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Largest 7-seg LED displays?

Discussion in 'Electronic Basics' started by DaveC, Nov 30, 2003.

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  1. Roger Gt

    Roger Gt Guest

    OKAY... Put together a RFQ including and ---
    How many?
    what size display?
    Voltage or current?
    Provisions for how many ranges?
    Resolution?
    Environmental considerations?
    Operating voltage?
    Isolation requirements?

    Would a Blue white display behind a polarized panel be OKAY?
    OR Color preferred?
    LED or?


    Send to:

    They will quote in USD. From what you suggest here, Less than $862 USD How
    much? Fill in the data!
     
  2. I doubt that, because I have seen a great deal of 4", 6", and even 8"
    panel meters on the market. In fact, your local repair garage likely
    has quite a few of these on their diagostic and wheel alignment
    systems.

    Perhaps you don't really mean a "panel meter"?

    Harry C.
     
  3. How about a small one with a big fresnel lens in front of it. Does it need
    to be seen at angles?

    Regards,
    Bob Monsen
     
  4. DaveC

    DaveC Guest

    Hey, Bob. Creative answer. But yes, the display needs to be seen clearly at a
    minimum of 45 degrees off-axis, more if possible.
     
  5. Guy Macon

    Guy Macon Guest

    "Unlike incandescent or fluorescent lamps, neon tubes can be
    switched on and off continually, or dimmed, without affecting
    the lifetime."

    http://www.signweb.com/neon/cont/neon101b.htm

    That sounds like it would work.

    "There is such a thing as a cold-cathode fluorescent lamp.
    They resemble "neon" signs, but are generally of slightly
    greater diameter. They are not too standard nor widely
    available. They are also typically a bit less efficient
    than hot-cathode lamps. However, they can be dimmed to
    any degree without any chance of damaging them or causing
    any excessive wear. It should also be noted that starting
    these frequently does not cause any excessive wear. (If
    excessive current flows during the first half-cycle of
    operation, a generally insignificant amount of extra
    wear occurs.)

    There are also miniature cold cathode fluorescent lamps,
    often used for backlighting LCD screens and in image
    scanners. (LCD screens sometimes use other means of
    lighting such as a white electroluminescent panel.)
    Like the large ones, miniature cold cathode fluorescent
    lamps are dimmable.

    Many "neon" signs are actually a variation of cold-cathode
    fluorescent lamps!

    There are dimmable, electrodeless compact fluorescent
    lamps generally known as induction lamps. These are now
    available from electrical/lighting supply shops. These
    use an even different way to get electricity from metal
    to gas. These lamps work at a very high frequency,
    which lets current flow capacitively through the glass
    or use induction to get power from a coil to the mercury
    vapor discharge. No metal electrodes touch the mercury
    vapor discharge. These lamps should work at least
    reasonably well with ordinary light dimmers."

    http://members.misty.com/don/f-dim.html

    (This page also has some discussion about heating the
    filaments with a lower wattage. This might work for
    flashing as well.)
     
  6. Not nessesarily. If you use relays in place of old-fashioned
    starters, keeping them closed will keep the lamp off, and opening them
    will allow the lamp to start instantly. [Dunno what that does to lamp
    or ballast lifetimes, but flourescents _can_ switch quickly.]
     
  7. I think he wants digital panel meters with 2" high seven segment
    displays.
    --
    24 days!


    Michael A. Terrell
    Central Florida
     
  8. You can do it like some old scoreboards. Use a piece of thick red
    Plexiglas. Cut it into strips for each segment, paint the sides black,
    and drill holes in the back side for small lamps like the old 1847, or
    the newer automotive side marker bulbs that push in. If you use four or
    five lamps you can loose a lamp or two and still see the segment.
    --
    24 days!


    Michael A. Terrell
    Central Florida
     
  9. Tim Jackson

    Tim Jackson Guest

    To clarify. Earlier this year I was looking for industrial wall-mounting
    digital meters to bolt on top of the machines that could be read at up to
    about 20m, say from the production office or anywhere on the shop floor, and
    would indicate the current value of a 0-10V process signal. I had in mind
    multi-LED seven segment displays. Sure, wheel balancers, car park counters
    and such, incorporate suitable displays, but they are built in, I can't bolt
    them to the factory wall or connect them to the process. I was looking for
    something installable by the resident electrician - a contractors bill for
    panel building or interfacing would push the cost back into the stratosphere
    (where accountants can't breathe).


