Adjust rheostat ohm range

Discussion in 'General Electronics Chat' started by Djchumpguy, Apr 12, 2016.

  1. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Going to keep this simple I want to know if there is a way to get a 0-90 ohm rheostat to a range of 3-64 ohms? I can provide more information for application purposes if necessary.
     
  2. #12

    Expert

    Nov 30, 2010
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    Put a 3 ohm resistor in series with it and 189 ohms in parallel with it.
     
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  3. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Thanks AAC going to try and upload this picture to make sure I got it right in my head
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Looks good. Trimming rheostats to get the range you want is almost an art. I can get you more detailed links if you like but for now it looks like you've got it.
     
  5. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Thanks again I tried asking our master electrician at work if it would be possible he just told me it would be to much work its not worth the time but it seems pretty straight forward to me. Hey wayneh if you don't mind me asking I understand the 3 ohm resistor raising the range to three ohms but for the life of me I can't understand how the 189 ohm resistor takes the rheostat down to 64 ohm
     
  6. #12

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    It doesn't. It takes the potentiometer down to 61 ohms so that plus the 3 ohm resistor makes 64 ohms. If you want 64 ohms, use 222 ohms in parallel.
     
  7. #12

    Expert

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    But here, you get the answer in 4 minutes.:rolleyes:
    Would you like some irony with that?
    I'm not qualified to sit for the test to become an electrician.:D
     
    Last edited: Apr 13, 2016
  8. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Exactly why is it easier to get random strangers to help with personal projects but, people you work with on a daily basis can't give you the time of day?
     
  9. #12

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    It is likely he doesn't know how to do this. Electrical and electronics are very different fields. Electricians install wires. Electronics people design circuits.;)
     
  10. crutschow

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    Mar 14, 2008
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    One possible down side to that technique is that the resistance change is not completely linear with wiper position, but that's not usually a problem in most applications.
    Below is an LTspice simulation showing the non-linear curve of resistance versus pot position
    (Edit) for the 90Ω compensated pot and the linear plot for a 61Ω pot.
    For example, the resistance at the 0.5 (50%) compensated pot position (cursor) is 40.7Ω whereas the linear pot gives a total resistance of (61/2+3) = 33.5Ω. The difference at midpoint is thus about 21% high.

    upload_2016-4-13_12-16-11.png
     
    Last edited: Apr 13, 2016
  11. wayneh

    Expert

    Sep 9, 2010
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    Just to expand your perspective on what can be done to tailor a variable resistor, you might find these links useful.

    tailoring potentiometers.pdf

    This and this, "The Secret Life of Pots", too.
     
  12. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Thank you crutschow that will cause problems but not enough to stop me from pursuing further it will only skew data received a little but it's only for reference to position high and low needing to be most accurate And thank wayneh you have been more then helpful
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    No randomness... If you call for a 12 at allaboutcircuits.com, you get
    #12
    here every day (or night).
     
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  14. wayneh

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    Maybe not random, but no comment about the "strangeness" of #12! :D
     
  15. #12

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    No problem. My best friends call me, "weird".
    I'm OK with that. Weird is in the eye of the beholder.
    The fact that most of the people on this site can do electronics makes them strange to most other people, so, welcome to the club.
     
  16. wayneh

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    Absolutely correct. My family and friends would not hesitate to use adjectives like strange, weird, nerdy and so forth. Being able to repair a string of Christmas lights pretty well separates you from polite society.
     
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  17. Djchumpguy

    Thread Starter New Member

    Apr 12, 2016
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    Yep until you finish a project and they all look at you like your a genius but, only for a brief second until they ask how. Then they stare at you like your speaking Vulcan and your back to ground zero......
     
  18. crutschow

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    Mar 14, 2008
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    When I was a kid I had to know how everything worked, both mechanical and electrical.
    (It bugged me no end that I couldn't figure out how an automobile rear-end differential worked, since I couldn't see how that could be done with gears. It was a eureka moment when I saw one taken apart in a repair shop and realized the trick was that two of the gears rotated with the drive gear.)
    It wasn't until many years later that I realized most people couldn't care less about how things work.
    I was a nerd.
     
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  19. MaxHeadRoom

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    Jul 18, 2013
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    My relatives gave me the title Cliff Claven!:(
    Max.
     
  20. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A mother brings here awkward child to the doctor and says, "He reads text books instead of plays with his friends, actually he doesn't make friends, he would rather take the radio apart than listen to it - in fact, he has taken apart and re-assembled nearly every expensive device in our house. What is wrong with him, doctor?" Doc says, "let me run a few tests". He come back a few minutes later and says, "I'm sorry ma'am, there is nothing we can do about it. Your son will be an engineer."

    These are the kinds of kids I strive to find when we recruit. Unfortunately, we end up hiring more of the kids that say, "well, I was good in math so I majored in Engineering. How long before I can transfer to a business position" The kids who were not born to be engineers don't know which way to turn a wrench (actually they don't know that things come apart). At my little chemical factory, one engineering intern (that someone else selected) didn't even get the concept that we were making liquid chemicals from different chemicals until he was on the job for three weeks. He just thought we were bringing blue barrels into one side of the factory, somehow turning them into black barrels and shipping black barrels to customers. He had no idea why the big stainless steel reactors and distillation columns were there.
     
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