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    [Help] 1.75V sink voltage reference

    Hello there,

    I need a voltage reference which can be used as sink and 1.75V. The stock ICs I know can't operate under 2.2. Do you have any advice ?
    Cake is a lie

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    Re: [Help] 1.75V sink voltage reference

    Hi,

    as so often: a picture tell more than a lot of words.

    Itīs not clear to me:
    * whether you need a "shunt reference"
    * whatīs the load (current range or resistance range)
    * if 1.75V is the ouput voltage or the input voltage (in case of non shunt reference)
    * whatīs the circuit in your mind, at all.

    Klaus
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    Re: [Help] 1.75V sink voltage reference

    1.75v is about the same as a plain red led. It sinks current (safe limit is 20mA).

    You can add a resistor inline to obtain a slightly greater voltage. Or, put a potentiometer across it and dial it to obtain a lesser voltage.



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    Re: [Help] 1.75V sink voltage reference

    Sounds like a shunt regulator to me. But as Klaus points out, it could be just about anything. PLEASE provide a better description of what you're trying to do.



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    Re: [Help] 1.75V sink voltage reference

    Sorry for being late.
    - I need shunt regulator.
    - load is 2.5V and have 680 ohm series.
    That's why my new voltage reference must be able to sink.
    Voltage reference must be able to draw (2.5-1.75)/680=1.1mA.

    To be more clear in details I need to change the reference voltage of a CKSR sensor.
    https://www.lem.com/sites/default/fi...ksr_series.pdf
    (Page 14 Fig 31 for schematic,more details in Page 15)

    In the mean time I've found LM385 http://www.ti.com/lit/ds/snvs741f/snvs741f.pdf
    (Page 2 Figure 6)
    Vout=1.24x(R3/R2+1)
    With market resistances R3=39k+10k=49k ,R2=120K all with metal film.
    Vout=1.24x(49k/120k+1)=1.7463V is good enough. I can correct accuracy in software side.
    Does anyone experienced if this IC can sink ?
    It has 9V with R1=50k ohm. Do I need to change R1 to 27k or 30k for 5V supply.

    I'm open for any advices. I need to complete the PCB this weekend so I can order it monday.
    In where I live it takes 2 weeks to get a component and PCB cargo takes 30 days from china.

    - - - Updated - - -

    Red led voltage changes with temprature too quickly. I tried a series circuit (Source - trimmer - 3x1n4001). Diodes assumed to be 0.7Volts, I was ok if they were 0.6V but they dropped to 0.45-0.50 volts surprisingly.

    Voltage follower opamp application is not an option since my source must sink.
    Last edited by Antor; 30th March 2019 at 21:19.
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    Hi,

    - load is 2.5V and have 680 ohm series.
    The rest of your description lets assume that:
    Input voltage is 2.5V
    But what is your load then?

    Does anyone experienced if this IC can sink ?
    Nothing else...

    ******
    I wonder what's your complete circuit... why you need to change reference voltage.
    I assume there are simpler or better solutions.

    Klaus
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    Re: [Help] 1.75V sink voltage reference

    Unfortunately there is not much a circuit but an explanation. I'm working on an matrix converter implementation. I got a new theory, mathematical background, matlab simulations and rest.
    I'm at the phase where I need a measurement board for input-output voltage and current. I got the voltage measurement with AD-620 and AD-8542. It is fast and secure. I didn't want to measure the current with series resistor method because resistance changes with temprature, I wanted to use hall effect current sensor (LKSR) but it is not avaible in my region so I had to choose fluxgate current transducer CKSR

    Zooming in my problem now.
    I'm measuring AC Ip=10A current for 3 phase.
    I have CKSR 6-N sensors. (Ipn=6A Ipmax=+-20A)
    Sensor have an internal Vref=2.5V, 0.625V/Ipn sensitivity and an 0.375V-4.625V output. For Ip=0 it is 2.5Volts (Vref).
    I need to change the Vref to 1.75 so it can oscillate between 0.5V-3V which is suitable for my MCU adc input.

    Maximum output 10A/6A*0.625V+1.75V=2.8V
    Minimum output -10A/6A*0.625V+1.75V=0.75V
    For worst case scenario (AC Ip=12A) output becomes 0.5V-3V.

    I need an 1.75V reference voltage source which can draw current from 2.5V 680ohm internal reference.

    The internal schematic of sensor added as image and the schematic I'm using added as pdf.

    (As supply for reference voltage IC, I have 5Vdc. But if it really needed I can get 15Vdc too.)
    Last edited by Antor; 30th March 2019 at 22:39.
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    As you already have found out, an adjustable shunt reference can well work for your purposes. If you also read the data sheet, you'd know that it can sink up to 20 mA.



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    Re: [Help] 1.75V sink voltage reference

    I've read the datasheet, but I may be having a blindness on selective perception. Could you please point where it is ?
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    Hi,

    Unfortunately there is not much a circuit but an explanation.
    I wonder why so many people are afraid of using a paper and a pencil..

    I didn't want to measure the current with series resistor method because resistance changes with temprature,
    I don't think you find a more precise way than using a good quality shunt resistor. In my eyes the temperature drift of any other method is higher...but they add extra errors like unlinearity, hysteresis, noise...

    I need to change the Vref to 1.75 so it can oscillate between 0.5V-3V which is suitable for my MCU adc input.
    This is what I was looking for.
    If you use an ADC, then this ADC needs a reference voltage.
    Using a second reference voltage just degrades overall performance, no matter how precise it is.
    --> use the ADC_ref to generate the 1.75V .... this way you not only avoid to add extra errors, but you partly compensate some errors.
    ADC_ref drift will not cause that large output offset drift anymore..it just will generate unavoidable gain drift.

