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  1. #1
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    Best position for trimpots in adjustable regulator or reference feedback paths

    Hi,

    Is there a "best" position for trimpots in adjustable voltage regulator and adjustable voltage reference feedback paths?

    Asking because thinking of feedback current that is seen across R1 and that in datasheet and app note calculations is considered important/relevant for calculating Vout.

    I would have thought that B or C as I understand R2 sees less current than R1, but maybe it makes no difference whatsoever whether the trimpot is placed in position A, B or C. Is there any difference between B or C in practical terms, too?

    Click image for larger version. 

Name:	vreg or vref adj feedback trimpot best position.JPG 
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    The question(s) is (are) related to issues such as temperature drift, perhaps noise if that is affected or in any way pertinent, and other minutae beyond my level of implementation successfully but of interest all the same.

    Just slip in a bonus question: Using 100ppm resistors in conjunction with 200ppm trimpots - (for someone who makes little circuits at home...) it's a trivial concern design-wise and irrelevant to the circuit long-term stability of the accuracy and not worth worrying about, isn't it?

    Thanks.
    Last edited by d123; 26th May 2019 at 07:55.

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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    Hi,

    There is no "best" solution.... because it depends.

    If the pot is in the upper path you get a linear resistance_to_outputvoltage behaviour
    If the pot is in the lower path you get an 1/x behaviour.
    It depends on you whether you want the one or the other.

    Also the resistance at the feedback node varies.
    Pot in the upper path: The node resistance becomes higher at higher output voltages. Also the power dissipation in the pot becomes higher with higher output voltage. Both may be seen a the "better" solution.

    Regarding noise (when you place the electrostatic charged screwdriver to the pot) I'd prefer the shematic where the pot is at a low impedance node (GND, output voltage) ... I don't like the situation where the pot is somewhere inbetween (your circuits A and C).

    Another noise source is when the whiper is moved. To avoid this noise you may add a parallel capacitor to the pot. This safely can be don when the pot is in the upper path. If you add a capacitor when the pot is in the lower path you should expect output voltage overshot at power up.

    Klaus
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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    Hi Klaus,



    Super, thank you, a very helpful, informative answer (I have run out of helped me votes today, sorry - tomorrow). Could you explain 1/x in other words, please, not understanding what that means in practical terms. Does that mean logarithmic behaviour, as in a curve?

    Depending on needs, B is the least bad choice (for me) by the sounds of it. Definitely don't want higher power dissipation in the pot, if avoidable. I see trimpots as a necessary evil, not a component of choice.

    Thanks very much!



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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    Maybe not applicable in your instance but 'open track' trimpots tend to get dust settling on the track which can cause irregular resistance when they are adjusted. The dust inevitably increases the resistance between wiper and track so it is wise to design defensively to prevent higher resistance causing over-voltage when in a voltage regulator circuit like yours.

    Brian.
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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    Hi Brian,

    Quote Originally Posted by betwixt View Post
    Maybe not applicable in your instance but 'open track' trimpots tend to get dust settling on the track which can cause irregular resistance when they are adjusted. The dust inevitably increases the resistance between wiper and track so it is wise to design defensively to prevent higher resistance causing over-voltage when in a voltage regulator circuit like yours.

    Brian.
    Very good point, thanks, all info. welcome as my mind is very much on "What I confidently designed is not the end result and over the years too many values have changed/drifted and I wasn't expecting any of that". I only have the little blue (and a couple of beige SMD) Bourns W X Y sealed types with the screw on the top. And some (appalling/next-to-useless) sealed round one-turn ones that came in a learners kit.

    Thanks!

    - - - Updated - - -

    Hi,

    Just realised that applying "B" version of original question to a negative voltage reference with a 431, I'm a bit flummoxed due to frankly still not grasping elementary things about ground and (negative) potential - which is the highest potential here, ground or -V? I think that B in the image below is the right way to implement that version with the trimpot on GND, is - or isn't - it (and A definitely not the right way)?

    Click image for larger version. 

Name:	vreg or vref adj feedback trimpot best position negative.JPG 
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    Thanks.



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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    You get rid of the additional error introduced by potentiometer wiper resistance if you use a true potentiometer circuit.



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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    "True potentiometer circuit" refers to a simple point. Connect the TL431 reference input to the wiper, avoid additional current through the wiper.


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    Re: Best position for trimpots in adjustable regulator or reference feedback paths

    I agree and was also wondering why you didn't show the reference connected to the wiper.

    One more option would be to put the pot in parallel. Pin A on Vout, pin B on GND with a resistor between the wiper and TL431 ref pin. This does two things: gives you another degree of freedom for setting the dynamic range of the trim pot adjustment and it isolates parasitics of the pot from your feedback loop. May not matter here but I've done this with digital pots which have poor frequency response.

    Regardless might want a small compensation cap between output and ref pin also.


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