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voltage controlled resistor

Discussion in 'Electronic Components' started by [email protected], May 16, 2007.

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  1. Guest

    Does anyone know of an IC chip or discrete component intended to be
    used as a variable resistor whose resistance value may be controlled
    by varying its voltage input/signal
  2. Franc Zabkar

    Franc Zabkar Guest

    About 20 years ago I encountered a device consisting of a light source
    (LED ?) and a light dependent resistor. IIRC the part came in a
    package that was not much bigger than a TO92 transistor. I think
    similar schemes are used to provide noiseless volume controls for
    musical instruments, eg the expression pedals for pianos and organs.

    - Franc Zabkar

  3. The "Vactrol"?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  4. And the poster hasn't really given enough detail.

    Those things, and you could make them yourself if you had the light
    dependent resistors, tend to be the simplest solution, because the
    existing circuit only sees a variable resistor. I gather though
    that they tended to be not so linear vs control voltage, and I don't
    recall them coming in a wide range of values.

    Since the original poster didn't specify what he was trying to do,
    getting a result is not going to be easy. The FETs mentioned by
    some, those work, but you have to retrofit them onto the existing
    design, and I thought there were linearity issues too, as in adding
    distortion to the circuit if not done right. Linearity in the control
    voltage versus resistance too, I think it was Siliconix who had a whole
    application note about that.

    Someone mentioned 4066s. The trick would be to pulse modulate those
    to get the varying resistance. The problem then is that you end up
    with junk on the signal, unless, as Don Lancaster pointed out decades
    ago, you are controlling a low pass filter that takes out the junk.

    One could use an attenuator made up of a forward biased diode, I've
    seen those but I don't know how well they worked.

    One could always change the circuit, so instead of a stage of amplification
    and a volume control, you put in a gain control stage. So any sort of
    stage intended for AGC or voltage controlled amplificaton would fit in.

  5. It really does depend massively, on the linearity you require, whether you
    need to handle voltages that are unipolar or bipolar, and the actual
    current levels involved.
    The simple FET, is the commonest solution.
    For AC, a pair wired as a 'bilateral switch', is the normal solution (the
    4066, contains a small pair of FETs that can be used this way).
    Historically, the Cds photo cell, was also sometimes used.
    If noise is not a problem, for higher powers, simple PWM control of a
    on/off switch is probably a 'better' solution.
    Generally, most solutions go 'non linear' at some point. You can achieve
    much wider control ranges, using something like the IL300 opto (or it's HP
    equivalent). The 'point' about this type of opto, is that two of the sense
    elements, are integrated into the single package, allowing accurate
    feedback control of the resistance achieved.
    For pure AC, the 'ecstatically wound choke', can be used, where a DC
    voltage fed through one winding, moves the choke into saturation.
    The number of possible solutions, is 'myriad', and a bit more data on what
    is required, might help in making a better suggestion.

    Best Wishes
  6. Franc Zabkar

    Franc Zabkar Guest

    The name doesn't ring a bell, but some research suggests that may be
    it. Thanks.

    - Franc Zabkar

  7. That was the brand I used to replace in guitar amps, back in the '60s
    and '70s. They were a neon lamp and photoresistor in a hard black
    plastic tube. Some could be cut open and replace the neon when it
    failed. I have no idea if the brand is still around. Its changed hands
    several times over the years.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  8. Franc Zabkar

    Franc Zabkar Guest

    I would hazard a guess that these devices were mostly used in volume
    control applications where the user provided the feedback. Hence
    linearity may not have been a big issue.

    - Franc Zabkar

  9. There were incandescent versions for remote volume control
    applications. Some Public Address amplifiers had sockets to add remote
    volume controls.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  10. Filament lamp (flashlight) and LDR - often just in a piece of shrink tubing.
  11. Franc Zabkar

    Franc Zabkar Guest

    "PerkinElmer Optoelectronics has been a leading manufacturer of analog
    optical isolators for over twenty years and makes a broad range of
    standard parts under its trademark VACTROL®."

    "The analog optical isolator (AOI) also uses an optical link between
    input and output. The input element is an LED and the output element
    is always photoconductive cell or simply photocell. Together, the
    coupled pair act as an electrically variable potentiometer. Since the
    output element of the AOI is a resistor, the voltage applied to this
    output resistor may be DC and/or AC and the magnitude may be as low as
    zero or as high as the maximum voltage rating."

    - Franc Zabkar
  12. Ray King

    Ray King Guest


    If you use a comparator or opamp output into the gate of a mosfet. ( do not
    forget the 10k or higher series gate resistor).
    Add a small value ( compared to the resistance range you are looking for )
    resistor in the source of the fet. The negative comparator input terminal to
    the source of the fet and the positive terminal to a reference voltage. The
    variable resistance will be across the series connection of the fet and
    source resistor.

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