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[Help] 1.75V sink voltage reference

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Antor

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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 ?
 

Hi,

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

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

Klaus
 

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.
 

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.
 

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/files/products_datasheets/cksr_series.pdf
(Page 14 Fig 31 for schematic,more details in Page 15)

In the mean time I've found LM385 https://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.
 
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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
 

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.)
 

Attachments

  • measurementboardv11.PDF
    21.4 KB · Views: 85
  • cksr6-np.JPG
    cksr6-np.JPG
    31.8 KB · Views: 91
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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.
 

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

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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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.

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.
https://www.ti.com/lit/ml/sprui73a/sprui73a.pdf
All I can get is sharing same DC neutral with MCU and measurement equipment.


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.)

cdn.sparkfun.com/datasheets/Components/General 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_393_n_mot_152377_en.pdf
(Original at Page 35 left bottom corner)

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

Thanks all.
 

Attachments

  • sigure4-4.JPG
    sigure4-4.JPG
    55.3 KB · Views: 101
Last edited:

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"
 

Hi,

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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    Antor

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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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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.

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.
 

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  • shuntres.PNG
    shuntres.PNG
    70.9 KB · Views: 81

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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.

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.
 

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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    Antor

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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 ?
 

Attachments

  • tlv431calc.pdf
    191.2 KB · Views: 102

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