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Why are transparent trimpots so rare?

Discussion in 'Electronic Components' started by Joerg, Dec 10, 2006.

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  1. Joerg

    Joerg Guest

    While stuffing a BOM with Mouser part numbers for a prototype order I
    found that pretty much all the multi-turn trim-pots were in stock.
    Except for the transparent ones. None, zip. Arrgh.

    How can that be? I was wondering why anyone would want to use a
    non-transparent trim-pot where you cannot see the position it is in. So
    the demand for transparent ones should be a lot higher.
     
  2. Jim Thompson

    Jim Thompson Guest

    "So rare. So rare."

    Wasn't that a top ten tune hit from the late '50's ?:)

    ...Jim Thompson
     
  3. Duuno, but I never figured out why US manufacturers could get away
    with shipping trimpots apparently set at random from one end to the
    other. The Japanese ones were always set very close to the middle
    right from the factory. If your design was good the pot would hardly
    have to move, since most of the time the variations will (mostly)
    cancel out. Also helps identify outliers in case something else is
    wrong.


    Best regards,
    Spehro Pefhany
     
  4. Bob Eld

    Bob Eld Guest

    Most designs adjust the pots to some measured parameter, voltage, current,
    time, pulse width, amplitude, etc. If the physical position is important,
    why do you need a trim pot? Just use a fixed resistor of the corresponding
    value.
     
  5. If you need a fixed gas pressure in a pipe and have a regulator, does this
    mean you don't need a gauge?

    The idea of seeing the position of a multiturn pot IS a good one, have you
    ever tried tweaking a device when direct physical feedback is worth more
    than the hassle of attaching some kind of metering to do it? If not, then
    you haven't done much tweaking.
     
  6. John  Larkin

    John Larkin Guest

    Transparent, unfilled plastic melts too easily. We've had that same
    problem with surface-mount LEDs; they tend to fail when reflowed, and
    if you try to replace one, your tweezers just squish them flat when
    you try to solder them.

    The surfmount led's we use now are either on a ceramic substrate, or
    have a clear plastic inner region and an opaque overmold that can take
    the heat. Osram has some great side-firing surface-mount led's that
    are rugged, have great optics, and come in all colors.

    But why use multiturn pots? You can see the position of single-turns,
    and they have about the same working resolution and maybe even better
    stability.

    Last few boards we've done had no pots at all; everything is in serial
    eeprom or flash. Well, I did use some pots to calibrate gains in a
    couple of optical/electrical converters. It's surprising what
    bandwidth you can use a pot at.

    John
     
  7. John Fields

    John Fields Guest

    ---
    Yup.

    Here's a link to a sample:

    http://www.cleverocity.ca/sorare.htm

    Andy Williams also did it, but I don't know when.

    From another web site, but I lost the link:


    "Words ands Music by John Rufus Sharpe and Jerry Herst

    Charted in 1937 by Gus Arnheim (# 2) and Guy Lombardo (# 3) and a #
    2 hit for Jimmy Dorsey in 1957. Andy williams also did it, but I
    don't know when.


    (So rare, so rare)
    (So rare, so rare)

    So rare, you're like the fragrance of blossoms fair
    Sweet as a breath of air fresh with the morning dew
    So rare, you're like the sparkle of old champagne
    Orchids in cellophane couldn't compare to you

    You are perfection, you're my idea
    Of angels singin' the Ave Maria
    Or you're an angel, I'd breathe and live you
    With every beat of the heart that I give you

    So rare, this is a heaven on earth we share
    Caring the way we care, ours is a love so rare

    (So rare, so rare)
    (So rare, so rare)

    (You are perfection, you're my idea)
    (Of angels singin' the Ave Maria)
    Or you're an angel, I'd breathe and live you
    With every beat of the heart that I give you

    So rare, (so rare) this is the heaven on earth we share (so rare)
    Caring the way we care (so rare) ours is a love so rare

    (So rare, so rare)
    (So rare, so rare)"
     
  8. There are several clear epoxies that could be used. The expense might be
    higher, needing a two-part moulding that has to be cemented together, but I
    think the current construction of multiturn pots isn't much different.
     
