easiest asjustable precision reference...

Discussion in 'Homework Help' started by dtvonly, Nov 4, 2013.

  1. dtvonly

    Thread Starter Member

    Dec 14, 2012
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    Hi. there are so many reference voltage circuits available, I just want to use the easiest one - to allow me to adjust from 0 to 3.3V. Please advise on circuit. Thank you.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    There are many choices because different applications call for different things. Current at the output? Temperature stability? Degree of accuracy and precision? How close to zero?
     
  3. dtvonly

    Thread Starter Member

    Dec 14, 2012
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    Current output: 25mA; accuracy: 5%; 0.1V to 3.3V output.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    With that much current and a need for 5% accuracy, I would rule out a simple voltage divider (which requires a reference voltage anyway).

    You've posted this in Homework. Is it?

    What things have you been considering?
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the input voltage from which the reference is to be derived?
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    For 5% accuracy you might (depending on the temperature range experienced) get away with a zener followed by a pot followed by an emitter-follower or source-follower.
     
    Last edited: Nov 5, 2013
  7. ScottWang

    Moderator

    Aug 23, 2012
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    You may using a resistor + 4V zener to be a reference voltage, and using a VR1 10K potential meter cross on the two pin of Zener, and using one stage of transistor to amplify the Base current, and Vout from E, and E connecting a 1K resistor to GND to be a false LOAD.

    You can also in series with two resistors, one with pin1 and another one with pin3 of VR1 to set the low limited voltage 0.1V and high limited voltage 3.3V.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I seem to recall that a 6.2 volt zener in series with a 1n4001 diode make a relatively stable voltage reference because the temperature coefficients cancel each other. I'm not sure if it's the 6.2 volt or one of the other close values (like 5.6 or 6.8 volt) zeners.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    A 5.6V zener has very little temperature coefficient because the zener and avalache modes are in balance and offset each other. But for reasons I haven't bothered to look up, a 6.2V zener is apparently more stable over time and has a tempco of about +2mV/°C which is equal and opposite the normal tempco of a forward biased silicon diode, so you can put them in series and get comparable tempco performance while keeping the long-term stability.
     
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  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    There are tons of references like LM336 or LM385 and others with all ranges of precision.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    The notion of a "precision" and "adjustable" reference are almost contradictory.

    Precision means repeatable. So what does that mean in the context of something that is adjustable? About the only thing that it can mean (or at least that I can think of right off the top of my head) is that whenever you turn the adjustment pot (or whatever you are using to adjust it) to the same point that you get the same reference voltage.

    Somehow I don't think that is what YOU are looking at when you use the term "precision" for this. So what do you mean?

    In a subsequent post, you indicated you wanted this thing to have an accuracy of 5%. Again, what does that mean in the context of an adjustable reference. It also needs to be given within the context of operating ranges over which you need this accuracy. Over what temperature range? Over what time frame? Over what supply limits? Over what range of current draws? Etc.

    Is this 25mA current draw constant, or is it anything from 0mA to 25mA?

    You really want a reference voltage down to 100mV and while delivering 25mA?
     
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