    Tim
     
  10. Roger Gt

    Roger Gt Guest

    The lamps would work, but not always start reliably, since the off times
    would vary and the plasma path would cool at different rates. The ballasts
    would be stressed, being current limited by inductance (assumption - not
    electronic) but would run at full power all the time, thus reducing or at
    least not improving life expectancy. If you were displaying data which
    changed in minutes, the fluorescents would work just fine.... Readable for
    a great distant.
     
  11. Roger Gt

    Roger Gt Guest

    Actually any florescent can be used as a CCFL by providing much higher
    starting voltages. Even so called burnt out tubes will light, although at
    that late in life the output is much lower. The trick is to use them on
    HVDC. The color changes too, so I haven't seen it done much!
     
  12. Some years ago, I designed some with 3" displays but we found the
    marketability to not be all that great. It's one of those things that
    a lot of people *say* they want but when you go and make one, and
    figure out how much it costs, many lose interest. There are up to
    200mm (8") displays available, but the MOQ $, power required, PCB cost
    and the housing cost goes up pretty fast at those sizes (with the
    square of the height). Of course if you're actually into making
    scoreboards..

    Best regards,
    Spehro Pefhany
     
  13. Guy Macon

    Guy Macon Guest

    I wasn't aware of that. Thanks!
     
  14. There is no "off" time, the tubes are either "ON" or in "Start" mode,
    where the current through the filaments in the ends keeps them warm.
    This is the old-fashioned 'short the ends to start the tubes'
    paradigm, where the starter is a separate device....
     
  15. Ian Stirling

    Ian Stirling Guest

    What about cold cathode tubes?
    I'm expecting delivery tomorrow of 2 CCFL tubes+drivers
    (http://www.ebuyer.com/) for around $8(us) total.
     
  16. Tim Jackson

    Tim Jackson Guest

    I am pretty sure that what erodes the electrodes is ionic sputtering. This
    is basically cratering of the cathode by impact of positive ions accelerated
    by the end-to-end electric field. This process, and electron impacts at the
    anode, are what keeps the electrodes hot when the heater turns off, but the
    heavier positive ions do more damage (same energy (= e * electric field *
    mean free path) but more momentum). I used to know all this stuff in the
    days of valves (vacuum tubes) but I haven't revisited it for decades. In
    high school at every opportunity I played with vacuum pumps and electrical
    discharges.

    Of course both the electrodes are alternately cathode and anode when using
    AC supply, so both wear equally.

    So to keep it running you need to supply a reduced heater current during
    'off' to maintain the same heat input as the ion flow normally produces.
    The rate of electrode ablation will certainly be reduced in the absence of
    plasma current, not increased, so there is no abnormal ageing problem.

    If the gas wouldn't conduct straight away then how would the lamp normally
    start? We heat the electrodes to get electron emission, then apply working
    voltage and "hey presto" it conducts and lights. With hot electrodes you
    don't need any HV striking pulse like say a sodium lamp - try it, it fires
    just like a neon lamp. Inverter driven fluorescents like for caravans don't
    have any circuit for an HV pulse, they just fizz a bit then get brighter as
    the electrodes heat up.

    The gas doesn't have to be ionised to 'conduct' the electrons, they just
    ionise it in passing, and while the gas is being ionised, it glows in the UV
    as the battered atoms recombine and return to ground state. When the
    current ceases the glow dies pretty instantly on a millisecond timescale, so
    the excited population must have collapsed in that time. Most of the light
    decay time ( a few ms) comes from the phosphor - the lower energy (visible
    light) excitation states are longer lived.

    You need a lot more current density for ionisation to make a significant
    difference to conductivity. Think arc welder.

    What we don't get at cold start with any metal-vapour tubes is full vapour
    pressure, so we don't get full brightness for a while, and presumably a
    somewhat higher striking voltage. Brightness might be a problem for the
    7-segment display. You'd have to keep the tubes in a warmed enclosure to
    get around it. Or use noble gas (eg neon) tubes like someone said.


    Tim Jackson
     
  17. Guy Macon

    Guy Macon Guest

    Excellent analysis, and much more accurate than my attempt was. Thanks!
     
  18. Graham W

    Graham W Guest

    Then how about a CRT with a purpose buillt driver? Even works in
    the dark!
     
  19. Tim Jackson

    Tim Jackson Guest

    Tried that. As manufacturer they would only quote for an MOQ of 50 units.
    As and end user we only need a few. Hence the remark "off the shelf".
    There's the rub.


    Tim
     
  20. Tim Jackson

    Tim Jackson Guest

    There's a thought.

    It would be cheaper than some of the options proposed just to buy a
    last-year's-model PC on the surplus market and program it to display the
    digits.

    Not exactly the most reliable or energy efficient solution though.


    Tim Jackson
     
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