buy five at
    once, typically for more than a buck apiece).
  20. Guest

    Except that the classic circuit does squegg in real life, if you make
    the inductor too big. The first one I ever built back on 1968 squegged
    continuously, until I took off a lot of the turns that that I'd
    tediously handwound onto the inductor.

    Don't remember ever making that claim. I can tune a DPI control loop,
    but state-variable control theory and self-tuning control loops I've
    so far been able to palm off onto people who do think they have
    mastered control theory.

    You seem to share their enthusiam for applying their expertise to
    situations that they didn't really comprehend.
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