Center Tap Pot problem

Discussion in 'Electronics Resources' started by Simon_m74, May 20, 2014.

  1. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    Hi there all,

    I need to make up a X/Y Joystick that uses a center-tapped 3k Pot for each axis.
    I cant find pots online to suit this, so does anyone know how to get round the problem?

    Thanks

    Simon
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    What are the joysticks connected to?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    That's not easy. What load is the pot driving? How is the pot connected?
     
  4. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    Its for a Virtuality video game machine from the 1990`s.
    The joysticks connect direct to a custom `I/O` board, that has a set of MAX,CEN,MIN trimming pots on it for each axis, then ultimately heads down a connection to a Commodore Amiga 3000 computer`s joystick port.
    It looks like the wiper goes to the Amiga via a resistor, and the tap and outer connections go to the trimming circuit part of the PCB.
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Perhaps I made an incorrect assumption at first. What do you mean by "center tapped"? What difficulties have you experienced when searching for a replacement? What electrically distinguishes the desired pot?
     
  6. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    Hi,

    What I mean is I need a 3K pot, that also has an extra connection to the resistor, half-way along its length,

    So really instead of a 3k Pot, it could be referenced like two 1.5K pots in series, with a single wiper that spreads across both resistors.

    The use for this kind of pot has almost become obsolete, as have the components.

    Hence I`m asking if there is a way of `faking` the output from one, as my VR system requires that kind of input.
     
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Good clarification. Unfortunately, I don't have any ideas. I searched myself and was also unsuccessful. Most of the results were for audio pots and much greater resistances.
     
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  8. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    yeah I know...

    thats why I want to `fake` it.
    Other than that, I have recently found information on how to mod a standard Pot to a have a center Tap, so I`ll try that :)
    Then I have design and build a pair of joysticks :)
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could try this fake. Might need to tinker with component values so that both the C and W impedances/voltages are convincing to the Commodore.
     
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  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    I did see a suggestion for using this circuit on an audio forum. Thanks, Alec
     
  11. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    Thanks Alec,

    it being a 3K pot,

    I then assume I`d need R1 & 2 to be 1.5K ?
    and U1 as the 3K ?

    or at least something like that?
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. With the component values I showed, R1 + R2 = 9.4k. That sum is in parallel with 5k, giving a combined resistance of 5k x 9.4k / (5k + 9.4k) = 3.26k between points A and B.
    So the Commodore will see an apparent '3.26k pot' between points A and B, and the voltage at 'centre-tap' C will be half the AB voltage. The voltage at wiper W will be a variable fraction of the AB voltage.
    This arrangement should fool the Commodore, providing the impedances of its inputs which connect to C and W are considerably higher than 5k.
     
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  13. Simon_m74

    Thread Starter New Member

    Jun 17, 2006
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    ah yes..... resistors in series / parallel ! - missed that when looking at it - DOH!!!

    I will give that a try soon! :) thanks :)

    I have a test program that is used to trim the pots, using a graph, so I`ll use that to see how it reacts.

    Simon
     
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