Trying to replace variable resistor with switch and fixed resistors

Discussion in 'General Electronics Chat' started by evicknair, Oct 17, 2011.

  1. evicknair

    Thread Starter New Member

    Oct 17, 2011
    3
    0
    I’m trying to modify a circuit that uses a 10kOhm variable resistor. I would like to replace it with a 4 position slide switch and fixed resistors to get 3 discrete steps and an off position.

    Unfortunately, my EE and circuit design knowledge has about 20 years of rust on it and I can’t visualize how to do this. Anyone have a suggestion?

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you can think of the pot as two variable resistors, the sum of which is always 10k Ohms. The two variable resistors are joined in the center. There are three terminals; the center one is almost always the wiper. The resistance between the two end terminals won't change.

    If you turn one side to 9k Ohms, the other side will measure 10k-9k=1k Ohms.

    Are you talking about having several fixed resistances that get larger and finally infinite for OFF? That would be a rheostat-type function; or a simple variable resistor.

    If you know what resistances you need, we can draw something up for you.
     
  3. colinb

    Active Member

    Jun 15, 2011
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    A simple variable resistor (which NOT a potentiometer, as the Sgt. pointed out), it is as simple as
    [​IMG]
     
  4. evicknair

    Thread Starter New Member

    Oct 17, 2011
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    Thank you. What I want is 3 fixed resistances starting at 10K and going down equally, so I was thinking 6.8k and 3k.

    What is messing me up is the third leg of the variable resistor. Not sure how to reconcile that with the switch and fixed resistors.

    The section of my current circuit using the 10k variable is attached.

    [​IMG]
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if you really wanted to divide two up evenly and still have a full ON position, you could use several 3.3k resistors. 3*3.3k = 9.9k, which is plenty close enough.

    But, are you certain that evenly spacing the resistance will give the desired results?

    I suggest that before making permanent changes, you measure the temperature at several different settings, and measure what the resistance of the pot is at those settings. That way you'll have a better idea of what resistances will correspond to what temperature range.
     
  6. evicknair

    Thread Starter New Member

    Oct 17, 2011
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    0
    Thanks. Yeah, I was going to put in the basic circuit and then do measurements to dial in what temperature levels I need.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    OK, do you have the switch already?
    If so, do you know the manufacturer and part number, or do you have a link to where you bought it, or a link to the datasheet?

    Just so you know, the potentiometer IS the basic circuit. It will go a lot faster/easier if you use the pot to set various temps, record the temp, and then the resistance of the pot.

    Otherwise, you'll be swapping around fixed resistors like mad trying to figure it out.
     
    Last edited: Oct 17, 2011
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