431 shunt regulator help

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.

-- Bill Sloman, Nijmegen

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.

Ah, the ever helpful and sociable Sloman.

Hasn't himself designed anything useful in decades, but professes enough expertise to insult people who ask sincere questions.

Speaking of "work"...

John

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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,

1. 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=3D4096 samples

5v/4096=3D1.22mV per sample. 0.666V/1.22mV=3D545samples. log545/log2=3D9 bits

so with this setup, your 11-13v measurement resolution is about

9bits.which is not bad.

hope it helps.

yours Ren

As in "piece of" ?:-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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 I love to cook with wine     Sometimes I even put it in the food

--
He may have known, once upon a time, but It seems Mr. Sloman\'s bent,
over the past few years, has been on grinding out uncalled for
nastiness.

JF

If I understand you correctly, you simply want to map...

+9V to +14V into 0V to +5V

Is that correct?

Simplest solution:

OpAmp whose output range includes negative rail (or you have a negative supply).

OpAmp powered +12V/0V

Plus input of OpAmp to +7V (divider from +12V supply?)

R from signal to minus input of OpAmp

R (same value) from minus input to OpAmp output

This maps +9V to +14V "signal" into +5V to 0V OpAmp output

In other words a phase reversal, but I'm sure your uC can cope ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

Is this what you're suggesting? ...

(view diagram in Courier font)

Vin >----------O-------.

9-15v | | | .-. | | | R1 | | | | '-' | | ,---' | R1-R2 set TL431 to desired TL431 / \\

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desired

gnal to

431.

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.

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signal to

TL431.

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.

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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 sharp.

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.

-- Bill Sloman, Nijmegen

n

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.

-- Bill Sloman, Nijmegen

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.

-- Bill Sloman, Nijmegen

--
Not at all, when you so infrequently do.

In this case, however, the "practical advice" you offered was to try to
get the OP to shut down by demeaning him.

(snip unhelpful response)

This is where your approach started.

and now the crux of the matter.

Faced with your requirement - as distinct from a learning project - I'd build a dead-simple isolated switchmode supply for a DVM or DPM, and then reference the neg line to whatever offset point you require on the DUT output.

Wrong, as usual. I'm off skiing at Lake Tahoe.

John

As in, 'not capabale of'.

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I'm just finishing up the "frame compiler" firmware part of the most kick-ass digital delay generator anybody has ever made. I think I'm still getting better at electronic design. But even if I plateau one of these years, I'll still be designing kick-ass electronics for a good while.

And you aren't even content to be useless. Pity.

John

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.

Ed