Potentiometer to Variable Resistor

Discussion in 'The Projects Forum' started by bong, May 20, 2013.

  1. bong

    Thread Starter New Member

    May 20, 2013
    5
    0
    Hi,

    i want to change the exisiting resistor circuit (potentiometer) to a variable
    type so that i can control the speed of the crane machine. I have attachment for clear illustrations and i would like to get ideas/advise on how to do it.
    I would appreciate for any assistance.
    regards,
    bong
     
  2. Shagas

    Active Member

    May 13, 2013
    802
    74
    By the resistor circuit are you talking about the 3,6k 10k and 5k?
    You want to put variable resistors instead of the fixed ones?
     
  3. bong

    Thread Starter New Member

    May 20, 2013
    5
    0
    yes. i want to change those three resistor if applicable to
    variable type to limit the current flowing thus reducing the
    speed of the motor.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Be sure to retain a fixed R1, to limit current and avoid a short at one end of the pot's rotation.
     
  5. bong

    Thread Starter New Member

    May 20, 2013
    5
    0
    Thanks for the reply. So I should retain R1 (5kohms) and only
    R2 and R3 to be variable type.
    Also, this is the advise from the crane maker "Yes, you can replace the potentiometer to variable (volume) resistor.
    However, that resistor will be required larger capacity (larger than 4 watt) because large current flows through it."
    How can I calculate for the correct wattage? Can you please show me
    (sketch) on how the variable resistor can be wired on the circuit?
    I appreciate for any reply. Thank you very much.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    That makes no sense until you consider those "bypass" switches. If the pot happened to be set near one end of its travel and the switch sends full voltage to the short, low ohms end, a high current might be possible. But even that depends on where Vout goes.
     
  7. bong

    Thread Starter New Member

    May 20, 2013
    5
    0
    thanks for the reply.
    will the attached makes sense?
    please correct me if i am wrong.
    any advise would be greatly appreciated.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Your polarity reversal switch drawing doesn't make sense but yes, I think the way you have replaced the fixed resistors+switches with pots makes sense.

    A single pot to replace the two would give the same functional result with just one knob to turn.

    I really don't think the manufacturer's comment about high current makes any sense. But I'd sure want to be careful when experimenting, on the chance I'd misunderstood what they were trying to tell me.
     
  9. bong

    Thread Starter New Member

    May 20, 2013
    5
    0
    "A single pot to replace the two would give the same functional result with just one knob to turn." ------
    Hi Mr Wayneh. I appreciate all your inputs. May i request for a sketch to show single pot to replace the two that will yield the same result. Thanks a lot.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    In your schematic, Vout depends on the sum total resistance of the two pots, not on the position of either of them specifically. So there's no need for two of them.
     
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