    Klaus
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    Re: [Help] 1.75V sink voltage reference

    TL431 IC (programmable precision reference) provides 2.5v in its basic simple configuration, can sink. (In order to source I believe current comes from the power supply). (When I read about it I set my meters to it.)

    http://cdn.sparkfun.com/datasheets/C...IC/TL431-D.pdf
    Last edited by BradtheRad; 31st March 2019 at 01:46. Reason: Revised claim about ability to source



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    Re: [Help] 1.75V sink voltage reference

    Quote Originally Posted by KlausST View Post
    Hi,
    I don't think you find a more precise way than using a good quality shunt resistor. In my eyes the temperature drift of any other method is higher...but they add extra errors like unlinearity, hysteresis, noise...
    A good quality shunt resistor which can sustain 10A and with low tolerance is 130$ + shipping + taxes (taxes are %20 + 20%). I can't afford 6 of them. Fluxgate is 13$. Fluxgate have a good di/dt and response comparing to op-amps and in-amps i know. If I fail I'll ask for your 3 component method you mentioned earlier.

    Quote Originally Posted by KlausST View Post
    Hi,
    If you use an ADC, then this ADC needs a reference voltage.
    Klaus
    The MCU board I'm using has no ADC reference pin.
    http://www.ti.com/lit/ml/sprui73a/sprui73a.pdf
    All I can get is sharing same DC neutral with MCU and measurement equipment.


    Quote Originally Posted by BradtheRad View Post
    TL431 IC (programmable precision reference) provides 2.5v in its basic simple configuration, can sink. (In order to source I believe current comes from the power supply). (When I read about it I set my meters to it.)

    http://cdn.sparkfun.com/datasheets/C...IC/TL431-D.pdf
    Thanks mate, but it seems like TL431 IC best usage is 2.5V and above.


    I checked the circuit I shared as an image but materails can not be found in region (not even 33.2k is around)
    https://conrad.ru/doci/komparator_lm..._152377_en.pdf
    (Original at Page 35 left bottom corner)

    I guess best is LM385 at this point. I hope it works.

    Thanks all.
    Last edited by Antor; 31st March 2019 at 09:41. Reason: thanks
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    I've read the datasheet, but I may be having a blindness on selective perception. Could you please point where it is ?
    Seriously?

    1. Title page, "Features"
    2. Electrical characteristics, "Reference voltage change with current"
    3. Respective diagram "Reverse characteristics"



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    Re: [Help] 1.75V sink voltage reference

    Hi,

    Quote Originally Posted by Antor View Post
    it seems like TL431 IC best usage is 2.5V and above.

    I guess best is LM385 at this point. I hope it works.
    TLV431 is (yet) another 1.24V adjustable shunt reference. Datasheet says minimum Iq is (55uA to) 80uA, so LM385 would be a better choice as it says minimum required is 10uA.


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    Re: [Help] 1.75V sink voltage reference

    Hi,

    A good quality shunt resistor which can sustain 10A and with low tolerance is 130$ + shipping + taxes
    This is nonsense. I don't know what special shunt you are looking for.
    I never paid more than 3€ for a 10A shunt. Add a good Opamp for additional a couple of €..

    The MCU board I'm using has no ADC reference pin.
    How can this be? Hopefully it does not use VCC as reference...if, then it's useless to talk about precision and accuracy....

    Klaus
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    Re: [Help] 1.75V sink voltage reference

    Quote Originally Posted by KlausST View Post
    This is nonsense. I don't know what special shunt you are looking for.
    It is about region and products we can get. Cheapest THT is 70$ with very few watts.

    Quote Originally Posted by KlausST View Post
    How can this be? Hopefully it does not use VCC as reference...if, then it's useless to talk about precision and accuracy....
    I haven't started programming MCU but as it's datasheet there is only external Vrefhi and Vreflo. I choose that MCU because it has a lot of ADC's. If you can suggest me a MCU/DSP with 12 ADC and fast at calculating trigonometry functions I'll gladly consider getting one. I need to drive 18 IGBT/Mosfet with MCU or DSP.
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    Hi,

    Shunt:
    An expensive one.
    But you noticed this is the "MPQ" price. But what is MPQ? 1 piece?

    Why not use a cheaper two terminal shunt?
    I hope this link works: https://de.farnell.com/webapp/wcs/st...4963,310091680

    Klaus
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    Re: [Help] 1.75V sink voltage reference

    Quote Originally Posted by KlausST View Post
    Hi,
    Shunt:
    An expensive one.
    But you noticed this is the "MPQ" price. But what is MPQ? 1 piece?
    Klaus
    Hi KlausST

    Sadly MPQ means price for one. I can't buy components directly from farnell nor mouser. I need to buy from a middle dealer. As it seems they don't want to import cheap stuff because they don't profit as much as expensive ones. In the past I did a shunt myself by bending motor wound/transformer wound copper but they tend to have a big induction effect.

    Quote Originally Posted by d123 View Post
    Hi,

    TLV431 is (yet) another 1.24V adjustable shunt reference. Datasheet says minimum Iq is (55uA to) 80uA, so LM385 would be a better choice as it says minimum required is 10uA.
    I can't find LM385-adj and TLV431. There is no stock and distributors doesn't like to import them.

    I'm running off my time and I really need your help about reference voltage.
    Cake is a lie



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    Re: [Help] 1.75V sink voltage reference

    LM4041-N-Adj

    How about a voltage reference made using the LM334?, e.g. page 13, Figure 26. 1.2V Regulator with 1.8V Minimum Input?


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    Re: [Help] 1.75V sink voltage reference

    Seems like there is TLV431 in stock here. Thanks for great advice. I guess my resistance choices are ok (pdf attachment).
    Now question is should I build 3 different voltage references or share 1 reference for 3 sensors ?
    Cake is a lie



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