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How do I replace this pot with 2 knob pots, coarse fine.

Discussion in 'General Electronics Discussion' started by Brad Hruska, Jul 7, 2017.

  1. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Hello all I am trying to remove the CC pot (the one in the center, the 200k) and wire in 2 knobs one 200k for the coarse, and one 20k for the fine.

    The photo below shows a general layout of the LTC 3780
    LTC3780_WD2002SJ.jpg

    The photo below shows the schematic of the circuit and the pot I am trying to replace.
    CC pot diagram.jpg
    The photo below is a high def photo (front view) with notes of the circuit board with the pots removed. Please note the left hand pot was removed and already has been replaced with a fine and coarse pot. I am trying to replace the center pot (value 200K) I drew the green lines, they are the traces on the front of the board, the red lines (best I can deduce) are the ones I drew for the traces on the back of the board.
    LCT 3780 Front traces.jpg

    This last photo is a high def of the back of the board.
    LTC 3780 Back .jpg

    If anyone can please draw me a diagram of how to add a coarse and fine pot to this.

    Thank you

    Ps. this is a completely different circuit and layout from the 1500w boost circuit I was working on. Please admin do not take down.
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Don't.

    Just fit a 10-turn potentiometer in its place.
     
  3. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    I want to use 270 degree pots only. I do not like multi turn pots nor do i have the space for a venier pot.

    270 degree linear pot only please

     
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    You could try this :
    FineTune.PNG
     
  5. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    What am I looking at Alex? 1mega? (Ohm resistor)? And a 1mega ohm pot
     
  6. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Can it be done with just a 20k for the fine?

     
  7. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Brad Hruska . . . . . . . . . . ( Wanna' buy a vowel ? . . . . that then would be Huruska)

    Vernier Pot Mod.png

    73's de Edd


    .







     
    Last edited: Jul 7, 2017
  8. AnalogKid

    AnalogKid

    2,285
    647
    Jun 10, 2015
    The problem is that that pot is not a signal adjustment, but a gain adjustment. Adjusting it varies both the series and shunt legs of the negative feedback loop.

    The gain range of the original circuit is from 9 to 1801. Without getting into the issues attendant to asking an LM358 to deliver over 65 dB of gain, your problem is getting the added fine control to have the same effect all through the range of the original (coarse) control. That cannot be done with passive components only and no mods to the SMT parts or traces. It could if the original pot were not configured the way it is. I've used that circuit technique, and it is more tricky than it appears.

    Basically, what you want to do is take the signal from the center of the original pot, buffer it, run it through the fine adjust pot, and return that signal to the original circuit. A better way is to take the output of the original LM358 circuit through the fine adjust pot to a buffer to drive whatever is downstream. Either way, if you want the fine control to be some percentage of the coarse setting there needs to be some kind of staging between the two.

    Edit: 73 posted while I was typing. His suggestion is the first choice of something to try, but I'd simulate it first to see the effect of a variable impedance between the feedback divider and the virtual ground.

    ak
     
  9. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Unless it is a micro pot or controlled by a chip somehow, there is no 2k pot I can find.

    ED can you share the link where you found this I have not seen this schematic.

    The only pots on board are a 500k CV, a 200k CI, and a 500k for the voltage adjust

     
  10. Alec_t

    Alec_t

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    693
    Jul 7, 2015
    Yes, a 1 megOhm resistor and a 1 megOhm pot.
    I think you've already tried the option below and found it didn't work?
    FineTune2.PNG
     
  11. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Would this work? Coarse N Fine Pots (2).png
     
  12. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    If you read my initial #7 I had erroneously put in 2K instead of 20K it now has been repoked and reposted.
     
  13. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    No I have not tried this one yet

     
  14. 73's de Edd

    73's de Edd

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    1,067
    Aug 21, 2015
    BY JOVE . . . . I DO believe that you gots it ! . . .now.
    (Your drawing 34975 attempt, had the 20K pot installed in the wrong leg, of the 200K pot.)
    CAVEAT:
    With your 200K pot, its rotation to the right increases the voltage.
    On your newly added 20K pot, you have shown 1-2 joined vice 2-3 so therefore, you then might fully expect the pot to work " sdrawkcab " and decrease voltage on its CW rotation.
     
    Last edited: Jul 7, 2017
  15. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    The blue square, is that off of pin 1 or 3?
     
  16. Minder

    Minder

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    Apr 24, 2015
    Deja-vu all over again!:rolleyes:
    M.
     
  17. Alec_t

    Alec_t

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    Jul 7, 2015
    Either would work, but would determine the direction of adjustment needed for increase/decrease.
     
  18. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Thank you Alec for yours and 73' Eds drawing, I will wire it exactly the same way as drawn. if it does not work, if it does not work then if not I will revert back to the drawing I posted, Dually noted about the reversals, I came across this with the last circuit board.
     
    Last edited: Jul 7, 2017
  19. Brad Hruska

    Brad Hruska

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    Jan 26, 2017
    Ed you are a nutter and a card, I am not sure what to do thank you or cry from laughing so hard.
    So is your last name Huruska? or a variant there of?
    If I was to buy a vowel I would buy an O and remove the r Houska

    I liked the presto chango magic you did. but, but, but but but... there is one already there.
    You had me looking all over the original schematic to see if I missed it originally. LOL...

    I will give yours drawing a try in a couple of hours, if it does not work I will try Alecs then my fall back to the drawing I used on the other board, (YES reversing the output) And if that doesn't work my ultimate fix, a blast from my 12g shotgun, that fixes damn near everything. LOL

     
  20. Brad Hruska

    Brad Hruska

    60
    0
    Jan 26, 2017
    LOl ok Minder, you got me on age, only by a few years.

     
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