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How do you connect a variable resistor?

Discussion in 'General Electronics Chat' started by Haru, May 14, 2005.

  1. Haru

    Haru New Member

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    How do you connect a variable resistor?...It has 3 pins, I only want to use 2 pins
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Use the centre pin and one of the outside ones, if you want you can connect the other outside one to the centre as well - but it doesn't actually affect it's operation, just stops it going O/C if the slider stops making contact.
     
  3. bloody-orc

    bloody-orc New Member

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    don't use one of the side pins then
     
  4. dave

    Dave New Member

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  5. mstechca

    mstechca New Member

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    Why do you care?

    I suggest using all three pins. Connect the center pin to one end. this forms pin #1. The remaining pin is pin #2.

    You should connect them if you dont want to risk having infinite resistance, and instead want the resistance to be at pot value if the middle pin goes open (after excessive abuse).
     
  6. Dr.EM

    Dr.EM New Member

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    I often see it connected in that way you described on schematics, but I usually just use the 2 pins, unless its a level pot on something, where one end goes to ground, the other to the signal source and the centre taps off at any point between (thats how I like to look at it :) ). Has anyone actually managed to break a pot like this, through excessive abuse? Perhaps in a DJ mixer it could happen, but even that seems unlikely.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Dr. EM,
    Pots frequently get dirty or wear out. If you don't connect the unused pin to the slider it will go open-circuit and the circuit won't work anymore. Ifyou connect the unused pin to the slider, it won't affect normal operation in any way but if the slider loses contact it will go to max resistance and probably still work but at a higher or lower level or frequency. :lol:
     
  8. eblc1388

    eblc1388 Active Member

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    Hi Dr. EM,

    Tie up the third pin is actually recommended by the trimmer manufacturer.

    Apart from the reasons that Audioguru and others had said, connecting the third pin would effectively put the wiper contact resistance in parallel with the unused portion of the trimmer thus helps to lower the contact resistance and improves circuit reliability even when your trimmer is new.
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The 'improvement' is going to be EXTREMELY negligible, and won't make the slightest bit of difference.

    The only real reason to do it is to prevent the entire circuit going O/C if the wiper contact stops making contact.

    This 'may' be an advantage?, or it could even be a disadvantage?, depending entirely on it's exact use, and how you would like it to fail in case of a preset failure.

    Commercially you see both methods used, presumably based on what they want to happen?.
     
  10. eblc1388

    eblc1388 Active Member

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    Well I tested one trimmer in my circuit and there is a 2mV difference(out of 2V) across the trimmer with and without the third pin shorted to the wiper.

    I don't know about the circuits you normally use but I do care about such improvement, especially when they are recommended by trimmer manufacturers and does not cost me an extra penny.

    An advantage to fail to open circuit? Strange argument.

    I'm not surprised. Even the trimmer manufacturer remarked that 50% of users do not aware of the reasons of connecting up the third pin, that's why they specially mentioned it in their literature.
     

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  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That's a 0.1% difference! - do you thing that's not negligible?.

    For that matter, would it make any difference anyway?.

    I really see no improvement in the figures you mentioned, can YOU adjust a preset to 0.1%? - a pretty pointless exercise, particularly when the spec on the track is probably +/-20%!.

    Going completely O/C could make the circuit 'fail safe', or if not that, at least fail so obviously you know something is wrong. Failing so it's just out of adjustment could cause damage, as the fault could then continue for extended periods.

    I've never said not to do it, only to be aware of the reasons for doing so, and not just doing so for no good reason.
     
  12. eblc1388

    eblc1388 Active Member

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    To you yes, to me no.

    For PIC and general applications, no. For metering and instrumentation, yes.

    You have mixed up adjustability and device tolerance. These are two different things. An easy mistake for many people to make.

    I do not agree on this line of reasoning.

    Similarily, I never said people must do it. What I said in the above posts was why I choose to do it.

    In fact my first post is just stating the reason of why some people is doing it.

    Now our positions are clear this argument can RIP.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is interesting that Bourns actually admits that their products are failing.
    Something like the disclaimer from semiconductor manufacturers on their datasheets: "Not for use in applications involving life-support. When our product fails and kills someone or causes an expensive disaster, you can't sue us!" :lol:
     
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Again WHY? - how does it affect the adjustment?.

    I'm fully aware of the differences, that's why I consider the possible 0.1% change totally irrelevent - it's for an adjustment, which you can't possibly adjust to anywhere near 0.1% accuracy. Assuming you 'could' adjust to that accuracy?, how does shorting the wiper and other end make any difference to your adjustment? - simply adjust the wiper 0.1% further along the track!.
     
  15. eblc1388

    eblc1388 Active Member

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    The 0.1% difference is what I measured. I have no idea whether it is typical or not because in real life the difference depends on the current through the trimmer.

    I hate wiper contact resistance gets in the way of my gain network and using the third pin at least helps in reducing the effect. That's my reason and you are free to form your judgement about it.

    It seems any more justification on whether using the unused pin would brings benefits is not fruitful so this would be the best time for me to stop. For that matter you have won the argument. 8)
     

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