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Isolated variable resistor function?

Discussion in 'Electronic Design' started by Joerg, Aug 25, 2012.

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  1. Joerg

    Joerg Guest

    Hi Folks,

    Does anyone know a part that is small, can be used to command a certain
    resistance and can be talked to across the usual mains-isolated barrier
    of office equipment and the like? I need to set resistor values on the
    mains side from a uC on the low voltage side. Has to be:

    Linearity error from 0V to about 1V excitation should be <5%.
    Absolute resistance tolerance <20% if possible.
    Bandwidth for resistor value changes up to a few kHz.
    No limit on the number of changes (meaning no EEPROM).
    Addressable across isolation barrier.

    So far I've considered:

    a. FET optocouplers such as the H11F1. A bit pricey at over $1 but most
    of all has the problem that the linearity goes to pots as soon as there
    is more than 20-30mV across the FET, plus it needs a ton of drive power
    for low resistance:

    b. Digital potmeters such as the MCP4011 series. These are nice, tons of
    bandwidth, cheap and small. But they do need an additional two-channel
    optocoupler to get the command data across and the absolute resistance
    value tolerance is borderline at 20%:

    c. Servoed FET plus opto. The servo gets rid of the non-linearity. Done
    it before but this gets old because it needs a lot of parts and real estate.

    d. LDR plus LED: Very nice, but fairly large and most of all LDRs are
    banned in some countries on account of their Cadmium content.

    Any other miracle parts out there?
  2. Guest

    Does it have to be four-quadrant? Floating? There are some cute fake resistor
    circuits around but the ones I've tried using need one end of the pot at
    "ground". That and an isolated DAC (DAC on the far side of an isolator, even)
    will solve your tolerance problem. You probably won't like the cost, though.
  3. Joerg

    Joerg Guest

    Would be nice but the part where the resulting signal goes into is
    blazingly fast (has to be). So the PWM would cause an undesired modulation.

    But it is an idea, maybe I can slow something down in there. And go in
    with several meggeehoitzes. The uC we have would moan and groan though.
  4. Joerg

    Joerg Guest

    That's similar to what John suggested. Thing is, this variable resistor
    is going to be the bottom resistor of a voltage divider and has to be
    varied between zero and 10k or so.
  5. Joerg

    Joerg Guest

    Actually single quadrant is ok and one side of it can be grounded. Cost
    is a factor but not like in most of my other design. Meaning a buck is
    acceptable but two bucks would be a stretch.
  6. Joerg

    Joerg Guest

    You really live your PICs :)

    I don't need a control voltage but a control resistance. That's the
    challenge. Getting a voltage across is way easier: You'd just send PWM
    out of the uC that we already have, onto an optocoupler, and then
    RC-filter this into a DC voltage one the other side. Could be done for
    <30c. Of course they'll throw coffee mugs and paper wads at me because I
    would have to file a request for the 2nd timer in there. Analog guys
    always want the timers. Well, those plus beer and marzipan.
  7. Guest

    How good of a resistor do you need? Is an opamp fast enough?
  8. Joerg

    Joerg Guest

    Yes, it is fast enough. But it has to behave like a real resistor, up to
    several kHz.
  9. Joerg

    Joerg Guest

    That would require shipping a trained mouse with every unit. Too
    expensive because of the food, veterinary bills and so on.
  10. Joerg

    Joerg Guest

  11. Jon Kirwan

    Jon Kirwan Guest

    Some of the MSP430's have Timer D, which will run PWM at
    256MHz -- if I read correctly.

  12. Jon Kirwan

    Jon Kirwan Guest

    I should have said "clocked at 256MHz," but you already know
    that the count per period would divide this down for the PWM
    frequency. Sorry about not being precise in writing that.

  13. Joerg

    Joerg Guest

    So have I, but tough to achieve a sustained 5% linearity over production
    runs. I've done it in servo fashion but with all that you quickly reach
    a point where a 30-40c digital potmeter is simply the better solution.
  14. Joerg

    Joerg Guest

    The uC has to do the twiddling.
  15. Joerg

    Joerg Guest

    A MSP430 clocked at 256MHz? Shazam! That sounds like a Suzuki Alto with
    a Hayabusa engine in there.
  16. Guest

    Why is linearity an issue (feedback, ya' know)? Drive it with a good (5%
    isn't "good") multiplying DAC.
  17. Joerg

    Joerg Guest

    Well, of course not while at 10k :)
  18. Joerg

    Joerg Guest

    The signal going into this divider can't be distorted more than 5% and
    with a FET-optocoupler it would be.

    Sure, but then I might as well just use my solution "b", a digital
    potmeter. Reduces that function to one part costing 30-40c, outside the
    optocoupler to xfer the data lines which I'd need either way.
  19. Jon Kirwan

    Jon Kirwan Guest

    Hehe. I was just needing a faster PWM (wanted to use a very
    cheap RC, 20db/decade rolloff, low-pass, but needed an audio
    spectrum. Couldn't get there with 1/62500 PWM period of
    standard MSP430. So I looked and... cripes!! They had
    something else!

    "Timer D also includes a dual capture mode reducing loading
    of capture operation by half. In addition, the combining
    compare blocks help to control both rising and falling edges
    of the PWMoutput signal. The Timer-Event-Control-block offers
    external triggering options as well as internal
    synchronization of timer instances."

    See the MSP430F51x2 parts.

  20. Joerg

    Joerg Guest

    In this case several MHz. In the world of switch mode power supplies
    that counts as "blazingly fast" :)
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