  9. If you trim the current of a DPSS laser pump diode, the last thing you want
    is some sticky one-turn pot to jerk through the max current point and fry
    the diode. I've used both types of preset, and I find it's worth using
    multiturn types even where single turn types might do, they work much
    better when setting accurately. I take the point about stability, but I
    don't think it's any better. Even single-turn pots have spring tension that
    can upset it after you leave it. With a multiturn you can turn back a
    fraction to release this, there is enough hysteresis there to allow it
    safely. Try doing that with a signle-turn pot, and you'll be tweaking for
    half an hour trying to repeat the same value, let alone make it stay there.
     
  10. Bob Eld

    Bob Eld Guest

    there.

    So, do you set the current in the laser diode by the position of the pot
    wiper or do you actually measure something? Something tells me you don't use
    the wiper position as an indicator of the diode current.
     
  11. Joerg

    Joerg Guest

    I still had to have my diapers changed back then :)

    BTW, Jim, when I answered your 2nd email on Friday it came back with
    some kind of spam refusal message. Just wanted to let you know since
    that might also happen with client email.
     
  12. Joerg

    Joerg Guest

    This is for prototyping and we need some fine adjustment capability for
    experiments. It's a one-off board, basically to see whether the concept
    I came up with is feasible and recovers nicely and fast enough when
    kicked in the shins. My designs for production never contain pots or
    variable caps either (we used to call that "rubber engineering" in the
    80's).
     
  13. No, because I can't see it. Seriously, the Chinese DPSS lasers often have
    IC names ground off and reverse engineering is made to be difficult, so
    instead of measuring an exact current when you don't know what it should
    be, you just note the current position and the number and size and
    direction of steps taken in turning. A lot of laser adjustments are made
    this way, being optical so the habit forms.

    The point could be debated beyond inanity but there's no denying that basic
    visual feedback of position of a moving object is a sensible requirement.
     
  14. Joerg

    Joerg Guest


    T'is excactly the situation here. A laser diode where the delta between
    smooth sailing and a bzzzt-poof to the tune of a few hundred Dollars is
    rather small.
     
  15. Jim Thompson

    Jim Thompson Guest

    I caught it right away. I run a self-checking loop because of all the
    problems I've had with Cox.

    <Begin Personal Opinion>

    Cox is such a bunch of mean-spirited "Cox-suckers" they hung a spam
    block on my SMTP relay (operated by GoDaddy).

    <End Personal Opinion>

    But GoDaddy is owned by Bob Parsons. He'll kick Cox' ass if they do
    that again.

    ...Jim Thompson
     
  16. Jim Thompson

    Jim Thompson Guest

    I was already "testing" the back seats of cars ;-)

    ...Jim Thompson
     
  17. I think that is rather strange.
    I remember Tek scopes with many many many (one turn) trimpots, form
    after the eighties (lets say nineties), also the digital storage ones.
    I also remember prof video equipment with even more.
    Because analog _has_ tolerances, trimpots serve a good purpose.
    To make it a religion to avoid these at all cost is bad practice in
    my view.
     
  18. Joerg

    Joerg Guest

    Can you dump them? I don't know who your telco provider is but good old
    Ma Bell has been pretty good to me.

    It can be worse: About a month or so ago I had all my emails to
    t-online.de recipients bounce. Then I began to receive some intervention
    requests from biz friends over in Germany. They didn't get reponses from
    US vendors. T-Online is a major ISP in Germany. Turned out they had
    verbatim blocked a whole SBC server and that cut the guys over there
    off. To add insult to injury a customer rep of this ISP "service" was
    quite cocky about it IMHO. If I had to run a biz over there I certainly
    would know which service not to use.

    And I am glad I still have a fax :)
     
  19. Joerg

    Joerg Guest


    Amen! It is somehow a good thing to come back from lunch and be able to
    verify that nobody had turned the trim-pot.
     
  20. Jim Thompson

    Jim Thompson Guest

    Cox is the only cable provider here. Qwest DSL is slo-o-o-ow by
    comparison.
    This wave of anti-spam hysteria is almost as bad as the global-warming
    hysteria.

    Anyone complains the ISP shuts you off... so malicious crap is
    happening... and I have enemies galore... leftist weenies can't cope
    with feeling inadequate ;-)

    But after a few law suits it'll stop ;-)

    ...Jim Thompson
     
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