Pot with Taps

Discussion in 'General Electronics Chat' started by DexterMccoy, Apr 15, 2014.

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  1. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    I fixed the schematics so they aren't sideways


    Why do these pots have TAPS in them?

    The TAP creates a different voltage Divider for the wiper?

    I'm not sure what a equivalent circuit it of a pot with a TAP

    The TAP goes to ground , so the middle of the Pot is tapped off to ground, this must change the voltage divider of the wiper

    The inputs to the Pot on both sides of the pot are 26VAC

    If the Pot didn't have a TAP to ground, How would this change the wiper?

    Having the TAP to ground , the wiper is changed how so?
     
  2. daviddeakin

    Active Member

    Aug 6, 2009
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    It's like having two pots sharing a single wiper. The wiper sweeps over one pot (one side of the tap), then eventually crosses over onto the other pot (the other side of the tap) leaving the previous side as a fixed resistance.

    In your second drawing, the left-hand diagram is correct.
     
  3. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    But why use a pot with a tap? If I remove the tap to ground what or how would this change the variable pot?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    In 40 years I have never seen this convention. My first thought was the case of the pot (which is electrically isolated from the pot itself) is grounded to quieten noise.

    Most dual pots show two pots with a dashed line connecting the wiper.
     
  5. ericgibbs

    Senior Member

    Jan 29, 2010
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    There are a couple of applications I know of, one is,
    If the tap is connected to say Common/0V and the pot ends to a +V and -V source respectively , turning the post from min to max, means the wiper would sweep from +V decreasing to 0V and increasing to -V.
    I have used this type of pot as motor speed/direction control signal.

    http://www.potentiometers.com/potcomFAQ.cfm?FAQID=18

    E
     
  6. #12

    Expert

    Nov 30, 2010
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    I have seen taps on audio controls, a very long time ago, and never figured out what they were for because the problem I was fixing was not concerned with the pots. You know, "Just fix it and be done, the customer's cash is waiting for you to fix it, not educate yourself".
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    he taps on the audio pots were to compensate volume and tone, the more you turned up the pot, the tone was adjusted too. but there never were any diodes connected to those pots.
     
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  8. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    So the tap is a reference point? And yes its set to zero volts . When a pot is adjust from a + supply voltage on the top side of the lug and the - negative voltage on the bottom side of the pot, the pot is a voltage divider or something else?
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    Looks like you have this one figured out, almost! In the first drawing, when the wiper is positioned exactly over the position of the tap, ZERO volts out. Moving the wiper up produces ever increasing NEGATIVE voltage. Check the diode direction. Likewise, when the wiper is moved in the downward direction, it will output an ever increasing POSITIVE voltage.

    The position of the tap may or may not be a the half way point of the total potentiometer resistance. That would allow a different rate of voltage change for the same number of degrees of pot shaft rotation.
     
  10. DexterMccoy

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    Feb 19, 2014
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    Yes true it depends on the tap, some are half way others aren't. My question is why didn't they use a regular pot? The pot with a tap is a different kind of voltage divider? How so
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Could it be a bidet? Isn't that a pot with a tap on it?
     
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  12. BillB3857

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    As has been explained many times before, in order to answer the WHY, you would need to talk with the engineer that made the determination that a given component is the right thing to use in that particular application. Sometimes, a standard part will not meet the requirement so a SPECIAL part must be designed,manufactured and procured. That part will be very expensive since only a few will be needed.
     
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  13. GopherT

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    Nov 23, 2012
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    If they did that, then any fluctuation on either power rail will change the mid-point voltage vs. ground. This way there is less fluctuation in the reference voltage from the voltage divider.
     
  14. DexterMccoy

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    Feb 19, 2014
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    So you're saying that is there is a Raise or drip in the power rails, it will change the mid point of the Pot to ratio degree turns?

    So that's why they had to have a TAP off lug to ground, in between the voltage divider
     
  15. GopherT

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    That is my guess - since the reference will usually be close to ground, then fluctuation will be very small.
     
  16. DexterMccoy

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    Feb 19, 2014
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    I'm not understand why the fluctuation would be better with a Pot with a TAP referenced to ground , or the fluctuation is very small with a POT with a TAP
     
  17. GopherT

    AAC Fanatic!

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    Then, if it is important to you, you should make some assumptions about possible power rail fluctuations and resistor values and then calculate the possible errors. If it is not important, keep asking questions until you get an answer you like.
     
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  18. vk6zgo

    Active Member

    Jul 21, 2012
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    Back in the late 1950s,& well into the 1960s these were common.
    They were used both in tone controls,& in some TV circuitry.
    In my first job,back in 1959,as a Storeman at an Electronics shop,around 50% of our stock were some version of tapped pot.

    Dexter,in your drawing when the wiper is at the centre tap,there is no output voltage.
    When it is moved either side of the centre,the output voltage becomes positive or negative,adjustable between zero & a maximum value.

    Your circuit does not show any common connection of the other end of the supply,but I think,despite my comments about assumptions in another thread,we can assume it is meant to be there.

    This circuit looks a lot like an old style model train controller,which used a massive specially made "pot" in this configuration.

    PS, To better understand how it works,redraw the circuit with two batteries connected "back to back" with their common point connected to the tap.
     
  19. DexterMccoy

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    Feb 19, 2014
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    Yes i have seen them in tone circuit in radios

    The TAP pot , TAP lug goes to a Capacitor to ground mostly or a RC network to ground

    But each side of the pot is not tied to ground, so it's variable like a balancing between two different inputs

    input#1 goes to the Top lug of the pot
    Input#2 goes to the Bottom log of the pot

    This is not a voltage divided

    A voltage divided goes to ground and the wiper picks off the divided voltage drop
     
  20. vk6zgo

    Active Member

    Jul 21, 2012
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    You can use a regular pot in a similar circuit.
    In that case,the centre position on the pot looks like a "virtual tap".
    It will work fairly well,but usually there is some uncertainty as to the "zero volts" setting.

    The tapped pot gives you a real "zero volts" point,& as you noted,the tap need not be central,which may be important in some circuits.

    The tapped pot can be looked at as a voltage divider -IF you are looking at the total voltage present across its outer terminals.

    It is more convenient to consider the positive & negative voltages separately.
     
    Last edited: Apr 16, 2014
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