Variable resistor or voltage divider?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TweeDLX, Dec 16, 2009.

  1. TweeDLX

    TweeDLX Member

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    OK, I'm trying to wrap my aging mind around this with little success. What's the difference between these two, and how are they used (application)? I came across an old post that spoke of changing a pot wired as a variable resistor to a voltage divider, and after looking around the net at various descriptions, diagrams, etc, still can't see a difference. Your help in increasing my knowledge is greatly appreciated.

    Mike
     
  2. phsyconoodler

    phsyconoodler Member

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    A variable resistor is a potentiometer and a voltage divider is two resistors used to control voltage.
    Electricity takes the easiest path.Your inputs on an amp are a voltage divider.The 1 meg resistor lets all the guitar signal go to the input grid,while on the low input some signal bleeds to ground because there is no resistor.
    Any pot in the amp is a variable resistor.Whether it goes to ground or another area of the amp,it does the same thing.
    There is a formula for choosing voltage dividers based on what you need.
     
  3. torquil

    torquil Member

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    A pot has three terminals, one each side of a fixed resistor, and one that connects at an adjustable position along the resistor.

    To wire a pot only as a variable resistor, you would need to use the terminal in the middle, together with a terminal on one side of the fixed resistor inside the pot, and leave the terminal on the other side of the resistor disconnected.

    When adjusting the position at which the middle terminal connects to the fixed internal resistor in the pot, the resistance between the two connected terminals changes. So it is a variable resistor.

    When wiring a pot as a voltage divider all three terminals are connected. The incoming signal is applied to each side of the fixed resistor inside the pot, and the outgoing signal is "extracted" from the middle terminal together with one of the terminals on one side.

    http://en.wikipedia.org/wiki/Voltage_divider
    http://en.wikipedia.org/wiki/Potentiometer

    Hope this helps!
    - Torquil
     
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  4. Keyser Soze

    Keyser Soze Member

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    I picture the pot's internals, mainly the strip of material that forms the resistive element, consider how the wiper moves along this element and how the variations in wiring the wiper changes the behavior of the circuit.

    The two 'outside' lugs are fixed connections, one at each end of the resistive element within the pot. If you only wire these two ends into the circuit it will function as a fixed resistor of a value equivalent to the stated value of the pot. The wiper is the middle lug and it is a contact inside the pot that can be physically moved from one end of the resistive element to the other.


    Variable Resistor

    If you connect the wiper to a single 'end' it becomes a short circuit to that end of the pot. So if the wiper is connected to terminal 3 and then you turn the wiper all the way up to terminal 1 you have a short circuit from 1 to 3 - yielding an effective resistance of zero. As you turn the wiper down towards terminal three the connection slides along the resistive element, adding back in resistance until you reach the far end, at terminal three, at which point the resistance is again equal to the total stated resistance of the pot (within tolerance.)


    Voltage divider

    A voltage divider is formed when one end terminal is connected to a voltage source, the other end terminal is connected to ground, and the wiper is then used as a means of dividing the pot's single resistive element into two separate resistors. One going from source voltage to wiper - known as R1, the other going from wiper to ground - known as R2. The ground connection is the key here - it is what changes a variable resistor into a voltage divider.

    The exact amount of voltage reduction is determined by the ratio of these two portions of the resistive element. The formula being:

    Vout = Vin * (R2 / R1 + R2)


    For example, if the wiper as at the halfway point between the two ends then the resistive element has been divided into two equal portions. So half of the voltage available at the source will be available at the wiper (the other half being 'lost' to ground.)

    As you move the wiper you change the ratio of the two resistors, thereby altering the amount of voltage reduction, from a minimum of zero reduction when the wiper is all the way up at the source voltage end to full reduction when the wiper is all the way down at the other end (and is effectively shorted to ground)
     
  5. TweeDLX

    TweeDLX Member

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    Thanks for all your replies! This is the part in question (Gain knob on a Concert II), and I take it to be a variable resistor, although it looks like one end goes to ground...
    [​IMG]
     
    Last edited: Dec 16, 2009
  6. mark norwine

    mark norwine Member

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    Close...

    Vout = (Vin * R2) / (R1 + R2)
     
  7. CitizenCain

    CitizenCain Member

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    Straight from Aiken's text...

    Vout = Vin*R2/(R1+R2) ...Same thing that Mark wrote minus the top parens.

    Vout = Vin * (R2 / R1 + R2) ...Whole different equation.
     
  8. mark norwine

    mark norwine Member

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    actually, on reflection.....either one will yield the same result.

    I'm sorry for the confusion.
     
  9. reaiken

    reaiken Member

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    No, they won't.


    Vout = Vin*R2/(R1+R2) is correct.

    Vout = (Vin * R2) / (R1 + R2) is also correct and is the same equation as above.

    Vout = Vin * (R2 / R1 + R2) is incorrect and will yield a different result from the above two equations. You must group the R1 + R2 terms in the denominator to get the correct result. Without the parenthesis in the denominator, the division operator will take precedence over the addition operator.

    RA
     
  10. Miles19

    Miles19 Member

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    To answer your specific question on the Concert's gain knob: it's a pot, wired as a variable resistor, and it plus the 220K resistor below it form a voltage divider. This particular circuit is a really unusual one for a gain control (more typically a pot alone is used, and the tap - the arrow on the symbol - is connected to the grid of the following tube). The practical effects of wiring it the Concert way are:
    - you can't go right down to zero gain (although it does go very low)
    - at low gains more low frequencies are passed through, comparatively, than at high gains. Probably this is to compensate for our lowered sensitivity to bass notes at low volumes. Net effect: it sounds brighter (you lose some bass) as you turn up the volume.

    Oh, and Mr. Aiken is correct about the formula. But the formula deals only with the pot and the 220K resistor, and does not account for the 0.0047 cap. The cap is what produces that brightening effect I mentioned.

    Miles
     
  11. mark norwine

    mark norwine Member

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    Again, my friend keeps me honest!

    Thinking I may have initially jumped too fast, I mentally ran some numbers through the formula both ways and came up with the same result.

    Having used this formula (my way) a million times, I [instinctively] added R1 + R1 first, overlooking the algebraic law of "multiply first" [which the "other way" demanded]

    This error came to me at 1AM in a "reverse eureka" moment. Happily, RA corrected me.....
     
  12. TweeDLX

    TweeDLX Member

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    PEMDAS
    Thank you to all for some incredibly informative answers.

    Mike
     
  13. hasserl

    hasserl Member

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    Well, I suck at math.
     
  14. mark norwine

    mark norwine Member

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    Aunt Sally....PEMDAS.....I had never heard any of that before!

    I figured out what PEMDAS must have meant, based on my knowledge of math, but "Aunt Sally"?

    So last night I asked my daughter, "Have you ever heard of P-E-M-D-A....."

    She cut me off and said, "Of course! Please excuse my dear aunt Sally!"

    I about fell over! Who knew? Not me!

    Thanks, TweeDLX!
     
  15. reaiken

    reaiken Member

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    That makes two of us...I guess Aunt Sally wasn't born yet when I took my math courses.

    Of course, I also learned the "politically incorrect" resistor color code mnemonic, too...

    RA
     
  16. mark norwine

    mark norwine Member

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    (we are old(er), I suppose)
     
  17. donnyjaguar

    donnyjaguar Member

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    A variable resistor is a rheostat (two-wire); a voltage divider *could* be a potentiometer (three-wire).

    I use the term rheostat for the VR because there's no confusion. :)
     

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