First Post - Potentiometer Problems

Discussion in 'General Electronics Chat' started by mbga9pgf, Nov 25, 2011.

  1. mbga9pgf

    Thread Starter New Member

    Nov 25, 2011
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    0
    Hi,

    I recently purchased a basic temperature controller for my incubator project. Unfortunately, it isnt exactly a "Quality" Item! Its a 12v controller with basic pot controlling resistance to control temperature. The issue I have at the moment, When I go to adjust the pot, it seems to be affecting the resistance of the pot! Its weird though, because when the sensed temperature is near the set point, the tip of the screwdriver seems to be affecting the resistance of the pot, even though the screwdriver is not in contact with the adjustment screw on the pot.

    The pot itself is a small circular section metal container type.

    Any tips before I scrap it? If I do, anyone know of a more reliable controller that has accurate temperature control suitable for an icubator (around 37.7 degrees +/- 2 degrees c?

    Thanks in advance!

    mbga

    One of the big problems I seem to be having
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Sounds to me that you may have it mis-connected--check your wiring.
    When a novice, I blew out a number of expensive heliopots thinking that the wiring was physically 1,2,3 when the wiper was actually on the end of the assy--I had it across a battery that could source many amps...
     
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  3. mbga9pgf

    Thread Starter New Member

    Nov 25, 2011
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    0
    Hi, thanks for the fast reply! Will go back to check the connections, it may be a case unfrtunately of desoldering and remounting onto another PCB :(
     
  4. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    One neat thing about this forum is that you generally get a fast response.
     
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  5. mbga9pgf

    Thread Starter New Member

    Nov 25, 2011
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    0
    I was just curious though as the resistance seems to be being affected even when there is no contact with the pot - in fact, just putting my finger within 1 inch of the pot seems to be affecting the resistance and thus the set point!
     
  6. Pencil

    Active Member

    Dec 8, 2009
    271
    38
    Maybe your finger/screwdriver is not affecting
    the pot. What other components are close to the
    pot on the board?
     
  7. mbga9pgf

    Thread Starter New Member

    Nov 25, 2011
    4
    0
    2 caps, a few resistors and the power supply cable (DC) the only other item at the other end of the pcb is the relay. But, this should be DC (the only AC in the board is the 12v AC for the downlighter) on the other side of the relay.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    This sounds very like a hand capacitance effect, perhaps coupling in unwanted interference e.g. from AC line voltages, or possibly related to oscillations within the circuit itself. Is the unit mains powered?

    Another possibility might be that some part of the circuit (such as the heat sensor) is responding to the approach of your hand altering light falling onto it.
     
  9. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    I agree with Adjuster that something is making an interference--is this a bang-banb control or does it PWM? PWM can generate tons of noise that can do this sort of thing--determine is the circuit is grounded--if not, try grounding the circuit--perhaps via a capacitor to see what happens. I have seen this when the control is tied to line voltage--poor practice.
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    I photo of your board would be helpful.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Most electronic stores sell "Twiddlers" (not sure of official name), essentially, all plastic screwdrivers used for tuning CRT color controls.

    Maybe using one of those for adjusting would fix your issue without doing everything over? With the metal bladed screwdriver, you add capacitance and possibly a magnetic field into the circuit, which can change the behavior, which is why the twiddle sticks were invented (that, and to isolate tuner from high voltage).
     
  12. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    I once had a perplexing problem like this in an electric vehicle--it turned out to be a ground fault in the motor that made all circuitry to radiate serious interference.

    Check for ground faults.
     
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