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What is the difference or similarity between these 2 pot circuits?

Discussion in 'General Electronics Discussion' started by Brad Hruska, Jul 5, 2017.

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  1. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    Hello Everyone I am trying to wire 2 pots together to get a fine and coarse adjustment. The pots are used (I believe) as a rheostat because legs 2 and 3 are connected together and power comes in on pin 1.

    So can anyone tell me what are the differences or similarity between these two schematics. and wether one will work better than the other (or work at all)

    I am trying to replace a PCB 500k pot with 2 knob pots one of 500 (coarse) and a 50k pot (fine)
    I am also trying to replace a 200k PCB pot with 2 knob pots one of 200k (coarse) and a 20k pot (fine)
    Please let me know which one will work best.
    Thank you
    Brad

    Untitled.png Coarse N Fine Pots (2).png
     
  2. Minder

    Minder

    3,144
    679
    Apr 24, 2015
    The bottom one is the correct one, although I would have expected them to be labeled (fiine/coarse) opposite.
    M.
     
  3. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    I would have gone for the top circuit, it only has two connections which is what you want. If you have three connections, what do they do?

    If replacing a 500kΩ pot, then a 500Ω pot will do very lttle.:)
     
  4. Minder

    Minder

    3,144
    679
    Apr 24, 2015
    The bottom example would be selected if it intended for such as a speed control into a analogue input for example, when intended for VFD or motor controller..Comparitor input etc
    Ensure you use linear not logarithmic type.
    M.
     
    Last edited: Jul 5, 2017
  5. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    So the fine should be 200k and the coarse should be 20k?? or do you mean that the top should be wired in where the bottom is wired in?

    But if replacing a 200k PCB pot, would you not put the 200k pot at top so the circuit sees 200k total resistance between pin 1 and 3? and then use pin 2 for the fine because it is 1/10 the total resistance of the original pot?




     
  6. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    On the board there are 3 holes, with 3 pins coming out, on the board pin 2 and 3 are linked together, so in effect there are two terminals yes, but 3 pins.

    as far as replacing the 500k pcb pot, I am going from a multi turn pot to a 270 degree 500k pot (for coarse adjustment), I need the circuit board to see a total of only 500k, across the pots. Wouldn't the board if wired in the top photo see a total of 550k??

    After the unit sees the 500k pot, then I need to add a fine adjustment (the 50k pot)


     
  7. AnalogKid

    AnalogKid

    2,538
    722
    Jun 10, 2015
    Not knowing the rest of the regulator circuit, I'd start with the top schematic. The lower one is a variable resistance in series with a setpoint, and may have very little effect if the input impedance of the rest of the circuit is high. The upper one works as a coarse/fine resistance no mater what it is connected to.

    ak
     
  8. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    "as far as replacing the 500k pcb pot, I am going from a multi turn pot to a 270 degree 500k pot (for coarse adjustment), I need the circuit board to see a total of only 500k, across the pots. Wouldn't the board if wired in the top photo see a total of 550k??"

    I should not think that 550kΩ will hurt anything. You could set the coarse pot to 450kΩ, then the fine pot could take you up to 500kΩ.
    If you are worried about a slighly high resistance then a shunt resistor could be added.
    1/R1 -1/R2 = 1/R, R = 5.5MΩ
     
  9. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    Could you explain how and where to wire in a shunt resistor please.


     
  10. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    The circuit is on a 1500w boost converter, the
    The CV (third photo) has pin 2 and 3 combined and has continuity to the DC+ output, pin 1 has the power in. up to 120v.

    The CC (second photo) has pin 2 and 3 combined and has continuity to the DC- output, pin 1 is the power in
    about 4.5v

    Originally the CV had a 200k PCB pot and the CC had a 5k PCB pot. I tried to use 2 pots as voltage dividers and they will not work in any combination. I have wired the 1500w boost converter with both of the circuits shown here and they both work. but which one is correct????
    Although the 5k and 500ohm as in schematic 1 seems to affect the voltage greater because the higher resistance??

    The 200k and 500k are for use on the LCT3780 which I will post later.

    This diagram is to beused with the 1500w just changing the values to match the original pot.

    1500w converter top side.jpg CC pot back with continuity test DC-.jpg CV pot back side and continuity test DC+.jpg

     
  11. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A shunt resistor could be connected across the whole circuit that is to the two connections on the board.

    I cannot understand what you are doing. Is the large resistance 500kΩ or 200kΩ?

    Edit: If this is a regulator, the pots are possibly the voltage sense feedback and excess resistance could give excess voltage - boom.
     
    Last edited: Jul 6, 2017
  12. Brad Hruska

    Brad Hruska

    60
    1
    Jan 26, 2017
    Hey Duke,
    Originally on the pictured 1500w boost converter, it had 2 pcb pots. The original pot on the CV had 200k
    the original pot on the CC was 5k

    I have taken out the multi turn PCB pot on the CV (200k) and want to replace them with 2 dial knob pots with 270 degree turn to be mounted remotely. I want a 200k coarse, and a 20k fine pot.

    I have taken out the multi turn PCB pot on the CC (5k) and want to replace them with 2 dial knob pots with a 270 degree turn, to be mounted remotely. I want to use a 5k coarse and a 500 ohm fine pot.


    I have another smaller buck boost converter with a 200k and a 500k PCB pot that I will remove and do the same to (but the 200k pot on the lct 3780 has a different configuration). I will post this one to figure out at a later time.


     
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